siprotec - optimum motor protection en

Contents Page

Introduction to thePrinciples of Asynchronousand Synchronous Motors 3

Overview of ProtectionFunctions and ProtectionRelays for Motors 11

Thermal Stress ofMotors and NecessaryProtection Functions 17

Motor Protection:Requirements forCurrent Transformers 25

Protection ofLow-Power Motors 31

Protection ofMedium-Power Motors 43

Protection ofHigh-Power Asynchronous Motors 51

Protection ofSynchronous Motors 63

Synchronization and Protectionof Synchronous Motorswith the 7UM62 69

Appendix 73

Optimum Motor ProtectionwithSIPROTEC Protection Relays

© Siemens AG 2006

Siemens PTD EA · Optimum Motor Protection with SIPROTEC Protection Relays 3

Introduction

This chapter is an introduction to three-phasemotors. On the basis of fundamental physicalrelationships and their structure, it is particularlythe characteristic operating modes of motorswhich are discussed. Essential mathematical rela-tionships, as are needed when deriving character-istic quantities of motors or for protectionsettings, are also presented.

�1. Basic principle of the three-phase machineElectric machines convert electrical energy intomechanical energy. The energy conversion proc-ess is based on the interaction of magnetic fieldsand windings. In its electrically active part, a ro-tary electric machine consists of a stator with arotating field winding and the rotor. The rotor isconnected to the shaft, through which mechanicalenergy is dissipated (in the case of the motor). Arotating magnetic field (rotating field) is the fun-damental prerequisite for the operation of three-phase machines. This is generated by connecting athree-phase winding to a symmetrical three-phasesystem. For a sinusoidal profile of the rotatingfield, the three phases must be placed at a 120°offset and there must be a 120° offset between theindividual phase currents. The rotating field’s ro-tation speed is called the synchronous speed nsyn

and is defined by the machine’s number of polesand the frequency of the supplying power system.The following applies:

nsyn = f . 60/p [rpm]

where: p = number of pole pairs

Operating principle of the asynchronous machineA voltage is induced in the rotor’s conductors ifthere is a difference between the rotation speed ofthe rotating field and the rotor (induction law). Ifthe conductors are part of a closed winding, theinduced voltage produces a flow of current. Thisleads to tangential forces according to the forceeffect of a current-carrying conductor in the mag-netic field. The sum of all tangential forces gener-ates a resulting torque over the rotor’s radiusacting as a fulcrum.

According to Lenz’s Rule, the force effect betweencurrents and the rotating field is oriented suchthat it counteracts the cause of induction. If therotating field of the machine still at standstill runsbeyond the rotor, it begins to turn in the rotatingfield’s direction to reduce the relative speed be-

tween the rotating field and the rotor. The fre-quency (speed) of the rotor can never reach therotating frequency of the stator, because a voltageis then no longer induced in the rotor and so theforce effect becomes zero. The deviation betweenthe frequency of the rotating field and the fre-quency of the rotor is referred to as slip s. Slip setsin so that a rotor current that is still just adequatefor the motor’s load arises.

sf f

f

n n

n= = =

−ω ωω

rf r

rf

rf r

rf

syn r

nsyn

– –(1)

ωrf angular frequency of the stator's rotatingfield ( rf = S)

ωr angular frequency of the rotorf frequencyn speed

The resulting speed of the motor (rotor) resultsfrom the number of pole pairs p (p =1 signifies 2poles, as shown in Fig. 1) and the slip.

nr = nsyn (1 – s) (2)

Introduction to the Principlesof Asynchronous andSynchronous Motors

Fig. 1 Principal components of the three-phase asynchronousmachine (U, V, W: conductor terminals UL1, UL2 and UL3)

Siemens PTD EA · Optimum Motor Protection with SIPROTEC Protection Relays4

Introduction

It was the imbalance between the frequencies ofthe rotating field and the rotor that led to thename asynchronous machine. Due to the voltageinduced, the term induction machine is fre-quently used, too.

Structure of an asynchronous machineFig. 1 shows, in a longitudinal section and across-section, the principal components of athree-phase asynchronous machine with a slip-ring rotor. The number of pole pairs is p = 1.

StatorThe stator core secured on the housing accom-modates the isolated three-phase stator windingin mostly half-closed slots. The stator core con-sists of mutually isolated electrical steel sheets.Lamination of the stator and rotor is necessary tominimize the eddy current losses arising due tothe alternating magnetic fields.

The three phases of the stator winding are con-nected in either star or delta form. Therefore it ispossible to operate the machines with differentrated voltages. Thus, machines in a star connec-tion operated at 400 V can also be operated at230 V with unchanged power output by changingover to a delta connection. Star-delta starting isalso possible if machines are operated at ratedvoltage in a delta connection.

RotorThe three-phase rotor core, which is also isolated,is housed in half or fully closed slots of the rotorcore. The ends of the rotor winding phases areconnected in a star point. The starts of the wind-ings are routed to sliprings so that the windingcan be shorted directly or with the aid of resistors.

Contrary to the slipring rotor, the winding of thesquirrel-cage motor consists of simple conductorbars without additional insulation. These conduc-tor bars are distributed concentrically around theshaft on the rotor’s circumference and are shortedby rings at the face ends of the rotor core. Theresulting winding cage led to the name of squirrel-cage rotor. The starting response of the motor(see also Section 3) can be influenced by specialcross-sectional shapes of the rotor bars (see Fig. 2)or by using two cages (double squirrel-cage rotor).

With a view to minimizing the magnetizationcurrent as far as possible, the air gap between thestator and rotor cores must be kept very small. Asall slot openings along the rotor or stator circum-ference act like an enlargement of the air gap andlead to distortions of the air gap’s field (voltagedistortions and additional losses), the slots aregenerally half-closed, to some extent entirelyclosed and only designed as open slots in the caseof high-voltage machines.

Operating principle of the synchronous machineIn the case of the synchronous machine, therotating field and rotor frequencies are identicaland so the slip is equal to zero. Both rotatingfields run synchronously. As already explainedabove, the principle of the induction machinetherefore fails. The magnetic field required forenergy turnover is generated with the aid of anexcitation winding. In the case of electrically ex-cited synchronous machines, an adjustable directcurrent is needed for the excitation winding. Thisis supplied with the aid of an excitation system.In the case of brushless excitation, as takes placeespecially in large synchronous machines, theexcitation power is fed by means of an excitationmachine fitted on the motor’s shaft and with asynchronously rotating three-phase bridge circuit.This directly generates the excitation voltage.As an alternative, the excitation power can alsobe fed via sliprings.

One positive characteristic of the synchronousmachine is the possibility of synchronous com-pensator operation. A reactive power can begenerated in a required form, thus improving apower system’s power factor (cos ϕ).

Fig. 2 Bar shapes of a squirrel-cage rotora) Round bar rotorb) Squirrel-cage rotor with parallel-flanked teeth

for cast cagesc) Deep-bar rotor with rectangular barsd) Deep-bar rotor with tapered barse) Double squirrel-cage rotor with round barsf) Double squirrel-cage rotor with parallel-flanked

teeth for cast cages


Introduction

�2. Equivalent circuit diagram of theasynchronous machine and mathematicalrelationships

Essential characteristic quantities are to be de-rived with reference to the example of the equiva-lent circuit diagram. We can find analogies to thetransformer, and so the asynchronous motor’sequivalent circuit diagram (see Fig. 3) can bederived from that of the transformer. The rotorvariables are converted to the variables of thestator via magnetic coupling. Similarly to thetransformer, this is done with the square of thenumber of turns per unit length of the rotor andstator windings (see equation (3)). To simplifymatters, the iron losses are ignored.

RW

WR'r

S

rr= ⎛

⎝⎜

⎞⎠⎟

2

(3)

It becomes clear that the effective rotor resistanceis low during starting (s = 1). The high startingcurrent with a low cosϕ is the result of this. Dur-ing normal operation, at s� 0 the effective resist-ance is high and the cosϕ approaches the value 1.

The necessary mathematical relationships areshown below. Equation (4) shows the stator cur-rent as the sum of the rotor and magnetizingcurrents (Im). If we neglect the idle current, thestator current is approximately equal to thetransformed rotor current.

I S m rm

rf m

m

rrf r

S

Sr

S

= − = −+

≈

+ + +

I IV

j L

VR

sj L

V

RR

sj X X

''

'

'

ω ω

( )'r

(4)

The power factor results from the real portion ofthe rotor current in equation (4) and leads to therelationship in equation (5).

( )cos

'

''

ϕ r

r

rr

=⎛⎝⎜ ⎞

⎠⎟ +

R

s

R

sX

22

(5)

A further characteristic quantity is the machine’storque, which can be derived from the power bal-ance. The mechanical power given off Pmech is theresult of the difference of the electrical effectivepower Ps supplied minus the losses in the statorPloss-s and rotor Ploss-r of the machine.

P P P Pmech s loss-s loss-r= − − (6)

The following applies if we neglect the statorlosses and introduce the air gap power Pσ:

P P Pmech loss-r= −σ (7)

The mechanical power can be expressed with theaid of the torque M and the mechanical angularvelocity ωmech of the rotor.

( )P M Mp

smech mechrf= = −ω ω

1 (8)

By analogy, the torque can be calculated fromboth the air gap power and the rotor’s dissipatedpower using the following equation.

MP

p= σ

ω rf

or MP p

s= loss-r

rfω(9)

After neglecting the stator losses, we can calculatethe rotating field’s power from the stator voltage(phase voltage) and the real portion of the statorcurrent which, for a three-phase machine, resultsin the following:

( )P U e Iσ = ⋅3 S SR

where

( )( )

Re S

Sr

Sr

S r

IU

R

RR

X X

=⋅

+⎛⎝⎜ ⎞

⎠⎟ + +

'

''

s

s

22

(10)

Inserted in equation (9), equation (10) leads tothe final relationship for the machine’s torque

( )M =

+⎛⎝⎜ ⎞

⎠⎟ + +

=

+

3

3 1

22

pUR

s

RR

sX X

pU

R

S2

rf

r

Sr

S r

S2

rf S2

ω

ω

'

''

( )X X

Rs

R

sRS r

r

rS

++ +

'

'

'2

2

(11)

By derivation of the denominator of equation(11), which is set to zero, we can calculate the slipat which the torque reaches the extreme value.This slip is referred to as the pull-out slip spull.Consequently, the torque developing with thisslip is the pull-out torque.

Fig. 3 Equivalent circuit diagram of the asynchronousmotor


Introduction

( )s

R

R X Xpull

r

S2

S r

=+ +

'

'2

(12)

( )M

pU

R X X Rpull

S2

rfS2

S r S

=+ + +

3

2

12ω '

(13)

If we relate equations (11) and (13) and if we ne-glect the ohmic stator resistance, we arrive at theso-called “Kloss relationship”. This formula(equation (14)) describes the torque-slip charac-teristics for the entire slip range (-∝ < s < +∝).The relationship for the pull-out slip is also sim-plified.

M

M s

s

s

spull

pull

pull=

+

2where s

R

X Xpull

r

S r

'=+ '

(14)

Thus, we have derived essential mathematical re-lationships for the operating variables of the asyn-chronous motor. These help when interpretingthe curves and data supplied by the manufacturerof the motor. In quality terms, Fig. 4 shows thecourse of the characteristic quantities. The typicalcharacteristics are also plotted.

Equation (4) shows that the starting current de-pends on the applied voltage and particularly onthe resistance of the rotor. The rotor’s equivalentresistance is designed low so as to keep losses inthe rotor likewise low. This leads to relatively highstarting currents, which can certainly reach valuesof five to eight times the rated current. As demon-strated in equation (11), the low equivalent resist-ance of the rotor limits the starting torque.

There are limits to reducing the starting currentby decreasing the stator voltage. Equation (11)clearly shows the square dependence of the torqueon the applied stator voltage (M ∼ V2), whichleads to a drastic reduction in the starting torque(half the terminal voltage leads to a quarter of theoriginal torque).

Increasing the rotor resistance by means of start-ing resistors (slipring rotor motor as shown inFig. 1) leads to reduction of the starting current.As a result, the cosϕ and thus the starting torqueincrease. During normal operation, the resistorsare shorted. The drawback of this solution is thegreater effort involved.

From the square voltage dependence of the torquewe can also derive that, as demonstrated in equa-tion (13), the pull-out torque is also clearly re-duced. Thus, when operating a motor with areduced voltage, we run the risk of the requiredmechanical torque exceeding the pull-out torque,resulting in a possibility that the motor will liter-ally stand still.

Fig. 4 Operating quantities of the asynchronous motoras a function of the rotor’s speed or the slip(Mst starting torque, Mpull pull-out torque; MN ratedtorque, npull pull-out speed, nsyn = nrf speed of thestator’s rotating field, spull pull-out slip, sN rated slip)


Introduction

�3. Startup of asynchronous motorsFrom the point of view of the power system, ap-propriate proof must be provided of reliable start-ing (from standstill) and restarting (from aparticular speed up to rated speed) in the event ofpossible residual magnetism of the magnetic field.The following impacts are possible if the startupconditions are not met:

– Extreme delaying of startup and thus high ther-mal stressing of the motors and the upstreampower system elements

– Standstill of the drive– Tripping of the motor protection and possibly

tripping of the upstream protection– Thermal stressing of all elements in the relevant

conducting path– Synchronism loss of motors in operation down

to standstill

As an extension of Fig. 4, Fig. 5 shows the enve-lope curve of the starting current. Similarly toactivation of an inductance, on activation ofthe motor with the slip s = 1 a transient inrushcurrent arises which then changes over to thequasi-stationary starting current. This current ini-tially drops very slowly and does not decrease fastto the normal operating current until near therated slip.

The starting factor (kst = 3 …6…10) is often spec-ified to characterize starting of machines. It essen-tially depends on the design (see Section 2 andFig. 2). The restart factor (kr-st = 1…3…10) de-

pends not only on the design, but considerablyalso on the speed and the still remaining magneticfield at the time of reactivation.

kststart

N

= I

Ior kr-st

restart

N

= I

I(15)

Fig. 6 shows the dependence of the restart factoron the slip by way of example with reference toasynchronous motors with different starting fac-tors.

For every drive, the starting or restarting condi-tion is met if, at any time, the motor’s torque M isgreater than the resistance torque Mr (opposingtorque of the machine, which is the sum of frictionmoments and other losses). A safety factor of 10 %is reckoned with. The required minimum voltagecan be determined by applying equation (16).This lies in the order of magnitude of Vmin =(0.55 … 0.7) VN, M.

V Vn

n

M

Mmin N,M

pull

N

N

pull

= 11.(16)

where:

VN,M rated motor voltageMN rated resistance torqueMpull pull-out torquenpull pull-out speednN rated speed

Restart factor kr-st as a function of slip and fordifferent starting factors kst

Envelope curve of the starting current


Introduction

An equivalent power system circuit, an exampleof which is shown in Fig. 7, is needed to calculatethe terminal voltages.

The equivalent power system impedance ZN iscalculated on the basis of the short-circuit power,the transformer impedance ZT from the trans-former’s rated data, the cable impedance Zcfrom the specific cable parameters and themotor impedance ZM from the starting factor kst(see equation 17).

( )Z ZM M A= +cos sinϕ ϕΑ j(17)

where ZV

SM

N,M2

st N,Mk=

where:

VN,M rated motor voltageSN,M rated motor apparent power

For high-voltage motors, a phase angle of 90° canbe used for calculation to simplify matters, so thatZM ≈ j XM is set.

�4. Operating modes of asynchronous motorsThe mechanical and thermal stresses depend onthe load state of motors. During continuous opera-tion, the temperature rise must not exceed thepermissible overtemperature limit for the insula-tion system used. It is obvious that a motor maybe subjected to a higher load for a certain periodof time without the winding overtemperatureexceeding its permissible value. After that, it isimperative to take a break for cooling downbefore the motor is loaded again. In IEC 60034,such operating cases have been standardized inan idealized form as so-called duty types S1 toS10.

S1 Continuous running dutyS2 Short-time duty (e.g. S2 60 min)S3 Intermittent periodic duty

(operation with similar loading cycles)S4 Intermittent periodic duty with startingS5 Intermittent periodic duty with electric

brakingS6 Continuous operation periodic duty

S7 Continuous operation periodic duty withelectric braking

S8 Continuous operation periodic duty withrelated load/speed changes

S9 Duty with non-periodic load/speedvariations

S10 Duty with discrete constant loads and speeds

The predominant duty type of motors is continu-ous duty (approximately 90 % of all applications).

Fig. 8 shows a selection of duty types. Besides theovertemperature curves, it also shows the lossesand the speed, with the result that the characteris-tics of the individual duty types can be derivedfrom the depictions alone.

�5.

Equation (18) describes the power for stable oper-ation of a synchronous three-phase motor.

P PEV

XV

X X

X Xmech synd

d q

d q

= = +−

33

222sin sinδ δ

where:

Pmech mechanical power outputPsyn active power consumed electricallyE rotor voltageV terminal voltage (phase voltage)Xd synchronous direct-axis reactanceXq synchronous quadrature-axis reactanceδ rotor angle

Equivalent power system circuit on activation of a motor

Losses Ploss, speed n and the overtemperatureϑ for the duty types S1, S2, S3 and S6(TB loading duration, TP pause duration, Tdc

duty cycle, Ti idle duration)


Introduction

If, in accordance with equation (18), the terminalvoltage or the excitation voltage is reduced at agiven constant mechanical power, the rotor angleδ increases until the motor falls out of step whenthe limit angle (δ > 90° at Xd = Xq) is exceeded.The motor falls out of step, no longer runs atsynchronous speed.

If the power system is undisturbed, this case isencountered whenever the excitation is too lowfor the required mechanical power or fails com-pletely. In this case we speak of “loss of synchro-nism due to underexcitation”. However, loss ofsynchronism can also occur as a result of a powersystem fault in which the terminal voltage dropsfor a short time or disappears entirely and thenreturns. When the voltage returns despite full ex-citation, the synchronizing torque cannot sufficeto re-establish synchronism of the motor. In thiscase we speak of “loss of synchronism due to apower system fault”.

We can distinguish between two possibilitieswhen a motor is not running synchronously:

1. Motor operation is stable

– with the excitation circuit closed like an asyn-chronous motor with two rotor windings (exci-tation and damper circuit) in the longitudinalaxis (double-cage characteristic), but only onerotor winding (damper circuit) in the trans-verse axis (single-cage characteristic)

– with the excitation circuit open like anasynchronous motor with one rotor winding(damper circuit) with a single-cagecharacteristic

2. The pull-out point for stable asynchronousrunning is exceeded and the motor is braked de-pending on the torque characteristic of the loadmachine. It slips and generally comes to a stand-still.

Equation (18) no longer applies to mathematicaltreatment of non-synchronous operation.Instead, the complete equation of motion must beapplied for an oscillatory machine.

Below, the response of motors to the two failuremechanisms is discussed on the basis of a dy-namic power system calculation program:

a) Loss of synchronism due to underexcitationIf the excitation voltage of a fully loaded synchro-nous motor is deactivated, synchronous runningis no longer guaranteed and the motor loses syn-chronism.

Fig. 9 shows the measured values in the admit-tance diagram. Due to the adequately high asyn-chronous pull-out torque, the motor is capable oflargely stable output of the required mechanicalpower as asynchronous power with low slip. Bycontrast, the differing reactance values in the lon-gitudinal and transverse axes result in consider-able oscillations of the conductance andsusceptance or the active and reactive power. Inthis specific case, the speed of the motor oscillatesaround an average of 0.99 nsyn with an amplitudeof 0.02 nsyn.

Oscillation is repeated cyclically and the pull-outlimit of the “asynchronous motor” is not ex-ceeded. In all cases, the susceptance remainshigher than 1/Xd.

Excitation failure (shorted excitation circuit) wasalso simulated on a second motor with other data(see Fig. 10).

Admittance in the event of excitation failure(2.22 MVA motor)

Fig. 10 Admittance in the event of excitation failure(9.88 MVA motor)


Introduction

In this case the asynchronous pull-out torque isexceeded and the motor comes to a standstill at aspeed of about dn/dt = 7 %/s as the slip rapidlyincreases. The rotor angle change rises from600°/s to ever increasing values.

Due to differing input reactance values in the lon-gitudinal and transverse axes, the admittancemoves almost in circles. The circle diameters be-come increasingly smaller because, in terms oftheir synchronous values, these reactance valuesdecrease towards their sub-transient values with in-creasing slip. The final point of this phenomenon isreached at a conductance of almost zero and asusceptance of approximately 2/(Xd" + Xq").

b) Loss of synchronism due to short-timepower system voltage failure

In the event of a three-phase power systemshort-circuit and a related breakdown of thepower system voltage, a synchronous motor canno longer obtain the mechanical power requiredby the load as electrical power from the powersystem. The lacking power is covered by the(negative) acceleration power. As this is propor-tional to the change in the rotor angle’s rate ofrise (d2δ/dt2), a fast rotor angle change is en-forced. In this case, the rotor angle is understoodto be the angle between the rotor voltage and thepower system voltage at an unchanging fre-quency. After short-circuit tripping and the re-lated return of voltage, either resynchronizationor a loss of synchronism can occur depending onthe motor constant, the mechanical load and theshort-circuit duration.

Fig. 11 shows the circle diagram, with the motorjust not having lost synchronism. Although thespeed drops to 5 % and the rotor angle has almostreached 180° in the time without voltage, the syn-chronizing torque after the voltage returns is highenough for the motor to resynchronize.

