applications and...instrument and control transformers: applications and selection15/531 15.1...

Contents

15.1 Introduction 15/531

15.2 Types of transformers 15/53115.2.1 Voltage transformers (VTs) 15/53115.2.2 Current transformers (CTs) 15/531

15.3 Common features of a voltage and a current transformer 15/53115.3.1 Design parameters 15/531

Section I: Voltage transformers 15/531

15.4 General specifications and design considerations for voltagetransformers (VTs) 15/53115.4.1 Instrument voltage transformers 15/53115.4.2 Electromagnetic voltage transformers 15/53415.4.3 Residual voltage transformers (RVTs) 15/53415.4.4 Capacitor voltage transformers (CVTs) 15/53815.4.5 Control transformers 15/54015.4.6 Summary of specifications of a VT 15/543

15.5 Precautions to be observed while installing a voltagetransformer 15/543

Section II: Current transformers 15/544

15.6 Current transformers (CTs) 15/54415.6.1 General specifications and design considerations for

current transformers 15/54515.6.2 Measuring current transformers 15/54915.6.3 Interposing current transformers 15/55015.6.4 Summation current transformers 15/55015.6.5 Protection current transformers 15/55115.6.6 Special-purpose current transformers, type ‘PS’ 15/55315.6.7 Core-balanced current transformers (CBCTs) 15/563

15.7 Short-time rating and effect of momentary peak or dynamiccurrents 15/563

15.8 Summary of specifications of a CT 15/564

15.9 Precautions to be observed when connecting a CT 15/564

Section III: Testing of instrument and control transformers 15/566

15.10 Test requirements 15/566

15.10.1 Voltage transformers 15/566

15.10.2 Current transformers 15/568

15

Instrument andcontroltransformers:applications andselection

15/529

Section IV: Non-conventional methods of currentmeasurement 15/569

15.11 Current Sensors 15/569

15.11.1 Resistive shunts 15/56915.11.2 Hall effect current sensors 15/57015.11.3 Faraday effect optical sensors 15/57015.11.4 Zero flux current sensors 15/57115.11.5 Rogowski coil current transducers 15/57115.11.6 Digital optical instrument transformers 15/573

Relevant Standards 15/574

List of formulae used 15/574

Further Reading 15/575

Instrument and control transformers: applications and selection 15/531

15.1 Introduction

Transformers are used in an auxiliary circuit, linked to apower circuit, to indicate, measure and control its voltagesand currents. They find application in a switchgear or acontrolgear assembly and a switchyard. It would beimpracticable to produce indicating and measuringinstruments or protective devices to operate at high tovery high voltages or currents. The universal practice,therefore, is to transform the high voltages, say, 415 Vand above, and currents above 50 A to reasonably lowvalues, as discussed later, for these applications. Indicatingand measuring instruments and protective devices aredesigned for these reduced values. The transformers usedto transform voltages are known as voltage transformers*and those to transform currents as current transformers.Below we discuss their classifications, basic requirementsand design parameters.

15.2 Types of transformers

15.2.1 Voltage transformers (VTs)

These may be classified as follows:1 Instrument voltage transformers

(i) Conventional two-winding, electromagneticvoltage transformers

(ii) Residual voltage transformers (RVTs) and(iii) Capacitor voltage transformers (CVTs). These

may be used for metering or protection, withvery little difference between the two as notedlater.

2 Control transformers

15.2.2 Current transformers (CTs)

These may be classified as:1 Instrument current transformers

(i) Measuring current transformers(ii) Protection current transformers and(iii) Special-purpose current transformers, class ‘PS’.

2 Interposing current transformers3 Summation current transformers4 Core balance current transformers (CBCTs)

15.3 Common features of a voltageand a current transformer

15.3.1 Design parameters (service conditionsand likely deratings)

These are similar to parameters for a switchgear assemblyas discussed in Section 13.4. Since they are directlyassociated with the same power system and interruptingdevices as a switchgear assembly, they should generally

meet the requirements for a switchgear assembly, exceptfor small variations in the test requirements. For moredetails refer to the following publications:1 For voltage transformers

IEC 60044-2 (for two-winding transformers such asCVTs)

2 For current transformersIEC 60044-1 and IEC 60044-6

SECTION I: VOLTAGETRANSFORMERS

15.4 General specifications anddesign considerations forvoltage transformers (VTs)

These transformers develop a voltage on the secondary,substantially proportional to the voltage on the primary(there being no knee point saturation, as is sometimesrequired in CTs (Section 15.6.1(viii)).

15.4.1 Instrument voltage transformers

1 Rated primary voltageThis will generally be the nominal system voltage, exceptfor transformers connected between a phase and the groundor between the neutral and the ground, when the primaryvoltage will be considered as 1/ 3 times the nominalsystems voltage (Vr).

2 Rated secondary voltageIn Europe and Asian nations this is generally 110 or 110/

3 V, (63.5 V) for phase-to-phase or phase-to-groundauxiliary circuits respectively. In the USA and Canadathese voltages are 120 or 120/ 3 V for distributionsystems and 115 or 115/ 3 V for transmission systems.

3 Rated frequencyThis may be 50 or 60 Hz as the system may require. Thepermissible variation may be considered as ±2% formeasuring as well as protection VTs. These limits arebased on the recommended variations applicable for aswitchgear assembly (IEC 60439-1) or an electric motor(Section 1.6.2).

4 Insulation systemsThese transformers may be PVC taped, thermoplastic(polypropylene) moulded, fibreglass taped, polyester resincast or epoxy resin cast depending upon the system voltageand the surroundings. HV indoor transformers, for instance,are generally polyester or epoxy resin cast, and areeconomical with good dielectric properties. They are resistantto humid, chemically contaminated and hazardous areas.Outdoor HV transformers, however, may be epoxy resincast, oil or SF6 insulated and oil or SF6 cooled. Epoxyinsulation provides better mechanical and constructionalqualities. They are resistant to humid, contaminated andcorrosive atmospheres and are suitable for all HV

* Potential transformer (PT) is not the appropriate word to identifyan instrument voltage transformer.

15/532 Electrical Power Engineering Reference & Applications Handbook

systems. They are mechanically strong and can bearshocks and impacts.

5 Creepage distancesFor outdoor installations the recommended minimumcreep distances for all types of voltage or currenttransformers are given in Table 15.1, according to IEC60044-1 or IEC 60044-2.

6 TappingsTappings are generally not necessary, as a transformer isdesigned for a particular voltage system. If and whensuch a need arises (as in a control transformer (Section15.4.5)) they can be provided on the primary side of thetransformer.

7 Rated outputThe standard ratings, at 0.8 p.f. lagging, may be 10, 15,25, 30, 50, 75, 100, 150, 200, 300, 400 or 500 VA or asthe auxiliary circuit may demand. The procedure todetermine the total VA burden of a circuit is described inSection 15.4.5. Typical values of VA burden for a fewinstruments are given in Table 15.2 from data providedby the manufacturers.

8 Rated burdenThis is the maximum burden the transformer may haveto feed at a time. The preferred values will follow seriesR-10 of ISO-3 (IEC 60059) and as noted in Section13.4.1(4).

9 Short-time ratingThis is not material in voltage transformers, as neitherthe voltage measuring instruments nor the protective relayswill carry any inrush current during a switching operationor a fault. No short-time rating is thus assigned to suchtransformers.

The electromagnetic unit, however, as used in a residualVT (Section 15.4.3) or a capacitor VT (Section 15.4.4)should be suitable for carrying the heavy discharge orinrush currents during a capacitor discharge or switchingrespectively.

10 Accuracy classThe accuracy of a VT depends upon its leakage reactanceand the winding resistance. It determines the voltageand the phase errors of a transformer and varies with theVA on the secondary side. With the use of better corematerial (for permeability) (Section 1.9) and better heatdissipation, one can limit the excitation current and reducethe error. A better core lamination can reduce the coresize and improve heat dissipation.

• Measuring voltage transformers Standardaccuracy class may be one of 0.1, 0.2, 0.5, 1 or 3.The recommended class of accuracy will depend uponthe type of metering and generally as noted inTable 15.3.

• Protection voltage transformers Generally, ameasuring voltage transformer may also be used forthe purpose of protection. A protection transformer,however, is assigned an accuracy class of 3 or 6, whichdefines the highest permissible percentage voltage errorat any voltage between 5% of the rated voltage up tothe voltage obtained by multiplying the rated voltageby the rated voltage factor of 1.2, 1.5 or 1.9. Andwhen the secondary has a burden between 25% and100% of the rated burden at a p.f. of 0.8 lagging. Thisaccuracy class is followed by a letter ‘P’ such as 3Pand 6P etc. The voltage and phase displacement errorsshould not exceed the values noted in Table 15.6.

Notes1 A low voltage of 5%, at which the transformer is required to

maintain its accuracy limit, is of great significance. A protection

Table 15.1 Recommended values of minimum creepagedistances for a VT or a CT

Pollution level Minimum creepage distance between phaseand ground mm per kV (r.m.s.)(phase to phase)

Light 16Medium 20Heavy 25Very Heavy 31

Table 15.2 Typical values of VA burdens of instruments

Instruments Maximum burden(VA)a

1 Voltmeter 52 Voltage coil of a watt-meter or a power

factor meter 53 Voltage coil of a frequency meter 7.54 Voltage coil of a kWh or a kVAr meter 7.55 Recording voltmeters 56 Voltage coils of recording watt-meters and

power factor meters 7.57 Voltage coil of a synchroscope 15

aThese VA burdens are for moving iron instruments. For electronicmeters these values would be of the order of 0.1 to 0.5 VA.

Table 15.3 Recommended class of accuracy for VTs for differenttypes of meters

Application Class of accuracy

1 Precision testing or as a standard 0.1VT for the testing of other VTs

2 Meters of precision grade 0.53 Commercial and Industrial meters 1.04 Precision industrial meters (Indicating 0.2 or 0.5

instruments, recorders and electronicintegrating meters)

5 General industrial measurements 1 or 3(Indicating instruments and recorders)

6 Purposes where the ratio is of less 3importance

NoteTo choose a higher class of accuracy than necessary is not desirable.


transformer is required to operate under a fault condition, duringwhich the primary voltage may dip to a value as low as 5% ofthe rated voltage.

2 It is possible to have two windings in the secondary circuit ofa VT when it is required to perform the functions of bothmeasurement and protection.

11 Rated voltage factorThis is the multiplying factor which, when applied to therated primary voltage, will determine the maximumvoltage at which the transformer will comply with thethermal requirements for a specified time as well as withthe relevant accuracy requirements. This factor carries agreater significance, particularly on a fault, when healthyphases may experience an over-voltage and the protectionVTs may all the more be required to accurately sensethis and activate the protective circuit. Such a situationmay arise on a ground fault on an isolated neutral systemor a high impedance grounded system (Sections 20.6and 21.7). Table 15.4, following IEC 60044-2, suggeststhe recommended voltage factors and their permissibledurations for different grounding conditions.

12 Circuit diagramTo illustrate the important features of a VT, let us analyseits equivalent circuit diagram. Refer to a simple diagramas in Figure 15.1 which is drawn along similar lines tothose for a motor (Section 1.10, Figure 1.15). For easeof analysis, the ratio of primary and secondary turns hasbeen considered as 1:1. Then from the circuit diagram,the following can be derived:

V I R X V1 1 1 1 1– ( + ) = and¢

¢ ¢ ¢ ¢ ¢V I R X V1 2 2 2 2– ( + ) =

and this is drawn in the form of a phasor diagram(Figure 15.2). The phase displacement between phasors V1

and ¢V2 is the phase displacement error ‘d’ as discussedlater.

13 Voltage error or ratio errorThis is the error in the transformed secondary voltageas generally caused by the excitation current I1, and asshown in Figure 15.1. It is the variation in theactual transformation ratio from the rated and is expressedby

Voltage error = –

100%n 2 1

1

K V V

V

◊ ¥* *

*

whereKn = rated transformation ratioV1 = actual primary voltage (r.m.s.)V2 = actual secondary voltage (r.m.s.)

Figure 15.1 Equivalent circuit diagram for a voltage transformer

R1 X1

Z1

I 1 ¢I 2 ¢R2 ¢X 2

¢Z 2

V1

¢Im I m

¢V1 ¢V2

I n�

V1 – Primary voltage

¢V1 – Primary induced emf

¢V2 – Secondary induced emf referred to the primary side(V2 being secondary induced emf not shown)

I n� – Excitation or no-load current

¢Im – Loss component of current supplying the hysteresis and eddycurrent losses to the voltage transformer core (it is the activecomponent)

I m – Magnetizing component producing the flux ‘f’ (it is the reactivecomponent)

R1 – Primary circuit resistance

¢R2 – Secondary circuit resistance referred to the primary sideX1 – Primary circuit reactance

¢X 2 – Secondary circuit reactance referred to the primary sideZ1 – Primary circuit impedance

¢Z 2 – Secondary circuit impedance referred to the primary sideZ – Load (burden) impedance

Z

Table 15.4 Rated voltage factors

Sr. no. Rated voltage factor Rated time Method of system grounding Method of primary connections

1 1.2 Continuous All types of system grounding (i) Between lines or(ii) Between transformer

star point and ground2 1.2 Continuous An effectively grounded system Between line and ground3 1.5 30 seconds An effectively grounded system Between line and ground4 1.2 Continuous An ineffectively grounded system Between line and ground5 1.9 30 seconds An ineffectively grounded system Between line and ground6 1.2 Continuous (i) An isolated neutral system or Between line and ground

(ii) A resonant grounded system7 1.9 8 hours A resonant grounded system Between line and ground

*NoteOnly the r.m.s. values and not the phasor quantities are consideredto define the voltage error. The phase error is defined separately.Together they form the composite error. Refer to Table 15.5 formeasuring and Table 15.6 for protection VTs.


14 Phase displacement error, dddddThis is the difference in phase between the primary andthe secondary voltage phasors (d ). The direction of thephasors are so chosen, that the angle is zero for a perfecttransformation. Refer to the phasor diagram, Figure 15.2,and Table 15.5 for measuring and Table 15.6 for protectionVTs.

15 Limits of voltage and phase displacement errors• At rated frequency, these should not exceed the values

given in Table 15.5 for measuring VTs, at any voltagebetween 80% and 120% of the rated voltage and aburden of 25–100% of the rated burden at a p.f. 0.8lagging.

• For protection VTs these should not exceed the valuesgiven in Table 15.6 at any voltage between 5% of therated, up to the voltages obtained by multiplying therated voltage by the rated voltage factor as in Table15.4, and a burden between 25% and 100% of therated load at a p.f. 0.8 lagging. At voltages lower than5% of the rated, the limits of error may increasedisproportionately and become up to twice the specified

errors at about 2% of the rated voltage, the limits ofVA burden and p.f. remaining the same.

15.4.2 Electromagnetic voltage transformers

These are single-, double- or three-phase wound-typetransformers with windings on both primary and secondarysides (Figures 15.3(a) and (b)).

Application

They are used for both measuring and protection purposes.As a measuring VT, they are used to feed a voltmeter,kW, kWh or a kVAr meter, a power factor, frequencymeter or a synchroscope. As a protection VT they areused to feed a protective circuit, incorporating voltagesensing protection relays. To save on cost and mountingspace, they may also be wound for one common primaryand two secondary windings, one for metering and theother for protection. For markings, see Section 15.10.1(2)and Figure 15.35.

15.4.3 Residual voltage transformers (RVTs)

When the primary of a three-phase two-windingtransformer, having its secondary wound for a three-phase open delta, is connected across an unbalanced supplysystem, a residual voltage across the open delta will appear.This is the principle on which this transformer is based(Figure 15.4(a)). As discussed in Section 21.2.2, andillustrated in Figure 21.7, the phasor sum of all the threeline to ground voltages in a three-phase balanced systemis zero, i.e.

V V VR Y B+ + = 0

When this balance is disturbed, due to either an unbalancein the loads or due to a ground fault, a residual or zerophase sequence voltage in the neutral circuit will appear.When one of the phases in the secondary of a three-phase transformer is open circuited and a three-phasesupply is applied to its primary windings, there will appear

¢V1¢V2¢ ◊ ¢I Z2 2

¢ ◊ ¢I R2

2

¢ ◊¢

I X

22

¢I 2

d ¢Im

d

f

I� I m

I 2

I �I m

¢Im

¢V1I1·Z1

V1

I1 ·R1

I 1·X

1

d = Phasor displacement error

Note The phasor diagram is drawn taking the applied voltage V1 as the reference phasor. It can also be drawntaking the primary induced emf ¢V1 as the reference. The logic to the diagram and the subsequent results shallhowever, remain the same.

Figure 15.2 Phasor diagram for a voltage transformer

I 1

Table 15.5 Recommended limits of voltage and phasedisplacement errors, applicable for all types of measuring VTs(only electromagnetic and capacitor VTs).(A residual VT is basically a protection VT)

Class of accuracy % voltage (ratio) Phase displacement (d)error ±a ± minutes

0.1 0.1 50.2 0.2 100.5 0.5 201.0 1.0 403.0 3.0 Not specified

As in IEC-60044-2

a These errors are valid only when the voltage is between 80% and120%, burden 25–100% of the rated burden and p.f., 0.8 lagging.


a residual or zero phase sequence voltage across theopen terminals at the secondary. This represents theresidual or the zero phase sequence voltage, whatevermay exist in the main supply system. This voltage willbe zero when the main primary system is balanced andhealthy.

Important parameters

1 Residual voltage The residual voltage appearingacross the secondary windings will be three times thezero sequence voltage, if it existed in the primary windings.This is due to an open magnetic circuit in the secondaryopen delta winding having no return path through the

Table 15.6 Recommended limits of voltage and phasedisplacement errors, applicable for all types of protection VTs(electromagnetic, capacitor and residual VTs)

Class of accuracy % voltage (ratio) Phase displacement (d)error± a ± minutes

3P 3.0 1206P 6.0 240

As per IEC-60044-2

aThese errors are valid only when the voltage is between 5% to‘rated voltage factor ¥ 100%’, burden 25–100% of the rated burden,and p.f., 0.8 lagging.