Extension of the short-circuit duration by 50 msleads to a greater speed drop (8 %) and the motorloses synchronism (see Fig. 12). Similarly to theunderexcitation shown in Fig. 10, the circle dia-gram shows circular motions around the point1/Xd". As the full excitation is effective, the circlediameters are accordingly larger. Here also, thesusceptance always remains higher than 1/Xd.

c) Conclusion

The examples clearly show that loss of synchro-nism of synchronous motors can be coped withvery well by means of underexcitation protectionwith an admittance characteristic (see 7UM61 or7UM62 manuals). Only one protection character-istic is needed, which must be set to 1/Xd. An ade-quate delay should be set to give the motors anopportunity to resynchronize. During simulationruns, this resynchronization took no more than0.5 s in the worst case of motor stressing at ratedload. Therefore, a tripping delay of 1 s is recom-mended for loss of synchronism.

As the motor terminal voltage changes onlyslightly due to the mostly low input reactance,reactive power monitoring can also be used as analternative.

References:[1] Weßnigk, K.-D.: Kraftwerkselektrotechnik.

VDE-Verlag GmbH, Berlin, Offenbach, 1993[2] Brendler, W.; Roseburg, D.: Elektrische

Maschinen. 4. Lehrbrief fürs Hochschul-fernstudium in der ehemaligen DDR, 1988

[3] Müller, G.: Grundlagen elektrischerMaschinen. VCH VerlagsgesellschaftmbH, Weinheim, 1994

[4] Clemens, H.; Rothe, K.: Schutztechnik inElektroenergiesystemen. Verlag TechnikGmbH, Berlin, 1991

[5] IEC 60034-1 – Rotating electrical machines.Part 1: Rating and performance, Release 2004

[6] Fischer, A.: Außertrittfall und Außer-trittfallschutz von Synchronmotoren.Etz Bd. 111 (1990) H. 6, S. 306 – 309

Fig. 11 Circle diagram of the admittance dueto a three-pole short-circuit lasting 0.15 s(2.22 MVA motor)1 – Initial value2 – Value during the short-circuit3 – Value after short-circuit tripping

Circle diagram of the admittance due to athree-pole short-circuit lasting 0.2 s (9.88 MVA motor)a) Power circle diagramsb) Admittance circle diagrams1 – Initial valus2 – Value at the start of the short-circuit3 – Value on short-circuit tripping4 – Value after short-circuit tripping


Protection Functions and Protection Relays

In this chapter, suitable protection functions arediscussed on the basis of possible faults in asyn-chronous and synchronous motors. An overviewfollows, showing how protection functions aredistributed over individual protection relays. Toconclude, the basic use of protection facilities forthe various motor types and power classes isdiscussed.

�1. Faults and protection functionsWith motors, various causes of faults are possible.Table 1 shows an overview of essential causes offaults and their influences. For both the stator andthe rotor, thermal overloading plays a centralrole. Such inadmissible stressing has a general in-fluence on the useful life of a motor and leads topremature aging. This, in turn, has a detrimentalinfluence on insulation capacity and simulta-neously increases the probability of follow-upfaults which can consist of earth faults and shortcircuits.

Cause Fault type

Thermaloverload

Interwindingfault(2-pole, 3-pole)

Earth fault Interturnfault

Mechanicaldestruction

Insulation aging � � �

Stationary and transientovervoltages

� � �

Undervoltage �

Asymmetrical voltage �

Conductor discontinuity,phase failure

�

Asynchronous changeover � �

Bearing damage � �

Mechanical overload duringcontinuous operation

�

Inadmissibly long starting time �

Rotor locking during starting �

Inadmissibly short pause time,too frequent reclosing

�

Soiling �

Inadmissibly high ambienttemperature

�

Cooling failure �

Overview of causes of faults and types of faults in asynchronous motors

Overview ofProtection Functions andProtection Relays for Motors



Fault Protection functions ANSI

Thermal overloading of the stator due toovercurrent, cooling problems, soiling, etc.

– Thermal overload protection with completememory

– Measurement of stator temperature bytemperature sensor (e.g. PT100)

49

Thermal overloading of the rotor during starting– Too long, rotor locked– Too frequent

Overload protection by two principles– Starting time supervision– Restart inhibit

4866, 49R

Voltage asymmetry, phase failure Negative-sequence protection(definite-time; thermal stage)

46

Overloading and thus overtemperature of thebearings

Measurement of bearing temperature bytemperature sensor (e.g. PT100)

38

Overstressing of drives running idle(pumps or compressors)

Minimum power consumption– Undercurrent I<– Active power P <

3732U

Earth fault – Earth-fault signaling via displacement voltage– Earth current measurement

(motor connected by short cables)– Earth-fault direction

59N51Ns

67Ns

Short circuits – Time-overcurrent protection– Differential protection

50, 5187M

Undervoltage Undervoltage protection(definite-time, inverse time)

27

Excitation failure in the case of synchronousmotors

Underexcitation protection 40

Asynchronous running of synchronous motors(falling out of step)

Underexcitation protection 40

Allocation of protection functions to types of faults

Causes of faults such as hidden manufacturingfaults, assembly errors (e.g. motor distortion orone-sided bearing pressure when connecting themachine) and inadequate maintenance are notlisted.

The task of protection consists of

– safeguarding the motor against destruction inthe event of a thermal overload and thusagainst a reduction of its useful life.

– sufficiently fast deactivation in the event of anyshort circuits, earth faults and interturn faults,both counteracting expansion of the damage tothe motor (destruction of the core assembly ormotor burning) and limiting the impact onother connected loads (voltage symmetry, volt-age drops and current overloads).

Table 2 lists the relationships between possiblefaults and protection functions that detect thesefaults.

An overcurrent protection that detects mechani-cal faults, above all in the drive machine, is notlisted. Here, for example, it is conceivable thatbulk goods in coal mills get stuck, induced draftducts clog up or mechanical damage occurs. Theresulting overcurrent inevitably leads to overload-ing of the motor and calls for tripping by theoverload protection. If heating time constants arelong, tripping is delayed accordingly.

To additionally reduce stress and strain on themotor, an overcurrent protection with a timerelement (approximately 1 s delay) is used in thecase of current clearly above the rated level (e.g.2 IN, M). This protection function is inactive dur-ing starting. If short circuits occur during normaloperation, this function also acts as short-circuitprotection in a clearly shorter time.

A detailed discussion of the individual protectionfunctions is dispensed with because these are dis-cussed in other chapters.



Protection function ANSI 7SJ602 7SJ61 7SJ62 7SJ63/64 7UM61 7UM62

Analog inputs 3I, Iee 3I, Iee 3I, Iee3V

3I, Iee3V/4V2TD/-

3I, Iee4V

6I, 2Iee4V3TD

Stator overload protection 49 � � � � � �

Starting time supervision,locked rotor protection

48 � � � � � �

Restart inhibit 66, 49R � � � � � �

Negative-sequence(unbalanced-load)protection I2 >

46 � � � � � �

Thermal unbalanced-loadprotection (I2

2 t)46 � �

Temperature monitoring(via RTD-box)

38 � 1) � � � � �

Undercurrent protection 37 � � � � � �

Active power protection (P <) 32U � �

Earth-fault protection

Non-directional

Directional

59N,

51Ns

67Ns�

� 2) �

�

�

�

�

�

�

�

�

�

�

�

�

Overcurrent protection 50, 51 � � � � � �

Current differential protection 87M �

Undervoltage protection 27 � � � �

Overvoltage protection 59 � � � �

Underexcitation protection 40 � �

Rotor earth-fault protection 64R � �

Frequency protection 81 � � � � �

Breaker failure protection 50BF � � � � � �

Freely programmablelogic (PLC/CFC)

� � � � �

Control function � � � � �

Graphics display � �

Flexible interfaces 1 2 2 2/3 2 3

Frequency operating range(11 Hz – 69 Hz)

� �

Operational measured-values � � � � � �

Event log and trip log � � � � � �

Oscillographic records � � � � � �

Protocols MODBUSPROFIBUS-DPIEC 60870-5-103

MODBUSPROFIBUS-DPDNP 3.0IEC 60870-5-103IEC 61850



MODBUSPROFIBUS-DPDNP 3.0IEC 60870-5-103


Table 3 Motor protection functions in the SIPROTEC protection relays(Iee – sensitive current input, TD – measuring transducer);Note on 7SJ602:

1) Depending on the order – not available if communication protocol needed2) Optional ordering also possible with Ve

�2. Scope of protection functions provided by the protection relays



Scope of protection functions provided by theprotection relaysIn particular, numerical protection is noted for itsmultiple functionality. Depending on the applica-tion, various protection functions can run on oneitem of standard hardware. Regardless of whethera cable, an overhead line, a generator or a motoris being protected, we are always dealing with thesame device type. This simplifies engineering bymeans of an identical amount of hardware. Appli-cations can be standardized and the need forspare parts can also be reduced.

In addition to choosing the hardware, users mustorder the appropriate combination of protectionfunctions.

Table 3 provides an overview of the functions formotor protection applications integrated in theprotection units and a selection of additionalfunctions is listed.

�3. Use of the protection relaysThe scope of protection to be applied dependsessentially on the rated power of the motor, itsmode of operation and its role and importance interms of the technological process linked to it. Itgoes without saying that costs also have to be con-sidered. Total costs should be considered because,to some extent, the costs of repairing a motor arerelatively low in comparison with the repair costsof other electrical equipment. In case of a failure,however, the follow-up costs for the productionsystem often amount to a multiple of the repaircosts.

The overview in Fig. 1 shows a breakdown ofprotection units according to power classes ofmotors and applies to asynchronous motors.The preferred types of protection relays are listedin the left-hand column.

If additional functions are required, for examplefreely programmable logic, more scope for serialcommunication, a wide frequency operatingrange and much more, refer to the units in theright-hand column. As the majority of protectionunits are installed directly in the medium-voltagecubicle, increased use is being made of controlfunction. Here, the relays with graphics displayoffer advantages in the form of illustration of afeeder with the switching devices. The conven-tional mimic diagram can be dispensed with.

Table 4 describes the advantages of the optionalrelays from Fig. 1.

Classification Options

Low(100 – 500 kW)

7SJ61: more functions(e.g. PLC/CFC), more communica-tion scope, and more binary I/Os

Medium(500 kW – 2 MW)

7SJ63/64: graphics display andthus better local control, scalablebinary I/O quantities; 7SJ64 hashigher PLC/CFC performance andone additional interface.

7UM61: larger volume of binaryI/Os, wider frequency range (reli-able protection functionality evenduring running down of a motor)

Explanation of the options



Fig. 1 Allocation of protection relays to motor power classes

The power of synchronous motors is generallyclearly higher than 2 MW. This is why it is imper-ative to provide differential protection.A SIPROTEC 7UM62 with the “Generator Basic”option is recommended as the protection unit.This option features underexcitation protectionto detect excitation problems (failure or regula-tion problems) and out-of-step cases (loss ofsynchronism).

You will find further notes on application, con-nection and setting of the protection relays in thefollowing chapters.

LSP2

001.

eps

LSP2

299.

eps

LSP2

171.

eps

LSP2

361.

eps

LSP2

299.

eps

LSP2

176.

eps

Preferred Options7SJ60 7SJ61

7SJ62 7SJ63, 64 7UM61

7UM62


Thermal Stress of Motors

Thermal stress of motors is discussed in this chap-ter. The various operating states are consideredand the thermal influence is derived from them.The second part contains discussions of suitableprotection principles and a basic explanation oftheir implementation in the numerical protectionunit. Taken from motor data sheets, notes on set-ting the protection functions are provided.

�1. Heating phenomenaIn the process of conversion from electrical tomechanical energy, losses occur which leads toequipment heating up, i.e. a motor. Their designpermits certain thermal stress.

Heating sets in when the heat sources become ef-fective. The fed energy is first of all stored in eachvolume element as heat. The initial curve of thetemperature rise ϑ = f(t) is independent of thethermal resistors through which the heat is laterdissipated. Moreover, it is in no way identicaleverywhere because different loss density valuesprevail in the various windings and also in theindividual segments of the magnetic circuit. Themotor can therefore generally not be looked uponas a homogeneous-body system.

Temperature differences develop between theparts of the motor and with respect to the sur-rounding coolant, and heat begins to flow in thedirection of the coolant. The temperature differ-ence between the coolant (which can also be air)and the machine part is called temperature rise (orovertemperature). After an adequately long time,the heat flows reach a state of equilibrium. Then,heat is no longer stored. The temperature hasreached its temperature-rise limit value. Fig. 1shows how the temperature rise (or overtempera-ture) develops for characteristic points within thestator of a motor.

The permissible temperature rise of an electricmachine is primarily limited by the relatively lowthermal resistance of the insulators for the wind-ings. The insulation systems are subdivided intotemperature classes A to H in relation to their ther-mal resistance. The permissible temperature riseof a winding at its hottest point differs from thepermissible temperature of the insulation systemused by the agreed maximum temperature of thecoolant (ambient temperature), which is definedas 40 °C. A safety factor of 5 to 15 K is generallyalso added.

Two modes of operation must be distinguishedwhen considering a motor from the point of viewof thermal influences.

a) Normal operation under loadDepending on the torque to be applied, the corre-sponding current is picked up from the powersystem which, depending on the design, leads tothe heat development described above. We canrefer mainly to the stator. The motor manufac-turer defines a machine thermally for the permis-sible temperature class. Operation under ratedconditions leads to a temperature rise that cangenerally be assigned to a lower temperatureclass. For example, if you find the letters F/B (de-signed/operated) in the motor data, the motor isdesigned for the temperature class F, but is usedin accordance with temperature class B. Thesethermal reserves results in a continuous overloadcapacity of about 10 %. A higher overload is pos-sible for a brief time, but the manufacturer doesnot specify any data because of the various influ-encing parameters. The thermal withstand curve(see Fig. 2) is to be referred to.

Thermal Stress ofMotors and NecessaryProtection Functions

Fig. 1 Temperature rise at three characteristic pointson the stator



The curves are specified for when the motor isboth cold and warm. Warm means that the motorhas been operated with rated quantities and anoverload then occurred. The graphic above showsthe worst case, the curve of the copper time con-stant for short-time duty. The time constant forcontinuous duty must be used for the protectionsetting. The continuously permissible current canalso be found in the characteristic. According toFig. 2 this is approximately 1.15 I/IN,M.

In the American (NEMA) region, the overload isdescribed by the service factor (SF), which speci-fies the permissible overload for the rated powerin the S1 mode. This overload refers to the overallsystem (electrical and mechanical parts). Accord-ing to NEMA MG1, the permissible limitovertemperatures of the relevant temperatureclass may be 10 K higher. The values for the ser-vice factor lie, for example, at 1.1, 1.15, etc. How-ever, if SF = 1 is specified, this means that themotor may only be operated at the rated power.Thermal overload protection refers only to themotor winding. If SF = 1 is specified, a continu-ously permissible winding overcurrent of 10 %can be assumed because the insulation is generallydesigned in accordance with temperature class Fand the motor is mainly operated in accordancewith temperature class B. If SF = 1.1, this may bemore than 10 % (e.g. quite possibly 15 %). Referto the thermal characteristic for the exact value ofthe continuously permissible overload current(see Fig. 2).

For detection of temperature rise, two tempera-ture sensors (preferable PT100) are usually in-stalled for each phase in the stator winding.Motor manufacturers have placed them at thethermally critical points. The tripping tempera-ture can be derived from the temperature class.F signifies a maximum permissible temperatureof 155 °C and for B the value lies at 130 °C. Thelatter temperature ought to set in approximatelywhen the motor is loaded with rated quantities.Minus a safety factor of 10 K, the tripping value isapproximately (155 °C -10 K = 145 °C).

The rotor additionally heats up when the motor isoperated on an asymmetrical voltage. The mostcritical case is discontinuity in a phase. Voltageasymmetry means that there is a negative-sequence voltage system that is driving a negative-sequence current. This negative-sequence rotatingfield leads to an AC current in the rotor (100 Hzrelative velocity) in the opposite direction of ro-tation. Due to the skin effect, a larger rotor re-sistance takes effect which, in turn, leads toincreased heating up. It can be assumed that1 % negative-sequence system voltage leads toabout 5 % to 6 % negative-sequence systemcurrent.

Fig. 2 Maximum permissible thermal overload of a motor

(Thermal copper time constant (short-time duty): 2.7 minThermal time constant (continuous duty (S1)): 15 minCooling time constant at standstill: 105 minTemperature class (designed/operated): F/B



The stator current can be estimated by way of theequivalent circuit. It must be split into the positivephase-sequence and negative phase-sequence sys-tems. As the rotating field of the negative-sequencesystem runs in the negative direction at the synchro-nous speed, the slip for the negative-sequence sys-tem is 2-s. The influence of the different rotorresistance values is very easily recognizable in Fig. 3.

Unless otherwise specified, it can be approxi-mately assumed that about 2 % negative-sequence system voltage does not lead to anyadditional heating of motors. This correspondsto a continuously permissible negative-sequencesystem current of approximately 10 %. If thecurrent rises above this value, tripping must takeplace after defined times. Unfortunately, motormanufacturers seldom specify any data in thisrespect.

b) StartingDuring starting, thermal stress occurs in the rotordue to the starting currents. Typical startingphenomena are dealt with in the first chapterentitled “Introduction to the principles of asyn-chronous and synchronous motors”.

The cases of the locked rotor and the acceleratingrotor must basically be considered. In the case ofmotors with critical rotors (the majority of them),the locked rotor case constitutes the higher stressfrom the thermal point of view. This is why motormanufacturers frequently specify the thermal lim-its of the locked motor in the thermal limit curve.These curves are shown on the right in Fig. 2. Ifthe motor accelerates, the thermal limits are gen-erally above them.

The motor’s starting time, for both the rated volt-age and the 80 % case, is also found in the techni-cal data. These times apply on connection of themachine.

For example, the motor’s starting time is 24.0 swith a starting current of 5.6 I/IN under ratedvoltage conditions (VN) and 52 s at 4.17 I/IN and0.8 VN. In Fig. 2, we read a time of about 40 s forthe warm condition at 5.6 I/IN, with the resultthat the characteristic of starting time monitoringcan lie below the locked rotor characteristic.A speed check is not imperative here.

A further thermal stress results from the fre-quency of successive startups of the motor. Here,manufacturers distinguish between warm andcold. We frequently find the typical value of threestartups from the cold state and two startups fromthe warm state. This statement also applies tostarting with a reduced voltage (80 %).

A further important characteristic quantity is thenumber of permissible startups per year. Thisamounts to 1000, for example, and must not beexceeded.

�2. Implemented protection configurations2.1 Stator thermal overload protection

(normal duty)In Section 1, we worked out that temperatureresponse is complex and really ought to be de-scribed by a thermal network. This descriptionleads to differential equations of a higher order,for which the input parameters are missing, how-ever. Due to the limited availability of data frommotor manufacturers, the simplified descriptionbased on a homogeneous-body model has estab-lished itself. Fig. 4 shows the model with the heatsource, realized as the current source I2, heat stor-age in the capacitor Cth, heat dissipation throughthe resistor Rth and the ambient temperature ϑa.The model is based on a reference temperature ϑ0

of 40 °C. The temperature ϑ describes the motor’sovertemperature.

Fig. 3 Equivalent circuit diagram of a motor as positive and negative-sequence systems(left: positive-sequence system, right: negative-sequence system)

Fig. 4 Equivalent circuit diagram of the thermal model



The following differential equation (1) can be de-rived from the equivalent circuit diagram, whichis simplified further when we assume a constantreference temperature ϑ0.

I C2 0 0= +tha

th

d

d

( – ) – ( – )ϑ ϑ ϑ ϑ ϑt R

→

I R R Ct

20th th th a

d

d= +ϑ ϑ ϑ ϑ– ( – ) (1)

When a constant ambient temperature is as-sumed, the maximum permissible current willlead to the maximum permissible temperature(I2

max · Rth � ϑmax). If we scale equation (1) withthe maximum permissible quantities, we arrive atthe differential equation to be implemented in theprogram. The differential equation must be calcu-lated separately for each phase current. Trippingoccurs if the value 1 is reached in at least onephase (reason: calculation is based on scaledquantities).

It

2p.u. th a

d

d= +τ Θ Θ Θ– (2)

with the following scaling

II

I

I

Ip.u.

Nk= =

max

Θ ϑϑ

ϑϑ

= =max k2

N

τth th th= R C Θ ϑ ϑϑ

ϑ ϑϑa

a 0 a 02

Nk= =– –

max

The solving of the differential equation leads tothe well-known exponential expression. If thecurrent is increased abruptly, the temperatureexponentially approaches the stationary value.

( )Θ Θ Θ Θ(t) 1 – ep.u. a t =0–t /

t 0th= + + =( – )I 2 τ (3)

As all values are scaled, the tripping time whenΘ(t) =1 is set can be calculated on the basis ofequation (3). If we also use the scaling quantitiesof equation (2) and assume a measured ambienttemperature, the tripping time can be calculatedwith the following equation. If no ambient tem-perature is measured, ϑa, measured = 40 °C must beinserted and this leads to further simplification ofequation (4).

t

I

I

I

I= ⋅

⎛⎝⎜

⎞⎠⎟

⎛⎝⎜

⎞⎠⎟

τth

2N

2

previous load

Nn

1

k

1

k

1l

2 2

–

k

1

k2N

2a,measured

N

I

I

⎛⎝⎜

⎞⎠⎟ + ⎛

⎝⎜

⎞⎠⎟

⎛

⎝

⎜⎜⎜⎜⎜

⎞

⎠

⎟

20 1

ϑ ϑϑ

––

⎟⎟⎟⎟

A quasi-stationary state (constant over 5 τth) ispresupposed for the previous load and the meas-ured ambient temperature.

Therefore, for overload protection, two character-istic quantities of the motor are needed, thefactor k, which describes the continuously maxi-mum permissible current referred to the ratedcurrent, and the thermal time constant τth. If theambient/coolant temperature is measured, thetemperature that sets in under the rated condi-tions is also needed as a scaling quantity. If this isnot available, it can be measured via the tempera-ture sensors.

During operation with a different load, the heat-ing time constant is equal to the cooling timeconstant. Longer cooling (no forced cooling bythe fan) must be assumed if the motor is shutdown, however. If, in this process, the currentfalls below a minimum value, standstill can beassumed and it is possible to switch over to thelonger cooling time constant (in the exampleshown in Fig. 2, this is 105 min).

The behavior of the thermal model on activationof the motor must now also be considered. Here,however, the thermal load acts on the rotor. Thissignifies restricted validity of the stator model orof the setting values. Two strategies are currentlypursued:

– Freezing the thermal memory during startingThis is intended to avoid overfunction of theoverload protection during starting.

– Internal limiting of the starting currentThe current fed to the thermal model is limitedduring starting. This “slows down” heat devel-opment. Current limiting should lie at approxi-mately 2.5 I/IN,M (see also Fig. 2). Furtherreduction of limiting is recommended ifstarting times are long and successive warmstarts are possible. In the example, a value of2 I/IN,M makes sense for a starting time of 52 s(at 0.8 VN).

Different operating states were assumed to pro-vide an insight into the thermal behavior of thethermal model. Fig. 5 encompasses the followingsequence: starting with rated voltage (startingtime = 24 s), continuous duty under rated condi-tions, deactivation and direct reactivation at areduced voltage (starting time = 52 s), brief dutywith a reduced load (0.9 I/IN,M) and then shut-down of the motor. After shutdown we recognizethe effect of the longer cooling time constant verywell. During starting, the starting current for thethermal model was limited to 2 I/IN,M.



The tripping threshold for the thermal model is1 (corresponding to 100 %). The higher, but al-ready reduced, current leads to a visible tempera-ture increase during starting. However, thetripping threshold is not reached despite the longstarting time. During operation under rated con-ditions (I = IN,M = 1), the thermal memory has a“filling level” of about 76 % (rule of proportion:1.152 / 100 % like 12 / x %).

2.2 Thermal protection with an asymmetricalvoltage (unbalanced-load protection)

Voltage asymmetry leads to a current asymmetry,which can be described by the negative-sequencesystem current. On the basis of the phase cur-rents, the protection algorithm calculates (corre-sponding to the definition equation of thesymmetrical components) the negative-sequencesystem current that is to be assessed in protectionterms. In the simplest case, thresholds are queriedand tripping is initiated after a set delay.

To take the heat development process into ac-count, an I2

2t characteristic must be simulated,which is a well-known characteristic (e.g. I2

2 t =10 s) in synchronous machines. The characteristicis released when the permissible negative-sequence (unbalanced-load) threshold (e.g. 10 %)is exceeded. Below the threshold, the motor mustbe allowed to cool down. An inverse curve resultsas the tripping characteristic (see also Fig. 9).

2.3 Starting time supervisionThe rotor is thermally overloaded if starting is toolong. The thermal limit (see Fig. 2) is described byan I2t characteristic. This characteristic must besimulated. The following equilibrium condition(see equation (5)) can be set up and the permissi-ble starting or blocked-rotor time can be deter-mined on the basis of it.

I2 t = I2start tstart → =t

I

Itstart

start⎛⎝⎜ ⎞

⎠⎟

2

(5)

where:

Istart maximum permissible starting currenttstart maximum permissible starting timeI measured starting current

Tripping occurs if the actual time is longer. Thetripping characteristic has an inverse characterand adapts very well to the starting conditions(with rated voltage and reduced voltage).

The calculation in accordance with equation (5) isnot released until starting is detected. For typicalmotors, the necessary current threshold amountsto approximately 2.5 IN,M. The threshold must belowered accordingly for motors with reducedstarting currents.

By way of example, Fig. 6 shows starting cur-rents/times measured for a motor. Adaptation ofthe protection characteristic to the starting condi-tions is easily recognizable.

2.4 Restart inhibitInadmissible heating of the rotor occurs when themotor is started too many times in succession. Inthe simplest case, the permissible limits can bemonitored by counter and the specified pausetimes can then be kept to.

Another approach is to model heating of the rotorto determine the thermal limit for the permissiblerestarts. In this way, we get closer to the physicalconditions and can generally load the motorbetter.

A homogeneous-body model is also expedient formodeling. The two necessary parameters consist-ing of the k factor for the rotor (kr) and the rotortime constant τr must be found. Both values aregenerally not given. However, they can be derivedfrom data of the manufacturers such as the num-ber of cold starts (nc) and warm starts (nw), thestarting time and the associated starting current.We create an equation system that describes the

Fig. 5 Behavior of the model in different load cases(data: k = 1.15; τheating = 15 min; τcooling = 105 min)

Fig. 6 Measured starting currents and tripping characteristic



cold and warm states with the known quantitiesand we determine from it the two unknown val-ues (kr and τr). Equation (6) shows the approxi-mated solution.

krc

c w

≈ n

n n–τr c w≈ ( – )n n I2

start tstart (6)

These parameters calculated internally in thefunction are incorporated into the thermal model(by analogy to equation (2)). Restart inhibit be-comes active if the thermal memory has reached acorresponding filling level. This threshold lies at(nc – 1)/nc.

At standstill, the slower cooling is taken into ac-count by extending the time constant. Beforethen, the thermal replica is frozen for an adjust-able time to take internal transient phenomenainto consideration. Only then does exponentiallydecaying cooling take place. Renewed restart isprevented for this time to allow the motor to rundown.

To meet demands for a minimum standstill time,an additional delay time is started after blockingby the thermal restart inhibit function. This timetakes effect only if the thermal model has enabledit beforehand.

Fig. 7 shows the thermal behavior for a possibleoperating case. The motor is started from the coldstate and runs under rated conditions for a certaintime. After that, it is restarted twice from thewarm state and operated at 90 % of the ratedcurrent. The number 1 describes the thermallimit for the rotor and 0.67 the restart inhibitthreshold.

The first restart from the cold state leads to rotorheating, which will also set in during rated opera-tion. Two restarts are permitted from this warmstate. When the warm motor restarts a secondtime, the restart inhibit threshold is exceeded andthe temperature approaches limit value 1. Thisvalue represents the maximum permissible rotortemperature. Subsequent continued operationunder load conditions leads to correspondingcooling of the motor. If the motor were now de-activated, it could be restarted immediately againbecause there is a sufficient thermal reserve.

If, by contrast, it were deactivated immediatelyafter the second start from the warm state, therestart inhibit function would immediately takeeffect and would prevent renewed restarting.From the thermal point of view, restart is onlypermitted when the temperature falls below the67 % threshold.

Fig. 7 Behavior under different loads according to thethermal rotor model(tstart = 52 s; Istart = 4.17 I/IN,M; nc = 3 and nW = 2)



�3. Protection settingTo summarize, the setting parameters for thethermal protection functions are gathered to-gether in a table and are commented on briefly.

a) Stator overload protection

b) Negative-sequence protection

c) Starting time supervision

Setting parameters Value Comment

Heating time constant 15 min Take this from the motor data sheet (see Fig. 2) or calculate iton the basis of a specified characteristic (equation (4) can beused to do this).

Cooling time constant 105 min Take this from the motor data sheet (see Fig. 2) or set anexperience-based value, e.g. 7 times the heating time constant.

k factor 1.15 Take this from the thermal characteristic; if this is not available,a k factor of 1.1 can be used (see Section 1 – temperature class F/B).

Thermal alarm stage 90 % During operation under rated conditions, the thermal warning stageis always below 90 %. With the default setting of 1.1, this is 83 % andin the case of k = 1.15, only 76 %.

Current limiting 2 I/IN,M A slightly lower value has been chosen due to the long starting time,in particular with a reduced voltage. The value 2.5 IN,M can be usedfor a short starting time.

Current alarm stage 1.15 IN,M This should be set to equal the k factor.


Continuously permissibleunbalanced-load or alarm stage

0.1 I2/IN,M Unless otherwise specified, this value should be set.

If this stage should also trip, it must be delayed appropriately(approximately 15 - 30 s); suggestion: 20 s.

Tripping stage 0.4 I2/IN,M Two-phase operation under rated conditions leads to a negativephase-sequence system current of about 66 %. Due to the power tobe produced, it will rise and will certainly reach values of 100 %.

A tripping delay of about 3 s is recommended to allow for transientphenomena.

Thermal unbalanced-load time((I2/ IN,M)2 t = K)(Internal designation:FACTOR K)

2 s The conservative setting of 2 s is chosen if the motor manufacturerdoes not specify any details. The time really ought to be longer if aneffect comparable to synchronous machines is presupposed.

Cooling time of thethermal model

200 s Use the following relationship:

t cooling

2,permiss.

K ss=

⎛

⎝⎜

⎞

⎠⎟

= =I

IN, M

2 2

2

0 1200

.


Starting current (pickup current) 5.6 I/IN,M Take this from the motor data sheet (starting curves).

Maximum permissible startingtime

35 s To this end, the specified starting times must be compared with thecharacteristic curve times. Go below the thermal characteristic ifthere is an adequate safety clearance. According to Fig. 2, a value of40 s is permissible in the case of 5.6 I/IN,M in the warm state. For thiscurrent, the motor’s starting time was specified as 24 s.

Chosen permissible starting time: 35 s

Start detection 2 I/IN,M With the voltage reduced, the starting current amounts to 4.17 I/IN,M

and current limiting for unbalanced-load protection was alsodefined at 2 I/IN,M. The typical value of 2.5 I/IN,M can be used forstandard motors.



d) Restart inhibit

e) Characteristics resulting from theprotection setting values

By analogy to Fig. 2, Fig. 8 shows the trippingtime of the thermal characteristics. The thermaloverload protection characteristic can be seen onthe left and the starting time supervision charac-teristic can be seen on the right. The locked rotorcharacteristic of the manufacturer is also shown(curve just above it).

Fig. 9 shows the tripping response to a negative-sequence. In addition to the thermal characteris-tic, the two definite-time characteristics are alsoshown.

Fig. 7 adequately explains the response of therestart inhibit function for the chosen settingparameters.

References[1] IEC60034-1 – Rotating electrical machines.

Part 1: Rating and performance,Release 2004

[2] IEC62114 – Electrical insulation systems(ISM). Thermal classification, Release 2001


Starting current (pickup current) 4.17 I/IN,M The current in the case of the longest starting time is taken from themotor data sheet. This is the value at the reduced voltage.

Starting time 52 s Choose the time that belongs to the current; if a motor starts clearlyfaster, e.g. in 2.6 s, you can be a little more generous with the setting.Here, we choose the lowest setting of 3 s.

Cold starts 3 Take this from the motor data; if values are missing, assume thevalue 3.

Warm starts 2 Take this from the motor data; if values are missing, assume thevalue 2.

Rotor temperatureequalization time

1 min This time is considered practicable and also takes running down tostandstill into account.

Note: Restarting of the motor is not possible during this time. Thetime must be set to zero if only blocking is to take place when thethermal limit is reached.

Extension of time constantat stop

5 This value is recommended for long starting times. It leads to releaseof the restart inhibit function after approximately 30 minutes. Atleast the value 10 must be chosen if the starting times are clearlyslower.

Minimum restart inhibit time 30 min A time between 15 min and 30 min is recommended if no data isavailable. The longer time must be chosen for longer starts.

Fig. 8 Thermal characteristics according tothe setting

Fig. 9 Negative-sequence protection characteristics


Current Transformer Requirements

This chapter discusses the design of current trans-formers for motor protection applications. Vari-ous operating cases and typical protectionprinciples are considered. Design is explainedwith reference to examples, based on a choice oftwo approaches, checking of existing currenttransformers and new design. These consider-ations are based, among other things, on the rele-vant IEC standards (IEC 60044-1, IEC 60044-6).

� Formula symbols and definitions used

Kssc = factor of the symmetrical rated short-circuitcurrent(example: transformer CI. 5P20 ) Kssc = 20)

K'ssc = effective factor of the asymmetricalshort-circuit current

Ktd = transient rated dimensioning factor

Ipn = primary rated transformer current

Isn = secondary rated transformer current

Rct = secondary winding resistance in � at75 °C (or another specified temperature)

Rb = rated resistive burden in �

R'b = Rlead + Rrelay = connected burden in �

Rrelay = relay burden in �

Rl

leadA

= ⋅ ⋅2 ρ

where:

l = single conductor length between currenttransformer and device in m

q = specific resistance = 0.0175 � mm2/m(copper) at 20 °C (or another specifiedtemperature)

A = conductor cross-section mm2

Istart = motor starting currentIstart trans = transient starting currentktrans factor for the transient starting current

For current transformers defined via the symmet-rical rated short-circuit current factor Kssc and therated resistive burden Rb (e.g.: 5P,10P), theeffective factor of the symmetrical short-circuitcurrent K'ssc can be calculated in accordance withthe following formula:

K Ksscct b

ct bssc'

'= ⋅ +

+R R

R R

Starting is crucial to stability when current flowsthrough and with regard to the design of thetransformers.

Requirements arising out of motor startingThe starting current Istart is in the range from fourto seven times IN with a DC time constant TA ofabout 40 ms (<1 MW) to 70 ms (>1 MW). This issuperimposed with a rush current of approxi-mately the same order of magnitude which, how-ever, decays in two to three cycles in accordancewith a time constant Trush of about 20 ms.

The starting current is therefore:

Istart trans = ktrans· Istart , where ktrans = 2 - 2.5

The minimum required factor of the symmetricalshort-circuit current K'ssc can be calculated inaccordance with the following formula:

K' Kssc tdstart trans

pn

≥ ⋅I

I

the following condition must be met:K'ssc (required) ≤ K'ssc (effective)

Motor Protection:Requirements forCurrent Transformers



Example 1 Definite-time overcurrent-time protection, check of the existing current transformer

IP

VN

N

N

kW

kVA=

⋅ ⋅=

⋅ ⋅=

3

1000

3 6 3 0 85107 8

cos . ..

ϕ

I Istart N A A= ⋅ = ⋅ =5 5 107 8 537 5. .

I Istart trans start A A= ⋅ = ⋅ =2 2 537 5 1075.

Setting value I >>: 1.3 · Istart trans = 1.3 · 1075 A = 1397 A

Note:For t >> = 0 ms.With a time setting of t >> = 50 ms, the setting valuecan be reduced.(Refer to Chapter “Protection of medium-power motors”)

Requirement: KA

Assc

pn

' .≥ >> = =I

I

1397

1509 3, but at least 20

The CT's rated burden in Ω amounts to:

RS

Ib

n

sn2 2

VA

A= = =5

15 Ω

The actually connected burden (cable + device) amounts to:

R R Rl

' ..

.b lead relay

A

mm

mm

m= + = ⋅ ⋅ + =

⋅ ⋅2

012 0 0175 10

2 5

2

ρ Ω

Ω

m 201 0 24+ =. .Ω Ω

This results in the effective factor:

K' Kssc sscct b

ct b

= ⋅ ++

= ⋅ ++

=R R

R R'

.

. ..10

1 3 5

1 3 0 2440 9

Ω ΩΩ Ω

K'ssc required = 20, K'ssc effective = 40,9� The dimensioningof the CT is correct.

Fig. 1



Example 2 Differential protection, design and checking of a current transformer

2.1 Design of the current transformer – T1

IP

UN

N

N

kW

kVA=

⋅ ⋅=

⋅ ⋅=

3

11165

3 10 0 85758

cos .ϕ

A CT with Ipn=1000 A is chosen.

I Istart N A A= ⋅ = ⋅ =5 5 758 3790

I Istart trans start A A= ⋅ = ⋅ =2 2 3790 7580

Requirement: K' KA

Assc td

start trans

pn

≥ ⋅ = ⋅ =I

I5

7580

100037 9.

where Ktd = 5 for the generator/motor differential protection(Catalog SIP 2006)

The CT's class should be 10P with 20 VA rated burdenand an internal resistance R ct≤ 4 Ω

The CT's rated burden in Ω is:

RS

Ib

n

sn

VA

A= = =

2 2

20

120 Ω

The actually connected burden (cable + device) is:

R R Rl

' ..

b lead relayA

mm

mm

6= + = ⋅ ⋅ + =

⋅ ⋅2

012 0 0175 200

2

ρ Ω

Ω

mm 2+ =01 1 266. .Ω Ω

This results in the rated factor:

K K'sscct b

ct bssc≥ +

+⋅ = +

+⋅ =R R

R R

' .. .

4 1 226

4 2037 9 8 3

Ω ΩΩ Ω

Kssc = 10 is chosen

With this, all necessary CT data are available:

1000 A/1A, 10P10, 20 VA, R ct≤ 4 Ω

Fig. 2



2.2 Checking the existing transformer – T2

K'ssc = 37.9 (from design of transformer – T1)


RS

Ib

n

sn2

VA

A= = =15

115

2Ω


R R Rl

' ..

.b lead relay

A

mm

mm

m= + = ⋅ ⋅ + =

⋅ ⋅2

012 0 0175 10

2 5

2

ρ Ω

Ω

m 201 0 24+ =. .Ω Ω


K Kssc sscct b

ct b

'' .

.= ⋅ ++

= ⋅ ++

=R R

R R10

4 15

4 0 2444 8

Ω ΩΩ Ω

K'ssc required = 37.9, K'ssc effective = 44.8 � The dimensioningof the CT is correct.

Note: Contrary to the star point CT (-T1),the required apparent power can be slightly lowerbecause the distance from the protection is clearlyshorter. If the final distance is not clear, the worstcase must be reckoned with and identical currenttransformers must be provided.

Example 3 Differential protection with bushing-type transformers

3.1 Current transformer – T1

The classic motor protection functions (overload, starting supervision,negative-sequence (unbalanced-load), etc. operate with this current transformer).

IP

VN

N

N

kW

kVA=

⋅ ⋅=

⋅ ⋅=

3

11165

3 10 0 85758

cos .ϕ

Fig. 3



A CT with Ipn = 1000 A is chosen.

I Istart N A A= ⋅ = ⋅ =5 5 758 3790

I Istart trans start A A= ⋅ = ⋅ =2 2 3790 7580

Setting value I I>> ⋅ = ⋅ =: . .1 3 1 3 7580 9854start trans A A

Note:For t >> = 0 ms.With a time setting of t >> = 50 ms, the setting valuecan be reduced.(Refer to Chapter “Protection of medium-power motors”)

Requirement: K'A

Assc

pn

≥ >> = =I

I

9854

10009 85. , but at least 20

The CT's class should be 10P with 5 VArated burden and an internal resistance Rct ≤ 4 Ω.The CT's rated burden in Ω amounts to:

RS

Ib

n

sn2

VA

A= = =5

15

2Ω

The actually connected burden(cable + device) amounts to:

R R Rl

' ..

.b lead relay

A

mm

mm

m= + = ⋅ ⋅ + =

⋅ ⋅2

012 0 0175 10

2 5

2

ρ Ω

Ω

m 201 0 24+ =. .Ω Ω


K Ksscct b

ct bssc≥ +

+⋅ = +

+⋅ =R R

R R

''

..

4 0 24

4 520 9 42

Ω ΩΩ Ω

Kssc = 10 is chosen

With this, all necessary CT data are available:

1000 A/1 A, 10P10, 20 VA, Rct ≤ 4 Ω

3.2 Bushing-type transformer – T2

For machines connected via cables, reliable differentialprotection with high response sensitivity can be realizedwith this method. The prerequisite is that the threephases are returned separated from the star point sideand are routed through the bushing-type transformerin the opposite direction. If the machine winding is fault-free, the currents in the transformer cancel each otherout and no current flows into the (differential) currentrelay. The current comparison is highly precise withoutsaturation problems, taking place as a magnetic(self-balancing) operation.No stabilization is necessary, i.e. simple current relayscan be used. The pickup value can be adjusted to 2 to 5 %of the machine’s rated current.One 7UM62 can also be used instead of two 7SJ6 units,in which case the overcurrent function must be assignedto the bushing-type transformer.



CT requirement for -T2The (differential) current relay mustrespond to high-current internal faults.

As the setting value is very low and the total currentis zero during fault-free operation,a CT with 500 A/1 A, 10P10, 5 VA,Rct = 2Ω is chosen, with the requirement:K'ssc W 20 (analogous to the definite-timeovercurrent-time protection)


RS

Ib

n

sn

VA

A= = =

2 2

5

15 Ω


R R Rl

' ..

b lead relayA

mm

mm

mm= + = ⋅ ⋅ + =

⋅ ⋅2

012 0 0175 200

6

2

ρ Ω

Ω

201 1 266+ =. .Ω Ω


K' Kssc sscct b

ct b'

= ⋅ ++

= ⋅ ++

=R R

R R10

2 5

2 1 226214

Ω ΩΩ Ω.

.

K'ssc required = 20, K'ssc effective = 21.4 � The dimensioningof the CT is correct.

� SummaryThe requirements for current transformers are definednot only by the starting operation, but also by therequirements of the protection principle. Furtherimportant defining variables are the burden through thefeeder and also the current transformer’s internal burden.The current transformer types chosen in the examplescan certainly be used for orientation. However, a generalcheck is nevertheless recommended.


Low-Power Motors

Protection ofLow-Power Motors

Principles and settings for the protection of asmall (asynchronous) motor are discussed below.Particular attention is devoted to design of theearth-fault protection for earthed, isolated and

�1. Protection functions

The following protection functions are available:

7SJ600:

Time-overcurrent protection phase/earth,thermal overload protection, starting timesupervision, restart inhibit and negative-sequence(unbalanced-load) protection

7SJ602 (from V3.5) additionally:

Sensitive earth-fault protectionDirectional earth-fault protection for isolatedand compensated systemsTime-undercurrent protectionTemperature monitoring by RTD-box

resonant-earthed power systems, representative ofall kinds and sizes of motors. The 7SJ600 or the7SJ602 is considered a preferred protective relayfor small motors.

* Required depending onstar point connection andsystem topology

** With 7SJ602 only

Besides the SIPROTEC relays 7SJ600 and 7SJ602presented here, it goes without saying that the7SJ61,62,63,64 relays in the SIPROTEC 4 familywith a wider scope of functions are also suitable.Refer to the chapters on protection of medium-and high-power motors.