At voltages lower than 5%, the limits of error may increase. Theybecome up to twice the specified errors at about 2% of the ratedvoltage, the limits of VA burden and p.f. remaining the same.

NoteThe choice of class 3P or 6P will depend upon the application andthe protection scheme of the system. The following may be consideredas a rule of thumb when making this choice.

(i) Class 3PThis class may be selected for protective devices that operateon the basis of phase relationship between the voltage and thecurrent phasors, such as in a directional overcurrent protection,reverse power or directional distance protection.

(ii) Class 6PThis class may be selected for protective devices where theiroperation does not depend upon the phase relationship betweenthe voltage and the current phasors, such as for an over-voltage, over-current or an under-voltage protection. Forinstance, a residual VT should have this accuracy class.

(iii) When a residual VT is employed for capacitor discharges itrequires no accuracy class.

Figure 15.3(b) Typical HV instrument voltage transformers(Courtesy: Prayog Electricals)

(a) 33 kV single-phase outdoor (b) 11 kV three-phase indoor

Transformers with HV fuse

Figure 15.3(a) Typical indoor epoxy resin cast instrument voltage transformers up to 11 kV (Courtesy: Kappa Electricals)


third magnetic limb. This phenomenon does not existwhen three single-phase transformers are used, as eachtransformer core winding will form a closed magneticcircuit of its own.

In a normal three-limb transformer the resultant flux,on a ground fault, of the two healthy lines limb willreturn through the transformer limb of the grounded line,inducing a heavy short-circuit current in that winding.This will induce a voltage which will be reflected in thecorresponding secondary winding, and the voltage acrossthe open terminals of the delta will not be a true residual.This situation is overcome by providing a low reluctancereturn path, suitable for carrying the maximum value ofunbalanced flux without saturation. This is achieved bythe use of a five-limb transformer (Figure 15.4(b)). Thetwo additional outer limb are left unwound.

2 Residual voltages under different operatingconditions To extend the ease of application of thisdevice, consider the following circuit conditions todetermine the quantum of residual voltages:

• Healthy system: System neutral, grounded orungrounded.

• Ground fault on one phase– System neutral grounded– System neutral isolated

Healthy system

In this case, all the three phases would be balanced andthe residual voltage, Ve , will be zero. The three voltage

phasors in the open delta windings will be as illustratedin Figure 15.5(a). The phasor sum of these phasors iszero. Therefore Ve = 0.

Ground fault on one phaseSystem neutral grounded

Consider a ground fault on phase R. The voltage acrossthis phase will become zero and the phasor diagram willbe as illustrated in Figure 15.5(b). The other two phasorswill remain the same as in a healthy system and add togive the residual voltage Ve, i.e.

V V V V Ve T2

T2

T T= + + 2 cos 120◊ ∞

where VT is the phase voltage across the secondarywindings

= 2 – T2

T2V V = VT

= 3 ¥ zero phase sequence voltage drop.

The voltage across open delta is thus the same as the fallin the voltage of the faulty phase. It will lead the currentcaused by the ground circuit impedance.

System neutral isolated

When the system neutral is isolated, the voltage acrossthe faulty phase R will be the same as the ground potentialand the ground potential will become equal to the phasevoltage VT as illustrated in Figure 15.5(c). The voltageacross the healthy phases will become 3 TV , i.e. 3times more than the normal phase voltage. The phasorsVY and VB will thus be 60∞ apart than 120∞ and 180∞from the primary.

\ V V V V Ve T2

T2

T T= ( 3 ) + ( 3 ) + 2 3 3 cos 60◊ ◊ ◊ ∞

= 3 + 3 + 3T2

T2

T2V V V

= 3VT

i.e. three times the healthy phase voltage.

Important requirements• Grounding Based on the above, it is essential that

the primary windings of the transformer have agrounded neutral, without which no zero sequenceexciting current will flow through the primarywindings. Although the open delta will develop somevoltage on an unbalance in the primary, it will onlybe the third harmonic component, as would becontained by the primary windings’ magnetic fluxand not the zero sequence component.

• Voltage factor Since this transformer may have toperform under severe fault conditions, it should besuitable for sustaining system switching surges as wellas surges developed on a fault. A voltage factor ashigh as 1.9 (Table 15.4) is generally prescribed forthese transformers.

• Short-time duty When this transformer is required

R BY

G

a1 b1 c1

a2 b2 c2

Residualvoltage

Figure 15.4(a) Connections of a residual voltage transformer

Figure 15.4(b) A five-limb transformer to carry unbalanced flux

a1 c1b1

1 432 5

a2 c2b2

Limbs


RR

Y

B

R ¢ B ¢Y ¢

N ¢

VPPrimarywinding

Tertiarywinding

VT

÷3 ·VT

R ≤ B ≤Y ≤

Ve = 0

Ve=

0

Secondarywinding

VS

R ¢≤ B ¢≤Y ¢≤

N ¢

R ¢≤

B ¢≤Y ¢≤

N ¢

VT

R ≤

B ≤

VT

2Y≤

VT

R ¢

B ¢ Y ¢

N ¢

N

YB

N

Figure 15.5(a) An RVT in a healthy system

V T sin 30∞

= VT

2

V V V Ve r y b = + + ¢¢ ¢¢ ¢¢= 0

x

RR

Y

R ¢ B ¢Y ¢

N ¢

VPPrimarywinding

Tertiarywinding

VT

÷3 ·VT

R≤ B≤Y≤

Ve = V T

Secondarywinding

VS

R� B�Y�

N ¢

R �

B �Y�

N ¢

R ≤

B≤ Y≤

30∞

VT s

in30

∞

R ¢

Y ¢

N ¢

N

Y

B

VP

B

B ¢

VT = 0

VTsi

n30∞ 30∞

V TV

T

Ve

V V V V V

V

e T2

T2

T T

T

= + + 2 cos 120

=

◊ ◊ ∞

Figure 15.5(b) An RVT under ground fault on a 3-f four-wiregrounded neutral system

N

Figure 15.5(c) Ground fault on a 3-f, three-wire delta or 3-f, four-wire ungrounded star system

RR

Y

R ¢ Y ¢

N ¢

Primarywinding

Tertiarywinding

÷3V

P

R ≤ B ≤Y ≤

Secondarywinding

R � B �Y�

N ¢B �

Y ¢N

N

Y

B

VP

B

60∞

÷3

Vs

÷3

Vs

÷3 ·V s

÷3 ·Vs

÷ 3 ·V sN

120∞

a ¢ a ≤

÷3· V

T

÷3·V

T

Ve = 3VT

b≤b¢

÷3 ·V T

÷3·V

T

Ve

÷3Vp

B ¢÷3Vp

120∞V V¢ ¢y b p = 3

÷3V

p

B ¢

÷3Vp

N

N

Y�

V V V V V

V

e a b = = ( 3 ) + ( 3 ) + 2 ( 3 ) cos60

= 3

T2

T2

T2

T

¢ ¢¢ ÷ ÷ ◊ ÷ ∞

¢¢¢ ¢¢¢ ◊V b Vy = 3 s


to discharge a charged capacitor bank it should becapable of withstanding heavy inrush dischargecurrents (see below for its application). The followingmay be considered when designing a transformer fordischarging purposes:

1 The size of the capacitor banks, their voltage andthe impedance of the capacitor circuit.

2 Rate of discharge of the trapped charge.3 The temperature of the primary windings after the

discharge.4 The magnitude of electrodynamic forces on the

primary windings, which may be developed bythe discharge currents.

• Applications They may be used to carry out thefollowing functions:

1 To detect a ground fault or operate a directionalground fault relay (Section 21.6.4).

2 To operate a neutral displacement relay (Figures26.4 and 26.9).

3 To detect an unbalance in a three-phase normallybalanced capacitor bank (Section 26.1.1(8)).

4 To discharge a charged capacitor bank over a veryshort period, particularly when a fully chargedcapacitor is interrupted. See also Section 25.7.

5 To discharge an interrupted HV circuit before areclosing. An HV system, say, a transmission lineor a cable when interrupted, develops high transientvoltages as discussed in Sections 23.5.1 and 20.1.Unless these transients are damped to a reasonablylow level so that they are not able to endanger theterminal equipment and devices on an automaticreclosing, the equipment and devices installed inthe system may become damaged due to theresulting switching transients. The normal practiceto deal with this is to damp the transients throughan electromagnetic transformer such as this. Thetransformer, however, may have to be designedfor such a duty to sustain the electrostatic stressesarising from such transients, the discharge timeand impedance of the interrupting circuit up to thetransformer.

15.4.4 Capacitor voltage transformers (CVTs)

This type of voltage transformer is normally meant for ahigh to an ultra-high voltage system, say, 110 kV and above.While a conventional wound-type (electromagnetic) voltagetransformer will always be the first choice, it may becomecostlier and highly uneconomical at such voltages.

The size and therefore the cost of a conventionallywound voltage transformer will be almost proportionalto the system voltage for which it is wound. As a costconsideration, therefore, a more economical alternativeis found in a capacitor voltage transformer (CVT) (Figure15.6(a)).

A CVT consists of a capacitor divider unit in which aprimary capacitor C1 and a secondary capacitor C2 areconnected in parallel between the line and the ground(Figure 15.6(b)). A tapping at point A is provided at anintermediate voltage V1, usually around 12 to 24 kV,

which is reasonably low compared to the high systemvoltage. This helps restrict the phase error, on the onehand, and facilitates an economical intermediate woundtransformer Tr, on the other, to perform the same duty asa normal wound voltage transformer. The purpose ofline capacitors is thus to step-down the high to very highsystem voltages to an economically low value. Throughthe tapping point A is connected a conventional and aless expensive wound-type intermediate or auxiliaryvoltage transformer Tr, rated for the intermediate voltageV1 in association with a reactor L (Figure 15.6(b)). Theuse of the reactor is to almost offset the heavy capacitivevoltage component. If possible, the reactor and thetransformer may be combined in one unit to make iteasier to operate. The secondary of the transformer israted for the required standard voltage, say, 110/ 3 (63.5V), to feed the auxiliary devices and components fittedin the auxiliary circuit.

The inductive reactance of the combined transformerand the reactor is chosen so that it will balance thecapacitive reactance of the line capacitors at the ratedfrequency and thus achieve a near-resonant circuit. Sincea frequency variation may cause a de-tuning of the resonantcircuit, tappings are generally provided on the intermediatevoltage transformer to facilitate adjustment of the circuitreactance at different frequencies, to achieve a near-resonant condition even on other frequencies. There is avoltage drop across both the capacitor units VC and thereactor VL. Figure 15.7 illustrates a simple equivalentcircuit for the CVT of Figure 15.6(b) for more clarity.These voltage drops, being 180∞ out of phase, aredetrimental in influencing and adding to the phase errorof the intermediate voltage transformer Tr. At higherfrequencies, the summation of these voltages ( + )C LV Vmay become very high and cause high phase errors, leadingto erratic behaviour of the instruments, devices andcomponents connected on the secondary of theintermediate voltage transformer. It is therefore, imperativethat these voltage drops be contained as low as possible,on the one hand, and must offset each other, i.e. ( + )C LV V� 0, on the other, to remain almost ineffective even athigher frequency variations, in influencing the phase errorof the intermediate VT.

Frequency variations are usually caused on a fault ora switching operation (Sections 20.1 and 23.5.1) andalso during the changeover of the tapping of theintermediate VT or the reactor. When the voltage dropsVC and VL are not large enough compared to V1, the contentof phase error is contained. An intermediate voltage ofalmost 12–24 kV is found to be realistic in restrictingthe voltage drops across C and L, to a reasonably lowvalue compared to V� during normal operation. Further,it is essential to offset the reactances XC and XL througha variable reactor to achieve a near-resonant circuit whenthe CVT is in service. This makes the whole systembehave like a normally wound VT in terms of its ratingand class of accuracy for both metering and protectionpurposes. The same error limits will apply as for a normalVT (Tables 15.5 and 15.6). The output for a given accuracyis dependent on the range of frequency variation overwhich the voltage transformer is required to operate.


NoteFerro-resonance: This phenomenon may occur in an isolatedneutral system employing a CVT, similar to an RVT (Section20.2.1(2)). The core of the non-linear electromagnetic unit (EMU)may saturate momentarily during a ground fault or even during ahealthy operation, under certain circuit conditions. For instance,low-frequency transients or a fault on the secondary side maycause momentary saturation of the magnetic core of the EMU,which may, in turn, resonate with the ground capacitive reactancesXcg and give rise to sub-harmonic oscillations. These may bedetrimental to the insulation of the EMU of the CVT as well asthe terminal instruments and devices connected to it. Theseoscillations must be damped as far as possible. This can be achieved

by inserting a damping resistance R in the EMU circuit, as illustratedin Figure 15.6(b).

Application

A CVT may be used to carry out the following functions:

1 To measure as well as protect a high-voltage system,generally 110 kV and above. To save on cost andmounting space, the electromagnetic unit may bewound for two secondary windings, one for meteringand the other for protection.

Figure 15.6(a) Capacitor voltagetransformer (CVT) rated voltage36–420 kV and above (Courtesy:ABB) Figure 15.6(b) Schematic diagram of a basic capacitor voltage transformer (CVT)

Primary voltageterminal

Capacitordivider unit

Intermediatevoltage terminal

C1

C2

A

V � /÷ 3

V1/÷3

SA

L

Tr

G GLow voltage or ground terminal

Z

R

1103

VSecondaryvoltage

Secondaryvoltagecircuit

1S1

1S2

2S1

2S2

3S1

3S2

V� Line voltage

V1 Intermediate voltage

=

+ 1

1 2V

CC C� ◊

C1 Primary line capacitance

C2 Secondary line capacitance

L Variable tuning inductance

SA Surge suppressor

Z Load (burden) impedance

R Damping resistor to preventferro-resonance effects

Tr Conventionally woundintermediate VT(electromagnetic unit, EMU)

1S1–1S22S1–2S2 Secondary tappings3S1–3S2

EHV line


2 To feed the synchronizing equipment.3 As a coupling unit for carrier signals (Section

23.5.2(D) and Figure 23.9(b)).4 To damp the transient voltages on the primary side.

For markings refer to Section 15.10.1 and Figure 15.35.

15.4.5 Control transformers

Refer to Figures 15.8 and 15.9. These transformers arequite different from a measuring or a protectiontransformer, particularly in terms of accuracy and short-time VA ratings besides their application. They are installedto feed power to the control or the auxiliary devices/components of a switchgear or a controlgear assemblynot supposed to be connected directly to the main supply.

These transformers do not require a high accuracy andcan be specified by the following parameters:

1 Rated primary voltage The normal practice for anHV system is to provide a separate LV feeder for theauxiliary supplies. The primary voltage will be thenormal system voltage, Vr, when the transformer isconnected line to line or Vr / 3 when connected lineto neutral.

2 Rated secondary voltage This is 24, 48, 110, 220,230, 240 or 250 volts, or according to the practice ofa country. Tappings, if required, can be provided onthe primary side.

3 Rated burden This is the maximum load thetransformer may have to feed at a time. The preferredratings will follow series R-10 of ISO-3 (Section13.4.1(4)).

4 Short-time VA burden This accounts for themaximum switching inrush VA burden of the variousauxiliary devices connected in the switching circuit

V 1/÷ 3

C = C1 + C2

VC VL

L R1 ¢R2

I n�

l m¢ l m

Z

R

During resonance, when: XC =XL the circuit would behave like a normal transformer

VC : Voltage drop across the line capacitorsVL : Voltage drop across the inductanceR1 : Primary resistance representing losses across ‘C ’ and ‘L’ and

the intermediate voltage transformer (EMU)

¢R2 : Secondary resistance of the intermediate VT referred to theprimary side.

¢Im : Loss component

Im : Magnetizing componentZ : Load (burden) impedanceR : Damping resistor to prevent ferro-resonance effects

Figure 15.7 Equivalent circuit diagram of a CVT

Figure 15.9 A typical outdoor type oil-filled 11 kV controltransformer

Figure 15.8 Typical single-phase and three-phase control transformers (Courtesy: Logic Controls)


such as contactors, timers and indicating lights. Unlessspecified, the short-time VA burden of the transformerwill be a minimum of eight times its rating at 0.5 p.f.lagging. It can be expressed in terms of VA versuscos f and drawn in the form of an inrush curve, foreasy selection of a transformer rating (Figure 15.10).

5 Voltage regulation In view of heavy currents duringthe switching of an auxiliary circuit, the reactanceand the resistance drops of these transformers shouldbe designed to be low to ensure a high degree ofregulation during a switching operation. Regulationof up to 6% for control transformers rated for 1.0kVA and above and up to 10% for smaller ratings isconsidered ideal (NEMA Standard suggests thesevalues as 5%).

For brevity, only the more relevant aspects are dis-cussed here. For more details, refer to IEC 60044-2for instrument voltage transformers and IEC 60076-3/IS 12021 for control transformers.

Application

These may be used to feed the solenoid or the motor ofan interrupting device (such as an electrically operatedbreaker), indicating lights and annunciation circuits,auxiliary contactors or relays, electrical or electronictimers, hooters or buzzers, and all such auxiliarycomponents and devices mounted on a controlgear or aswitchgear assembly requiring a specified control voltage.

Procedure to determine the VA rating of a controlcircuit

The total VA burden of a control or an auxiliary circuitis the phasor sum of the VA burdens of each individualcomponent and device connected in the circuit, andconsuming power. It is advisable to add the VA burdensvectorially rather than algebraically. Since a control

transformer may feed more auxiliary components anddevices consuming power compared to an instrumentVT, the VA rating of such transformers is generally higherthan of an instrument, metering or protection VT.

Algebraic summation will lead to a higher VArequirement than necessary. The transformer should notbe too small or too large to achieve better regulation inaddition to cost. From Figure 15.11 the following maybe derived:

VA W VArT = +

or VA W VArT2 2= +

= (( cos ) + ( sin ) )2 2VA VAf f

whereVAT = Total VA burden

VA = VA burden of individual component

W = W1 + W2 + …

and VAr = VAr1 + VAr2 + …

W1, W2, VAr1 and VAr2 are the active and reactivecomponents respectively of the VA burden of a device ata p.f. f1 and f2.

The following may be ascertained when selecting therating of a control transformer:

• Maximum hold-on (continuous) VA burden and thecorresponding p.f. of all the devices likely to be inservice at a time.

• Pick-up VA or short-time VA: An electromagneticdevice such as a contactor or a timer carries a highinrush current, also known as ‘sealed amperes’, duringa switching operation and it is associated with a highmomentary pick-up VA burden on the circuit and thefeeding control transformer. The effect of the maximummomentary pick-up VA burden and the correspondinginflow p.f. of all the components likely to be switchedat a time must be calculated.

• Maximum lead burden of the connecting wires underthe above conditions.

The control transformer to be selected may have arating nearest to the maximum hold-on VA burden socalculated and must be suitable to feed the requiredinrush current at the p.f. so calculated without affectingits regulation. So long as these two points fall below theinrush curve of the control transformer, its regulationwill be maintained within the prescribed limits. Figure

Figure 15.10 Inrush characteristics of a control transformer

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Cos f (of control circuit)

% V

A r

atin

g of

the

con

trol

tra

nsfo

rmer

1600

1400

1200

1000

800

600

400

200

W = VA cos f

f

VAT VAr = VA sin f

Figure 15.11 Phasor representation of a load (VA burden).


15.10 illustrates this requirement and Example 15.1demonstrates the procedure to determine the requiredVA of a control transformer.

Example 15.1Consider the control scheme of an auto-control capacitorpanel as shown in Figure 23.37. The scheme shows thecontrol voltage as being tapped from the main bus. But forour purpose, we have considered it through a controltransformer 415/110 V.

The following data have been assumed:

System: 415 V, three-phase, four wire.

Control voltage: 110 V a.c.

Control wire: 2.5 mm2 (resistance of wire = 7.6 W/1000 m, asin Table 13.15).

Approximate length of wire for each feeder up to the powerfactor correction relay (PFCR): 35 m.

(1) A study of the control scheme

Component Total quantity nos Maximum hold-on occurs when Maximum inrush occurs when five steps ofall the six steps of the PFCR the PFCR are ON and the sixth isare ON switched ON

Hold on Inrush

Main contactor 125 A 6 6 5 1Auxiliary contactor 6 A 2 1 (auto or manual) 1 –On indicating light 8 6 (auto or manual) 5 1PFCR 1 1 (6 steps) 5 1

(2) Approximate VA burden and cos fffff for each component, as available from the manufacturers’ catalogues

Component VA cos f W = VA cos f VAr = VA sin f

125 A contactorHold-on 65 0.31 20.15 61.79Inrush 900 0.42 378 817

6A contactorHold-on 15 0.33 4.95 14.16Inrush 115 0.60 69 92

Indicating lightHold-on 7 1 7 –Inrush 7 1 7 –

PFCR (each step)a

Hold-on 5 1 5 –

aPFCRs are available in both static and electromagnetic versions. Their VA levels therefore vary significantly due to inbuilt switching relays,LEDs (light emitting diodes) and p.f. meter etc. For illustration we have considered an average VA burden of 5 VA at unity p.f. for each step.For static relays, this may be too low

(3) Computing the maximum hold-on (steady state) and inrush burden values and their cos fffff

Maximum hold-on values Maximum pick-up (inrush) values

Hold-on for five steps already ON Inrush for the sixth step

Component Qty Total Total Qty Total Total Qty Total TotalW VAr W VAr W VAr

Main contactor 6 6 ¥ 20.15 6 ¥ 61.79 5 5 ¥ 20.15 5 ¥ 61.79 1 378 817= 120.90 = 370.74 = 100.75 = 308.95

Auxiliary contactor 1 4.95 14.16 1 4.95 14.16 – – –Indicating light 6 6 ¥ 7 = 42 – 5 5 ¥ 7 = 35 – 1 7 –PFCR (steps) 6 6 ¥ 5 = 30 – 5 5 ¥ 5 = 25 – 1 5 –

Total 197.85 384.90 165.70 (a) 323.11(b) 390(c) 817(d)

\ VA = 197.85 + 384.902 2 \ Total inrush, W ( a + c) = 555.7and VAr (b + d) = 1140.11

� 433 without considering the \ VA = 555.7 + 1140.112 2

burden for wire leads � 1268 without consideringthe burden for wire leads


Control circuit current

Ic = 1268/110 = 11.53A

and lead burden = 11.532 ¥ 2.035

= 270.53 W

\ Total maximum inrush burden

W = 555.7 + 270.53

= 826.23

and VAr = 1140.11

\ Maximum short-time VA = 826.23 + 1140.112 2

= 1408.0

at an inrush (short-time) cos f = 826.231408.0

= 0.587

Rating of control transformer

Select a continuous rating = 500 VA

at a cos f = 0.51

and short-time rating = 1500 VA

at a cos f = 0.587

The actual values as worked out above must fall belowthe inrush curve of the selected control transformer of500 VA, as illustrated in Figure 15.12.

15.4.6 Summary of specifications of a VT

In Table 15.7 we list the data that a user must provide toa manufacturer to design a VT for a particular application.Some of the data chosen are arbitrary to define thespecifications.

15.5 Precautions to be observedwhile installing a voltagetransformer

1 Since a VT forms an inductive circuit, it generatesheavy switching current surges which should be takeninto account when deciding on protective fuses. Afuse with an appropriately high rating should be chosento avoid a blow-up during switching.

2 As a result of the generally high rating of protectivefuses they provide no adequate protection against aninter-turn fault. For critical installations, and for HVVTs particularly, a separate protection may be providedfor inter-turn faults.

3 When an HV-VT develops an inter-turn fault on theHV side, there is no appreciable rise in the primarycircuit current and which may not be detected. But,

it may cause local discharge and heating up of theinter-turns, leading to dangerous fault currents andionization of oil in an oil-filled VT. It is advisableto provide a Bucholtz relay to protect oil-filled VTsby detecting the presence of gas in the event of afault.

4 For lower voltages (<33 kV), any fault on the VT willbe detected by the protective devices installed in themain circuit.

5 Temperature detectors may also be provided in thewindings of large VTs as are provided in a motor(Section 12.8).

1000

900

800

700

600

500

400

300

200

100

00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.587cos f (of control circuit)

>1500 VA

% V

A r

atin

g

Inrush characteristic of a500 VA control transformer

This point shouldlie at more than

1500 VA

Figure 15.12 Checking the suitability of the 500 VA controltransformer for the required duty for Example 15.1

Control circuit currentIc = 433/110 = 3.94A

and lead burden = c2I R◊

where R is the resistance of the connecting wires at theoperating temperature (90∞C, as in Table 14.5)

= 6 35 7.61000

[1+ 3.93 10 (90–20)] –3¥ ¥ ¥ º W

(for details refer to Table 14.4)

= 2.035 W\Lead burden = 3.942 ¥ 2.035

= 31.59 W

\ Total maximum steady-state hold-on burden

W = 197.85 + 31.59 = 229.44

And VAr = 384.9

\Maximum VA = 229.44 + 384.92 2

= 448.1

at a steady-state cos f = 229.44448.10

= 0.51


SECTION II: CURRENTTRANSFORMERS

15.6 Current transformers (CTs)

These may be one of the following types:

• Ring (Figures 15.13(a)–(d))• Bar primary (Figures 15.14(a) and (b) and 15.15(a)

and (b))• Wound primary (Figures 15.15(a) and 15.16)

In ring-type CTs the primary current-carrying conductoris passed through the ring and the ring forms the secondarywinding (Figure 15.13). Generally, all LV CTs areproduced thus, except for very small ratings of the primarycurrent, say, up to 50 A, when it becomes imperative todesign them in the form of a wound primary due to thedesign constraints discussed in Section 15.6.5(iv).

For special applications, such as for a motor protection

over-current release (Figure 12.15) where the use of aring-type CT may appear crude and the connectionscumbersome, a bar primary CT may be used, as shown inFigure 15.14. HV CTs, as a matter of necessity and tomaintain correct clearances and dielectric strength betweenthe primary current-carrying conductor and the secondarywindings, are made in the form of a bar primary or woundprimary only, depending upon the primary current rating(Figures 15.15 and 15.16). In a wound primary, the primaryis also wound the same way as the secondary.

Types of insulation

An LV CT may be insulated in the following ways,depending upon their location and application.

• Tape insulated (Figure 15.13(c)) For normalapplication and generally clean atmospheric conditions.

• Epoxy resin cast (Figures 15.13(a) and 15.14) Toprovide greater mechanical strength and a betterinsulation system. They are more suitable for humid,

Sr. no.

1

2

3

4

5

6

7

8

9

10

11

12

Specifications

System voltage

Insulation level (peak)

Class of insulation say,

Frequency

Nominal voltage ratio

Output (VA) say,

Short-time output (VA burden)and corresponding p.f.

Class of accuracy

Grounding system(Section 21.7)

The rated voltage factor andthe corresponding rated time

Service conditions

Marking of VTs

Measuring VTs

E

50 or 60 Hz

500

–

0.1, 0.2, 0.5, 1 or 3

Not applicable

∑ Indoors or outdoors∑ Whether the system is

electrically exposed∑ Ambient temperature∑ Altitude, if above 1000 m∑ Humidity∑ Any other important features

(a) 6.6 kV/110 V, 500 VA,class 1a

Protection VTs

B

50 or 60 Hz

500

–

3P or 6P

Depends upon the systemgrounding. Refer to Table 15.4


electrically exposed∑ Ambient temperature∑ Altitude, if above 1000 m∑ Humidity∑ Any other important features

6.6 kV/110 V, 500 VA,class 3P

Table 15.7 Summary of specifications of VTs

Control transformers

E

50 or 60 Hz

1500

Say, 8 times the rated VA at0.2 p.f.

Not applicable

Depends upon the system faultconditions and generally as 1.9Table 15.4


electrically exposed∑ Ambient temperature∑ Altitude, if above 1000 m∑ Humidity∑ Any other important

features

6.6 kV/48 V, 1500 VA andshort-time VA: eight times therated VA at 0.2 p.f.

As in Table 13.1 (1/ 3 V for line to neutral transformers)

Generally as in IEC 60044-2 or Tables 13.2, and 14.3 for series I and Tables 14.1 and 14.2 forseries II voltage systems

e.g. 6.6 kV/110 V for two phase or three phase transformers and 13

times this for line to neutraltransformers

Whether an isolated neutral system, an effectively grounded system or a non-effectively groundedneutral system

aWherever two separated secondary windings are provided, say, one for measuring and the other for protection, the markings will indicateall such details as are marked against (a) for each secondary winding.For more details and voltages higher than 66 kV, refer to IEC 60044-2.

(b) System voltage and insulation level, class of insulation and frequency etc. for all types ofVTs


contaminated and corrosive atmospheres and for allHV systems. They are mechanically strong and canbear shocks and impacts.

NoteAll HV CTs are normally manufactured with epoxy resin cast.

• Fibreglass tape (Figure 15.13(b)). To make it morecompact.

• Polypropylene

15.6.1 General specifications and designconsiderations for current transformers

The rated frequency, insulation systems and therequirement of creepage distances will generally remainthe same as for a voltage transformer (Section 15.4.1).For the remaining parameters, the following may be noted.

(i) Rated secondary current

This will be 1, 2 or 5 A, 1 and 5 A being more common.All measuring and protection devices are alsomanufactured for these ratings only as standard practice.

A 1 A secondary is not recommended in higher ratiosdue to increased induced voltage on the secondary sideduring an accidental open circuit on load. It may damagethe inter-turn insulation or cause a flashover, besidesbeing dangerous to nearby components or a human body,if in contact. This we have illustrated in Example 15.8,under Section 15.9.

(ii) Rated output

The standard VA values may be one of 2.5, 5, 7.5, 10, 15and 30 generally, depending upon the application, althougha value beyond 30 VA is also acceptable.

(iii) Rated burden

This is the value of the impedance of the secondarycircuit (the impedance of all the devices connected toit), expressed in ohms and power-factor or volt-amperes,at the rated secondary current. The CTs will be selectednearest to the computed total VA burden in the circuit.A CT with a higher VA burden than connected willhave a slightly higher error besides size. A slightly less VA

(b) LV fibreglass taped (c) taped LV PVC (d) Ground leakage relay with CBCT

Figure 15.13 Ring type CTs for measuring or protection (Courtesy: Prayog Electricals)

CBCTRelay

(a) LV epoxy resin cast


rating than the connected may normally be permissible,subject to confirmation by the manufacturer. Typicalvalues of VA burdens at the rated current for the devicesthat a measuring CT may have to usually feed are asfollows:

Instruments and measuring devices*Moving iron ammeters 1.5 to 5 VARecording ammeters 2 to 10 VACurrent coils of watt-meters 5 VARecording watt-meters 5 VAkWh and kVAr meters 5 VAThermal demand ammeters 3 VAThermal maximum demand ammeters 4 to 8 VAPower factor (p.f.) meters 5 VA

Protective devices In view of the large variety of thesedevices such as static or electromagnetic, VA burdensmay be obtained from catalogues or their manufacturers.

Copper flexible leads (wires) The approximate resistances

(a) Single-phase HV CT

Figure 15.14 Typical indoor-type bar primary epoxy resin castCTs (Courtesy: Kappa Electricals)

(b) Three-phase LV CT

11 kV CT 33 kV CT

Figure 15.15(a) Typical outdoor-type oil-filled bar primary orwound primary HV CTs (Courtesy: Kappa Electricals)

Figure 15.15(b) 400 kV bar primary outdoor current transformer(Courtesy: BHEL)

*These values are typically for moving iron instruments and devices.For electronic instruments and devices they would be of the orderof 0.1 to 0.5 VA and less.


of such conductors at 20∞C are provided in Table 13.15.They can be estimated at the operating temperature (90∞C,as in Table 14.5 or as desired).

VA burden = I2R

e.g. the VA burden of a CT having a rated secondarycurrent of 5 A with the length of the 2.5 mm2 connectingleads as 10 m.

VA = 5 7.6 101000

[1 + 3.93 10 (90–20)]2 –3¥ ¥ ¥

= 2.42 VA (for details refer to Table 14.4)

Computing the VA burden1 The VA values of some of the devices used in the

circuit may be available at a different current ratingthan the actual rated secondary current (1 or 5 A)chosen for the CT circuit. To compute the VA burdenof a circuit when selecting the correct VA level of aCT, the VA values of all the devices not correspondingto the rated current of the circuit must be first convertedto the rated current and only then added. This isessential because the VA level of a CT varies in asquare proportion of the current passing through it,i.e. VA µ I2. As a result, at lower operating currentsits VA capacity to feed a circuit would also decreasesharply while the VA requirement of the instrumentsor the relays connected in the circuit will remain thesame. It is therefore important that the VA level of theCT is raised in the same inverse square proportion ofthe current to maintain at least the same level of VAto make it suitable to activate the measuring orprotective devices connected in the circuit, i.e.

VA =

VA1

12

2

22I I

where VA1 and VA2 are the VA levels of a circuit atcurrents I1 and I2 respectively.

Example 15. 2Consider a 5 A secondary CT circuit connected to the followingdevices:

Device I = 0.3 VA at 1 ADevice II = 5 VA at 5 ADevice III = 7.5 VA at 5 A

Then the total burden at 5 A will be

= 0.3 ¥ 51

2ÊË

ˆ¯ + 5 + 7.5

= 7.5 + 5 + 7.5

= 20 VA

Therefore, one should select a 20 VA CT.Similarly, if this value was required at the 1 A secondary, thenthe total burden would be

= 0.3 + 5 ¥ 15

+ 7.5 15

2 2ÊË

ˆ¯

ÊË

ˆ¯

= 0.3 + 0.2 + 0.3

= 0.8 VA

In this case one can select a 2.5 or 5 VA CT.

2 The current element of a relay is wound for a widerange of current settings in terms of the rated secondarycurrent of the CT, such as 10–80% for a ground faultprotection, 50–200% for an over-current and 300–800% for a short-circuit protection. At lower currentsettings, while the VA requirement for the operation ofthe relay will remain the same, the VA capacity of theCT will decrease in a square proportion of the current.A CT of a correspondingly higher VA level wouldtherefore be necessary to obtain the reduced VA level,at least sufficient to operate the relay. At a 40% setting,

(a) Single phase HV CT

(b) HV epoxy resin cast (c) HV epoxy resin cast

(d) LV epoxy resin cast (e) LV tape wound

Figure 15.16 Typical indoor-type wound primary CTs formeasuring or protection (Courtesy: Kappa Electricals)


for instance, the CT must have a VA of (I/0.4I )2 or6.25 times the VA of the relay and at a setting of 20%,(I/0.2 I )2 or 25 times of the relay. Therefore when therelay setting is low this must be borne in mind and aCT of a higher VA burden be chosen. Such aconsideration, however, is more pertinent in the caseof electro-magnetic relays that have a high VA levelthan in electrostatic (electronic) relays that have anear negligible VA level at only around 0.005 VA.

Where three CTs for unrestricted or four CTs forrestricted ground fault or combined O/C and G/Fprotections are employed in the protective circuit, theVA burden of the relay is shared by all the CTs in paralleland a normal VA CT may generally suffice. Such is thecase in most of the protective schemes discussed inSections 21.6 and 15.6.6(1), except for those employingonly one CT to detect a ground fault condition, such asfor a generator protection with a solidly grounded neutral(Figure 21.12).