Protection functions Abbreviations ANSI

Time-overcurrent relay, phase I >>, I >, Ip 50, 51

Non-directional/directionalearth-fault protection *, **

IEE dir.>>, IEEdir.>, VE> 51N, 51Ns,67Ns, 59N

Displacement voltage protection 59N

Thermal overload protection I2 t > 49

Starting time supervision Istart2 t 48

Restart inhibit I2 t 66, 49R

Negative-sequence(unbalanced-load) protection

I2 >, t = f (I2) 46

Temperature monitoring ** � (Thermobox) 38


Low-Power Motors

�3. General power system dataThe current and voltage transformer data and thetype of connection are entered in the unit under“1100 Power System Data”.

Here are the DIGSI parameters of a 7SJ602:

1118: Via this setting, the entries for the “startingtime supervision” and “restart inhibit” motorprotection functions are referred to rated motorcurrent. 1119: Motor starting or locked-rotorcurrent. 1120:Tthe rotor locking time for 6.5 IN,Mis 18 s, and the startup time for the 422 kW pumpused in the example is 2.1 s. An interme- diatevalue of 8 s was chosen (see also comments onstarting time supervision).

No. Parameter Value

1100 POWER SYSTEM DATA

1101 Rated system frequency fN 50 Hz

1105 Primary rated current 75 A

1106 Secondary rated current 1 A

1111 Matching factor Iee/Iph forearth current

0.800

1112 CT nom. current IEEp, sec 1 A

1113 Nominal VT voltage, primary 6.00 kV

1114 Nominal VT voltage, secondary 100 V

1115 Reverse power direction off

1116 Threshold circuit breaker closed 0.10 I/In

1118 Nominal current of motorin relation to Tr.C.

0.6

1119 Startup current in relationto nominal current

6.5

1120 Maximum startup time 8.0 s

1121 Reset thermal image on startup yes

1134 Minimum tripcommand duration

0.15 s

1135 Maximum closecommand duration

1.00 s

Connection of the 7SJ602 for an isolated orresonant-earth system with earth-fault directiondetection.

�2.

A 75 A/1 A current transformer is chosen for thephases and a core-balance current transformer60 A/1 A, as well as 6 kV/100 V voltage trans-formers.

Fig. 1 Connection with 3 current inputs IA(L1)-IB(L2)-I4(EE)

for sensitive earth-fault protection anddisplacement voltage input Ve-n

Size Value

Power 460 kW

Rated voltage 6 kV

Rated motor current 48 A

Max. starting current (at 100 % VN) 312 A = 6.5 · IN, M

Starting time (at 100 % VN) 2.1 sec

Heating time constant 40 min

Cooling time constant at standstill 240 min

Max. continuous thermal rating current 58 A = 1.2 · IN, M

Max. locked rotor time 18 sec at 6.5 · IN, M


Low-Power Motors

�4. Protection functions4.1 Time-overcurrent protection, phase

Dynamic thresholds are used, i.e. during theincreased current in motor starting, the over-current stages I>>> (1303) and I>> (1305)are raised appropriately (1304: I>>> dyn,1306: I>> dyn) and are lowered again after start-ing (hold time 1302: 10s). Advantage: a sensitivesetting with a short tripping time is possibleduring normal operation. As an additional reservestage, stage I> is operated with a tripping time(1310) longer than the starting time.All stages are set to 1.5 times the maximum operat-ing current, e.g. I>>, dyn = 1.5 · 6.5 · 0.64 = 6.24;6.5: max. starting current,0.64: motor/transformer rated current conver-sion.I>> = 1.5 · 1.2 · 0.64 = 1.15.1.2: max. continuous thermal current,0.64: motor/transformer rated current conver-sion.

�4.2 Negative-sequence protectionNo data is available from the motor manufactur-er, and so the following settings can be used asrecommended values:

No. Parameter Value

1300 O/C PROTECTION PHASE FAULTS

1301 O/C protection for phase faults on

1302 Duration oftemporary pickup value c/o

10.00 s

1303 Pickup value of the high-set inst.stage I>>>

1.2 I/In

1304 Pickup val. of high-setinst. stage I>>> (dyn)

6.3 I/In

1305 Pickup valueof the high-set stage I>>

1.2 I/In

1306 Pickup valueof the high-set stage I>> (dyn)

6.3 I/In

1307 Trip time delayof the high-set stage I>>

0.05 s

1308 Pickup valueof the overcurrent stage I>

1.2 I/In

1309 Pickup valueof the O/C stage I> (dyn)

1.2 I/In

1310 Trip time delay of theovercurrent stage I>

10.00 s

1311 Measurement repetition no

1319 Manual close I>>undelayed

The values refer to the rated transformer current.1502 and 1504 are set to approx. 10 % or 40 % ofthe rated motor current. Further information canbe found in the chapter entitled “Thermal stressof motors and necessary protection functions”.

4.3 Thermal overload protectionThermal overload protection serves to protect thestator against inadmissible temperature rise.

All values are referred to rated transformer current.2702: The figure can be obtained from the mo-tor’s thermal withstand curve and is the asymp-totic value of the characteristic at the bottom.In the example,

kN, M

= ⋅1 2.I

I

Referred to rated transformer current:

k N, M

N, CT

= ⋅ = ⋅ =1 2 1 2 0 64 0 77. . . .I

I

2703 and 2704 are taken from the data sheet;important: the thermal time constant duringcontinuous operation and not the copper timeconstant for short-time operation must be usedfor 2703.2705: The thermal warning stage picks up whenthe calculated temperature reaches 90 % of themax. permissible temperature.

No. Parameter Value

2700 THERMAL OVERLOADPROTECTION

2701 State of thermaloverload protection

on

2702 K-factor for thermaloverload protection

0.77

2703 Time constant for thermaloverload protection

60.0 min

2704 Multiplier of timeconstant at standstill

4.00

2705 Thermal warning stage 90 %

No. Parameter Value

1500 UNBALANCED LOADPROTECTION

1501 State of the unbalanced-loadprotection

on

1502 Pickup value of neg. seq.I low-set stage I2>

8 %

1503 Trip delay of neg. seq.I low-set stage TI2>

20.00 s

1504 Pickup value for highcurrent stage

26 %

1505 Trip time delay for highcurrent stage

3.00 s


Low-Power Motors

4.4 Starting time supervisionThis serves to protect the rotor during motorstarting. It is advisable to permit a maximumstarting time that lies between the real startingtime (2.1 s) and the maximum locked-rotor time(18 s), on the one hand to avoid responding toosensitively in the event of long starting times, andthus deactivating the motor unnecessarily, on theother hand, though, to also avoid reaching the ro-tor insulation’s absolute thermal limit. In the caseof the 7SJ602, both values are located in the sys-tem data parameter block, where they are de-scribed.

2803: If the measured phase current rises abovethis value, motor starting is detected and thefunction is activated. A reasonable value forstarting detection is 2.5 times the rated motorcurrent IA = 2.5 · 0.64 = 1.6. 2804 offers the possi-bility of blocking the overcurrent stages to avoidovercurrent tripping during motor starting.Our example, however, uses dynamic thresholdboosting during starting and a reserve stageI> with a tripping time longer than motorstarting, and so this must not be blocked.

4.5 Restart inhibitThis function prevents excessively frequent startingof the motor in succession. As starting time super-vision does not have a thermal memory, it wouldpermit several starts in direct succession providedthe maximum start-up time were not exceeded.Generally, though, motor manufacturers permit

only three starts from the cold operating state(4503 + 4504) and two from the warm state (4503).

Calculation is based on a thermal homogeneous-body model similar to the standard overloadprotection that operates for the stator winding.The time constant during operation is equal tothe value set in 2703, and an additional value 4505can be set for prolongation during motor stand-still. 4502 is an empirically based value and cangenerally be left set as it is. If the manufacturerspecifies a minimum inhibit time between twostarts, this can be set in 4507. It acts in additionto the blockage derived dynamically from thethermal image. Here, the k factor, which mustbe worked out in the case of overload protection,is determined automatically on the basis of thenumber of starts for cold and warm and is nor-mally considerably lower for the rotor than forthe stator.

4.6 Undercurrent monitoringUndercurrent monitoring serves to protect thedriven load and detects a load loss that might

cause damage, e.g. pumps running dry.The settings depend on the type and size of thedriven load.

4.7 Breaker-failure protectionThis serves to repeat (on another command relay)

a TRIP command that does not lead to openingof the circuit-breaker, thus tripping any secondrelease coil of the circuit-breaker or a higher-levelcircuit breaker.

The delay time must be greater than the circuit-breaker’s run time and the reset time of the BFPincluding safety reserve.

No. Parameter Value

2800 STARTING-TIME SUPERVISION

2801 Supervision of starting time on

2803 Base value Istrtof permissible start-up curr.

1.6 I/In

2804 Block of the I>/Ip stages duringstart-up

no

No. Parameter Value

4500 MOTOR START PROTECTION

4501 Motor start protection on

4502 Temperature equalization time 1.0 min

4503 Maximum permissible numberof warm-starts

2

4504 Difference between no.of cold and warm-starts

1

4505 Factor for tau during standstill 5.0

4506 Factor for tau during operation 2.0

4507 Minimum blocking timefor motor protection

6.0 min

No. Parameter Value

4600 UNDERCURRENT MONITORING

4601 Limit for undercurrentmonitoring

0.20 I/In

4602 Delay time forundercurrent monitoring

10.0 s

4603 Undercurrent monitoring on

No. Parameter Value

3600 BREAKER-FAILURE PROTECTION

3601 Circuit breaker failure protection on

3602 Delay time T-B/F 0.3 s

3603 Analysis of auxiliarycontacts for B/F

off


Low-Power Motors

4.8 Displacement voltage protection VE>

This function serves to detect an earth fault inisolated and compensated power systems and isused in addition to overcurrent sensitive earth-fault protection. Especially at the infeed of abusbar, single-pole faults cannot be detected dueto the zero sequence current and are indicated bythe VE protection. The fault location can be deter-mined by switching operations and the fault canbe cleared.

Below, earth-fault protection is described indetail, representative for all kinds and sizes ofmotors.

4.9 Earth-fault protectionEarth-fault protection mainly depends on thepower system’s star point earthing. An earth faultoccurs in isolated and compensated power sys-tems (Petersen coil at the system’s star point);i.e. the occurring fault currents are relatively lowand therefore do not need to be deactivated in theshortest of times. Power system operators cancontinue operation of their networks for a limitedtime and eliminate the fault through switchingoperations. By contrast, in impedance or solidlyearthed power systems, the fault current is so highthat is has to be switched off straight away.

a) Solid or low-impedance star point earthingThe fault currents are short-circuit currents andmust be eliminated fast. Any resistance installedat the star point serves to limit the earth-fault cur-rents in the event of earth faults (ph-e, ph-ph-e)to a certain maximum value.

• Selectivity criteriaThe short-circuit currents occur only in thefeeder containing the fault. Short-circuitcurrents can be detected that are lower thanthe motor’s rated current because this isapproximately symmetrical during normal

operation; i.e. it does not contain a negative-sequence and zero-sequence component. Thesetting sensitivity limit is defined by asymme-tries and measuring errors of the current trans-formers – especially in a Holmgreen circuit –and asymmetries of operation. It is thereforerecommended not to set more sensitivelythan 0.1 – 0.2 IN, CT.

Fig. 2 shows an overview of the usual star point earthing configurations (solid and inductiveearthing, however, is uncommon for motor protection applications).

No. Parameter Value

3300 DISPLACEMENT VOLTAGEPROTECTION VE>

3309 Displacement voltage level 0.10 U/Un

3311 Delay time forannunciation of VE>

1.00 s

3312 Delay time T-VE of the VE> stage 10.00 s

No. Parameter Value

1400 O/C PROTECTION EARTH FAULTS

1401 O/C protection for earth faults on

1402 Pickup value of thehigh-set stage IE>>

0.20 I/In

1403 Pickup value ofhigh-set E/F stage IE>> (dyn)

1.00 I/In

1404 Trip time delay ofthe high-set stage IE>>

0.05 s

1405 Pickup value ofthe overcurrent stage IE>

+ * I/In

1406 Pickup value ofdef. time E/F stage IE> (dyn)

+ * I/In

1407 Trip time delay of theovercurrent stage IE>

60.00 s

1408 Measurement repetition no

1416 Manual close IE>>undelayed


Low-Power Motors

• 1402: Earth-fault protection pickup value.1403: Due to the higher operating currentsduring starting, the pickup threshold isalso proportionally pulled up because of themeasuring errors.1404: Fast tripping is possible as no gradingtimes have to be considered.

b) Isolated power systemsThe earth-fault current is generated by theline-earth capacitances (Fig. 3).

Currents generated by line capacitances that liedownstream in the direction of the fault canceleach other out with the current returning in thefaulty phase (measurement point A). By contrast,capacitive currents behind the measurementpoint are detected by it via the fault phase(measurement point B). Applied to several feedersin an isolated system, this means that the capaci-tive currents of the healthy phases including theinfeed are measured in the faulty phase.The amount of the fault current that is measuredon the faulty feeder is therefore equal to the sumof the capacitive currents of the remaining, gal-vanically coherent healthy power system. Eachhealthy feeder carries a part of the fault current,which is also measured by the protection relaysinstalled there (Fig. 4).

In this case with a fault on feeder A, the zerosequence current

1

3IEA

⎛⎝⎜ ⎞

⎠⎟ (measured in point A) lags behind the

zero-sequence voltage by 90°; current IEB (meas-ured in point B) leads by 90°.The zero sequence current is determined by thetotal capacitance (not shown in the diagram) ofline B and possible further lines.

Remark: The direction of the earth currents IE

of the SIPROTEC relays is opposite to the zero-sequence currents. So the directional charac-terisitics in the manual differ from the abovedrawing in this respect.

Fig. 4 Earth fault in an isolated system, amount and phase angleof IEA, IEB and V0

I C V V V V V VE, B E, B 2E 3E 2E 2Ej 30

3E 3E-j 30j e= + = =° °ω ( ); ;

V V V V2E 2E 3E 3Ej j= °+ ° = °− °(cos sin ); (cos sin )30 30 30 30

V V V2E 3E N= =

�V V V V V2E 3E N N N+ = ⋅ ⋅ ° = ⋅ ⋅ =2 30 23

23cos

I C VE, B E, B Nj= ⋅ ⋅ω 3 I I C VE, A E, B E, B N= j= − − ⋅ ⋅ω 3

Fig. 3 Earth fault in an isolated system, fault currents

All currents cancel each other out (sum total is 0), whenthe zero current

Io =1

3(IL1 + IL2 + IL3) is measured in

point A.

In point B, however, only the fault currents of the faultyphase can be seen.

Earth fault in an isolated system


Low-Power Motors

• Selectivity criteriaIn contrast to the earthed system, a current ismeasured in each fault-free feeder whoseamount is proportional to the phase-earthcapacitance in the forward direction. The distri-bution of the capacitances determines whetheran overcurrent criterion alone is sufficient forfeeder-specific selectivity. Therefore, for eachfeeder the current in the event of an earth faulton this feeder (forward direction) must behigher than the current in the event of an earthfault in reverse direction. This is the case, whenfor each feeder of the galvanic connected powersystem the capacity of the remaining system(system without the faulted feeder) is muchhigher than the capacity of the faulted feeder.The current for an earth fault in forward direc-tion depends on the capacity of the remainingpower system while the current for an earthfault in reverse direction depends on thecapacity of the feeder.

Another requirement of the earth-fault protectionis the detection of earth-faults within the motor.Generally, a coverage amounting to 80-90 % ofthe stator winding is demanded.

The voltage driving the zero current I0 is thevoltage at the fault location V1,F. It arises due to avoltage divider

x 1M

1M

Z

Zi.e. V1F = – V0 = x · V1

If we neglect the resistances along the path ofthe current I0 ((1-x)Z0M, (1-x)Z2M), and thereactances of the feeding cable, which results in avery good approximation in view of the relativelyhigh resistance of the zero capacitance in compar-ison with it, then I0 (x) is proportional to x.

IV

C0 03

( )x x Nrev= ⋅ ⋅ ⋅ω

Therefore, in the case of fault coverage amountingto 90 % of the stator winding, the resulting zero-sequence current is only 10 % of the zero-sequen-ce current that arises in the event of a fault on thesupply line, i.e. outside the stator.

Fig. 6 Selectivity by current criterion, principle

Selectivity by currentcriterion

The fault current in thefaulty feeder is clearly higherfor each feeder than for thefault current in the healthyfeeders.

Fig. 7 Isolated system, earth fault in the stator of the motor

Fig. 5 Component equivalent circuit diagram in the caseof a single-pole fault. It can clearly be seen, that thefault current is mainly limited by the phase- earthcapacitance of the remaining system. When com-pared to Fig. 3, it can be seen, that the direction aswell as the amount of the fault current correspondwith the representation with natural quantities.

V1,L1: Component of positive-sequence systemof phase L1 (= V1)

V2, Vo: Negative-sequence system and zero-sequencesystem's voltage

RF: Fault resistance

Co,B: Zero-sequence system capacity of feeder B

Z1M, Z2M, Z0M: Positive, negative, zero sequence of motor

If the transfer resistance of the fault is neglected, it can beseen, that V2 = 0; V0 = – 1 and that the fault current isdetermined by capacity C0B>> C0A and voltage V1.

I I IV

R Z0A 1A 2A

F C0BFwhere= = =

+=1

30; R

� IV

Z

Vj C j C V0A

C0B

0B

1 0B 0B

j

= = = ⋅ = − ⋅1 101

ω

ω ω

C

V

I C V I0A 0B EAj= − ⋅ =ω 01

3

Earth-fault in isolated system, componentequivalent circuit diagram for fault on feeder A


Low-Power Motors

�1st example: Selectivity through overcurrentA power system is assumed with ten motors, eachof them connected by means of a cable measuring200 m in length to a busbar that is fed by a trans-former:

Transformer: VN20 kV

6 kV= ; SN = 6 MVA, uk = 0.08

The cables have a length-related capacitance toearth CE = 0.257 μF / km.The motors have a stator-earth capacitance of0.12 μFAs our motors have a rated current of 48 A, thenext largest phase current transformers of thepreferred type with 75 A/1 A are chosen.

The earth-fault protection of each feeder shouldnot only reliably find faults on the supplyingcable, but also faults in the motor up to a rangeof at least 80 % of the stator winding (calculatedfrom the outside).

1. Calculation of a feeder’s total capacitance:

C0, tot kmF

kmF F= ⋅ + =0 2 0 257 012 01714. . . .

μ μ μ

2. Zero-sequence current of a feeder in the eventof a fault in the reverse direction outside themotor:

I CU

0,rev 0, totN

3 s

s kV= ⋅ ⋅ = ⋅ ⋅ ⋅−

ω π100 01714

10 6

3

6

.Ω

I0,rev = 0.187 A (primary)IE,rev = 3 · I0,rev = 0.561 A (primary)

Each feeder protection relay detects this fault cur-rent in the event of a fault in the reverse direction.Under no circumstances may it pick up, i.e. thecurrent threshold should be approximately 50 %above this value, also due to the transient condi-tion on occurrence of the earth fault.

IEE> = 3 · 1.5 · I0, rev = 0.84 A (primary).

With these very low primary currents, preferencemust be given here to a core-balance CT over aHolmgreen circuit. To measure an adequatelyhigh current on the secondary side, preferenceis given to a transformation ratio of 60 A/1 Aover a higher ratio such as 100 A/1 A. With atranformation ratio of 60/1, we get a setting ofIEE> = 14 mA.

The following current is measured in the event ofa fault in the forward direction (factor x of thestator winding):

I0,fwd = 9 · I0,rev · x, because the nine healthyfeeders now supply the fault current.

I0,fwd = 9 · 0.187 A · 0.2 = 0.337 A

IE,fwd = 3 · I0,fwd = 1 A

This value is approximately 20 % higher than theIEE> primary setting of 0.84 A. This thereforeensures a range of 80 % into the motor.

7806: Direction determination can be deactivatedin this example.

No. Parameter Value

7800 SCOPE OF FUNCTIONS

7801 Characteristic of O/C protection Definite time

7802 Temporary pickup value changeover (O/C-st.)

Existent

7803 Unbalanced-load protection Existent

7804 Thermal overload protection With memory

7805 Supervision of starting time Existent

7806 Direction determination forsensitive earth

Existent

7807 Characteristics for O/C earth Definite time

7835 Breaker fail protection Existent

7839 Trip circuit supervision Bypassresistor, 1 BI

7840 Undercurrent monitoring Existent

7841 Motor start protection Existent

Nro. Parameter Value

3000 EARTH FAULT IN COMPENSATED/ISOLATED NETWORKS

3001 High-sensitivity earth-faultprotection

on

3013 IEE>> stage of high-sensitivityE/F prot.

0.014 I/In

3014 Delay time T-IEE>> of the IEE>>stage

0.50 s

3015 IEE> stage of high-sensitivityE/F prot.

+* I/In

3016 Pickup value of high-set stageIEE>> (dyn)

+* I/In

3017 Pickup value of O/C stage IEE>(dyn)

+* I/In

3018 Delay time T-IEE> of the IEE>stage

5.00 s

3019 Measurement repetition for E/Fpickup

no


Fig. 8 Example system with 10 identical motors


Low-Power Motors

3001: Sensitive earth-fault protection is activated.3013: 14 mA ensures that the non-directional stageIEE>> will not pickup in the event of reverse faults.3014: To prevent pickup or tripping due to tran-sients, e.g. on starting of the motor, the delay timeis increased to at least 0.5 s. In particular, a longdelay time is recommended for a Holmgreen cir-cuit because, in this case, transformer asymmetriessimulate a zero-sequence current on the secondaryside that does not exist on the primary side.