(iv) Circuit diagram

This can be drawn along similar lines to those for a VT(Section 15.4.1(12)). Refer to the simple diagram in Figure15.17, from which we can derive the following:

¢I I I2 1 n1 = – and

I I In1 m m = + ¢

and from this is drawn the phasor diagram (Figure 15.18).

The phasor difference between ¢I I I2 1 n1 and , i.e. resultsin a composite error ¢Im . The phase displacement between

¢I2 and I1 by an angle ‘d’ is known as the phase error.The current error will be important in the accurateoperation of an over-current relay and the phase error inthe operation of a phase sensitive relay. The compositeerror will be significant in the operation of a differentialrelay.

(v) Current error or ratio error

The error in the secondary current from the rated causedby the excitation current In1 or the variation in the actualtransformation ratio is expressed by:

Current error = ( – )

100%n 2*

1*

1*

K I I

I

¥ ¥ ( ¢I2 = Kn · I2)

(Kn being the rated transformation ratio.)Refer to Table 15.8 for measuring and Table 15.9 for

protection CTs.

Note*Only the r.m.s. values and not the phasor quantities are considered todefine the current error. The phase error is defined separately. Togetherthey form the composite error.

(vi) Phase error

As noted above, this is the phase displacement betweenthe primary and the secondary current phasors. Angle din Figure 15.18 is generally expressed in minutes. For aperfect transformer, the direction of phasors is chosen sothat this displacement is zero. Refer to Table 15.8 formeasuring and Table 15.9 for protection CTs.

(vii) Composite error

Refer to the phasor diagram in Figure 15.18 and Table15.8 for measuring and Table 15.9 for protection CTs.This error can also be expressed by

¢e2e 2e1

¢Im I m

I n�

R1 X1 I 1 ¢I 2 ¢R2 ¢X 2 Z

e1 – Primary induced emfe 2 – Secondary induced emf

¢e2 – Secondary terminal voltage for bar primary e1 � e2

R1 – Primary circuit resistance

¢R2 – Secondary winding resistance referred to the primary sideX1 – Primary circuit reactance

¢X 2 – Secondary winding reactance referred to the primary sideZ – Load (burden) impedance

I n� – Excitation or No load current

¢Im – Loss component supplying the hysteresis and eddy currentlosses to the CT core (it is the active component)

I m – Magnetizing component producing the flux ‘f’ (it is thereactive component)

¢I 2 – Secondary current referred to the primary side

Primary side Secondary side

Figure 15.17 Equivalent circuit diagram of a current transformer

Figure 15.18 Phasor diagram of a CT

e 2

¢Im

d

I n� I m

¢Im I1

I n�I m

¢I2

d = Phase displacement (phase error) between I I1 2 and ¢ .

e 1


Composite error = 100 1/ ( – ) d %1 0

n 2 12

IT K i i t

T

Ú ◊

whereKn = rated transformation ratioI1 = actual primary current (r.m.s.)I2 = actual secondary current (r.m.s.)i1 = instantaneous value of the primary currenti2 = instantaneous value of the secondary currentT = duration of one cycle

= 1/50 s or 20 ms for a 50 Hz system.

(viii) Knee point voltage

This is the point on the magnetic curve of the laminatedcore of the CT at which the saturation of the core willstart. It is defined as the point where an increase of 10%in the secondary voltage will increase the magnetizing(excitation) current Im by 50% (Figure 15.19). Beyondthis point, a very large amount of primary current wouldbe required to further magnetize the core, thus limitingthe secondary output to a required level.

(ix) Instrument security factor (SF)

This is the ratio of instrument limit primary current tothe rated primary current. Consequently a high SF willmean a high transformation of the primary current and

can damage instruments connected to its secondary. Formeasuring instruments therefore it is kept low, as it isrequired to measure only the normal current and not thefault current.

15.6.2 Measuring current transformers

These are employed for the measurement of power circuitcurrents through an ammeter, kW, kWh or KVAr andpower factor meter, or similar instruments requiring acurrent measurement. They must have a specified accuracyclass as in IEC 60044-1 and the secondary currentsubstantially proportional to the primary within a workingrange of about 5–120% of its primary rated current. Theyare required to commence their saturation beyond 120%of the primary rated current and saturate fully by 500%,as a system is not warranted to operate on an over-loador short-circuit, and will be interrupted through itsprotective devices. Thus a low knee-point voltage or alow saturation level is needed to protect the connectedinstruments from fault currents (over-current factor) on

Table 15.9 Limits of error for protection CTs

Accuracy Current error at Phase displacement Composite errorclass rated primary angle d (Figure at rated

current 15.18 at rated accuracy limitprimary current primary current

% minutes %

5 P ± 1 ± 60 510 P ± 3 – 1015 P ± 5 – 15

As in IEC 60044-1

Table 15.8 Limits of error for measuring CTs

Accuracy ± % Current (ratio) errora at % of rated primary current ± Phase displacement angle d (Figure 15.18)class in minutes at % of rated primary current

% rated. I1 5 20 50 100 120 5 20 100 120

0.1 0.4 0.2 NA 0.1 0.1 15 8 5 50.2 0.75 0.35 NA 0.2 0.2 30 15 10 100.5 1.5 0.75 NA 0.5 0.5 90 45 30 30

1.0 3.0 1.5 NA 1.0 1.0 180 90 60 603.0 – – 3 – 3 —̈————— Not specified ——————Æ5.0 – – 5 – 5 —̈————— Not specified ——————Æ

As in IEC 60044-1aThese errors are valid only when the CTs are loaded by a minimum 25% of the rated VA burden, for CTs of class 1 and 50% for CTs ofclasses 3 and 5 and a primary current of not less than 5% or more than 120% of the rated current. The measuring CTs may not transformcorrectly unless the above conditions are met.

Figure 15.19 Knee point of the excitation characteristic of acurrent transformer

Exc

itatio

n (s

econ

dary

) vo

ltage

(V

f)

10%

Knee-point

50%

Excitation current (Im)


the primary side. For example, in a measuring CT of1000/5 A, the secondary current will be in direct proportionto the primary current from about 50 to 1200 A and thecore will start saturating beyond 1200 A.

Over-current factor for instruments

As in IEC 60051, the measuring instruments are requiredto have an over-current factor of not more than 120% fortwo hours for instruments of all accuracy classes, 200%for 0.5 second for class 0.5 or less, and 1000% for 0.5second for class 1 accuracy and above. Over-currents ordurations longer than this may damage the instruments.

Accuracy class

This defines the maximum permissible current error atthe rated current for a particular accuracy class. Thestandard accuracy classes for the measuring CTs may beone of 0.1, 0.2, 0.5, 1, 3 and 5. The limits of error inmagnitude of the secondary current and the phase error,as discussed in Section 15.6.1 and shown in Figure 15.18,must be as in Table 15.8, according to IEC 60044-1,when the secondary burden is a minimum 25% of itsrated burden for CTs up to class 1 and 50% for CTs ofclasses 3 and 5.

The recommended class of accuracy will depend uponthe type of application and is generally as noted below:

VAI = VA of the interposing CTs at the primary ratedcurrent

VAL = VA of the load (instrument) connected on thesecondary of the interposing CTs, including theconnecting leads.

15.6.4 Summation current transformers

These are required to sum-up the currents in a number ofcircuits at a time through the measuring CTs provided ineach such circuit. The circuits may represent differentfeeders connected on the same bus of a power system(Figure 15.21(a)), or of two or more different powersystems (Figure 15.21(b)). A precondition for summationof currents on different power systems is that all circuitsmust be operating on the same frequency and must relateto the same phase. The p.f. may be different.

Each phase of these circuits is provided with anappropriate main CT, the secondary of which is connectedto the primary of the summation CT. Summation ispossible of many circuits through one summation CTalone per phase. The primary of summation CTs can bedesigned to accommodate up to ten power circuits easily.If more feeders are likely to be added it is possible toleave space for these on the same summation CT.

The summated current is the sum of all the CT secondarycurrents of the different circuits. The rating of the instrumentconnected on the secondary of the summation CT shouldbe commensurate with the summated current. The errorof measurement is now high, as the errors of all individualCTs will also add up vectorially. It is necessary that CTshave the same ratio, secondary resistance of magnetizingcurrent to minimize the error.

Application Class ofaccuracy

1 Precision testing or laboratory testing CTs 0.12 Laboratory and test work in conjunction with high 0.2

precision indicating instruments, integratingmeters and also for the testing of industrial CTs

3 Precision industrial meters (indicating instruments 0.5and recorders)

4 Commercial and industrial metering 0.5 or 15 Use with indicating and graphic watt-meters 1 or 3

and ammeters6 Purposes where the ratio is of less importance 3 or 5

15.6.3 Interposing current transformers

These are auxiliary CTs, and are sometimes necessary toalter the value of the secondary of the main CTs. Theyhelp to reduce the saturation level and hence the over-loading of the main CTs, particularly during an over-load or a fault condition. They are used especially wherethe instruments to which they are connected are sensitiveto over-loads. They have to be of wound primary type.So that the main CTs are not overburdened they have aVA load that is as low as possible. Figure 15.20 illustratesthe application of such CTs and their selection is madeon the following basis:

VAM = VAC + VAI + VAL

whereVAM = VA of the main CTsVAC = circuit losses between the main and the

interposing CTs at the primary rated current

I p I s1 I s2

Main CT(VAM)

Interposing CT(VAI)

VAC

VAC

(a) Schematic diagram

I p I s1VA I

Main CT

(b) Equivalent control circuit diagram

Figure 15.20 Use of interposing CTs

Load or(VAL) instrument

VAIIL

2s1

s2

ÊËÁ

ˆ¯̃


Any main CT that is under-loaded will also add to theerror in the measurement. Similarly, if provision is madein the primary of the summation CT to accommodatefuture circuits but is not being utilized it must be leftopen, otherwise it will also add to the error. The impedanceof the shorting terminals will add to the impedance ofthe circuit and will increase the total error.

As the currents of each circuit are summed by thesummation CT, the VA burden of each main CT is alsoborne by the summation CT in addition to its own. TheVA level of the summation CT, including its own, isshared proportionately by all the main CTs in the ratio oftheir primary currents. Referring to the three differentcircuits of Figure 15.21(b), having the ratings as shownin Table 15.10, the rating of the summation CT can bechosen as 3400/1 A. If we choose a VA level of this CTas 25 VA, making no provision for the future, then theVA burden shared by each main CT will be as calculatedin the last column, ignoring the losses in the connectingleads. Based on this, the VA burden of each main CT canbe decided.

15.6.5 Protection current transformers

These are employed to detect a fault, rather than measuringthe current of a power system or the connected equipment.There is a fundamental difference in the requirement of

a measuring and a protective transformer in terms ofaccuracy, saturation level and VA burden. Unlike ameasuring CT, a protection CT will have a high saturationlevel to allow the high primary current to transformsubstantially to the secondary as may be required,depending upon the current setting of the protective ortripping relays. For protection CTs, therefore, the accuracyclass is of little relevance up to the primary rated current,but a true reflection in the secondary is more importantof a fault condition in the primary.

Corollary

Both requirements of measuring and protection cannotbe met through one transformer generally. Thus two setsof transformers are required for a power circuit associatedwith a protection scheme, one for measurement and theother for protection.

(i) Accuracy limit primary current

This is the highest limit of the primary current that can

R BY

Main CTs

Circuit no. 1

Circuit no. 3

Circuit no. 2 Onphase B

Summation CT

VA burden

(a) Measuring the sum load of three circuits on phase B

R1 Y1 B1

R2 Y2 B2

Main CTs

R1

Y1

B1

R2

Y2

B2

SummationCTs

BurdenBurdenBurden

(b) Measuring the sum load of two circuits connected on different supply sources

Figure 15.21 Application of summation CTs

Table 15.10

Circuit whose Current Main CT VA burden sharedcurrent is being rating ratio by the mainsummed CTs

Circuit 1 1000 A 1000/1 A *25 10003400

¥

� 7.0 VA

Circuit 2 800 A 800/1 A *25 8003400

¥

� 6.0 VA

Circuit 3 1600 A 1600/1 A *25 16003400

¥

� 12.0 VA

Total load Ratio of *VA of= 3400 A summation summation

CTs = 3400/1 CTs = 25


be transformed to the secondary, substantially proportional,complying with the requirement of the composite error(Section 15.6.1). For example, a protection CT 2000/5Arepresented as 5P10 means that a primary current up toten times the rated (i.e. up to 2000 ¥ 10 A) will induce aproportional secondary current. The factor 10 is knownas the accuracy limit factor as noted below.

(ii) Accuracy limit factor (ALF)

This is the ratio of the rated accuracy limit primary currentto the rated primary current. For example, in the abovecase it is

2000 102000

= 10¥

The standard prescribed factors can be one of 5, 10, 15,20 and 30.

(iii) Accuracy class

This defines the maximum permissible composite errorat the rated accuracy limit primary current, followed byletter P for protection. The standard prescribed accuracyclasses may be one of 5P, 10P and 15P. A protection CTis designated by accuracy class, followed by accuracylimit factor, such as 5P10. The current error, phase errord (Figure 15.18) and the composite error with the ratedburden in the circuit are as in Table 15.9, according toIEC 60044-1. It should be chosen depending upon theprotective device and its accuracy requirement todiscriminate. Closer discrimination will require moreaccurate CTs.

NoteAn accuracy class beyond 10P is generally not recommended.

(iv) Output and accuracy limit factors

The capabilities of a protection CT are determined bythe primary inputs of a CT such as the primary ampereturns AT (primary current ¥ primary number of turns),core dimensions and the quality of laminations. All thisis roughly proportional to the product of the rated output(VA) and the rated accuracy limit factor of the CT. Fornormal use, the product of the VA burden and theaccuracy limit factor of a protection CT should not exceed150, otherwise it may require an unduly large and moreexpensive CT. For example, for a 10 VA CT, the accuracylimit factor should not exceed 15 and vice versa. Theburden and the accuracy limit factor are thus interrelated.A decrease in burden will automatically increase itsaccuracy limit factor and vice versa. In a ring or barprimary CT, which has only one turn in the primary, theampere turns are limited by the primary current only,thus limiting the accuracy and burden of such CTs. Thisis one reason why CTs of up to 50 A are generallymanufactured in a wound primary design, with a fewturns on the primary side to obtain a reasonably highvalue of VA burden and accuracy. For larger productsthan 150, it is advisable to use more than one protectionCT, or use low secondary current CTs, i.e. 1 A insteadof 5 A.

NoteFor similar reasons, a measuring CT of up to 50 A primary currentis recommended to be produced in a wound design.

Example 15.3Consider a protective scheme having a total VA burden of 15and requiring an accuracy limit factor (ALF) of 20:

\ VA ¥ ALF = 15 ¥ 20 = 300

which is too large to design a CT adequately. In such casesit is advisable to consider two sets of CTs, one for thoserelays that are set high and operate at high to very highcurrents (short-circuit protection relays) and the second forall other relays that are required to operate on moderateover-loads. For example, consider one set of CTs for short-circuit protection having

VA = 5

and ALF = 20

i.e. a 5P20 CT having a product of VA ¥ ALF of not more than150 and the other set for all the remaining protections having,say,

VA = 10

and ALF = 5

i.e. a 10P5 CT having a product of VA ¥ ALF of much lessthan 150.

NoteFor high set protective schemes, where to operate theprotective relays, the primary fault currents are likely to beextremely high, as in the above case. Here it is advisable toconsider a higher primary current than the rated for theprotection CTs and thus indirectly reduce the ALF and theproduct of VA ¥ ALF. In some cases, by doing so, even oneset of CTs may meet the protective scheme requirement.

Example 15.4Consider a system being fed through a transformer of 1500kVA, 11/0.433 kV, having a rated LV current of 2000 A. Theprotection CT ratio on the LV side for the high set relay maybe considered as 4000/5 A (depending upon the setting ofthe relay) rather than a conventional 2000/5 A, thus reducingthe ALF of the previous example from 20 to 10. Now only oneset of 15 P10 CTs will suffice, to feed the total protectivescheme and have a VA ¥ ALF of not more than 150.

(v) Other considerations when selecting aprotection CT

1 The accuracy limit factor (ALF) will depend uponthe highest setting of the protective device. For a 5 to10 times setting of the high set relay, the ALF will bea minimum of 10.

2 A higher ALF than necessary will serve no usefulpurpose.

3 It has been found that, except high set relays, allother relays may not require the ALF to be more than5. In such cases it is worthwhile to use two sets ofprotection CTs, one exclusively for high set relays,requiring a high accuracy limit factor (ALF), and theother, with a lower ALF, for the remaining relays.Otherwise choose a higher primary current than rated,if possible, and indirectly reduce the ALF as illustratedin Example 15.4 and meet the requirement with justone set of protection CTs.


15.6.6 Special-purpose current transformers,type ‘PS’

These are protection CTs for special applications such asbiased differential protection, restricted ground faultprotection and distance protection schemes, where it isnot possible to easily identify the class of accuracy, theaccuracy limit factor and the rated burden of the CTs andwhere a full primary fault current is required to betransformed to the secondary without saturation, toaccurately monitor the level of fault and/or unbalance.The type of application and the relay being used determinethe knee point voltage. The knee point voltage and theexcitation current of the CTs now form the basic designparameters for such CTs. They are classified as class‘PS’ CTs and can be identified by the followingcharacteristics:

• CTR = Ip /I s• Rated test winding current• Nominal turn ratio (the error must not exceed ± 0.25%)• Knee point voltage (kpv) at the maximum secondary

turns,

Vk ≥ 2Vft

where Vk = knee point voltage andVft = maximum voltage developed across the

relay circuit by the other group of CTsduring a severe most through fault.

• Maximum magnetizing (excitation) current at thevoltage setting (Vft) of the relay or at half the kneepoint e.m.f. to be < 30 mA for 1A CTs for most highimpedance schemes. The manufacturers select a properiron core to limit this to help reduce the effectiverelay current setting and improve its sensitivity.Magnetizing characteristics, Vf versus Im (Vf beingthe CT secondary voltage under rated conditions), asshown in Figure 15.19, are provided by themanufacturer to facilitate relay setting.

• Maximum resistance of the secondary windingcorrected to 90∞C or the maximum operatingtemperature considered. In fact, it should be substitutedby the actual operating temperature.

We discuss below a high-impedance differentialprotection scheme to provide a detailed procedure toselect PS Class CTs.

1 High-impedance differential protection scheme

The scheme primarily detects an inter-turn fault, a groundfault or a phase fault. It can thus protect a bus systemand windings of critical machines such as generators,transformers and reactors in addition to a ground fault.The differential system is a circulating current systembetween the two winding terminals of the equipment oreach section of a multi-section bus system being protectedas illustrated in Figure 15.22. The scheme is based onKirchhoff’s law, which defines that the phasor sum ofthe currents entering a node is zero, i.e.

I I I I1 2 3 4 + + + = 0

as illustrated in Figure 15.23.

Applying this law to a three-phase, three wire system,

I I IR Y B + + = 0

and to a three-phase four-wire system

I I I IR Y B n + + + = 0

When a three-phase four-wire system feeds non-linearor single-phase loads this balance is upset and theunbalanced current flows through the neutral. The samerelationship can be expressed as

I I I IR Y B n + + = .

F1, F2 – 2 sets of identical class PS CTsRelay – High impedance three element differential protection relay

Wp – Windings of a power equipment or section of a power systemto be protected

Figure 15.22 A circulating current scheme to provide a phaseand a ground fault differential protection

R

B

YL1

I p IpI p

F1I f1 I f1I f1

I p I pI p

Wp

I f 2 I f2I f2

G

G

L2

F2

Relay

I 2

I 3

I 4

I 1

Figure 15.23 Kirchhoff’s law – sum of currents entering a nodeis zero

I I I I1 2 3 4 + + + = 0


Similarly, the balance is disturbed in the differentialscheme on a fault of any type and a spill current, whichis the difference between the currents drawn by the twosets of CTs, flows through the relay. Since the schemefunctions on the principle of balance of currents, it isimperative that the two sets of CT parameters, such astheir ratio, secondary resistance and the magnetizingcurrent, should be identical, except for the permissibletolerances as discussed in Section 15.10.2. The secondarylead resistances, from the CTs to the relay terminals,should also be the same, otherwise, spill currents mayflow through the relay, even under healthy condition andcause an unwanted trip, or require a higher minimumsetting of the relay. A higher setting of the relay mayjeopardize its sensitivity to detect minor faults. Since itis not practical to produce all CTs to be identical, smallspill currents under healthy condition are likely and theminimum relay setting, Ist, must account for this. Belowwe consider three different cases to explain the principleof circulating currents, along with the procedure, to selectthe CTs and carry out the relay setting.

Equivalent circuit diagram and selection of classPS CTs

Refer to the control circuit diagram of Figure 15.24,drawn for the scheme in Figure 15.22. It is drawn on asingle-phase basis for ease of illustration, where

If1, If2 = If = CT secondary currentsIm1, Im2 = Im = CTs’ excitation currents (these will

vary with the CT secondary voltage;refer to Figure 15.19)

Ic1, Ic2 = Ic = circulating currentsIre = spill or differential current through

the relayVf1, Vf2 = Vf = CT secondary voltages under rated

conditions (these relays are definedby both the current and the voltagesettings)

Rr = resistance of the relay coil

= VAIst

2 where VA is the burden of the

relay. This may be specified in termsof its current rating 1 A or 5 Aor setting current Ist. Consideringthis to be 1 VA relay at a setting of0.05 A,

Rr 2 = 1(0.05)

= 400 W

Ist = relay settingR�1, R�2 = R� = maximum resistance of the

connecting leads from the CTterminals to the relay terminals. Forcalculating this, for an estimatedlength and size, refer to cable datain Table 13.15

XCT1, XCT2 = XCT = equivalent excitation reactances ofthe CT secondary windings. In ringtype CTs, they are generally very lowand can be ignored for ease ofderivation

RCT1, RCT2 = RCT = equivalent resistances of the CTsecondary windings

Healthy conditionRefer to Figure 15.24:

If1 = Im1 + Ic1

If2 = Im2 + Ic2

and Vf2 = (If2 – Ic2) · XCT2

= Ic2(RCT2 + RI2) + (Ic2 – Ic1) (Rst + Rr)

The two current through the relay are in opposite directionstherefore

Ire = Ic2 – Ic1

= (If2 – Im2) – (If1 – Im2)

Under healthy conditions

If1 = If2

and Im1 = Im2

\ Ire = 0

Hence, in a healthy condition there will be no spill currentthrough the relay and it will stay inoperative.

Through-fault condition

Refer to Figure 15.25(a). On a fault occurring outside

Strictly speaking, these areall phasor quantities butonly their magnitudes areconsidered for ease ofillustration, as quantitiesof similar parameters suchas If1, If2 and Im1, Im2 fallalmost in phase with eachother.

¸

˝

ÔÔÔÔÔ

˛

ÔÔÔÔÔ

Equ

ipm

ent/s

yste

mun

der

prot

ectio

n

L1

I p

F1 I f1 V f1

XCT1 I m1

R CT1

R �1

R st

I C1

R r

I re

Differentialrelay

I C2

I m2X CT2

V f 2I f2

L 2

I p

F2

R CT2

R �2

(1) Healthy condition, I re = I c2 – I c1 = 0

Figure 15.24 Equivalent control circuit diagram for a differentialground fault protection scheme of Figure 15.22


the protected zone, all the CTs that fall in parallel willshare the fault almost equally, depending upon the locationof the fault and the impedance of each CT circuit up tothe point of fault. The balance of the CTs secondarycurrents is therefore disturbed, but only marginally, asthe polarities of the two sets of CTs also fall in oppositionand neutralize most of the unbalanced current (Ic2 – Ic1)through the relay. The small spill currents may be takencare of by the minimum setting of the relay to avoid atrip in such a condition. Hence, the relay may remaininoperative on a moderate fault, as illustrated in Figure15.25(b).

But this may not always be true, as it is possible thatone or more CTs in the faulty circuit may saturate partiallyor fully on a severe through-fault and create a short-circuit (Vf2 = 0) across the magnetizing circuits of all theCTs that are saturated. Refer to Figures 15.26(a) and (b).The CTs’ resistances, however, will fall across the relaycircuit. Assuming that the other sets of CTs in the circuitremain functional, this would cause a severe imbalanceand result in a heavy unbalanced current through therelay and an unwanted trip. Under such a condition,

Vft = isc (Rct + RI)

whereVft = maximum voltage that may develop across the relay

circuit by the other groups of CTs during a severethrough-fault.

isc = maximum fault current through the secondary ofthe CTs, on a severe through-fault. This maycorrespond to the fault level of the machine or thesystem being protected, depending upon the machineor the system impedances that may fall in the faultycircuit.

The protection must be designed to remain inoperativein such a fictitious fault condition. This condition willalso determine the stability limit of the protection schemeand can be considered as the minimum voltage setting ofthe relay. In fact, this setting will have a sufficient safetymargin, as the knee point voltage, Vk, of the CTs isconsidered quite high, of the order of Vk ≥ 2Vft on theone hand, and the saturation of the CTs is possible onlyunder extreme conditions, on the other. Hence the levelof Vft developed by the CTs may not be as high as thoughtand when the relay is set at this voltage it will providesufficient stability.

NoteIt is advisable to choose the CTs with low secondry current, say, at1 A, to permit a lower relay setting for the voltage and the currenttrip coils. The reduced voltage across the relay will also improvethe stability level of the protection scheme.

Figure 15.25 A through-fault condition outside the protected zone in a differential scheme

R

B

YL1

1

F1 i f1i f1

Wp

L2

F2

Relay

F1, F2 – 2 sets of identical class PS CTs.Wp – Windings of a power equipment or section of a power

system to be protectedFault location 1:

The relay stays inoperative for a fault occurring outsidethe protected zone even if it is within the CTs zone

Fault location 2:It falls outside the CTs zone.

i f1i f1

i f2i f2

i f 2

2

G (Neutral of the winding grounded)

F

F

i p f

Differentialrelay

Equ

ipm

ent/s

yste

mun

der

prot

ectio

n

L1

I pf

F1 I f1 V f1

R CT1

X CT1

R �1

X CT2

V f2I f2

L2

I pf

F2

R CT2

R �2

Small spill current through the relay I re = I c2 – I c1

I m1

I C1

Rr

I C2

I m2

R st I re

(a) Principle of operation (b) Control circuit diagram


Sensitivity

This is the ability of the scheme to detect the weakestinternal fault.

Stability

This can be defined by the most severe external fault atwhich the scheme will remain inoperative. It should alsoremain inoperative in healthy conditions. That is it shouldbe immune to the momentary voltage or current transientsand normal harmonic contents in the circulating current.Series LC-filter circuits are generally provided with therelay coil to suppress the harmonics and to detect thefault current more precisely.

Use of stabilizing resistance

It is possible that the voltage Vft may become sufficientlyhigh to cause a spill current on a through-fault higherthan the relay pick-up current, Ist. To ensure that no spillcurrent higher than the relay setting, Ist, will flow throughthe relay circuit under a through-fault condition, theimpedance of the relay circuit is raised substantially. Itcan be obtained by using a stabilizing resistance, Rst,such that the differential circuit will act like a highimpedance path for this spill current, compared to thevery low magnetizing impedance of the saturated CT.This resistance is shown in Figure 15.26(b). It will allowa current of less than the relay pickup current, Ist. Tofulfil this condition, the impedance of the differentialcircuit must be a minimum to ensure

( + ) r stft

stR R

VI

≥

The normal practice is to choose Rst based on thesetting voltage required. In the above equation, Vft is theminimum voltage required across the relay branch(Rr + Rst) for pushing a current equal to Ist to ensure thatthe relay stays immune on a through-fault. During aninternal fault, the fault current is much more than Ist, andhence it is easy to detect. The equation also implies thatRst is chosen high to limit the relay current during athrough fault (assuming that one of the CTs is fullysaturated) to less than its pickup current. Solving theabove equation for Rst,

RVI

RVI

VAIst

ft

str

ft

st st2*

– or – ≥

Since the additional resistance will stabilize theprotective scheme during a maximum through-faultcondition without raising the relay setting, Ist, it isappropriately termed the stabilizing resistance. Figure15.27 shows an arrangement in a relay circuit and for thepurpose of illustration, it is shown separately in variouscontrol circuits (Figures 15.24, 15.25(b) and Figure25.26(b)).

R

B

YL1

F1 I f1I f1

Wp

L2

F2

Relay

I f1I f1

I f 2I f 2

If2

G (Neutral of the winding grounded)F

I p f

(a) Power circuit

L1

(b) Control scheme

Figure 15.26 Power circuit and control scheme during a verysevere external fault condition

Differentialrelay

Equ

ipm

ent/s

yste

mun

der

prot

ectio

n

I p f = n·i sc

F1 V f1

X CT1

R CT1

R �1

X CT2

V f 2 = 0

L 2

I pf up to I sc

F2

R CT2

R �2

V f t = i sc (R CT + R �)I sc = Fault level of the equipment under protection

I m1

I c1 = i sc

I re

I C2

R st R r

HealthyCT

V f t

Sat

urat

ed C

T

Magnetizingcircuit isshort circuitedduring thesaturationof CT

I f1 = i sc

*This is relay-specific. The manufacturer may specify VAcorresponding to its rated current of 1 A or 5 A or setting current Ist.


As standard practice, this resistance is supplied withthe relay by the relay manufacturer. It is of variable type,to suit system conditions and the actual fault level. Themaximum value of the stabilizing resistance to be suppliedwill depend upon the type of protection (ground or phaseor both) and the relay setting. Generally, it may varyfrom 50 to 1500 W.

Fault within the protected zone

Refer to Figure 15.28(a). The balance of the two sets ofCTs is disturbed again. The CTs now have the samepolarity and currents and the two sets add up to cause ahigh-imbalance spill current through the relay. Referringto Figure 15.28(b)

I I I I I

I I

I I I

re c1 c2 sf2 m2

sf1 m1

sf2 sf1 m

= + = ( – )

+ ( – )

= + – 2

These are all phasorquantities, but consideredlinear, for ease ofillustration and without much error

¸

˝ÔÔ

˛ÔÔ

The additive characteristic of the scheme now has high

stability and prevents the relay from operating on moderateexternal faults, while it is sensitive to small spill currentsfor all internal ground and phase faults, including windingfaults.

R NBYStabilizingresistors(1450 W eachfor Example 15.6)

Non-linearresistors

Figure 15.27 Three-element high-impedance circulatingcurrent relay scheme (shown with the front view of a differentialprotection relay for transformers, generators, motors and busbars)

R

B

YL1

Isf1 Isf1 + Isf2

Rel

ay

F1

WP

Isf1 Isf2

IpfF2 Isf2

Isf2

L2

Ipf

G

Relay – High impedance differential protectionrelay. It operates for the fault occurringwithin the protection zone

Ipf – Fault current through ground for faulton phase B

(a) Power circuit

Isf1

Figure 15.28 Fault within the protected zone

Differentialrelay

Equ

ipm

ent/s

yste

mun

der

prot

ectio

n

L1

Ipf = n·Isf

F1 I sf1

X CT1

R CT1

R �1

X CT2I p f

L2 I re = I c2 + I c1

F2

R CT2

R �2

I m1

I c1

R st

R r

V f1

I re

I c2

I m2

F

V f2I sf2

(b) Control scheme

(Courtesy: Siemens)


2 Current setting of the relay

The relay has voltage as well as current settings. Theformer defines the stability limit against through-faults,as discussed above, while the latter determines thesensitivity of the protected zone.

If Ipf = minimum fault current through the primary(chosen on the basis of the rated full-load currentof the machine or the system being protected)required to trip the relay. It may be termed theminimum primary operating current (POC) ofthe scheme. Ipf, in terms of the secondary

= n ¥ Isf

n = turn ratio of the CTsIm = corresponding to the Vft, to account for the most

severe through-faultIst = relay current setting, i.e. minimum spill current

required to operate the relay

Then

I I Isf m st = +

Since on a fault the p.f. is low (Section 13.4.1(5)) allthese quantities may be considered in phase with eachother, with little error,

\ Isf = Im + Ist

If there are N number of CTs connected in parallel, themagnetizing current will flow through all of them. In aGF protection scheme all the three CTs of all the feedersbeing protected together will fall in parallel, while incase of a combined GF and phase fault protection scheme,only one third of these CTs will fall in parallel. The CTin the faulty circuit must be able to draw enough currentto feed the magnetizing losses of all the CTs falling inparallel and the relay pickup current, Ist. The sensitivityof the differential scheme can therefore be expressedmore appropriately as

Isf = N ¥ Im + Ist

(N being the number of CTs falling in parallel) and interms of the primary

Ipf = n (N ¥ Im + Ist) (15.1)

Since it determines the sensitivity level of the protectionscheme, it must be kept as low as possible to detect evena small fault. To achieve a high degree of sensitivity it istherefore essential

• To have the CTs with a low Im• To keep the number of CTs in parallel as small as

possible, suggesting protection of individual feeders,rather than many feeders together, particularly whenthe equipment is critical and requires a higher levelof sensitivity for adequate protection.

As the relay will have only one current setting for alltypes of faults, it is recommended to keep it around 20–40% of the rated current of the machine or the system beingprotected. This setting will be sufficient to meet the CT’smagnetizing current requirements and also trip the relay.

For a ground fault scheme, it is recommended toconsider a still lower setting to ensure effective detectionof the ground fault current and rapid disconnection ofthe machine or the bus system being protected. A lowersetting may be desirable as the actual ground fault currentmay already be larger than is being detected by the relaydue to a higher impedance of the ground loop than assumedpreviously.

As a rule and as recommended in IEC 60255-6, thePOC may be chosen within 30% of the minimum estimatedground fault current. When the scheme is required todetect only a ground fault, a single-pole relay is connectedbetween all the CTs’ shorted ends (Figure 15.29). All theCTs now fall in parallel.

When the scheme is required to detect the groundfault as well as the phase faults, a triple-pole relay isused, each pole of which is connected between the shortedterminals of the two same phase CTs and the neutralformed by shorting the other terminals of all the CTs, asshown in Figure 15.22. The setting of all the poles iskept the same. In other words, the sensitivity level remainsthe same for all types of faults.

In the case of over-current and ground fault protectionthe sensitivity level becomes much higher than in a single-pole relay. Now the requirement of the minimum primaryoperating current, Ipf (Equation (15.1)), which is a measureof sensitivity, is greatly reduced. The CT on the faultyphase has to feed only one third of the CTs that fall inparallel of each relay coil rather than all the CTs, that fellin parallel in ground fault protection using only a single-pole relay.