Other similar power system configurations wherethe earth-fault current suffices as the selectivitycriterion may be:

• Motors and several transformers for lowvoltage in the same power system withapproximately identical cable length.

• Infeed without a transformer at the transferpoint so that the entire capacitance of the pre-vious power system feeds to faults in the motorfeeders. Depending on the size of the previouspower system, considerably higher fault cur-rents can arise here that are also mastered witha Holmgreen circuit. The secondary settingsshould not be below approximately 0.3-0.5 I/IN.

Faults on the busbar, however, are not detected bythe protection relay at the infeed and they mustbe signalled with the aid of a voltage relay or theV0 protection function of the 7SJ602.

2nd example: Selectivity through directionalearth-fault protection

The power system above consists of only twomotors because the other motors are now nolonger on the system.In this case, the current in each feeder is the samefor internal and external faults. Therefore, thecurrent as the sole criterion does not suffice forselective tripping. In addition to the zero current,the fault direction must be determined by way ofthe zero voltage.

In Fig. 4, we see that the angle of the zero currentand voltage deviates by 180° between the forwardand reverse directions. This is a reliable and rela-tively robust directional criterion. This is alsowhy no special accuracy demands are placed onthe transformers in the isolated system either.A Holmgreen circuit of the current transformersfrequently suffices for measurement of the earthcurrent. The statements above in relation tomeasuring accuracy apply.

As the fault currents are now very low (I0, min =0.187 · 0.2 A = 0.0374 A primary), they can nolonger be measured with the transformers com-monly used (e.g. core-balance current trans-formers 60 A/1 A or phase current transformers75 A/1 A). A common solution to this problem isto use an earthing transformer at the busbar thatincreases the zero-sequence current in the isolatedpower system.

Fig. 9 shows connection of an earthing trans-former in a Yd circuit. A load resistor is con-nected to the open delta winding and allows acurrent to flow through all secondary windings.On the primary side, a zero-sequence currentarises in each phase, i.e. also in the faulty phase,although there is no driving voltage on the pri-mary side here. This zero-sequence current flowsinto the fault location via the earthing trans-former’s star point and closes itself through therespective cables via the feeding transformer’s starpoint. The dimensioning of the load resistor mustbe such that the lowest zero-sequence current tobe measured, i.e. in the event of a fault in themotor, e.g. in the case of 20 % of the stator wind-ing, can still be measured with the associatedfeeder protection.

Generally, secondary settings below 5-10 mAshould be avoided (measuring tolerances of thecore-balance current transformers and digitalresolution of the protection).

3rd example Increase of the earth current byusing an earthing transformer

The zero-sequence current of the second exampleshould be increased so that an earth current of atleast 10 mA secondary can be measured in theevent of a fault on 90 % of the stator winding.

An earthing transformer with 500 V secondaryand a core-balance CT 60 A/1 A are used.Calculation of the load resistance:With the above-mentioned data, the correspond-ing minimum primary zero-sequence current is:

I0min =1

3· 60 · 10 mA = 200 mA

IEmin = 3 · I0min = 600 mA

In the event of a fault outside the stator, the faultcurrent is

IEp =1

x· IEmin = 6 A

Fig. 9 Increasing the earth current by an earthingtransformer


Low-Power Motors

IEs =1

3· TRET · IEp =

1

3·

6 3

500 3

kV

V

/

/

⎛⎝⎜

⎞⎠⎟ · 6 A = 41.57

A

RBs =Vs

ES

V

41.57 AI= =500

12 Ω

Power dimensioning:

At the full displacement voltage RBs consumes apower of

PV

R= = ⋅ =s

Bs

2V A

12 VkW

2 6

0 2510

20 8. .

The load resistor and earthing transformer mustbe designed for this power.

As earthing transformers including load resistorare loaded only during a fault, they are frequentlydefined only for an operating duration of 20 s,during which the fault must be cleared. If thistime is exceeded, a relay before the load resistorensures its clearing.

With now sufficient earth current, the necessarysettings for directional earth-fault protection canbe defined:

The displacement voltage VE is set to 0.1 VN, thusensuring that VE enables direction detection evenat the lowest possible fault current and zerovoltage. 3312: Normally, VE is only signalled.

Current transformer phase angle errors areentered at the addresses 3102 to 3105. This isparticularly important in compensated powersystems. In isolated systems like the one in thisexample, extremely precise measuring currenttransformers can be dispensed with. 3115 and3122 specify the tripping direction. The type ofdirection determination is specified with 3125.Generally, the following applies: “sin phi” forisolated systems and “cos phi” for compensatedsystems. Here, “cos phi” was set because theresistive amount in the fault current through theearthing transformer is about ten times as high asthe capacitive amount. Thus, the fault currentassumes an resistive-capacitive character. 3124 isa parameter for rotating the direction characteris-tic on the basis of Par 3125 by a specific angle.With a resistive current of IEp = 0.6 A and acapacitive current of IEC = 0.0561 A, the resultingangle is 5 degrees (arctan 0.0561/0.6). 3124 istherefore set to + 5°, which is the capacitivedirection. This angle is constant in a very goodapproximation and is not dependent on whetherthe fault is inside or outside the stator winding.

No. Parameter Value

3100 DIRECTION OF SENSITIVE EARTH

3102 Second. current I1 for max. errorangle of C.T. I1

0.010 I/In

3103 Error angle of C.T. at I1 0.3 deg

3104 Second. current I2 for max. errorangle of C.T. I2

0.100 I/In


3115 Dir. IEE>> stage ofhigh-sensitivity E/F prot.

Forwards

3122 Dir. IEE> stage of high-sensitivityE/F prot.

Forwards

3123 Operating direction of the IEE>or IEEp stage

0.010 I/In

3124 Correction angle for direc.determination

5.0 deg

3125 Measurement mode for direc.determination

Cos phi

3126 Drop-off delay for dir.stabilization

1 s

No. Parameter Value

3300 DISPLACEMENT VOLTAGEPROTECTION VE>

3309 Displacement voltage level Ve> 0.10 U/Un

3311 Delay time for annunciationof VE>

1.00 s

3312 Delay time T-UE of the UE> stage 10.00 s


Low-Power Motors

The IE>> stage is used for the current threshold(Par 3013). A tripping time that is not too short isrecommended to reduce inaccuracies as a resultof transients (starting current of the motor ortransients on the occurrence of an earth fault). Inthis case: 0.5 s (Par 3014)

The resulting direction characteristic with vectorsfor the zero current and voltage is shown in Fig. 10.

c) Compensated power systemIn the compensated or resonant-earthed powersystem, the fault current at the fault location isreduced with the help of a coil matched to thesystem’s phase-earth capacitances, the so-calledPetersen coil, so as to enable self extinction of thearc.

In a compensated power system the capacitivecurrent in a faulted feeder is already low andfurther reduced by the parallel-connected zeroreactance of the Peterson coil. Therefore no(selective) protection can be provided with theaid of a current criterion. Instead, the so-called“wattmetric residual current method” or thecos phi method is applied.

No. Parameter Value



on


0.010 I/In

3014 Delay time T-IEE> of theIEE>> stage

0.5 s


+ * I/In

3016 Pickup value of high-set stageIEE>> (dy)

+ * I/In

3017 Pickup value of O/C stageIEE> (dyn)

+ * I/In

3018 Delay time T-IEE> of theIEE> stage

5.00 s

3019 Measurement repetition forE/F pickup

no


Fig. 10 Adjustment of the direction characteristic to theresistive-capacitive fault current. It should be setso that the earth current in the forward directionis perpendicular to the direction characteristic.

Fig. 11 Schematic diagrams of compensated or resonant-earthed system

11a)

11b)

11c)

IL: Current flows through XL

ILR: Current flows through RL

ILB, ILRB: Coil current in point B


Low-Power Motors

As the total zero sequence current in B (Fig. 11)either leads or lags depending on compensation,which may also change during operation due todisconnection of grid parts, the phase angle of thecapacitive/inductive zero current is not a reliablecriterion. Instead, the resistive current is assessed.This is the amount of the fault current that iscaused by the resistive amount of the Petersencoil’s reactance and is only to be found in thefaulty feeder. As this current is very low in com-parison with the inductive coil current (in theorder of magnitude of a few percent), sometimesan ohmic resistor is connected in parallel with thecoil so that sufficient resistive current is flowingfor an earth fault at 80 % of the stator winding.

Since the resistive currents are very low, it isimperative to use a core-balance current trans-former. A Holmgreen circuit would falsify theresult of measurement to an inadmissible extent.To correct the phase angle error of the core-balance current transformer, it is possible to entera few fault-current pairs of the core-balancecurrent transformer in the 7SJ 602.

3013: It is assumed that the residual resistivecurrent amounts to at least 10 mA. If necessary,the Petersen coil must be connected in parallelwith an ohmic resistance to achieve this value.

3102 to 3105 serve to correct the aforementionedcurrent transformer errors. 3123 is the minimumcurrent threshold as from which direction deter-mination is performed. 3125 defines the cos phimeasurement, i.e. the residual resistive current inthe fault current is assessed. In this case, 3124 re-mains set to 0° because only the resistive amountof the fault current is assessed.

� SummaryThe earth-fault protection of the motor must bedesigned with the appropriate SIPROTEC 7SJ602according to the earthing of the system star point.With the use of earthing transformers or core-balance current transformers the sensitivity isadequate for sure detection of a fault, even if thefault currents are very low.

No. Parameter Value



on


0.014 I/In

3014 Delay time T-IEE> of theIEE>> stage

0.50 s


+ * I/In

3016 Pickup value of high-set stageIEE>> (dy)

+ * I/In

3017 Pickup value of O/C stageIEE> (dyn)

+ * I/In

3018 Delay time T-IEE> of theIEE> stage

5.00 s

3019 Measurement repetition forE/F pickup

no


No. Parameter Value

3100 DIRECTION OF SENSITIVE EARTH

3102 Second. current I1 for max. errorangle of C.T.

0.010 I/In


3104 Second. current I2 for max. errorangle of C.T.

0.100 I/In


3115 Dir. IEE>> stage ofhigh-sensitivity E/F prot.

Forwards

3122 Dir. IEE> stage of high-sensitivityE/F prot.

Forwards

3123 Operating direction of the IEE>or IEEp stage

0.010 I/In

3124 Correction angle for direc.determination

5.0 deg

3125 Measurement mode for direc.determination

Cos phi

3126 Drop-off delay for dir.stabilization

1 s


Medium-Power Motors

Protection ofMedium-Power Motors

Motor settings using the SIPROTEC relay 7SJ62is explained below. Information is given on howto use the motor’s existing technical data toderive meaningful settings. Among other things,dynamic parameter changeover to reduceovercurrent settings during normal operation isexplained. As an option, an external RTD-boxcan be used to directly monitor the stator andbearing temperatures via temperature sensors.

�1. SIPROTEC 7SJ62The SIPROTEC time-overcurrent relay7SJ62 offers motor protection functions formedium-power motors with the following scopein one of the available ordering variants:

Besides 7SJ62, the 7SJ63 or 7SJ64 can also be used.The differences between these devices and the 7SJ62 are:

7SJ63:

Same functions as 7SJ62 and also

• Large graphical feeder control display• Comfortable on-site device operation for

control• Key switch• Large number of binary inputs and outputs

7SJ64:

Same as 7SJ63 and additionally

• Higher PLC/CFC logic program performance• Flexible protection functions• Additional rear interface (separate connection

of the RTD-box and rear DIGSI interfacepossible)

• A fourth voltage input for measuring thedisplacement voltage or for synchro checkfunction

Protection functions Abbreviation ANSI

Time-overcurrent protection(phase/earth)

I >>, I >, Ip, IE >>, IE >, IEp 50, 51, 50N, 51N

Directional earth-fault detection IEE >>dir., IEE dir. >>, VE/Vo > 67N, 51Ns, 59N

Thermal overload protection I2 t > 49

Starting time supervision Istart2 t 48

Restart inhibit for motors I2 t 66, 49R

Negative-sequence protection I2 > 46

Undervoltage V < 27

Temperature monitoring � (RTD-box) 38

Fig. 1 SIPROTEC 7SJ62 and 7SJ63/7SJ64 relays

LSP2

361.

epsLS

P229

9.ep

s


Medium-Power Motors

�2. Available motor dataNot all data is available in our example. Unfortu-nately, this all too frequently reflects reality, forexample when a motor has been in operation fora very long time and complete documentation isno longer available or only the data on the mo-tor’s rating plate is available.

Given:Compressor motor for compressed air in anindustrial plant:

�3. General power system dataThe current and voltage transformer data and thetype of connection are entered in the unit under“System Data 1”.

Current transformer 75 A/1 ACore-balance current transformer 60 A/1 AVoltage transformer 10 kV/100 VInsulated power system

Size Value

Power 780 kW

Rated voltage 10 kV

Motor rated current 54 A

Max. starting current (at 100 % VN) 250 A

Starting time (at 100% VN) 5 sec

Heating time constant 40 min

Cooling time constant at standstill 140 min

Max. continuous thermal ratingcurrent

60 A

Max. blocked rotor time 10 sec

No. Parameter Value

0213 Voltage transformer connection U12, U23, UE

0204 Transformer rated current,phase primary

75 A

0205 Transformer rated current,phase secondary

1 A

0217 Transformer rated current,earth primary

60 A

0218 Transformer rated current,earth secondary

1 A

0202 Transformer rated voltage prim. 10 kV

0203 Transformer rated voltagesecondary

100 V


Medium-Power Motors

�4. Starting time monitoringThe setting parameters for monitoring the start-ing time require the starting current and the start-ing time. In our example, the blocked rotor timeis not critical because it is longer than the startingtime (10 sec in comparison with 5 sec). The asso-ciated setting parameter for the blocked rotortime is deactivated (key in a lower-case “o” twicefor infinite). The starting time monitoring func-tion can quickly respond to a blocked rotor andexternal wiring with a speed monitor is dispensedwith.As the guaranteed starting time (5 sec) is specifiedin the technical data, the protection setting mustlie above it. In our specific case, the value 7 secwas chosen. Taking the current transformer ratiointo account, the maximum starting current iscalculated as Istart = 250 A/75 A = 3.33.

�5. Overload protectionTypical characteristic quantities designate theoverload function, namely: maximum permissiblecontinuous operating current, heating (heat-gain)time constant and cooling time constant at stand-still. The maximum current is often not specifiedin the technical data. According to experience, asetting 10 % above IN, M can be used and, in ourcase, IM, max = 60 A is already specified. Underparameter 4202,k factor = IM, max /ICT. prim = 60 A/75 A = 0.8 is set.

The 40 min heating time constant can be adopteddirectly as the setting. The overload protectionfunction also takes cool-down behaviour intoaccount because different prerequisites for themotor (with or without fan) produce differingcool-down behaviour. The value is set underparameter 4207 A as a heating to cooling factor,i.e. 140 min/40 min = 3.5.

No. Parameter Value

4102 Maximum startup current 3.33

4103 Maximum startup time 7 sec

4104 Locked rotor time oo

The thermal warning stage, parameter 4204,offers an additional warning before tripping. Thisis why it should be set to below the 100 % trip-ping threshold, for example 90 %. If you want toknow how high the thermal value already is atrated current, calculate

1/k2 = 1/(1.1·1.1) =1

1 2.= approximately 0.83,

i.e. approximately 83 % atIN = (54 A/75 A) 1 A = 0.72 A sec.

The chosen current warning stage can be set toequal the maximum current.

Parameter 4208A, dropout time after emergencystarting, is active only if coupling in via a binaryinput has been realized, so as to allow emergencystarting for the overload protection functiondespite the presence of an overload TRIP signal.

Fig. 2 7SJ62 connection diagram

No. Parameter Value

4202 k factor 0.8

4203 Time constant 40 min

4204 Thermal alarm stage 90 %

4205 Current alarm stage 0.8

4207 A Kt time factor at motor standstill 3.5

4208 A Dropout time after emergencystarting

100 sec


Medium-Power Motors

Under System Data 2 there is an important para-meter that defines the interplay of overload pro-tection and starting time monitoring (parameter1107). The overload protection function is activeup to this setting, the device detects higher cur-rent values as motor starting, the starting timemonitoring function runs and the thermal over-load model does not increase any further. It is ad-visable to set the value to approximately 50 % ofthe starting current (in our case 250 A/2 = 125 A,taken into account as the secondary value with thecurrent transformer ratio of 75 A/1 A = 1.66 A).Thus, starting at low rated voltage is detectedand protection is provided against sufficientlyhigh overloads above the maximum current(110 % of IN).

Direct measurement of temperaturesUp to two RTD-boxes with a total of 12 measur-ing points can be used for temperature detectionand these can be detected by the protection unit.Thus, the thermal state can be monitored, in par-ticular on motors, generators and transformers.The bearing temperatures of rotary machines arealso checked to detect when limit values are ex-ceeded. Temperatures are measured by sensors atdifferent points on the protected object and arefed to the unit through one or two RTD-box (es)7XV566.

As an alternative, the ambient or coolant temper-ature can be fed to the overload protection func-tion via the RTD-box. To this end, the necessarytemperature sensor must be connected to sensorinput 1 of the first RTD-box. As all calculationsare run with scaled quantities, the ambient tem-perature must also be scaled. The temperature atrated current is used as the scaling quantity. If therated current deviates from the rated transformercurrent, the temperature must be adjusted withthe help of the following formula under parame-ter 4209, “temperature at rated current”.

ϑ ϑNsecondary N, MNprim

N, M

= ⋅ ⎛⎝⎜

⎞⎠⎟

I

I

2

For example ϑN, M = 80 °C(obtained by measurement),then ϑNsecondary = 80 °C (75/54)2 = 153 °C

No. Parameter Value

1107 Motor starting current(blk overload)

1.66 A

No. Parameter Value

4209 Temperature rise at ratedcurrent

153 °C

Fig. 3 RTD-box 7XV5662-xAD10

LSP2

411-

afp

.ep

s


Medium-Power Motors

Fig. 4 Connecting cable between SIPROTEC and RTD-box 7XV5103-7AAxx

�6. Restart inhibitThe second thermal model for motor protectionis created in the restart inhibit function for pro-tection of the rotor. The main focus is placedon the number of starts from the cold and warmstates. This data is not available in our case andso three cold starts (nc) and two warm starts (nw)are assumed.

The rotor temperature equalization time, para-meter 4304, is set to 1 min and defines the mini-mum dead time between individual starts.Parameter 4302, starting current/rated motorcurrent, results in 250 A/54 A = 4.6.The value at which the motor starts with ratedtorque and rated voltage is selected as startingtime. Often, the time is indicated in the startingcharacteristics supplied. In this specific case start-ing time is 5 seconds.

If data is not available, the cool-down behaviorfor the restart inhibit function can be assumedsimilarly to the cool-down behavior for overloadprotection. In the case of overload protection, thiswas 140 min/40 min = 3.5. For the restart inhibitfunction, this is entered under parameter 4308.

Parameter 4309, “extension of time constantat running”, takes effect when activation of themotor was successful and then the motor con-tinues to run in rated operation. Cooling for thisis clearly shorter than under parameter 4308 be-cause rotor operation involves intrinsic coolingand so we choose the factor 2.

After three starts in brief succession, the restartinhibit function then blocks (and issues a releasesignal after a calculated dead time to enablereconnection once again). The internally calcu-lated dead time depends on the respective loadand, accordingly, may differ in length. Alterna-tively, it is possible to consciously specify a mini-mum inhibit time (for example, if the customerinsists on additional safety factors). This is set un-der parameter 4310.

�7. Negative-sequence (unbalanced-load)protection

As no further information is available, recom-mendation-based values are used. In the case of adefinite-time tripping characteristic, these are:

10 % I2/IN, M for warning or long-time delayedtripping and approx. 40 % I2/IN, M for short-timedelayed tripping.The setting values have to be converted for thesecondary side by using the transformation ratioof the current transformer (75 A/1 A). Forparameter 4002 the lowest setting value is 0.1 A.

No. Parameter Value

4302 Starting current/ratedmotor current

4.6

4303 Maximum permissiblestarting time

5 sec

4304 Rotor temperatureequalization time

1.0 min

4305 Rated motor current 0.72 A

4306 Max. number of warm starts 2

4307 Difference between warm andcold starts

1

4308 Extension time constant at stop 3.5

4309 Extension time constant atrunning

2.0

4310 Minimum restart inhibit time 6.0 min

No. Parameter Value

4002 Pickup current I2 >> 0.1 A

4003 Time delay T I2 >> 20 sec

4004 Pickup current I2 >> 0.30 A

4005 Time delay T I2 >> 3 sec


Medium-Power Motors

�8. Time-overcurrent protectionOnly the phase time-overcurrent protection isconsidered; due to the “isolated power system”earthing, the earth function is covered by thesensitive earth-fault function.In relation to the phase time-overcurrent protec-tion settings, it must be noted that these must lieabove the motor starting values. Due to the short-time occurring motor inrush, the I>> stage musteven be set to >1.5 · starting current, and so wechoose 1.6 · (250/75) A = 5.33 A. Time delay TI>> (parameter 1802) is selected at 50 ms. Ini-tially the peak value of the starting current may beeven higher. With time delay T I>>, non-delayed,set at 0 ms, the I>>-stage should be set with 2.5 ·starting current. 1.1 · (250/75) A = 3.6 A is calcu-lated for the I> stage.

If a short-circuit current occurs which lies at2.5 · IN, M due to a contact resistance, thetime-overcurrent protection will not pickup but,as shown in Fig. 5, will trigger starting time super-vision. Tripping does not occur, however, untilafter a few seconds (> 7 seconds in our example),which can lead to enormous damage.

It is now possible to reduce the definite-timeovercurrent-time settings during rated motoroperation, so as to be able to respond more sensi-tively to all manner of current faults (see Fig. 6).To this end, the “Dynamic Parameter Change-over” option is used: during the normal state,i.e. when the motor is running, lower settings arevalid which may already trigger (with a shortdelay) in the event of faults as from 1.5 · IN, M

(depending on overload conditions).

Two criteria are optionally available for detectionof the deactivated system and thus changeover tohigh settings:

• The circuit-breaker position is communicatedto the unit via binary inputs (parameter1702 dynPAR.START = CB position).

• Falling below an adjustable current threshold(parameter 1702 dynPAR.START = currentcriterion) is used.

Fig. 5 Coordination of protection functions(without dynamic parameter changeover)

The active time parameter 1704 must be set tohigher than the motor starting time. The reducedvalue for I>, parameter 1204, results from1.5 · (54/75) A = 1.1 A.

Dynamic parameter changeover:

Additional programming with PLC/CFC logicis not necessary. Changeover takes place auto-matically. To avoid tripping prematurely with theI> stage 1204 in the event of short-time, substan-tial overloads (load jam), the tripping time is setto 1 sec.

No. Parameter Value

1703 Interruption time 0 sec

1704 Active time 8 sec

1801 Pickup current I>> 5.33 A

1802 Time delay T I>> 0.05 s

1803 Pickup current I> 3.6 A

1804 Time delay T I> 0.2 s

Definite-time overcurrent-time protection:

No. Parameter Value

1202 Pickup current I>> 5.33 A

1203 Time delay T I>> 0.05 sec

1204 Pickup current I> 1.1 A

1205 Time delay T I> 1 sec

Fig. 6 Reducing the time-overcurrent protection I> stageduring rated operation


Medium-Power Motors

�9. Sensitive earth-fault detectionSensitive earth-fault detection includes a largenumber of settings, for example to exactly deter-mine the displacement voltage or to correct trans-former errors. We will consider only the essentialsettings for detection of a directional earth fault.The other parameters can remain at default set-tings. The thresholds for the IEE current must bedetermined by way of the power system data. Tothis end, we need the cable lengths and types tocalculate the capacitive earth current ICE.

Sometimes a load resistor is also connectedupstream, so as to arrive at an increased earth-fault current in the event of a displacementvoltage. Let us assume a capacitive earth currentICE of 20 A. For a protection range of 90 %, theprotection should already operate at 1/10 ofthe full displacement voltage (parameter 3109with 0.1 · 100 V = 10 V), where also only 1/10 ofthe earth-fault current results.Therefore, (20 A/(60 A/1 A)) 0.1 ≈ 0.035 Α is setfor parameter 3117.

With regard to determining the direction, notethat the earth current flows in the direction ofthe protected motor when parameter 3122 set to“forward“ and parameter 0201 “current trans-former star point” in direction of “line” areselected and the earth transformer is connectedas shown in Fig. 2.

No. Parameter Value

3113 Pickup current IEE>> 0.5 A

3114 Time delay T IEE>> oo

3117 Pickup current IEE> 0.035 A

3118 Time delay T IEE> 5 sec

3109 Ven> measured 10 V

3122 Direction IEE> Forward

3125 Measurement method Sin. Phi

�10. Voltage protectionThe motor still has to cope with up to about 80 %of the rated voltage and values below that lead toinstability.

� SummaryThe motor protection functions of theSIPROTEC 7SJ62 derived from the current andvoltage inputs result in a combination that offersusers effective and low-cost all-round protectionand which is very frequently utilized for medium-voltage motors in industry. Steps for transferringthe motor data to 7SJ62 setting data were dis-cussed and the substitute settings suitable forcharacteristic motor variables were proposed.

No. Parameter Value

5103 Pickup current U< 75 V

5106 Time delay T U< 1.5 sec

5111 Pickup current U<< 70 V

5112 Time delay T U<< 0.5 sec


Medium-Power Motors


High-Power Asynchronous Motors

Protection of High-PowerAsynchronous Motors

To minimize impacts on the system to the great-est extent possible when starting motors, high-voltage motors are often switched in via a startingprocess. Various processes have an influence onthe design of protection. This chapter works outthe protection setting, taking reduced voltagestarting via a starting transformer into account.

�1. Device selectionAn expedient mix of functions is needed to pro-tect a motor against the various kinds of faults.Design should be based not only on the motor’spower output, but also on the importance of thedrive for the technological process, the operatingconditions and the requirements of the motormanufacturer.The motor in this example has a rated power of3 MW. For this size of motor, it is advisable to usefast short-circuit protection in the form of differ-ential protection, which is performed by aSIPROTEC 7UM62 for asynchronous motors.

Other protection functions recommended for amotor of this power rating are:

Transformer protection is not discussed in thischapter and must be set up separately with addi-tional hardware. This can be done with a second7UM62, for example, which is set as transformerdifferential protection. Savings in terms of spare-parts stocking can be achieved by using uniformhardware.

Transformer differential protection relay 7UT6 isanother possibility for protecting the transformer.In the case of a remote transformer outside theprotected zone, the line differential protectionrelay 7SD610 can be used with the additionalfunction (transformer in the protected zone).

�2. ConnectionAs shown in the following single-pole overviewdiagram (Fig. 1), the asynchronous motor is con-nected to a Yd5 transformer in unit configura-tion. With a view to the power system conditions,the starting current IA should be kept low and thisis why the motor is started via reduced voltagestarting by means of the so-called three-switchmethod or the Korndorfer circuit. The machinevoltage is stepped down during starting by meansof a starting transformer (autotransformer). Assoon as the motor has reached a certain speed, thetransformer is bypassed with Q2 and the motor isswitched to the full power system voltage.

Starting takes place as described below:

1) Q3 closed2) Via Q1, the starting transformer is switched

into the power system and runs at a reducedvoltage.

The motor voltage is taken off a center tap of thestarting transformer and is as follows:

Vn

nVM power system= 2

1

n

n2

1

= winding ratio of starting transformer

Via the starting transformer, the motor voltagecan be stepped down to such an extent as is per-missible with a view to the opposing torque dur-ing starting. The starting current from the powersystem also decreases in the same ratio as thestarting torque.

Protection functions Abbreviation ANSI

Stator thermal overloadprotection

I2t 49

Restart inhibit for rotors I2t 66, 49R

Motor starting timesupervision

Istart2t 48

Negative-sequence(unbalanced-load) protection

I2> 46

Earth-fault protection V0> 59N

Differential protection DI> 87M



The current consumption from the power systemduring starting is as follows:

In

nIpower system M= 2

1

and is limited via the starting transformer’s trans-formation ratio.

3) In the next starting phase, Q3 is opened andthe motor is powered from the system via thestarting transformer’s series inductance.

4) Once the motor has reached the specifiedspeed, Q2 is closed and the motor is operatedat the full power system voltage.

�3. Available data

Transformer

Motor

Starting transformer

Fig. 1 Single-pole overview diagram

Current transformer,star point end

650 A/1 A

Current transformer,feeder end

650 A/1 A

Voltage transformer3 3

3

100

3

100

3

. kV V V

Dimension Value

Rated power 3000 kW

Rated voltage 3300 V

Rated current 591 A

Idle current 93 A

Thermally permissiblecontinuous current

650 A

Thermal stator time constant 12 min

Cooling stator time constant 60 min

Permissible starts from thecold state

3

Permissible starts from thewarm state

2

100 % VN 57 % VN

Starting current 6.7 IN 3.7 IN

Starting time 5 sec 28 sec

Locked rotor time,cold machine

5 sec 29 sec

Locked rotor time,warm machine

4 sec 21 sec

Primary voltage 3300 V

Secondary voltage 2362 V

* Optionally, an additional current transformer can be usedat the starting transformer’s star point. This is discussed inthe section on differential protection.



�4. Protection functions appliedThe various protection functions are describedbelow. The settings are specified as secondaryvalues. The focus is on the differential and earth-fault protection functions. Protection functionsthat protect a motor against thermal overloadingare described in depth in the chapter entitled“Thermal stress of motors and necessary protec-tion functions”.

4.1 Differential protectionDifferential protection provides fast short-circuitprotection for the motor. The principle of meas-urement is based on a comparison of all currentsflowing into the protected object (Kirchhoff’scurrent law). The direction of the counting ar-rows is defined so that currents flowing in thedirection of the protected object are counted aspositive.

In Fig. 2, the following results for the differentialcurrent:

Δ =I I I Idiff = +1 2

If very large currents flow through the pro-tected zone at the moment of switching on, acorresponding differential current arises in themeasured element M with differing transferbehavior in the saturation range of the currenttransformers W1 and W2, and this can causetripping. A stabilizing current Istab is addition-ally introduced to avoid such over-functioningof the protection.

I I Istab = +1 2

A differential protection characteristic (Fig. 3)consisting of four branches a, b, c and d is de-fined via the differential and stabilization cur-rent.

Pickup of the differential protection takes placein two stages. In addition to the pickup thresh-old Idiff>, a second pickup threshold has alsobeen introduced. If this threshold (Idiff>>) isexceeded, tripping is effected regardless of theamount of the stabilization current.

The starting process of a motor via a startingtransformer must be considered in greater detail.Due to the transformation ratio of the startingtransformer during starting, the following applies:

IV

VI1

2

12= ⋅

I I I IV

VI

V

VIdiff = + = − ⋅ = −⎛

⎝⎜

⎞⎠⎟ ⋅1 2 2

2

12

2

121

I I IV

VI I

V

VIstab = + = − + = +⎛

⎝⎜

⎞⎠⎟ ⋅1 2

2

12 2

2

121

It is necessary to check whether the differentialcurrent caused by the starting transformer duringstarting leads to tripping. The differential currentprofile is as follows:

I

I

V

VV

V

diff

stab

V

VV

V

=−

+=

−

+=

1

1

12362

33002362

33001

0

2

1

2

1

.166

and is plotted into the characteristic diagram(Fig. 3) as a broken line. The characteristic showsthat the differential current is always in the char-acteristic’s inhibition range – despite the existingdifferential current, the differential protection is

Fig. 2 Basic principle of differential protection on themotor (single-phase depiction)

Fig. 3 Tripping characteristic of the differential protectionwith fault characteristic



stable during starting and can therefore also beswitched to the active state during starting.

Fig. 4 shows the value pairs from a recorded in-stantaneous value record from this motor in thetripping diagram of the differential protection.The curve reflects the calculated curve of the dif-ferential current – the fault current moves alongthe “straight line” into the quasi-stationary oper-ating point (blue circle).

Still during starting, current transformer satura-tion (see Fig. 5) occurs in the phase L2 (character-istic shown in +++) due to high starting currents,with an extreme rise in the differential current,the result being that the operating point movesinto the tripping zone above the tripping charac-teristic shown, in the worst case entailing unin-tentional tripping of the differential protection.

To avoid overfunction during this mode of opera-tion, the differential protection’s pickup value canbe increased at the moment of starting. For pro-tection of a motor, it is recommended to increasethe differential protection characteristic by a fac-tor of two during starting. In Fig. 4 we can seethat the differential current does not exceed theraised tripping characteristic. The differentialprotection remains stable.

Fig. 5 shows the instantaneous value curves of thecurrents for the phase L2. The saturation responseis clearly evident after the first two sinusoidalcurves; the reason being DC components in thestarting current. Even though identical currenttransformers are applied, different current trans-former burden cause divergent transmission per-formance with a differential current (see Fig. 4).

Differential currents during motor starting (approx. 5 sec )(recorded differential and stabilization currents evaluated with

Fig. 5 Depiction of the reason for the differential current transformersaturation in phase L2 of side 2 (evaluated with Mathcad)



In Fig. 6, the motor’s starting current (positive-sequence system) is shown in the top curveI1 as an r.m.s. value record of side 2 (motor’sfeeder side). The recording took place duringcommissioning of the motor with the protectionrelay 7UM62. The starting current follows a typi-cal curve. The current peak at approximately21.5 sec, caused by compensation phenomenaduring shorting of the starting transformer(closure of Q2), is conspicuous.

The duration of the pickup increase should bemaintained beyond this time. The sensitivity ofdifferential protection is reduced by activation ofthe pickup value increase. An additional currenttransformer at the starting transformer’s starpoint is needed (see note for Fig. 1) if the full sen-sitivity is not to be dispensed with. The currenttransformer’s secondary side is inversely con-nected in parallel with the current transformer ofside 2 (connection side), thus correcting the cur-rent on the connection side. As a result, the cor-rect measured values are fed to the differentialprotection, also during starting, and raising of thepickup characteristic can be dropped – the differ-ential protection also operates during startingwith the normal sensitivity.

In the present example, there was no additionalcurrent transformer at the starting transformer’sstar point and raising of the pickup characteristicwas chosen to ensure stability of the protection.

During commissioning, it is advisable to set thefault value recording mode to r.m.s. valuerecording, thus making it possible to record theentire starting operation (≤ 80 sec.) and to recog-nize such variations.

The 7UM62 begins to automatically record assoon as the stabilization current enters the addi-tional stabilization zone by more than 85 %, withthe result that a record is created automaticallyduring every starting operation and does not needto be triggered separately.

To complete commissioning, recording should bechanged over to instantaneous value logging, in-cluding recording of the differential and stabiliza-tion currents.

Fig. 6 Motor starting current (r.m.s. value record from the 7UM62)

LSP2

841.

tif



Below, the setting parameters for the differentialprotection function are listed and briefly com-mented on.

4.2 Earth-fault protection

An earth fault is caused by a fault on the insula-tion of a phase to earth.

The motor discussed in this paper is connected ina unit configuration. In this case, evaluation ofthe displacement voltage has proven a reliablecriterion for earth-fault detection. The full dis-placement voltage VE occurs in the event of aterminal earth fault (Fig. 7).

In the event of an earth fault on the winding, itdecreases in proportion to the winding voltageand becomes zero for of an earth fault at themachine’s star point.


I-DIFF>Pickup value of the trip stageIDIFF>

0.20 I/InO The setting is referred to the protected object’s rated current.The 0.20 I/InO default setting can be retained.

T I-DIFF>Delay time of the trip stageIDIFF>

0.00 s The delay time T I-DIFF> is started if an internal fault in the motorhas been detected. The differential protection should normally tripwithout delay, and so the default setting of 0 sec can be retained.

I-DIFF>>Pickup value of the trip stageIDIFF>>

7.5 I/InO The I-DIFF>> stage is an unstabilized fast tripping stage. As far asthe set value is concerned, it must be ensured that the value is notexceeded during normal operation.The setting should be oriented to the maximum starting current(in this case 6.7 InO).

T I-DIFF>>Delay time of the trip stageIDIFF>>

0.00 s The T I-DIFF>> time is an additional time delay and does not includethe relay’s inherent operating time delays. The differential protectionshould normally trip without delay, and so the default setting of0 sec can be retained.

CH-INCREASED-STARTINGPickup value increase duringstarting

On Increasing of the characteristic must be activated with theCH-INCREASED-STARTING parameter.

STARTING-STABPickup value ISTAB forstarting detection

0.10 I/InO The setting is referred to the protected object’s rated current.

STARTING FACTORPickup value increaseduring starting

2 The factor for increasing the pickup values during starting is definedwith the STARTING FACTOR. A factor of 2 has proven practicable.

MAX. STARTING TIMEMaximum starting time

30.0 s The maximum starting time is specified as 28 s in the motor data.The starting increase should be effective for the entire startingprocess and this is why a setting of 30 s is chosen.

Fig. 7 Displacement voltage VE as a function of the fault locationin the event of an earth fault in the machine winding



The chosen setting for the displacement voltageshould be very low, so as to achieve the greatestpossible protection range on the machine’s wind-ing. By contrast, the possible sensitivity of theprotection is limited by interference voltageswhich, in the event of earth faults in the upstreampower system, are transmitted to the motor volt-age side via the coupling capacitance of theunit-connected transformer.

The equivalent circuit in Fig. 8 shows that VEO issplit up into the voltages VC and VRprim . Via thevoltage transformer’s (earthing transformer’s)transformation ratio, VRprim is transformed to thesecondary side (VR) and represents the interfer-ence voltage measured by the relay on the loadresistor.

As VR has to be lower than the relay’s pickupvalue, the chosen load resistor R must have a lowresistance. A protection range covering about 80to 90 % of the motor winding is achieved in thisway. To achieve a protection range of 80 %, forexample, the relay must be set to 20 % of the fulldisplacement voltage VE.

Dimensioning earth-fault protection with an 80 %protection range

The capacitive resistance of the coupling capaci-tance CK is always very much higher than theohmic resistance of the load resistor R. The cur-rent transformed to the LV side in the event of anearth fault on the HV side therefore practicallydepends only on the amount of CK and VEO.

The dimensioning of R is such that VR is approxi-mately half the pickup value of the earth-faultfunction that is to be set. This achieves doubleprotection against spurious tripping.

I V CCprim EO KfkV

f nF A= ⋅ ⋅ = ⋅ ⋅ ≈211

32 10 0 02π π .

• ICprim Interference current on the primaryside of the earthing transformer(voltage transformer)

• VEO Displacement voltage on the HV sideof the unit-connected transformer

VEOkV= 11

3

• CK Total capacitance between the HV andLV sides of the unit-connected trans-former (total coupling capacitancebetween HV and LV) CK = 10 nF (Notransformer data sheet available =>standard value)

I I TRCsek Cprim ET A A= ⋅ = ⋅ ⋅ =1

3

1

30 02 571 0 38. . .

• TRET Transformation ratio of the earthingtransformer (voltage transformer)

TR

V

ET

N

V

V

= = =3100

3

3300

3100

3

571.

• ICsek Interference current on the secondaryside of the earthing transformer(voltage transformer)

RI

= ⋅ ⋅ = ⋅ ⋅SF KV V

ACsek

100 1

20 2

100

0 3826 3.

.. Ω

• SF Safety factor VR N 0.5 · relay setting• K 80 % protection range → K = 0.2• R Load resistor

PV

R= =

2

380 VA

• P Required power of the earthingtransformer under loading with R

IV

RtR max

VA for s= = = ≤100

26 3380 20

..

Ω

• IR max Current via load resistor R in thecase of 100 % VE

Fig. 8 Single-phase equivalent circuit(CE=Cgenerator+Cline+Ctransformer neglected)

VEO Displacement voltage on the HV sideCK Total coupling capacitanceRprim Load resistor R referred to the primary sideVRprim Interference voltage referred to the primary side of

the earthing transformerVR Voltage measured by the protection relay at R in the

event of an earth fault on the HV side(interference voltage)

TRET Limb transformation ratio of the earthing transformer



The following are chosen:

1) Load resistor (for example, with 30 Ω) and anadjustable intermediate tap

2) Voltage transformer with 380 VA for 20 secload duration

Below, the setting parameters for the earth-faultprotection function are listed and briefly com-mented on.

4.3 Thermal overload protectionOverload protection prevents thermal overload-ing of the asynchronous motor’s stator winding.

Below, the setting parameters for thermal over-load protection are listed and briefly commentedon.


V0 >Starting voltage V0>

20.0 V The range for earth-fault protection is defined with this setting.A protection range covering 80 % (20 V) to 90 % (10 V) of the statorwinding is usual. An increase in the sensitivity also means that theload resistance becomes smaller, and thus also the power of theearthing transformer has to become higher.

T SESDelay time T SES

0.30 s The maximum loading duration of the loading unit must be takeninto consideration when defining the time delay. A time delay of0.3 sec – 0.5 sec has proven practicable.


K-FACTORMaximum permissiblecontinuous current

1.00 The thermally maximum permissible continuous current Imax isdescribed as a multiple of the protected object’s rated current IN:Imax prim = k · INThe data sheet indicates a thermally permissible continuous currentof 650 A.Values are as follows with the following motor and transformer data:Rated current of the motor IN, M = 591 AThermally permissible continuous current Imax prim = 650 ACurrent transformer 650 A/1 A

K factor kA

Aset

N, M

N, CT. prim

= ⋅ = ⋅ =I

I11

591

6501 0. .

Heating time constant720 s Taken from the motor's data sheet – 12 min

K -FACTORCooling time constant

5.0 The machine’s cool-down behavior at standstill can be taken intoaccount with the K time factor.Taken from the motor’s data sheet – 60 min.The default setting of 1 can be retained if different thermal behavior isnot to be taken into consideration.

WARNThermal warning stage

90 % The thermal warning stage issues a warning before the trippingtemperature is reached.In the case of the motor discussed with k = 1.1 and adaptedrated machine current, the temperature-rise limit value(overtemperature value) is

ΘΘTRIP k

= = =1 1

1183

2 2.%

of the tripping temperature. A setting of 90 % therefore lies abovethe operationally expected overtemperature of 83 %, but below thetripping temperature of 100 %.



4.4 Restart inhibitEspecially when the motor is started, the rotor issubjected to very high thermal stress. Frequentstarting of motors can result in thermal overload-ing. The purpose of the restart inhibit function isto inhibit restarting of the motor.

As the rotor current cannot be measured directly,the rotor temperature is simulated by means ofavailable stator variables and restarting is inhib-ited if a limit temperature is exceeded.

Below, the setting parameters for the restart in-hibit function are listed and briefly commentedon.


I WARNCurrent warning stage

1.00 A A current warning stage can be realized in addition to the thermalwarning stage. The current warning stage should be set to equal thecontinuously permissible secondary rated current.

II

IIWarn

N, M

N, CT primN, CT secondarysecondary

k= ⋅ ⋅ = ⋅ ⋅ =11591

6501 1 0. .

A

AA A

I LIMITLimit current of the thermalreplica

2.27 A The limit must be chosen so that, even at the highest possible short-circuit current, the tripping times of the overload protection are reli-ably above those of the short-circuit protection functions. Limiting toa secondary current that corresponds to approximately 2.5 – 3 timesrated machine current is generally sufficient.


IStrt/IMot.ratedStarting current/motor rated current

3.7 The starting current parameter is entered as the ratio to the ratedmotor current (IStrt/IMot.rated). For correct interpretation, it is im-portant for the apparent power SN GEN/MOTOR and the rated volt-age VN GEN/MOTOR of the motor to be set correctly in the systemdata 1.