The CTs are designed for the worst conditions offault, even when the scheme is designed to detect only a

Figure 15.29 Scheme for only ground fault differential protection

R

B

Y

F1

WP

F2

G

G

Non-linearresistor

Stabilizingresistor

Relay

Relay – High impedance single element ground faultdifferential protection relay


ground fault. This may be a phase to phase and groundfault, causing a severe unbalance. The iron core of suchCTs must therefore possess near-linear magnetizingcharacteristics, to the extent of the fault level of themachine or the system being protected. This is to achievea near-replica of the magnitude of the fault in thesecondary, which may be 15 to 20 times or more of therated current. In generators, it can increase to 21· Ir (Section13.4.1(5)). For the CTs, a saturation level sufficient totransform the maximum primary fault condition to thesecondary is therefore considered mandatory to ensurethat the CTs do not saturate during the most severe faultcondition, and render the tripping scheme erratic. Thisalso ensures better stability of the relay, particularly duringsevere most through-fault conditions (outside the CTs’detection zone) such as a bus fault, as illustrated in Figure15.30. It is normal practice to define the secondary voltageof the CTs by its knee point voltage (kpv), Vk. Thisvoltage will depend upon the type of relay, its VA burdenand the required stability of the system. It is commonpractice to make this at least twice the relay setting voltageon the most severe through-fault, i.e. Vk ≥ 2Vft.

The most severe fault is the capacity of the machineor the system being protected to feed the fault, and isdetermined by its fault level as indicated in Tables 13.7and 13.10. To consider a higher fault level than this,such as of the main power supply, is of little relevance asit would fall outside the detection zone of the CTs andwould serve no useful purpose except to further improvethe stability level of the protective scheme.

Applying this scheme to system protection, wherethe number of circuits and hence the number of CTs are

high, will mean a high POC (Equation (15.1)). A highPOC may not be desirable, as it may under-protect thesystem. In such cases, it is advisable to divide the systeminto more than one circuit and apply the schemeindividually to all such circuits (Example 15.6).

3 Suppressing system harmonics

Such relays are normally instantaneous, highly sensitiveand operate at low spill currents. Since they detect theresidual current of the system, the current may containthird-harmonic components (Section 23.6(a)) and operatethe highly sensitive relay in a healthy condition. To avoidoperation of the relay under such conditions, it is a normalpractice to supply the relay coil with a tuned filter, i.e. aseries L-C circuit to filter out the third-harmoniccomponents. The capacitance of the filter circuit mayalso tame a steep rising TRV (Section 17.10.3) during amomentary transient condition and protect the relay.

4 Limiting the peak voltage

As this is a high-impedance scheme, it can result in veryhigh voltages across the CTs and the relay, particularlyduring internal faults, when the CTs have the same polarityand the spill currents are additive. As in IEC 60255-6, itmust be limited within 3 kV across the relay circuit toprotect the CTs and the relay. An approximate formulato determine the likely peak voltage across the relaycircuit is given by

V V V Vp k m k = 2 2 ( – ) (15.2)

where Vp = peak voltage across the relay and Vm = theoretical maximum CT secondary voltage

across the relay circuit at the maximuminternal fault current. (The maximum internalfault current is the level of fault of themachine or the system under protection.)This must also take into account any othersupply sources that may also feed the fault,such as more than one supply bus, as shownin Section 13.4.1(5) and Figure 13.18, andillustrated in Figure 15.30. If the cumulativefault current is Iscc, then the maximum CTsecondary voltage will be

Vm = Iscc ¥ impedance of the relay circuit.

This can be limited by using a non-linear resistance calledMetrosil* across the relay, as shown in Figure 15.27. Ifvoltage reaches a dangerous level, this resistance willprovide a low-resistance parallel path to the current andlimit the voltage across the relay to about 1 kV. Thecurrent I through the non-linear resistance is given by

Vm = K ¥ Ib (K and b are constants)

G1 G2

F1

F2 = I scc

R r

F3

F1, F3 Through faults which may be much higher than atF2 but outside the CTs’ zone

F2 Internal fault being fed by two sources althoughlimited by the equipment impedance

Figure 15.30 An internal fault being fed by more than one source

*This is a brand name given by the manufacturer of the non-linearresistor, a GEC group company in the UK. General Electric, USAcall it Thyrite, and similar names have been given to it by differentmanufacturers. Basically, it is a SiC non-linear resistance to providethe desired over-voltage protection. Refer to Section 18.1.1 formore details.


All these values are provided by the relay supplier whenthis resistance becomes necessary.

5 I Selecting class PS CTs

Ground fault protection of a machine and setting ofthe relay. The following example illustrates the procedureto select class PS CTs for a typical G/F scheme. In practice,this scheme would be more appropriate for phase andground fault protections, as illustrated in Figure 15.22.

Example 15.5Consider a generator, 10 MVA, 3.3 kV, for ground faultprotection having a sub-transient reactance ¢¢ ±x d = 12 10%(Figure 15.29).

Grounding method: solidly groundedOver-load capacity: 150% for 30 seconds

(as in IEC 60034-1)Relay type: differentialRating: 1 AVA: 1, at the setting current, I st

I r

3 = 10 10

3 3.3¥

¥

= 1750 A

The fault level of the system,

I sc = 1750 10010.8

¥ (Equation (13.5))

= 16.20 kA

(Assuming a lower value of ¢¢ ◊x d (12 – 12 = 10.8%) to be onthe safe side.)

Consider CTs with a ratio of 2000/1 A and having Rct = 7 Wand the magnetizing chracteristics as in Figure 15.31. Consider

a lead resistance from CT terminals to the relay to be 0.5 Wper lead.

\ Total lead resistance, R1 = 2 ¥ 0.5

= 1 W (presuming this to be at theoperating temperature)

Fault current in terms of the secondary,

i sc = 16200 12000

= 8.1 A¥

1 Relay voltage setting (stability limit)

V ft = i sc( RCT + R1) (considering the resistances at the operatingtemperature)

= 8.1 (7 + 1)

= 64.8 V

say, 65 V or nearest higher setting available on the relay.

\ Minimum kpv, Vk = 2 ¥ 65

= 130 V

2 Relay current setting

Considering a ground circuit resistance of, say, 2 W:

\ I g = 3.3 10003 2¥

¥where I g is the ground fault current

= 952.6 A (say 950 A)

Let us consider a setting of, say, 30% of I g:

\ I pf = 0.3 ¥ 950

= 285 A

Referring to Figure 15.28(a) the number of CTs that will fallin parallel,

N = 6

and I m corresponds to the relay voltage setting of 65 V fromthe curve of Figure 15.31 = 15 mA.

From Equation (15.1)

285 = 2000 (6 ¥ 0.015 + I st)

\ I st = 2852000

– 6 0.015¥

= 0.1425 – 0.09

= 0.0525 A

Therefore the relay can be set between 5–7.5% of 1 A.

3 Stabilizing resistance

Total desired relay circuit impedance

Rz =V ft

stI

= 650.0525

= 1238 W

Relay resistance

Rr = VAI st

2 2 = 1

(0.0525)Figure 15.31 Assumed magnetizing characteristic of2000/1 A class PS CTs

Knee pointVk = 130 V

143

130

65

0 15 I m 1.5 I m

Excitation current I m (mA)(Ampere-turns converted into Amps.)

RCT = 7 WCore material – CRGO silicon steel

Sec

onda

ry v

olta

ge,

(Vf)

(Flu

x de

nsity

con

vert

ed in

to v

olts

)


� 363 W

\ Required stabilizing resistance

Rst =1238 – 363 = 875 W

4 Peak voltage across the relay circuit

Vp = 2 2 ( – )k m kV V V (15.2)

whereVm = i sc ¥ Rz (considering that there are no other feedsto the generator internal fault from othersources)

= 8.1 ¥ 1238

= 10,027.8 V

\ Vp = 2 2 130(10,027.8 – 130)

= 3208 V

which is a marginal case. It is, however, advisable to providea non-linear resistance.

5 Specification for class PS CTs

CTR = 2000/1Quantity = 6 numbers (identical) Vk ≥ 130 V I m = maximum 15 mA at a Vk/2 of 65 V.

The CT manufacturer must provide the user with themagnetizing characteristics of the CTs, i.e. I m versus Vf.

II Protection of a feeder circuit

Example 15.6Consider a power distribution system as shown in Figure15.32, where a transformer of 50 MVA, 33/11 kV, having afault level of 750 MVA, is feeding a bus connected to sixfeeders of different ratings. All the CTs for a combined phaseand ground fault may be connected in parallel as illustrated.The CTs on the primary side of the transformer will be similar

Figure 15.32 Phase and ground fault differential protection scheme for a transformer and feeder bus protection

R BY

Sw

I/C feeder

N

7

Circuit breaker

G

50 MVA 33/11 kV750 MVA

transformer 3-element highimpedance instantaneous

differential relay

O/G feederswith CTs of

identicalcharacteristics

Control bus

1 2 3 4 5 6

R

BY

N

Sw SwSwSwSw Sw

Under healthy condition

RYB


to those on the outgoing feeders, except for the insulationsystem and the turn ratio, to provide identical secondarycurrent and magnetizing characteristics, as on the secondaryside of the transformer. The relay may be set for a slightlyhigher value to account for the slight error introduced and theconsequent spill currents to avoid an unwanted trip.

I r =50 10

1.732 11

3¥¥

= 2625 A

Consider a reasonably low value of I pf, say, 25% of I r, toachieve a high level of sensitivity and still feed the Im to all the

213

= 7 CTs

\ I pf = 0.25 ¥ 2625

= 656.25 A

Assume the CTs on the secondary side of the transformer tobe 3000/1 and on the primary to be

30001

1133

or 10001

A¥

and their magnetizing characteristics as in Figure 15.33.For all the CTs let Rct = 10 W and lead resistance

RI = 0.75 ¥ 2 = 1.5 W

The fault level of the system

I sc =750

1.732 11¥

= 39 (say 40 kA)

and in terms of the secondary

i sc = 40 103000

13

¥ ¥

= 13.3 A

1 Relay voltage setting (stability limit)

Vft = 13.3 (10 + 1.5) (considering the resistance at the operatingtemperature)

= 152.95 V

say, 155 V or nearest higher setting available on the relay.

\ Minimum kpv, Vk = 2 ¥ 155

= 310 V (Figure 15.33)

2 Relay current setting

I pf = 656.25 A

I m at 155 V = 20 mA from the curve of Figure 15.33

\ 656.25 = 3000 (7 ¥ 0.02 + I st)

or I st =656.253000

–7 0.02¥

= 0.788 A

Therefore, the relay can be set, say, at 10% of 1 A.The scheme is suitable to detect both a ground fault and aphase fault.

3 Stabilizing resistance

Relay circuit impedance, Rz =1550.1

= 1550 W

Relay resistance, R r 2 = 1

0.1= 100 W

(for the same relay as in the earlier example)

\ Required stabilizing resistance,

Rst = 1550 – 100 = 1450 W

4 Peak voltage across the relay circuit

Vk = 310 V

Vm = i sc Rz

= 13.3 ¥ 1550

= 20,615 V

\ Vp = 2 2 310(20,615 – 310)

= 7095 V

which is more than 3 kV. Hence, a non-linear resistance willbe necessary across the relay branch and must be orderedfrom the manufacturer with the relay.

5 Specification for class PS CTs

CTs : 33 kV

CTR = 1000/1 A Qty 3 numbers

CTs : 11 kV

CTR = 3000/1 A Qty 21 numbers

V k ≥ 310 V

I m = as low as possible, but not more than 20 mA at 155 V.The CT manufacturer must provide the magnetizingcharacteristics.

Figure 15.33 Assumed magnetizing characteristic of 3000/1 Aclass PS CTs

Knee pointVk = 310 V

Sec

onda

ry v

olta

ge,

(Vf)

(Flu

x de

nsity

con

vert

ed in

to v

olts

) 341

310

155

0 20 Im 1.5ImExcitation current I m (mA)

(Ampere-turns converted into Amps.)


Notes1 In the above case the incoming feeder would trip, even

when the fault occurs in any of the outgoing feeders,which may not be desirable. It is therefore recommendedthat this scheme be applied to individual feeders, so thatin case of a fault, only the faulty feeder is isolated ratherthan the whole system.

2 With the relay one should also order from the manufacturer(a) Stabilizing resistance of 1450 W and(b) A non-linear resistance to discharge the excess induced

e.m.f., across the relay circuit.Both these resistances are illustrated in Figure 15.27.

15.6.7 Core-balanced current transformers(CBCTs)

These are protection CTs and are used for ground leakageprotection. They are also a form of summation CTs, wherethe phasor sum of the three phase currents is measured.The phasor difference, if any, is the measure of a groundleakage in the circuit. They are discussed in Section 21.5.

15.7 Short-time rating and effect ofmomentary peak or dynamiccurrents

The normal practice of users when selecting a measuringor a protection CT has been to specify only the currentratio, the likely maximum VA burden it may have to feedand the class of accuracy for metering and accuracy limitfactor for protection CTs.

Fault level is normally not mentioned, nor it is requestedby the CT manufacturer. Generally, it should be sufficientto meet the likely fault level of the system and its durationin most cases, particularly on an LV system. For criticalinstallations, large feeders and all HV systems, however,it is recommended to check the suitability of the CTs forthe system fault level and its duration.

A short-circuit on a system will cause over-heating asa result of the short-time current, Isc, and its duration of1 or 3 seconds, according to the system requirementsand its protective scheme. It will also developelectrodynamic forces (Equation (28.4)) as a result ofthe momentary first peak of the fault current (in CTs it is2.5 times the short-time current, Isc, as in IEC 60044-1;see also Table 13.11). These forces may result in electricalas well as mechanical damage to the windings of a CTdepending upon the number of turns in the primarywinding and the configuration of the coil. For bar primaryCTs, having only one turn in the primary, such forces arethe least, hence the statement above. With a lower classof accuracy, a low VA level, and a lower accuracy limitfactor (for protection CTs) a CT can easily be built to bemechanically rugged. But higher requirements of suchparameters may necessitate a bulky CT, disproportionatein size and cumbersome to instal.

In most applications, a bar primary CT is generallyused and a normal CT may be suitable. But for too smallratings, where the use of a wound primary CT isimperative, short-circuit effects must be considered, exceptthe CTs for an LV system, where the fault level for suchsmall ratings may be very low and may not matter (Section

13.4.1(5)). For applications on an HV system, where awound primary CT is imperative, choice of a CT fromstandard wound primary CTs may still be possible, meetingthe minimum requirements of class of accuracy, VA burdenand short-time rating. IEC 60044-1 indicates for measuringand protection CTs the maximum short-time factors (STF)that can be obtained economically for a normal woundprimary CT where

Table 15.11 Maximum short-time factors obtainableeconomically corresponding to rated output, accuracy class,accuracy limit factor and rated short-time for wound primarycurrent transformers

Accuracy Rated output STF obtainable, corresponding to theclass VA rated short times up to

0.5 s 1.0 s 3.0 s

(A) Measuring CTs0.5 2.5 1100 775 450

5 750 525 30010 500 350 20015 375 275 15030 200 125 75

1 2.5 1100 775 4505 1000 700 400

10 675 475 27515 500 350 200

3 30 275 200 1102.5 1100 775 4505 1000 700 400

10 675 475 27515 500 350 20030 275 200 110

(B) Protection CTs5P 10 2.5 550 400 225

5 375 275 15010 225 150 9015 150 100 6030 – – –

5P15 2.5 325 250 1355 275 200 110

10 150 100 6015 85 60 3530 – – –

5P20 2.5 325 250 1355 200 125 75

10 100 75 4015 – – –30 – – –

10P5 2.5 1000 700 4005 750 525 300

10 425 300 17515 375 275 15030 150 100 60

10P10 2.5 600 425 2505 425 300 175

10 275 200 11015 200 125 7530 – – –

10P20 2.5 325 225 1255 275 200 110

10 125 75 5015 85 60 3530 – – –


Short-time factor (STF) = Rated short-time currentRated primary current

= sc

r

II

Some STFs for more important CTs are reproduced inTable 15.11.

Example 15.7Consider a 1.5 MVA 33/11 kV transformer having a fault levelof 750 MVA. The STF can be calculated as below with thecircuit of 11 kV:

I sc =7503 11¥

� 40 kA

and I r =1.5

3 11 1000

¥¥

� 79 A

\ STF = 40 100079

¥

� 506

Consider a CT with a ratio of 100/5 A and select a barprimary CT. If a bar primary is not practicable, then foran STF of almost 500, we can choose a wound-typemeasuring CT from Table 15.11 with an accuracy classof 0.5 and above and a corresponding VA burden of 5 fora short-time current of 1 second. If these parameters arenot suitable use the measuring CT with a higher-ratedprimary current to meet the requirement.

Similarly, for a protection CT from Table 15.11 choosean accuracy class of 10P5 with a VA burden of 5 for aone second short-time current. If this does not meet theneed, the protection CT may also have to be selectedwith a higher-rated primary current.

15.8 Summary of specifications of aCT

In Table 15.12 we list the data, that a user must provideto a manufacturer to design a CT for a particularapplication. Some of the data chosen are arbitrary todefine the specifications.

15.9 Precautions to be observedwhen connecting a CT

1 It is mandatory to ground the secondary circuit of theCTs (in a balanced 3f system, the current throughthe neutral will be zero; see Section 21.2.2, Figure21.7). It is required to eliminate the error due toaccumulation of electrostatic charge on the instrumentsthat may influence the readings. All the CTs in acircuit must be grounded at one point only otherwisecirculating currents may raise the potential of thecircuit, which is dangerous and may damage

instruments or give the operator a shock or even tripother relays connected in the circuit.

2 One should not allow the CT secondary to be opencircuited when it is energized, for it may inducedangerously high voltages. This phenomenon isexplained in Example 15.8.

Example 15.8To determine the terminal voltage of a CT during an accidentalopen circuit under an energized condition consider a meteringCT connected across a few instruments. Refer to the followingfigure based on Figure 15.17, showing an equivalent CTcircuit referred to its secondary side.

* Approx. impedance of the excitation circuit.

(b) Under energized but open circuit condition.

CT circuit referred to the Secondary side.

e2¢¢

1.0 W

393.7A

Z l = 103.23 kW

7.5 MVA,11/÷3kV J60W*

Excitation circuit* Typical parameters of the CT

** Resistance of instruments of 7.5VA

(a) Under energized and closed circuit condition.

7.5 MVA,11/÷3kV

1.0 WZ l = 103.23 kW

170 W J60W**

393.7A

e2¢ **0.3 We2 = 508.1 kV

We have assumed the following parameters,

Connected Load = 7.5 MVA

System Voltage = 11 kV

Burden of all instruments connected across the

CT = 7.5 VA

Lead resistance = 2 ¥ 0.5 W = 1W

Rated current, I r = 7.5 10

3 11 10

6

3¥

¥ ¥

= 393.7 A

Consider a CT ratio of = 400/5 A


Load impedance Z � = 11 103 393.7

3¥¥

� 16.13 W

Z � referred to the secondary side = 16.13 (400/5)2

= 103.23 kW

For ease of analysis we have ignored (without much error)

the CT’s own resistance and reactance.Total resistance of the instruments under rated condition

= 7.5/52

= 0.3 W

e2 = 113

4005

¥

� 508.1 kV

Table 15.12 Summary of specifications of a CT

Sr. no. Specifications Measuring CTs Protection CTs Special-purpose protectionCTs type ‘PS’

1 System voltage As in Table 13.1

2 Insulation level (peak) As in IEC 60044-1 or Tables 13.2 and 14.3 for Series I and Tables 14.1 and 14.2 for Series IIvoltage systems

3 Class of insulation E B B

4 Frequency 50 or 60 Hz 50 or 60 Hz 50 or 60 Hz

5 Nominal current ratio 600/5 A 2000/5 A 2000/5 A

6 VA burden 2.5, 5, 7.5, 15 or 30

7 Class of accuracy 0.1, 0.2, 0.5,1, 3 or 5 (5P, 10P or 15P)a –

8 Accuracy limit factor (ALF) – (5, 10, 15)b –

9 Short-time current Isc and its 25 kA for 1 second 50 kA for 1 second –duration

10 Dynamic current Minimum 2.5 times Isc Minimum 2.5 times Isc –(in the above case 62.5 kA) (in the above case 125 kA)

11 Nominal turns ratio – – 1/400

12 Limiting secondaryc – – 3resistance at 90∞C (W)

13 Knee point voltage (V) – – 950

14 Excitation current at knee – – 0.05point voltage (or at any otherrequired voltage or both) (A)

15 Service conditions ∑ Indoors or outdoors ∑ Indoor or outdoor ∑ Indoors or outdoors∑ Ambient temperature ∑ Ambient temperature ∑ Ambient temperature∑ Altitude, if above 2000 m ∑ Altitude, if above 2000 m ∑ Altitude, if above

for LV and above 1000 m for LV and above 1000 m 2000 m for LV andfor HV for HV above 1000 m for HV

∑ Humidity ∑ Humidity ∑ Humidity∑ Any other important ∑ Any other important ∑ Any other important

requirement requirement requirement

16 Marking of CTs (a) 600/5 Ad 2000/5 A 6.6 kV, 2000 A, 1/40010 VA, class 1 15 VA, class 5P10 950 ¥ 0.05 R3.

(b) System voltage and insulation level, class of insulation, frequency, short-time rating anddynamic current rating etc. for all types of CTs

¨ææææææææææææææ æææææææææææææææÆ

NotesaThe class of accuracy for protection CTs is recommended to be not more than 10P as far as possible.bProduct of VA and ALF not to exceed 150.cThe limiting secondary resistance is required to determine the secondary limiting e.m.f. which is = (FS) ¥ rated secondary current ¥ VA

¥ resistance of secondary windings at 90∞C or the highest operating temperature as in Table 14.5, where

FS = Instrument security factor

= Rated instrument limit primary current

Rated primary currentdWherever two separate secondary windings are provided, say, one for measuring and the other for protection, the markings shall indicateall such details that are marked against (a) for each secondary winding.

¸˝˛

magnetizingcurve to befurnished by themanufacturer


(a) Under energized condition when the CT’s secondaryis a closed circuit, voltage developed across the relay,

¢ ¥

¥¥

¥

ee

22

3

3

= circuit impedance

0.3

= 508.1 10(103.23 10 + 1 + 0.3)

0.3

= 1.48 V

(b) Under energized condition when the CT’s secondaryis accidentally open circuited, the current will haveonly the magnetizing path and the voltage inducedacross the CT open terminals will be the same asacross the magnetizing circuit. Under this situationthe magnetizing circuit shall carry the same currentas caused by the primary current, which is very high.

\ Voltage developed across the CT open terminals,

¢¢ ¥¥

¥e23

3e

e = 508.1 10(103.23 10 + Z )

Z

where Z = Impedance of excitation circuit,

1Z

= 1170

+ 1J60

e

e

È

Î

ÍÍÍ

˘

˚

˙˙˙

For simplicity, considering the approximate impedance of theexcitation circuit (without much error) as J60 W

\ ¢¢ ¥¥

¥e23

3 = 508.1 10

(103.23 10 + J60) J60

� J295 V

which is approximately 200 times that of voltage under normalcondition and hence highly detrimental for the insulation ofthe CT, the connecting leads and the human contact etc.

Depending upon the system loading at the instant of CTcircuit interruption, it is possible that the primary current isenough to cause a saturation of the CT core. When so, it islikely that the induced voltage across the CT open terminalsmay give a further momentary kick up to 2 ÷2 times thevoltage calculated above, as the current and hence the voltageshall undergo a rapid change from one peak to the otherwithin one half of a cycle, Section 1.2.1.

3 Provision is therefore made to short-circuit all the CTsecondary terminals not in use (for example, in threeenergized measuring CTs, connected to a commonammeter through a selector switch, when either noneor any one of the CTs only may be connected to theammeter at a time, the other CTs remaining out ofcircuit). In such cases, except for the CT in use, theremaining CTs should be shorted. The selectorswitches are therefore designed so that all the CTterminals not in use are shorted automatically throughthe switch, even during a change-over from one CTto another. A typical circuit diagram of the switch isshown in Figure 15.34, which illustrates the fulfilmentof this requirement. It may be observed that in theOFF position, all the CT secondaries are shorted.And when any one of them is in circuit, the remainingtwo are shorted. All such switches must be the ‘makebefore break-type’, so that the CT terminals are shortedbefore being connected to the load (ammeter) duringthe changeover.

4 One should select a lower secondary current, say,1 A CT, for installations requiring long lengths of

connecting leads, such as for remote measurement ofcurrent or other quantities. It is advisable to limit theextra VA burden on the CTs, on account of such leads.

SECTION III: TESTING OFINSTRUMENT AND CONTROL

TRANSFORMERS

15.10 Test requirements

The following tests are recommended on a finished voltageor current transformer:

1 Type tests These are conducted on a finished voltageor current transformer, one of each design and type,to verify their compliance with the design data andrelevant Standards.

2 Routine tests These are conducted on each finishedvoltage or current transformer to verify their suitabilityfor the required duty.

3 Field tests4 Special tests Any tests that are not covered above

and are considered necessary by the user may beagreed upon between the manufacturer and the user.

15.10.1 Voltage transformers

1 Type tests These will cover the following tests:(i) Temperature rise test(ii) Verification of dielectric properties on the primary

windings To check the insulation level, as in Table13.2 for series I and Tables 14.1 and 14.2 for seriesII and Tables 14.3, 14.3(a) and (b) common for seriesI and II voltage systems.(a) Power frequency voltage withstand or HV test.(b) Impulse voltage withstand or lightning impulse

test.Since a VT is associated with a switchgear, eitherwith its assembly or with the switchyard, the abovetwo tests are almost the same as those for the

Sel

ecto

r sw

itch

G

R

Y

B

CT1

CT2

CT3

R Y B

A1 A2

A

Figure 15.34 Shorting of all unused CT terminals in a CTsecondary circuit using a selector switch


switchgear assembly and as discussed in Sections14.3.3 and 14.3.4. The test requirements andprocedures are also similar.

(iii) Wet test for outdoor type transformers The outdoorVTs are also tested for dielectric properties underwet conditions. The procedure to create the wetconditions and to carry out the test are specified inIEC 60060-2. In wet conditions, the VT has the sametest voltages as specified above.

(iv) Verification of accuracy The test results obtainedmust comply with the values of Tables 15.5 and15.6 for a measuring and a protection transformerrespectively. For brevity, we have limited ourdiscussions as above. For more details, exact testvalues and test procedure refer to IEC 60044-2.

2 Routine tests These will cover the following tests:(i) Verification of terminal marking Refer to Table 15.7

and Figure 15.35, illustrating types of transformerconnections.

(ii) Power frequency withstand test on the primarywindings This must be conducted only on theelectromagnetic unit of a VT. For example, whentesting a capacitor VT it must be conducted only onthe secondary circuit, i.e. the electromagnetictransformer. The test values and test procedure willremain the same as discussed above.

NoteA repeat power frequency test, if considered necessary, mustbe performed at 80% of the prescribed test voltage. See alsoSection 14.5.

(iii) Power frequency withstand test on the secondarywindings This must also be conducted only on theelectromagnetic unit of a VT, as noted above, similarto the control and auxiliary circuit dielectric test(Tables 14.3 and 14.3(a) and (b)).

(iv) Verification of accuracy As under type tests above.

3 Field tests Power frequency withstand test on theprimary windings (for un-grounded VTs). The valueof the test voltage and the test procedure, is almostthe same as that for a switchgear assembly (Section14.5).

4 Additional tests on a capacitor VT The testsdiscussed above refer generally to the electromagneticunit only. To test the whole VT, the following testsare recommended. For the test procedure and resultsrefer to IEC 60044-2.

Type tests

1 Tests on capacitors(a) Self-resonating frequency test – applicable only

to carrier coupling capacitors.

Figure 15.35 Single and three-phase VTs with one and two windings in the secondary

R Y or N

R ¢ Y ¢ or N ¢

R Y or N

1R ¢ 1Y ¢ or 1N ¢

2R ¢ 2Y ¢ or 2N ¢

R N

R ¢ N ¢ R ¢ N ¢(1) Single phase VT

(2) Single phase VT with twosecondary windings

(5) Single phase residual VT

R NBY

R NBY R NBY

R ¢ N ¢B ¢Y ¢

R ¢ N ¢B ¢Y ¢

R ≤ N ¢B ≤Y ≤ R ≤ N ¢B ≤Y ≤

R ¢ N ¢B ¢Y ¢

R

NB Y

R ¢N ¢

R ≤

N ¢B ≤ Y ≤

(3) 3-phase VT

(4) 3-phase VT with twosecondary windings

(6) 3-phase residual VT


(b) Power frequency wet withstand test on outdoorcapacitors.

(c) D.C. discharge test.(d) Impulse voltage withstand test.(e) Partial discharge (ionization) test.

2 Temperature rise test.3 Impulse voltage withstand test.4 Ferro-resonance test.5 Transient response test.6 Verification of accuracy.

Routine tests

1 Tests on capacitors(a) Capacitance and tangent of the loss angle (tan d )(b) Power frequency dry withstand test(c) Sealing test

2 Verification of terminal marking3 Power frequency withstand test on the secondary

circuit4 Verification of accuracy

15.10.2 Current transformers

1 Type tests These will cover the following tests:(i) Short-time current (Isc) test

(ii) Momentary peak or dynamic current test. (Thismust be conducted at a minimum of 2.5 I sc)

(iii) Temperature rise test(iv) Verification of dielectric properties on the primary

windings to check the insulation level as in Table13.2 for series I and Tables 14.1 and 14.2 forseries II and Tables 14.3, 14.3(a) and (b) commonfor series I and II voltage systems.(a) Power frequency voltage withstand or HV test.(b) Impulse voltage withstand or lightning

impulse test.Since a CT is associated with a switchgear, eitherwith its assembly or the switchyard, the abovefour tests are almost the same as those for aswitchgear assembly and as discussed in Sections14.3.3 and 14.3.4. The test requirements andprocedure are also similar.

Figure 15.36 A CT wound in different combinations

P1 P2

S 1 S 2

P1 P2

S 1 S 2 S3

(1) Single ratio CT (2) CT with an intermediatetapping on the secondary winding

(3) CT with a primary windingin 2 sections which may beconnected in series orparallel

(4) CT with two secondary windings,each with its own magnetic core

C1 C2

P1 P2

S 1 S 2

P1 P2

1S1 1S 2 2S 1 2S2

Figure 15.37 Circuit to check the polarity of a bar primary CTat site

(v) Wet test for outdoor type transformer This testis similar to that for a VT and as discussed inSection 15.10.1(1).

(vi) Verification of accuracy The test results obtainedmust comply with the values of Tables 15.8 and15.9 for a measuring and a protection CTrespectively.

2 Routine Tests These will cover the following tests:(i) (a) Verification of terminal marking

Refer to Figure 15.36, illustrating types oftransformer connections.

(b) To check the polarity of a CT It is imperativethat the terminals of a CT are wired with correctpolarity, with reference to the primary in eachphase. A reversal in any phase will lead toincorrect meter readings, in metering CTs anderratic signals to the protective relays inprotection CTs. Although CTs are marked withpolarities by their manufacturers, such as P1P2 for primary and S1 S2 for secondary (Figure15.36) it is possible, that by sheer human errorat the time of fitting the CTs, care is not takento maintain the same polarity in all the threephases, or their connections are madeinadvertently, without ascertaining their correctpolarity. It is also possible that on areconnection, such as at site, while reassemblingthe modules of a switchgear or a controlgearassembly, such an omission is made. It istherefore advisable that the polarity of the CTsbe ascertained at site before commissioningthe equipment, such as a switchgear or acontrolgear assembly or a switchyard utilizinga few CTs.

D.C. voltage test to ascertain the polarity Asimple procedure to ascertain this is indicated inFigure 15.37. A low reading d.c. voltmeter isconnected across the CT secondary windings anda battery of 6–12 V through a switch across the

Voltmeteror

Ammeter

Primaryconductor(winding)

Secondaryterminals

S1

P1 P2

S 2

D.C. source(6V–12V)

Switch

– +


primary. On closing the switch, the meter needlewill give a momentary flicker. If the polarity iscorrect, the flicker will be positive on connectionand negative on disconnection. For HV CTs,mounted on transformer bushings, it isrecommended to short-circuit the main transformersecondary (LV) windings to reduce the overallimpedance of the transformer to achieve anappreciable deflection of the voltmeter needle.

(ii) Power frequency withstand test on secondarywindingsThe secondary windings should be capable ofwithstanding a rated power frequency, short-duration withstand voltage of 3 kV for 1 minute.

(iii) Power frequency withstand test between sectionsThis test is applicable when the CT’s primaryand secondary windings have two or more sections.Then the section in between will be capable ofwithstanding a similar voltage as noted in item(ii) above.

(iv) Inter-turn over-voltage testThis test is performed to check the suitability ofthe inter-turn insulation to withstand the highvoltage developed in the secondary circuit in theevent of an accidental secondary open circuit onload. The inter-turn insulation of the windingsshould be capable of withstanding an inter-turnover-voltage of 4.5 kV peak across the completesecondary winding. The test may be conductedby keeping the secondary winding open circuitedand applying a primary current less than or equalto the rated primary current for 1 minute, sufficientto produce a voltage at the secondary terminalsequal to 4.5 kV (peak).

(v) Power frequency withstand test on primarywindingsThis test is same as for item (iv) (a), under typetests.

NoteA repeat power frequency test, if considered necessary, must beperformed at 80% of the prescribed test voltage. See also Section14.5.

(vi) Partial discharge measurement(vii) Verification of accuracy

This test is the same as under type test item (vi).3 Special tests The following additional tests may be

conducted when considered necessary:(i) Chopped lightning impulse test. Refer to IEC

60044-1(ii) Measurement of the dielectric dissipation factor

(tan d ), applicable to only liquid immersed primarywindings, rated for 110 kV and above.

NoteFor lower voltage systems, say, 2.5 to 10 kV, measurement ofdielectric loss factor tan d, along similar lines, to those recommendedfor motors and discussed in Section 9.6.1 is advisable as a processtest to monitor the quality of an HV insulation system, during thecourse of manufacture and using the same value for future referencewhen checking the quality of insulation at the time of energizing orduring a field test.

SECTION IV: NON-CONVENTIONALMETHODS OF CURRENT

MEASUREMENT

Below we provide a brief description of the non-conventional CTs to give an idea of the new generationcurrent measuring devices, their applications and inherentadvantages over conventional CTs. In view of their distinctadvantages their applications for current measurementsis gradually on the rise. It it is possible that soon for veryhigh currents, voltages and special applications callingfor higher accuracy these devices may take overconventional CTs. Conventional CTs gradually limitingto simple current and energy measurements only. Someof these new devices can also be made digital IEDs(intelligent electronic devices) capable for serial datatransmission and power system automation from a remotecontrol station (SCADA systems Section 24.11). For moredetails one may refer to the manufacturers or the literatureson the subject provided under Further Reading.

15.11 Current sensors

A conventional CT is the most used device to measurecurrent of a circuit. But its magnetizing current causes asmall phase displacement (d) in its secondary current ¢I2with reference to primary current I1 as shown in Figure15.18. This displacement introduces a phase error (Table15.8) in the accurate measurement of primary current.Lower the p.f. of the circuit higher the error. It poses alimitation for instruments and devices that are currentoperated and demand for accurate measurement of circuitcurrent in turn to provide reliable measurements, such astesting instruments, measuring current and energy. Thislimitation of a conventional CT is overcome throughelectrically isolated (having no magnetic effect) currentsensors or transducers. Some prominent transducersdeveloped so far and available in the market are notedbelow,

– Resistive shunts– Hall effect current sensors– Faraday effect optical sensors– Zero flux current sensors– Rogowski current transducers or Rogowski coils– Digital optical instrument transformers

15.11.1 Resistive shunts

They are miniature but high precision copper shuntsconnected across the circuit whose current is to bemeasured. In high current circuits they are usually placedin a slot made in the current carrying conductor. Woundas low inductive coils they can have very low ohmicvalue in the range of milli and micro ohm to containvoltage drop across them. They can operate at anyfrequency zero to MHz according to the application ofthe device.