The current in the case of the longest starting time is taken from themotor’s data sheet. This is the value at the reduced voltage. On thedata sheet, the current and the starting time are specified at a volt-age of 57 % VN.

T Starting MAXMaximum permissiblestarting time

28 s Taken from the motor’s data sheet – the time chosen mustcorrespond to the entered starting current.

n-WARMPermissible numberof warm starts

2 Taken from the motor’s data sheet.

n-COLD<-> n-WARMDifference between warmand cold starts

1 Taken from the motor’s data sheet.

T COMPENSATIONRotor temperatureequalization time

1 min No values are specified on the motor’s data sheet. It is recommendedto leave the default setting as it is. See also the chapter entitled“Thermal stress of motors and necessary protection functions”.

K -OPERATIONExtension of the time constantτL during operation

2 While the motor is running, heating up of the thermal rotor replica iscalculated with the time constant τL worked out on the basis of themotor’s characteristic values. Requirements for slower cooling:cooling is generated with the time constant L · K –OPERATION.

The present motor’s data sheet does not specify any details of a rotortime constant. In this case, it is recommended to leave the defaultsetting K -OPERATION = 2, in which case cooling takes twice as longas heating up.



4.5 Starting time monitoringStarting time monitoring supplements the over-load function and the starting inhibit function toprevent inadmissible thermal stressing of therotor due to excessively long starting operationsas a result of a locked rotor, for example.

Below, the setting parameters for starting timemonitoring are listed and briefly commented on.


K -STANDSTILLExtension time constant τL atstop

5 To correctly take into account the lower heat dissipation ofself-cooled motors at motor standstill, the cooling time constant atstandstill L · K –STANDSTILL can be entered separately.

The present motor’s data sheet does not specify any details of a rotortime constant. In this case, it is recommended to leave the defaultsetting Kτ = 5, in which case cooling at standstill takes five times aslong as heating up.

To ensure correct functioning, it is important to correctly set thecurrent threshold to distinguish motor standstill/running LS I>(recommendation: 0.1 · I/IN, M).

T MIN.INHIBIT TIMEMinimum inhibit time of therestart inhibit function

12.8 min Independently of thermal models, the requirement for a minimuminhibit time after exceeding the number of permissible starts can befulfilled with the T MIN.INHIBIT TIME. The parameters are in confor-mity with the motor manufacturer’s specifications. As there are norequirements for the present motor, setting acc. to the thermal rep-lica is possible.

τL · Kτ–OPERATION 28 sec · (3-2) · 3.72 · 2 = 766 s


STARTING CURRENTMotor's starting current

3.07 A Starting time monitoring operates with the measured values ofside 2. Starting takes place via a starting transformer, this must betaken into account when setting the parameters.

I start reducedV

V

A

AA=

⎛⎝⎜

⎞⎠⎟ ⋅ ⋅ ⋅ =2362

33006 7

591

6501 3

2

. .07 A

MAX. STARTING TIMEPermissible starting timeof the motor

18 s The setting for the maximum starting time in accordance with thestarting characteristic shown in Fig. 6. Starting takes approximately14 sec. At 18 sec, a slightly higher setting is chosen.

I STRT. DETECTStarting detection currentthreshold

1.50 A The threshold must lie above the maximum load current (load peaksmust be taken into account) and below the minimum starting cur-rent. In the present example, starting detection is set to above thecontinuously permissible current and below the expected startingcurrent.1) Continuously permissible current:

I I Ithermperm N, CT secondary N, CT seconA

A= = ⋅650

6501 dary A= 1

2) Starting current at VM = 2362 VAssumption: Starting current decreases in a linear fashion

I start reducedV

V

A

AA=

⎛⎝⎜

⎞⎠⎟ ⋅ ⋅ ⋅ =2362

33006 7

591

6501 3

2

. .07 A

ISTRT.DETECT setting parameter is set tow 0.5 · 3.07 A = 1.53 A ≈ 1.5 A.

LOCKED ROTOR TIMEMotor's locked rotor time

4 s The setting was taken from the motor’s data sheet.As the maximum permissible locked rotor time tE is shorter than themotor’s starting time, a locked rotor must be detected via a speedmonitor and read into the protection relay via a binary input(“>locked-rotor”).



4.6 Negative-sequence protectionAn unbalanced load is the result of failure of aphase or asymmetry of the power system voltage.The motor frequently continues to run and, incomparison with three-phase operation, con-sumes an increased current, which may lead toexceeding of the permissible limit temperature.

The negative-sequence (unbalanced-load) protec-tion of the 7UM62 operates with the inverse cur-rent I2 – if a parameter-definable threshold isexceeded, the tripping time is started and a TRIPcommand is triggered after it has elapsed.

Below, the setting parameters for the unbal-anced-load protection function are listed andbriefly commented on.

� SummaryThe application-oriented function mix of theSIPROTEC 7UM62 in the order variant“asynchronous motor” can be used to advantagefor this application. The asynchronous motor isoptimally protected with just one multifunctiondevice.


I2 PERM.Continuously permissibleload unbalance

9.1 % For the thermal replica, the maximum permissible inversecurrent I2max.prim/IN is crucial. According to experience,it is between 6 % and 12 %.

I2I

I

I

Iperm

max.prim

N, M

N, M

N, CT prim

= ⋅2

The machine manufacturer does not specify any values for the maxi-mum permissible inverse current. This is why I2max.prim/IN = 10 % ischosen.

I2permA

A= ⋅ =10

591

6509 1% . %

FACTOR KAsymmetry factor K

1.65 s The asymmetry factor is machine-dependent and represents themaximum time in seconds during which the motor may be stressedwith 100 % load unbalance.The conservative Kprim = 2 s setting is chosen if the motor manufac-turer does not specify any details.

The factor Kprim can be converted to the secondary side with thefollowing relationship:

K K ssecondary primN, M

N, CT prim

= ⋅⎛

⎝⎜

⎞

⎠⎟ =

I

I

2

1 65.

T COOLCooling time of thethermal replica

165 s The T COOL parameter defines the period of time that elapses beforethe protected object cools to the initial value after stressing with apermissible unbalanced-load I2 PERM.

If the machine manufacturer does not specify any details, the settingcan be found by assuming the cooling and heating times of the pro-tected object as being equal. The following relationship then appliesbetween the asymmetry factor K and the cooling time:

TCOOLK

2

ss

perm

N

=⎛⎝⎜

⎞⎠⎟

= =I

I

2 1

1 65

0 1165

.

.

An asymmetry factor K = 2.4 s and a permissible continuousnegative sequence of I2/IN = 10 % result in a cooling time of 240 s.

I2>>Starting current I2>>

60 % Two-phase operation under rated conditions leads to a negative-sequence current of about 66 %. Due to the power to be produced,it rises and reaches values of 100 %.

T I2>>Delay time T I2>>

3 s A tripping delay of about 3 sec is recommended to allow for transientphenomena.


Synchronous Motors

Protection ofSynchronous Motors

This chapter focuses on protection systems forlarge synchronous motors. Several references aremade to other chapters in this volume in whichprotection functions also applied to synchronousmotors are presented in detail. Thus, this motorprotection volume must be studied in its entirety.

�1. Scope of a protection system for motorsSynchronous motors perform various tasks inplants in the raw materials and processing indus-try, in power generation systems and in infra-structure facilities. Synchronous motors with apower output up to 50 MW are preferably usedwhen high performance demands are placed onmotor operation. Examples are conveying systemsin mining or coolant feed pumps in power plants.

With regard to the fundamentals of operation anduse of various types of motors, refer to the chap-ter “Introduction to the principles of synchro-nous and asynchronous motors”. Selectedprotection functions such as overload or differen-tial protection are dealt with in detail in the chap-ters “Thermal Stress of Motors and NecessaryProtection Functions” and “Protection of High-Power Asynchronous Motors”. Their discussionsalso apply to large synchronous motors.

Fig. 1 Olikiluoto, Finland, nuclear plant

LSP2

840.

tif

Besides dealing with a few general considerationson the operation and availability of motors, thischapter focuses on further protection functionsor types of protection required especially for syn-chronous motors.

The motor protection function package wasimplemented specially for large motors in the7UM62 numerical protection relays. It containsall functions for comprehensive protection ofhigh-power motors.

Fig. 2 shows an example of connection of the7UM62 protection to a synchronous motor.


Synchronous Motors

Fig. 2 Connection diagram of motor protection 7UM62


Synchronous Motors

The following table contains a summary of theprotection functions mainly used for large syn-chronous motors.

Working out an adequate protection system for amotor is oriented to several factors. It goes with-out saying that the protection engineer strives toprotect a motor comprehensively against possibledamage, such as thermal overload, or to limitunavoidable damage, such as short circuits, to aminimum. By contrast, protection should notunnecessarily restrict the availability of a motordrive by premature deactivation. Justice is done tothis requirement by adapting, as exactly as possi-ble, the protection tripping characteristics to themotors’ operating diagrams. Last but not least,the cost-benefit calculation also plays an essentialrole when it comes to selecting individual protec-tion functions. There should be a healthy rela-tionship between the investment in a protectionsystem and the anticipated costs in the event ofmotor damage. Besides including the direct repaircosts, these also encompass indirect costs such asproduction outage or possibly even consequentialdamage to the process plant. This means that thetechnical scope of a protection system is meas-ured not only by the size and thus the procure-ment costs of a motor, but also by its importancefor a production process.

�2. Selected protection functions2.1. Short-circuit and overload protectionThe classic measured electrical value for detectinga fault is the motor’s stator current picked upfrom the feeding three-phase power system. Byevaluating the stator current, the electrical protec-tion facility detects a short-circuit in the statorwinding, an impending thermal overload of thestator winding or a thermal hazard for the rotorwinding due to an imbalance in the three-phasesystem.

While short-circuit protection limits the damageoccurring after an electrical fault, overloadprotection can avoid damage by detecting therisk early on. This applies equally to negative-sequence protection (unbalanced-load protec-tion) that detects an excessive load imbalance inthe feeding three-phase power system andswitches off the motor in good time before ther-mal damage arises.

Differential protection is generally used as fastand selective short-circuit protection. Thanks tothe strict selectivity of this measurement princi-ple, extremely short tripping times can beachieved with differential protection, thus attain-ing optimal damage limitation in the event ofshort-circuits. This protection facility is presentedin the chapter “Protection of High-Power Asyn-chronous Motors”.

Besides differential protection, overcurrent pro-tection, the pickup (response) value of which isabove the maximum permissible operatingcurrent, is also used for protection against short-circuits. Pickup values of 10 % to 20 % above themaximum operating current are customary.When use is made of definite-time overcurrent-time protection, tripping must be delayed by atime that is more than the motor’s permissiblestartup time. To limit motor damage in the eventof high-current short circuits, a second currentstage is provided, whose current pickup (re-sponse) value lies above the motor’s startup cur-rent. This high-current stage is provided with ashort tripping delay to operation during inrush.

Fault type Protection function ormeasurement method

Abbrevia-tion

ANSI No.

Short circuit Differential protection,time-overcurrent protection

D I >I > t

87, 51

Stator earth fault Directional earth-fault protectionDisplacement voltage measurement

IE >V0 >

64, 51N59N

Stator overload Overload protection as a thermalreplicaTemperature measurement via PT100

I2 t >

υ >

49

38

Rotor overload Starting time monitoringRestart inhibit via I2tNegative-sequence protection

t = IstartI2t >rotorI2 >

4866 (49R)46

Out of step Underexcitation protection 1/Xd 40

Power failure Undervoltage protection,undercurrent protection

V <I <

2737


Synchronous Motors

When use is made of inverse-time overcurrent-time protection, the pickup characteristic must bechosen so that it lies above the starting currentcurve in the current-time diagram. Fig. 3 showsthe motor startup current plotted over the start-ing time. The inverse-time overcurrent character-istic is chosen so that it does not intersect with thestarting current curve.

The advantage of an inverse-time characteristic isthat the short-circuit protection tripping time isreciprocal to the amount of the fault current, andso the damage caused by a short circuit on themotor can be better limited. Within the range oflow short-circuit currents under twice the ratedcurrent, the limit between short-circuit and over-load protection is blurred.

A small motor belonging to duty class S1 that isrun with a constant load torque can be satisfacto-rily protected by an overcurrent protection withinverse characteristic against thermal overload.For motors with changing load cycles or intermit-tent operating belonging to the duty classes S 3 toS 10, a thermal replica with a complete memory isindispensable for cost-effective, safe and reliableoperation. Such a thermal replica calculates themotor’s present thermal state from the stator cur-rent via the cumulated Joule heat in accordancewith the formula υ = k · I2t. The thermal replicaexactly records the heating and cooling phenom-ena linked to load cycles. Compared with simpleovercurrent protection, the thermal replica pro-vides optimum protection of the motor againstoverheating simultaneously with full use of themotor within its operationally permissible limits.

Asymmetries in the feeding three-phase powersystem can also be the cause of thermal damageon a motor. In accordance with the method ofsymmetrical components, each three-phase sys-tem can be split into a positively rotating positivephase-sequence system, a negatively rotating neg-ative phase-sequence system and a zero system.A symmetrical three-phase system consists of apositive phase-sequence system only. Asymme-tries in the three-phase system generate a negativephase-sequence system that induces a rotatingfield in the motor’s rotor circuit. This field ro-tates, relative to the rotor, at double the frequencyin the direction opposite to the rotor’s directionof rotation. This opposing rotating field causesstrong magnetic losses in the rotor core that canlead to inadmissible temperature rise.

Phase failure is an extreme case of an unbalancedload due to asymmetries. In the case of a two-phase supply, the motor can now only developsmall and pulsating torques. As the drive ma-chine’s torque requirement generally stays un-changed, in the remaining two phases the motormust accordingly pick up more current, whichthermally overloads the windings.

The thermal replica is presented extensively in thechapter on the subject of “Thermal Loading ofMotors and Necessary Protection Functions”.

2.2 Out-of-step protectionIn a protection system for synchronous motors,particular importance is attached to out-of-stepprotection (protection against loss of synchro-nism). The characteristic of a synchronous motoris that, during stable operation, the rotor rotatesin synchronism with the feeding rotating field.This synchronous operation can be disrupted bytwo causes.

For one, the synchronous motor can get out ofstep as a result of underexcitation. This occurswhen the exciter power is insufficient for the me-chanical power required of the machine. Depend-ing on its design, whenever the exciter power istoo low or absent completely the synchronousmotor continues to run as an asynchronousmotor or it is braked down to standstill.

Fig. 3 Starting current with overcurrent-timeprotection


Synchronous Motors

A second cause of out-of-step condition can be adisturbance in the feeding power system. In theevent of a power system short circuit followed bytripping of the fault feeder, the electric drivepower from the power system is missing. In ac-cordance with the reactance principle, the motorcompensates for this missing electric power fordriving the machine by means of a negative accel-eration power. It is braked and continues to runwith rising slip as an asynchronous motor. Modeltests with various synchronous motor construc-tion types have shown that a motor practically al-ways copes well with a brief power system failureof up to approximately 150 ms. During the deadtime without voltage, the speed drops to about95 % of the rated frequency, and thus only so farthat the synchronizing torque after the voltage re-covers is sufficient to return the motor to syn-chronous operation. If the motor remainswithout voltage for longer than 200 ms, in mostcases the speed drops so far that the motor fallsout of step even when the voltage returns.The recordings from the aforementioned modeltests indicate extreme active and reactive poweroscillations in the event of exciter or voltagefailure. See Figs. 4 and 5.

Especially at full exciter power, the power oscilla-tion is not attenuated, as can be seen in Fig. 5. Themechanical stress on the motor’s shaft, bearingsand foundation caused by the active power oscil-lation calls for deactivation of the motor that hasfallen out of step.

In all cases of asynchronous running of a motorstudied, be it due to underexcitation or a powersystem disturbance, the motor oscillation curvesin the conductivity diagram are above asusceptance of 1/Xd.

Consequently, adequate protection in the event ofan out-of-step condition, regardless of the cause,is underexcitation protection with a pickup char-acteristic that runs parallel to the ordinate in theconductivity diagram at 1/Xd.

In addition to the motor’s asynchronous pull-outtorque, it is the duration of the voltage failure thatcrucially influences whether the motor will resyn-chronize after the voltage returns. The model testsdemonstrate that successful resynchronization iscompleted 500 ms after a short circuit occurs.Returning of the motor to synchronous stableoperation can no longer be expected after expiryof this time period. The motor inevitably falls outof step. Therefore, a time setting of about 700 msis recommended for the out-of-step protectiontripping delay.

Fig. 4 Admittance in the event of excitationfailure (9.88 MVA motor)

Fig. 5 Circle diagram profile of the admittance due to a three-poleshort circuit lasting 0.2 s (9.88 MVA motor)a) Power circle diagramsb) Admittance circle diagrams1 – Initial value2 – Value at the start of the short-circuit3 – Value on short-circuit tripping4 – value after short-circuit tripping


Synchronous Motors

2.3 Undervoltage protectionBesides the protection of equipment in the faceof electrical faults, disturbances or disruptions,the protection engineer’s task also encompassesthe consideration of system operating states oractions by operating personnel that ought not tooccur at all if the applicable work instructions(safe isolation, earthing, etc.) are complied withexactly. In extreme exceptional circumstancessuch as the coincidence of several disturbances ina system, an operating error can never be ruledout entirely, be it due to a stress situation or be-cause such complex disturbances have not yetbeen described in the operating manual. Whensuch disturbances occur, it is the task of the pro-tection engineer – conscious of his responsibility– to employ suitable measurement methods andautomated switching and control commands, soas to counteract a risk for persons or to protectequipment against damage.

Undervoltage protection is an example of this. Ifthe upstream feed to a supply busbar is deacti-vated, the motors fed by it do not receive a voltageand stop. Undervoltage protection detects thatthe motor does not have a voltage and opens thecircuit-breaker directly assigned to it. This en-sures that, if the supply should suddenly return,the motor will not unexpectedly be connected to avoltage, which might possibly endanger servicepersonnel.

Moreover, commissioning of a drive system withseveral motors powered from one busbar gener-ally requires sequential startup of the individualmotors. In many cases, simultaneous startup ofseveral motors will overload the infeed due to thesum of the starting currents, which will lead torenewed pickup of a protection facility, e.g. theoverload protection for the feeding transformer.Here also, the undervoltage protection at eachmotor feeder performs the task of setting theoverall system to a defined basic state out ofwhich operating personnel can successively startup the drive units again.

A further task of undervoltage protection is toprotect a motor against damage that might arisefrom inadmissible starting when the voltage re-turns. This applies in particular to motors thathave to be started up via star-delta changeover.

In practice, a setting of 40 % of the rated voltagehas proven suitable for detecting power failureand opening the circuit-breaker. The tripping de-lay must be coordinated with the loss of synchro-nism protection. If the power system voltageshould drop only briefly, the motor can oftencontinue to run after the voltage returns. There-fore an undervoltage protection tripping delay ofabout 1 to 2 seconds is adequate.

� SummaryLarge synchronous motors perform importanttasks, that are sometimes relevant to safety, in anproduction process. The purpose of electricalprotection is to safeguard these motors againstdamage to the best extent possible or to limit theextent of occurring damage as far as possible.Strict selectivity of the protection facility is just asimportant. This means that a motor should re-main in operation for as long as its specified oper-ating parameters permit.

The 7UM62 numerical protection relay meetsboth requirements excellently. Electrical faultsand inadmissible operating states are reliably de-tected and remedied. The protection functions’pickup characteristics are adapted to the motors’operating characteristics, thus achieving high mo-tor drive availability.


Synchronization and Protection

Synchronization and Protectionof Synchronous Motorswith the 7UM62

Use of the 7UM62 as a control and protectionunit for synchronous motors is described in theapplication example. At the premises of the cus-tomer Petrobras (Brazil), the protection equip-ment was to be renewed and, furthermore, theproblem of synchronization of the motor afterstartup had to be resolved. This article focuses ondescribing the implemented synchronizationsolution, i.e. the controlled connection of staticexcitation.

�1.Synchronous motors are frequently started asasynchronous motors and the excitation is con-nected shortly before the synchronous speed isreached. Differing technical solutions can achievethis. In most cases, the excitation package pro-vided includes such a control facility. This is alsothe case with Siemens whenever a turnkey solu-tion (motor plus excitation) is delivered. Afterclosing of the circuit-breaker by the controller(often a SIMATIC), decay of the starting currentis checked. In parallel, the speed is monitored viaan external sensor. Excitation is connected oncethe starting current has dropped to below 120 %of the rated motor current and the speed is higherthan 96 % of the synchronous speed.

There were no speed sensors in the specific appli-cation discussed. However, there was access to thesliprings and the rotor frequency could be derivedvia the voltage induced in the rotor, thus makingit possible to indirectly derive the speed. Themotor has reached the synchronous speed if thevoltage is zero.

Now, the rotor begins to turn when the motor isactivated. The measured frequency of the rotorvoltage is proportional to the slip. The frequencyof the stator’s rotation field is measured when therotor is at standstill (s = 1). Continuous accelera-tion of the rotor leads to a reduction in the slipand thus to a drop in the frequency measured inthe rotor’s voltage. The amplitude of the inducedvoltage also drops at the same time. The excita-tion circuit-breaker must be closed shortly beforethe synchronous speed is reached and the motorthen pulls itself into synchronism. What is impor-tant in this technical solution is determining ofthe exact moment for connection of the excita-tion. Damage to the motor is risked if this takesplace purely randomly.

Fig. 1 shows the course of the rotor voltage duringstarting of the motor. Clearly recognizable are thechanging frequency, dropping of the voltage andapproximation to the synchronous speed. Theideal connection point for closing of the excitercircuit-breaker (contactor) is when the slip be-tween the stator and the rotor is 4 % to 3 % orless, which corresponds to a very low frequency.