Usually they have negligible burden. But not so athigher currents because of high resistance loss and hence


Figure 15.38 Figure illustrating Hall effect

not preferred at higher currents. Being low inductivecoils they may be associated with small contents ofparasitic L and R. The ratio of L/R determines the highfrequency limit of the measurement. Increasing R to reduceL/R can cause heat dissipation and insertion loss problems.The basic advantage of these transducers is isolationfrom the circuit of which it is measuring the current.

The current to be measured passes through a verylow value register with low temperature co-efficient. Shuntresistance (R) in the current path generates a voltageproportional to the path current. The potential drop acrossthe resistor is a measure of the current to be measured.Different manufacturers may adapt to different techniquesto measure ac and dc currents with the use of these shunts.

Hybrid sensors

When current or voltage sensors are not capable ofdemonstrating the values to be measured directly andare required to be augmented through some electronicintegrator to perform this job, the sensors are termed ashybrid sensors. Such as use of optic fibre or optic crystalsensors and obtaining the desired values throughinterferometric technology.

Optical sensors are EM compatible and ideal foraccurate measurements of currents and voltages in EMpolluted environments. EM pollutions are on the risewith the ever-rising use of electronic technology in allfields and corrupt measurements from conventional CTs.Moreover with digital technologies the new generationsensors act like IEDs and are capable of serial data transferfor remote control and automation. Due to costconsideration up to MV systems it is possible that theconventional devices may continue but for HV and EHVsystems where cost may be of little significance the newgeneration devices are gradually becoming state-of-the-art devices.

15.11.2 Hall effect current sensors

The function of Hall sensors is based on the principle ofthe Hall effect named after its inventor E.H. Hall. Itsuggests that a voltage (Hall voltage) is generatedtransversely to the current in a conductor if a magneticfield is applied perpendicular to the current as illustrated

in Figure 15.38 (a corollary to Fleming’s left hand ruleFigure 1.1).

This philosophy is widely used in sensing numerousparameters in semiconductor circuits such as flow, r.p.m.,commutation of brushless d.c. motors, UPS (un-interruptedpower supply), electric welding machines, numericalcontrol machine tools, electrolysis, rectifier andelectroplating plants. As position sensors for vane control,liquid level, magnetic position, throttle or air valveposition. Similarly, for various automotive applications(like measuring the ignition current). Measuring currentsin electric and electronic circuits is just a few of themany applications these devices can perform. Figure 15.39shows a current measuring circuit. The current throughthe circuit produces a magnetic field which can be guidedby a magnetic yoke to a linear Hall sensor. The output ofthe sensor is proportional to the electric current. It canmeasure the inverter loadside harmonic currents up to100 kHz. It is useful where an isolated measurement ofcurrent is required, which has a d.c. component. Thedevice can measure both ac and dc current componentsbut having magnetic core, is influenced by saturationphenomenon as in a conventional CT. They also requirea power source and precision. If the device is not isolatedfrom EM environment with the use of magnetic shieldingbetween the sensor and the electronic circuit, the sensormay be influenced by EM interferences.

15.11.3 Faraday effect optical sensors

Faradays’ law (Section 1.1) – Output signal is proportionalto the time derivative di/dt of the current ‘I ’ to bemeasured. To obtain the signal proportional to themonitored current (I ) the output signal ‘d i/d t’ is requiredto be electronically integrated. This is a drawback withthis transducer. Because of this feature it also falls in thecategory of hybrid sensors. For integration of signalsuse of optical crystals or optical fibres is made. In bothcases a light source is necessary. The optical signals arethen converted into electrical signals through lightpolarization using the technique of interferometry notedbriefly later. These measuring devices can be called asnew generation sensors and are becoming increasinglysought after devices for measurement of currents inelectrical circuits.

Optical sensors are more accurate being immune fromEM interferences. They are however, sensitive to

Figure 15.39 Current measurement through Hall effect

Magnetic yoke

HALLVoltage

Hall sensorI – Current to be measured Ammeter

20

A

0I


temperature variations and mechanical stresses. For moredetails one may refer to the available literatures on thesubject, a few mentioned under Further Reading.

15.11.4 Zero flux current sensors

They are employed to measure d.c. component such asfor error in a CT, current in a d.c. circuit like transmissionand distribution of d.c. power. Schematic of such a deviceis illustrated in Figure 15.39(a). The device is in theform of a toroidal magnetic circuit similar to aconventional CT. The flux caused by a.c. component ofthe current I1 in which the content of d.c. component isto be measured, is cancelled by current I2 as illustratedin Figure 15.39(a). Current I2 is specially created for thispurpose by providing a compensating secondary windingS2. I2 is adjusted automatically by an amplifier ‘A’.Accordingly, when I1 = I2 the resultant flux in the magneticcircuit (MC) is zero and hence the name of the sensor.The remainder is the d.c. component. Because the fluxesare nullified, the error of measurement is low. The devicebeing a magnetic core is highly susceptible to EMI andmust be well protected from EM interferences and madeEM Compatible.

15.11.5 Rogowski coil current transducers(isolated current probes)

Rogowski coils were invented in 1912. They are air coredtoroidal windings, which when wrapped around a currentcarrying conductor produce a voltage that is proportionalto differential of the current di/dt flowing in the conductor.The windings are spaced around the toroidal as shown inFigure 15.40(a). Since there is no magnetic circuit theycannot measure d.c. components and therefore outputvoltage is largely independent of d.c. distortions. Havingno magnetic core there is no hysteresis saturation effect(no non-linearity) and bandwidth is wide. Figure 15.40(b)illustrates the linear characteristics of such transducers.But they are susceptible to external electromagneticinterferences (EMI). To minimize sensitivity of the coilfrom external magnetic fields, the coil has a rigid coilformer and symmetrical coil windings. Small error inpositioning of the windings can render the Rogowskicoil (RC) susceptible to fields produced by other current

carrying conductors in the vicinity. To overcome EMIeffects a grounded di-electric shield to make itelectromagnetic compatible (EMC) is usually providedbetween the core and the winding. There are two mirrorimage coil layers, each consisting of two concentricsections wound in opposite directions such that the turnsarea of the inner and outer sections are equal. The currentcarrying conductor whose current is to be measured passesthrough the core of the coil. Electric flux generated bythe current in the busbars is coupled with the sensor coil.The output signals of the coil are used directly formeasuring, protection and control/monitoring units. Figure15.40(c) shows a few such coils. These coils can also bemade digital and used for serial data transfer andpower system automation from a remote control stationmaking use of Faraday’s effect and employing opticalsensors.

Usually two designs are available, one wound on arigid toroidal core former and the other wound on aflexible chord like core former to enable achieve anysize of core to fix it on to any size of a current carrying

Figure 15.39(a) Schematic diagram of a zero current sensor(Source: Merlin Gerin)

MC

I1

I2

S1

S2

AG

Z

I1 = Current to bemeasured

I2 = Secondary circuitcurrent

MC = Magnetic circuitZ = Load impedance

usually lowA = Current amplifierS1 = Secondary windingS2 = Zero flux detection

winding controllingamplifier A

G = Ground

Figure 15.40(a) Representing a Rogowski coil

I

C ¢C

e L it

= dd

3.5

3.0

2.5

2.0

1.5

1.0

0.2

0

Sec

onda

ry v

olta

ge (

V)

0

1600

3200

4800

6400

8000

9600

1120

0

1280

0

1440

0

1600

0

Primary current (A)

Figure 15.40(b) Linear characteristics of a few Rogowski coils(Source: Larsen & Toubro)


circuit. Both designs can be made openable enabling tofix it on to a line and measure current of a live circuit. Inthis way they can also be fixed at inconvenient locations.

Rogowski principle Rogowski coil is used for highcurrent measurements. The basic principle is to measurethe electric field produced by the primary current. It istherefore susceptible to external electromagneticinterferences (EMI) compared to a conventional CT. Butthe coil can be protected from EMI by proper design asnoted above. For details refer to the literature mentionedunder Further Reading. The induced voltage at the coilterminals

e = L dd

it

where L represents the mutual inductance of Rogowskicoil in henry (H). It is the signal level of the coil per unitdi/dt. The e.m.f. of the coil would vary with the variationin the primary current. Since the measurement is dependenton principle of mutual inductance by the primary current,this coil stays immune to the d.c. components that produceno induced field. These coils are now established as thetrue d i/d t sensors. Due to no non-linearity in themeasurement, this arrangement is advantageous to measurelarge to very large currents with the least error. Thismethod is quite popular for measuring large systemcurrents accurately at any voltage. Figures 15.41 and15.42 show a few applications.

Unique features– They are environment friendly as they use no oil or

SF6 as in conventional HV and EHV instrumenttransformers and therefore call for little maintenance.

– System voltage is no bar and they can be employedfor LV and all HV and EHV applications. They canbe encapsulated and fixed around bushings or cablesavoiding the need for high insulation.

– Since there is total electrical isolation they are usefulwhere a CT is not convenient or a high d.c. componentexists (high immunity to d.c. transients in primary).

– They are compatible under broadband high frequencies0.1 Hz–100 MHz.

– As a broadband measuring device they can measuresinusoidal and irregular current waveforms accuratelywithout electrical contact and also display it on anoscilloscope.

– Because of non-magnetic core they have high linearityand cause no saturation or ferro-resonance effects atfault currents. And as there are no iron losses theyare accurate and linear current sensing devices.

– They can measure currents from 50 mA–50 kA andmore.

– They cause low errors and have accuracy up to 1%and less.

Figure 15.41 Arrangement of a Rogowski coil (Source: PowerElectronic Measurements, UK)

Figure 15.42 A Rogowski coil probe to measure a.c. currents0.1 Hz–16 MHz (Source: Power Electronic Measurements, UK)

Figure 15.40(c) View of a few Rogowski coils (Source: Lilco, UK)


– Can also measure capacitive discharge currents ofany magnitude.

– Can monitor power semiconductors switchingperformance.

– Can be used as current monitors, current probes andcurrent sensors.

– Fault monitoring – by measuring breaker fault currents.

ApplicationsThe use of Rogowski coils is now on the rise on HV andEHV systems. They are being used on a live circuit forhigh current measurements which can be rich in harmonicssuch as capacitor circuit currents, busbar systems,circulating currents in beams, columns etc. As they canoperate accurately under broadband high frequencies theycan also be used for measuring transient currents andtesting of generators, circuit breakers, bus systems orany kind of high current system. A unique application isto measure arc currents of an arc furnace to monitor arcrestrikes to optimize heating.

Note1. A Rogowski coil is a low VA device of the order of 0.001 VA

during normal service and 0.25 VA on fault and can feed onlylow VA burdens. With the application of the state-of-the-artmicroprocessor based measuring and protective devices theVA demand of the measuring and protective circuits is nowvery low and these coils are fully capable to feed theseburdens.

2. Some manufacturers have combined the Rogowski coil andHall sensors to achieve a current probe to take advantage ofboth. Such as measurement of d.c. component (which is notpossible by RCs), large a.c. components (which is not possibleby Hall sensors) and measuring the current directly than usingan electronic integrator etc.

15.11.6 Digital optical instrument transformers(also known as electronic instrument currenttransformers)

A Rogowski coil (RC) by employing Faraday’s effectand using optical fibre or optical crystal technologyand incorporating digital current integrator can be madedigital to provide digital signals. They may also be calledas Faraday effect current sensors. As digital device theycan transmit data in digital form and can be termed asnew generation instrument transformers. They are capableof digital communication between power generation anddistribution systems (Section 24.11.5) and compatibleto interface with other IEDs (relays and measuringinstruments) for real time data transfer for power systems’monitoring and automation schemes like SCADA(Section 24.11). They can conform to various protocolrequirements like IEC 61850 now in vogue. Opticalfibre or crystal sensors are not sensitive to EMI influences

(Section 23.5.2). These transformers also conform toIEC 60044-8.

Principle of operation The same Faraday effect applies(to sense and integrate electric field). The current througha conductor induces electric field that affects propagationof light travelling through an optical fibre wrapped aroundthe conductor CC¢ (for simplicity not shown in Figure15.40(a)). The electric field changes with the change inthe conductor current – and that changes the velocity ofthe polarized light waves in the sensing fibre. Bymeasuring change in light velocity in an interferometric*scheme and processing the information so obtained, anextremely accurate measurement of current is obtained.

Digital current integrator Since the Rogowski coil sensormeasures the current in time derivative (d i/dt ), anelectronic integrator is essential to convert this to theprimary current ‘i(t)’ for further processing. Usually ahigh performance digital integrator is used to convertthe d i/dt signal output to the current output. It also takesaccount of EMI.

Additional features of a digital RC compared toan analog Rogowski coil

– They can combine with a capacitor voltage dividerunit (CVT), Section 15.4.4 and perform current andvoltage sensing for measurement and protectionthrough a single device.

– Accuracy is very high and can conform to IEC-Class0.2 and 0.3.

– As real-time monitoring sensors they can also sendout data for change in temperature and environmentalconditions.

– Can undertake extensive metering and data acquisition– Long-term trending analysis.– Use of optical or crystal fibre eliminates elaborate

insulation.

NoteAbove we have discussed Rogowski coil digital instrumenttransformers being the next generation instrument transformers.Conventional digital optical instrument transformers (they are alsoelectronic instrument transformers) are, however, already in use.For details see Cigre paper under Further Reading or contact themanufacturers of conventional HV and EHV instrumenttransformers.

* Radar interferometry system is a technique for measuringinterference phenomena with the use of a device called interferometer.Interferometer separates out a beam of light by means of reflectionin two beams to produce interference pattern to measure wavelengthand index of refraction to determine the circuit current.


List of formulae used

Differential ground fault protection

Current setting of the relay,

Ipf = n (N ¥ Im + Ist) (15.1)

Ipf = minimum fault current through the primary requiredto trip the relay

n = turn ratio of the CTsIm = magnetizing current of the CTs corresponding to

the Vft

Ist = relay current settingN = no. of CTs falling in parallel

To limit the peak voltage

V V V Vp k m k = 2 2 ( – ) (15.2)

Relevant Standards

IEC

60034-1/2004

60044-1/2002

60044-2/2003

60044-5/2004

60044-6/1992

60044-7/1999

Title

Rotating electrical machines.Rating and performance.

Specifications for current transformers.General requirements.Application guide for current transformers.

Application guide for voltage transformers.General requirements for voltage transformers.Measuring voltage transformers.

Instrument transformers.Capacitor voltage transformers.

Protective current transformers.Protective current transformers for special purposeapplications.

Instrument transformers – Electronic VTs

IS

4722/2001,325/2002

2705-1/2002,2705-2/2002,4201/2001

4146/2001,3156-1/2002,3156-2/2002

5547/2001

2705-3/20022705-4/2002

–

BS

BS EN 60034-1/1998

BS EN 60044-1/1999

BS 7729/1995,BS EN 60044-2/1999

BS EN 60044-5/2004

BS EN 60044-6/1999

–

ISO

–

–

–

–

–

–60044-8/2002

60051-1 to 9

60059/1999

60060-1/1989

60060-2/1994

60076-3/2000

60255-6/1988

60439-1/2004

––

Instrument transformers – Electronic CTs

Direct acting indicating analogue electrical measuringinstruments and their accessories.

Standard current ratings (based on Renald series R-10 ofISO-3).

High voltage testing techniques. General definitions andtest requirements.

High voltage test techniques. Measuring systems.

Power transformers. Specification for insulation levelsand dielectric tests.

Electrical relays for power system protection. Generalrequirements.

Low voltage switchgear and controlgear assemblies.Requirements for type-tested and partially type-testedassemblies.

Summation current transformers.

Specification for control transformers for switchgear andcontrolgear for voltages not exceeding 1000 V.

–

1248-1 to 9

–

2071-1/1999

2071-2/2001

2026-3/2001

3231/20013842/2001

8623-1/1998

6949/2001

12021/2000

–

BS 89-1 to 9

–

BS 923-1/1990

BS EN 60060-2/1997

–

BS EN 60255-6/1995

BS EN 60439-1/1999

–

–

–

–

3/1973

–

–

–

–

–

–

–

ANSI/IEEE-C57.13/1993 Instrument transformers (CTs and VTs) – Requirements.

Relevant US Standards ANSI/NEMA and IEEE

Notes1 In the table of relevant Standards while the latest editions of the Standards are provided, it is possible that revised editions have become

available or some of them are even withdrawn. With the advances in technology and/or its application, the upgrading of Standards is acontinuous process by different Standards organizations. It is therefore advisable that for more authentic references, one may consult therelevant organizations for the latest version of a Standard.

2 Some of the BS or IS Standards mentioned against IEC may not be identical.3 The year noted against each Standard may also refer to the year it was last reaffirmed and not necessarily the year of publication.


Vp = peak voltage across the relayVm = theoretical maximum CT secondary voltage across

the relay circuit at the maximum internal fault currentVk = knee point voltage

Further Reading

Protective Relays and Application Guide, GEC Measurement, GeneralElectric Co. Ltd, Stafford, UK.

Ray, W.F., Hewson, C.R., High Performance Rogowski CurrentTransducers, Power Electronic Measurements Ltd, NG96AD,UK.

Klimek, Andrew, Optical Technology: A New Generationof Instrument Transformers, Electricity Today Magazine,Canada.

Development of an electronic instrument transformer (active andpassive) Adolfo Ibero, Jose Miguel Nogueiras, ElectrotecnicaArteche, Hnos, S.A. (Spain) – 12/23/34-04 (Session 2000) © Cigre.

Teyssandier Christian, n∞170 – From current transformers to hybridsensors, in HV, Merlin Gerin – E/CT 170 first issued March,1995.

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