Fig. 1 Measured rotor voltage (induced field voltage)



� 2. Synchronization solution2.1 ConceptFig. 2 shows connection of the 7UM62. In thesolution presented here, there were no currenttransformers at the star point on the motor andso differential protection was not used. Therewere also requirements stipulating additionalmonitoring of the excitation voltage, which tookplace via the measuring transducer inputs. Therotor frequency was recorded via a resistor net-work connected to two binary inputs. This wasconnected in parallel with the de-excitationresistor. The voltage input Vin was also connectedthere to record the course of the rotor voltage onthe protection unit’s fault record and to enableassessment of connection quality. The busbarvoltage and the current of the core-balance cur-rent transformer were also processed.

To determine the connection time, two binaryinputs were used as the “sensor” whose logicalsignals were processed in an extensive PLC/CFClogic. Additional quantities such as

• Starting current and active power• Interlocking depending on system conditions• Slip between the rotor and stator as well as the

closing command of power system circuit-breaker were also included in the logic or addi-tionally monitored.

Fig. 3 shows sensing of the rotor voltage.

A voltage divider, to which the binary inputs areconnected, was connected in parallel with thede-excitation resistor. Via a diode, a positive volt-age, i.e. the positive half-wave of the rotor voltage,is fed to binary input (BI) 1 and, on the basis ofthe same principle, binary input 2 accordinglymonitors the negative half-wave. The binary in-put threshold was set to 19 V so that the binaryinput picks up at an adequate voltage during onesinusoidal half-wave and drops out again if thevoltage is below the threshold. The dropoutthreshold is slightly lower than the pickup thresh-old. During the measurements, the thresholdsamounted to approximately 19 V (pickup 19.1 Vand dropout 18.9 V).

Fig. 2 Connection of the 7UM62

Fig. 3 Circuit for evaluating the rotor voltage



2.2 Measurement methodFig. 4 shows a zoomed section of the voltageinduced in the rotor. Once the synchronous speedis approached, the frequency drops rapidly, whichis also evident in the flatter increase of the ACvoltage when it transits through zero.

The timing of the signal change between zerotransitions is monitored with the binary inputs.When a further enlargement of the rotor voltageis considered, Fig. 5 shows the trigger thresholds(ideal threshold of 19 V) with the cursors. In theevent of a change from negative to positive, BI 2will drop out and thus issue a “low” signal. BI 1will pickup at an adequate voltage amplitude andwill generate a “high” signal. Both signals are fedto a PLC/CFC logic, on the basis of which thevoltage rate of change “dV/dt” is determinedand monitored. The rotor frequency is thereforeworked out indirectly.

The time difference determined on the basis ofboth trigger thresholds is around 10.5 ms if thethresholds are assumed to be at about 19 V. Thistime difference is of a magnitude comparable tothe previous zero transition.

The logic is designed so that measurement of thezero transitions is begun after an adjustable time.This time must be determined during commis-sioning. It should be oriented to the decayingstarting current. In the specific example, monitor-ing was started after 4 s. To this end, a timer wastriggered after closing of the circuit-breaker viaan auxiliary contact. After expiry of the time, thecontact shown in Fig. 3 closes and the binaryinputs are active. The times between the zerotransitions can now be determined.

If the subsequent changes at the zero transitions(from positive to negative) are examined, it canbe seen that the increase reduces clearly and alonger time difference is measured. According toFig. 6, this time difference is 26.2 ms.

Fig. 4 Detailed view of Fig. 2

Fig. 5 First measured passage through zero(BI1 is active and BI 2 is inactive)

LSP2

843.

tif



The time difference measured is greater than inthe previous measurement and in all other meas-urements since starting the motor. We cantherefore conclude that slip is clearly reduced.The ideal time for closing the excitation circuit-breaker is approached. It is released when thetime criterion (adequately flat rise in the rotorvoltage) is met and a zero transition change hastaken place. These principal conditions are proc-essed in further logic blocks. Once all conditionsare met, a CLOSE command is sent to the exci-tation circuit-breaker and the motor is synchro-nized.

Load connection is enabled by a further relay twoseconds after applying of the excitation voltage(field voltage). General starting supervision alsotakes place, by means of which the starting andsynchronization operations are aborted if not allconditions have been met within a certain time.In this specific case, this time was 8 seconds induration. This time depends on the motor’sstarting time.

Numerous oscillographic recordings of motorstarting operations were carried out to check thechosen solution. The stator current and the statorvoltage and also the voltage induced in the rotorwere analyzed. The recorded operations resultedin almost identical curves, as can be seen inFigs. 1 and 4.

The PLC/CFC logic was tested in detail with therecords and logs.

“Live” commissioning on the motor then tookplace, which did not present any problems. Allsynchronous motors of the customer Petrobraswere then equipped with this solution.

� 3.The 7UM62 features the protection functionsneeded for motor applications. The protectionfunctions used are listed below with their ANSInumbers.

• 27 Undervoltage protection• 46 Negative-sequence

(unbalanced-load) protection• 47 Phase-sequence voltage• 49 Stator thermal overload protection• 50BF Breaker failure protection• 50G Earth-fault protection

(zero-sequence current)• 50/51 Time-overcurrent protection• 59 Overvoltage protection• 81O/U Overfrequency and underfrequency

protection

Further protection/monitoring functions wererealized by means of PLC/CFC. To this end,measured values were evaluated, including via thethreshold monitoring function. In detail, thesewere:

• 55 Power factor protection• 78(40) Out-of-step protection of the

synchronous motor (under-excitation protection can alsobe used to advantage here)

• 86 Lockout function/restart inhibit• Monitoring of the excitation current and

voltage• Calculation of the rotor resistance on the basis

of the excitation quantities• Display of the excitation voltage and current

and of the calculated rotor resistance• Load connection

It also ought to be mentioned that the excitationcurrent and voltage were recorded via sensors andthese were linked to the relay’s internal measuringtransducers via the 4-20 mA interfaces. The calcu-lated rotor resistance and the excitation voltageand current were provided as measured values onthe display and can be shown via the selectionmenu for measurements on the 7UM62.

Loss of synchronism (out of step) of thesynchronous motor is also supervised via powerfactor (cos ϕ) and excitation current monitoring.

�

The SIPROTEC 7UM62 features all necessaryprotection functions applied for motor protec-tion. Further protection and monitoring func-tions were realized by means of PLC/CFC.

Fig. 6 Measurement of the next zero transition

LSP2

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Appendix

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P283

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� Siemens PTD EA on the InternetFor product support in the use of SIPROTECprotection relays, the following information ordocumentation can be downloaded free of chargefrom our Internet page www.siprotec.de:

� Catalog sheets and documentation� Updates and bug fixes for DIGSI� Demo software and utilities� Firmware updates� Protocol updates

... and more

� PTD Customer Support CenterEfficient operation is crucially conditional on ahigh degree of availability of power transmissionand distribution systems. That is where CustomerSupport comes in. Our service is available free ofcharge; our experts will be pleased to handle yourenquiries. The PTD Customer Support Center canbe reached 24 hours a day, 365 days a year.

How to contact us:

Tel.: +49 180 524 7000*Fax: +49 180 524 2471*Email: [email protected]

Appendix

www.siprotec.de

www.siemens.com/energy-support

* Subject to charges, e.g. 12 ct/min


Appendix

Siemens Companies and Representatives of the Power Transmission and Distribution Group� EuropeAustriaSiemens AG ÖsterreichGudrunstr. 11A-1101 WienPhone: +43-5-1707-46530Fax: +43-5-1707-53075

Belarus (White Russia)Siemens Representative Officeul. Storozhovskaja 8220002, MinskPhone: +375-17-217-3484Fax: +375-17-210-0395

BelgiumSiemens S.A.Demeurslaan 132B-1654 HuizingenPhone: +32-2-536-7789Fax: +32-2-536-6900

Bosnia and HerzegovinaSiemens PTDHamdije Cemerlica 2BA-71000 SarajevoPhone: +387-33-276 647Fax: +387-33-661 279

BulgariaSiemens PTD2, Kukush Str.BG-1309 SofiaPhone: +359-2-8115 639Fax: +359-2-8115 622

CroatiaSiemens PG/PTDZagrebacka 143aHR-10000 ZagrebPhone: +385-1-6331 233

Czech RepublicSiemens s. r. o.Evropska 33aCZ-160 00 PrahaPhone: +420-2-3303 2132Fax: +420-2-3303 2190

DenmarkSiemens A / SBorupvang 3DK-2750 BallerupPhone: +45-4477 4477Fax: +45-4477 4034

EstoniaAS SiemensPärnu mnt. 139 c11317 TallinnPhone: +372-630 4794Fax: +372-630 4778

FinlandSiemens OsakeyhtiöMajurinkatu 6FIN-02600 EspooPhone: +358-10-511 3385Fax: +358-10-511 3770

FranceSiemens S. A.. S.9, boulevard FinotF 93527 Saint Denis, Cedex 2Phone: +33-1-4922 3972Fax: +33-1-4922 3091

GreeceSiemens A. E.Artemidos 8GR-151 25 Amaroussio/AthenPhone: +30-210-6864 771Fax: +30-210-6864 536

Siemens A. E. NorthGeorgikis Scholis 89GR-570 01 ThessalonikiPhone: +30-2310-479 217Fax: +30-2310-479 265

HungarySiemens Rt.Gizella út 51-57H-1143 BudapestPhone: +36-1-471 1668Fax: +36-1-471 1632

ItalySiemens S.p. A.Via Vipiteno 4I-20 128 MailandPhone: +39-02-243 63203Fax: +39-02-243 62213

IrelandSiemens Ltd.Leeson CloseDublin 2Phone: +353-1-216 2423Fax: +353-1-216 2458

LatviaSiemens SIA PTDLidostas 'Riga' teritorija,Marupes pagasta, Rigas rajons1053 RigaPhone: +371-7015507Fax: +371-7015501

LithuaniaUAB Siemens PTDJ. Jasinskio str. 16c01112 VilniuPhone: +370-5-239-1517Fax: +370-5-239-1501

NetherlandsSiemens Nederland N.V.Prinses Beatrixlaan 800NL-2595 BN Den HaagPhone: +31-70-333 3975Fax: +31-70-333 3225

NorwaySiemens A / SBratsberg veien 5 - Bygg 6N-7493 TrondheimPhone: +47-7395 9132Fax: +47-7395 9790

PolandSiemens Sp.z.o.o. Northul. Zupnicza 11PL-03-821 WarschauPhone: +48-22-870 9127Fax: +48-22-870 8627

Siemens Sp.z.o.o. Southul. Gawronow 22PL-40-533 KatowicePhone: +48-32-208 4250Fax: +48-32-208 4259

PortugalSiemens S. A.Rua Irmaos Siemens1-AlfagideP-2720-093 AmadoraPhone: +351-21-417 8361Fax: +351-21-417 8071

RomaniaCalea Plevnei nr.139Sector 6RO 060011 BucurestiPhone: +40-21-2077 390Fax: +40-21-2077 482

RussiaOOO Siemens, PTDul. Letnikovskaja, 11/10115114, MoscowPhone: +7-495-737 1815Fax: +7-495-737 2385

Serbia and MontenegroSiemens PTDOmladinskih brigade 21CS-11070 BeogradPhone: +381-11-2096 185Fax: +381-11-2096 055

SlovakiaSiemens PTD EAStromová 9SK-837 96 BratislavaPhone: +421-2-5968 2660Fax: +421-2-5968 5260

SloveniaSiemens PG PTDBratislavska cesta 5SI-1511 LjubljanaPhone: +368 (1) 4746-116Fax: +386 (1) 4746-106

SpainSiemens S.A.Ronda de Europa5-Tres CantosE 28 760 MadridPhone: +34-91-514 7545Fax: +34-91-514 4730

SwedenSiemens ABPower & TransportatJohanneslundsvägen 12-14Upplands Väsby (Stockholm)194 87Phone: +468-728-1248Fax: +468-728-1008

SwitzerlandSiemens-Schweiz AGFreilagerstr. 28CH-8047 ZürichPhone: +41-585-585 051Fax: +41-585-544 761

TurkeySiemens Sanayi ve Ticaret ASYakacik Yolu No: 11134870 Kartal-IstanbulPhone: +90-216-459 2721Fax: +90-216-459 2682

UkrainiaDP Siemens Ukraineul. Predslawynska 11-1303150, KievPhone: +380-44-201 2300Fax: +380-44-201 2301

United KingdomSiemens plcSir William Siemens HousePrincess RoadManchester M20 2URPhone: +44-161-446 6634Fax: +44-161-446 6476

� AfricaEthiopiaSiemens (Pvt) Ltd.Africa Avenue (Bole Road)Friendship city center 7th FloorP.O.Box 5505Addis AbabaPhone: +251-11-663 9922Fax: +251-11-663 9998

EgyptSiemens Ltd.Cairo, P.O.Box 775 / 1151155 El Nakhil and El Aenab St.El-Mohandessin, GizaPhone: +20-2-333 3644Fax: +20-2-333 3608

KeniaSiemens AGUnit A, Ngong RoadP.O.Box 50876NairobiPhone: +254-20-285 6204Fax: +254-20-285 6274

MoroccoSiemens S.A.Km1, Route de RabatAin-Sebâa20500 CasablancaPhone: +212-22-669 222Fax: +212-22-240 151

NigeriaSiemens Ltd. Nigeria98/100 ApapaOshodi ExpresswayP.O. Box 304 ApapaLagosPhone: +234-1-4500 137Fax: +234-1-4500 131

South AfricaSiemens Ltd., PTD EA300 Janadel AvenueHalfway HouseMidrand 1685Phone: +27-11-652 2953Fax: +27-11-652 2474

� North AmericaUSASiemens PT&D, Inc.7000 Siemens Rd.Wendell, NC 27591-8309Phone: +1-800-347 6657

+1-919-365 2196Fax: +1-919-365 2552Internet: www.siemenstd.com

CanadaSiemens Canada Ltd.1550 Appleby LineBurlington, ON L7L 6X7Phone: +1-905-315 6868

ext. 7223Fax: +1-905-315 7170

� Central AmericaCosta RicaSiemens S.A.P.O.Box 10022-1000La Uruca200 Este de la PlazaSan JoséPhone: +506-287 5120Fax: +506-233 5244

MexicoSiemens S.A. DE C.V.Poniente 116, no. 590Col. Ind. VallejoDelegacion Azcapotzalco02300 Mexico D.F.Phone: +52-5-328 2031Fax: +52-5-328 2192

� South AmericaArgentinaSiemens S.A.Ave. Pres. Julio A. Roca 516Capital FederalC1067ABN Buenos AiresPhone: +54-11-4738 7194Fax: +54-11-4738 7355

BrazilSiemens Ltd.Av. Mutinga, 3800Cap. 05110-901 Sao PauloPhone: +55-11-3908 3912Fax: +55-11-3908 3988

ChileSiemens S. A.Av. Providencia 1760Edificio Palladio, Piso 10Providencia150-0498 Santiago de ChilePhone: +56-2-477 1317Fax: +56-2-477 1030

ColombiaSiemens S.A.Santafe de Bogota,D.C.Carrera 65 No. 11-83Apartado 8 01 50Santafé de Bogotá 6Phone: +57-1-294 2272Fax: +57-1-294 2287

El SalvadorSiemens S.A., EnergiaAntiguo Cuscatlán,Apartado 1525CP 1137 San SalvadorPhone: +503-278 3333Fax: +503-278 3334


Appendix

Siemens Companies and Representatives of the Power Transmission and Distribution GroupEcuadorSiemens S.A.Calle Manuel ZambranoPanamericana Norte KM2.5QuitoPhone: +593-2-294 3900Fax: +593-2-294 3904

GuatemalaSiemens Electrotécnica S.A.PTD2a. Calle 6-76, Zone 10Ciudad de GuatemalaPhone: +502-379 2393Fax: +502-334 3670

PeruSiemens S.A.C.Avenida DomingoOrue 971Lima 34Phone: +51-1-215 0030Fax: +51-1-421 9292

VenezuelaSiemens S.A.Avenida Don Diego Cisneros/Urbanización Los RuicesCaracas 1071Phone: +58-212-203 8755Fax: +58-212-203 8613

� Asia PacificAustraliaSiemens Ltd., PTD-EA885 Mountain HighwayBayswater VIC 3153Phone: +613-9721 7360Fax: +613-9721 7319

BangladeshSiemens Ltd., PTDPlot 02, Block SW (1)Gulshan AvenueDhaka – 1212Phone: +88-02-989 3536Fax: +88-02-989 2439

ChinaSiemens Ltd., PTDA10, Jiu Xian Qiao Lu,Chaoyang District100016 BeijingPhone: +86-10-6476 8888Fax: +86-10-6476 4901

Hong KongSiemens Ltd., PTD58/F Central Plaza18 Harbour RoadWanchai, Hong KongPhone: +852-25 83 3362Fax: + 852-28 02 9908

IndiaSiemens Ltd., PTD6A Maruti Industrial AreaSector-18Haryana 12201 GuragonPhone: +91-124-204 6060Fax: +91-124-204 6261

IndonesiaSiemens Indonesia, PTD SJalan Jendral AhmadYani Kav. B67-68Jakarta, Pulo Mas 13210Phone: +62-21-472 9153Fax: +62-21-471 5055

JapanSiemens K.K.20-14, Higashi-Gotanda3-chome, Shinagawa-kuTokyo 141-8641Phone: +81-3-5423 6814Fon: +81-3-5423 8727

KazakhstanTOO Siemens, PTDDostyk Ave. 117/6050059, AlmatyPhone: +7-3272-58 37 37Fax: +7-3272-58 37 00

KoreaSiemens Ltd.10th Floor Asia Tower Bldg726, Yeoksam-dong, Kang-man-guSeoul 135-719, KoreaPhone: +82-2-3450 7347Fax: +82-2-3450 7359

MalaysiaSiemensElectrical Engineering Sdn Bhd13th Floor, CP Tower11 Section 16/11,Jalan Damansara46350 Petaling JayaSelangor Darul EhsanPhone: +60-3-7952 5324

(direct)Phone: +60-3-7952 5555

(switchb.)Fax: +60-3-7957 0380

New ZealandSiemens Ltd. PTD55 Hugo Johnson DriveAucklandPhone: +64-9-580 5500Fax: +64-9-580 5601

PakistanSiemens Pakistan EngineeringCo. Ltd. PTDB-72, Estate Avenue, S.I.T.E.Karachi 75700, PakistanPhone: +92-21-2566 213Fax: +92-21-2566 215

PhilippinesSiemens Inc., PTD169 H.V. De la Costa Street,Salcedo Village - Makati1227 ManilaPhone.: +63-2-814 9017Fax: +63-2-814 9014

SingaporePower Automation PTE28 Ayer Rajah Crecent05-02/03Singapore 139959Phone.: +65-6872 2688Fax: +65-6872 3692

TaiwanSiemens Ltd., PT & D3, Yuan Qu StreetNan Gang District, 8F-111503 Taipei, Taiwan R.O.C.Phone.: +886-2-2652 8659Fax: +886-2-2652 8654

ThailandSiemens Limited, PTD EACharn Issara Tower II,2922/283 New Petchburi RoadBangkapi, Huay KwangBangkok 10310Phone.: +66-2-715-4771Fax: +66-2-715-4701

VietnamSiemens Ltd. PTD-EAThe Landmark Building,2nd Floor5B Ton Duc Thang St.District 1Ho Chi Minh CityPhone.: +84-8-825 1900Fax: +84-8-825 1580

� Middle EastUnited Arab EmiratesSiemens LLC, PTD EALevel 16, Al Otaiba TowerSheikh Zayed 2nd StreetP.O.Box 47015Abu DhabiPhone: +971-2-6165 142Fax: +971-2-6393 444

Saudi ArabiaSiemens Ltd. PTDJeddah Head OfficeP.O. Box 462121412 JeddahPhone: +966 2 665 8420Fax: +966 2 665 8490

KuwaitSiemens Electrical andElectronic Services K.S.C.Jaber Al-Mubarak Street,Block 4Sharq, Kuwait CityPhone: +965-241-8888

ext. 211Fax: +965-246-3222

IranSiemens SSK, PTD EANo. 32, A. Taleghani Ave.P.O.Box 15875 - 4773TeheranPhone: +98-21-6614 2123Fax: +98-21-6646 2951

PTD EA 10.06


Appendix

Exclusion of liability

We have checked the contents of this manualfor agreement with the hardware and softwaredescribed. Since deviations cannot be precludedentirely, we cannot guarantee that the applicationsdescribed will function correctly in any system.

Copyright

Copyright © Siemens AG 2006. All rights reserved.

The reproduction, transmission or use of thisdocument or its contents is not permitted withoutexpress written authority. Offenders will be liablefor damages. All rights, including rights created bypatent grant or registration of a utility model ordesign, are reserved.

Registered trademarks

SIPROTEC, SINAUT, SICAM and DIGSI areregistered trademarks of SIEMENS AG. The othernames appearing in this manual may be tradenames the use of which by third parties for theirown purposes may infringe the rights of theowners.

If you have any questions aboutPower Transmission and Distribution,our Customer Support Center isavailable around the clock.Tel.: +49 180/524 70 00Fax: +49 180/524 24 71(Subject to charges: e.g. 12 ct/min)E-Mail: [email protected]/energy-support

Siemens AGPower Transmission and DistributionEnergy Automation DivisionPostfach 48 0690026 NuernbergGermany

www.siemens.com/ptdwww.siprotec.de

Subject to change without prior noticeOrder No.: E50001-K4454-A101-A1-7600Printed in GermanyDispo 31900KG 11.06 5.0 76 En 102013 6101/C6114

The information in this document contains general descriptions of the technical options available, which do not always have to be present in individual cases.The required features should therefore be specified in each individual case at the time of closing the contract.

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