mcag14 & mfac r6136b high imp rlys,0

8/11/2019 Mcag14 & Mfac r6136b High Imp Rlys,0

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The Application ofHigh Impedance Relays


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The Application of

High Impedance Relays

Figure 1:

Principle of high impedance

protection

Introduction

The application of the MCAG14/34, MFAC14/ 34 and MCTI14/ 34relays to the protection of machines,power transformers and busbarinstallations is based on the highimpedance differential principle,offering stabiliy for any type of faultoccurring outside the protected zoneand satisfactory operation for faultswithin the zone

A high impedance relay is definedas a relay or relay circuit whosevoltage setting is not less than thecalculated maximum voltage whichcan appear across its terminalsunder the assigned maximumthrough fault current condition.

It can be seen from Figure 1, thatduring an external fault the throughfault current should circulatebetween the current transformersecondaries. The only current thatcan flow through the relay circuit isthat due to any difference in thecurrent transfomer outputs for thesame primary current. Magnetic

saturation will reduce the output ofa current transformer and the mostextreme case for stability will be ifone current transformer iscompletely saturated and the otherunaffected. This condition can beapproached in busbar installationsdue to the multiplicity of infeeds andextremely high fault level. It is lesslikely with machines or powertransformers due to the limitation of

through fault level by the protectedunits impedance, and the fact thatthe comparison is made between alimited number of currenttransformers. Differences in currenttransformer remanent flux can,however, result in asymmetriccurrent transformer saturation withall applications.

Calculations based on the aboveextreme case for stability havebecome accepted in lieu of

conjunctive scheme testing as beinga satisfactory basis for application.At one end the current transformercan be considered fully saturatedwith its magnetising impedance

ZMB short circuited while the currenttransformer at the other end, beingunaffected, delivers its full currentoutput which will then dividebetween the relay and the saturatedcurrent transformer. This division willbe in the inverse ratio of RRELAY CIRCUITand RCTB+2RLand obviously, ifRRELAY CIRCUITis high compared withRCTB+ 2RL, the relay will beprevented from undesirable

operation, as most of the current willpass through the saturated currenttransformer.

To achieve stability for externalfaults, the stability voltage for theprotection (VS) must be determinedin accordance with formula 1 andformula 2 for the MCAG/ MFACand MCTI respectively. The settingwill be dependent upon themaximum current transformersecondary current for an external

fault (If) and also on the highest loopresistance value between the currenttransformer common point and anyof the current transformers(RCT+ 2RL).


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Iop = (CT ratio) x (Ir + nIe)

Ir < ( Iop - nIe)CT ratio

Ie < 1x( Iop - Ir)CT ration

protection, it is considered goodpractice by some utilities to set theminimum primary operating currentin excess of the rated load. Thus, ifone of the current transformersbecomes open circuit the highimpedance relay does notmaloperate. The MVTP11/ 31(busbar supervision relay) shouldgive an alarm for open circuitconditions but will not stop amaloperation if the relay is set belowrated load.

In the case of the high impedancerelay, the operating current isadjustable in discrete steps. Theprimary operating current (Iop) will bea function of the current transformerratio, the relay operating current (Ir),the number of current transformers in

parallel with a relay element (n) andthe magnetising current of eachcurrent transformer (Ie) at the stabilityvoltage (Vs). This relationship can beexpressed in three ways:

To determine the maximum currenttransformer magnetising current toachieve a specific primary operatingcurrent with a particular relayoperating current.

Vs >If(RCT+ 2RL) 1Vs > 1.25If(RCT+ 2RL) 2

where RCT= current transformersecondary windingresistance

RL= maximum leadresistance from thecurrent transformer to

the common point

Note:When high impedance differentialprotection is applied to motors orreactors, the external fault currentwill be low. Therefore, the lockedrotor current or starting current ofthe motor, or reactor inrush current,should be used in place of theexternal fault current.

To ensure satisfactory operation of

the relay under internal faultconditions the current transformerkneepoint voltage should not be lessthan twice the relay voltage settingi.e. VK2VSfor the MCAG/ MFACand 1.6VS for the MCTI.

The kneepoint voltage of a currenttransformer marks the upper limit ofthe roughly linear portion of thesecondary winding excitationcharacteristic and is defined exactly

in British practice as that point onthe excitation curve where a 10%increase in exciting voltageproduces a 50% increase inexciting current.

The current transformers should beof equal ratio, of similarmagnetising characteristics and oflow reactance construction. In caseswhere low reactance currenttransformers are not available andhigh reactance ones must be used.

It is essential to use in thecalculations for the voltage setting,the reactance of the currenttransformer and express the currenttransformer impedance as acomplex number in the formRCT+ jXCT. It is also necessary toensure that the exciting impedanceof the current transformer is large incomparison with its secondaryohmic impedance at the relaysetting voltage.

The MCAG14/ 34

The MCAG14/ 34 is anelectromechanical current

calibrated relay with setting rangesof:

0.025 - 0.100A0.050 - 0.200A0.100 - 0.400A0.200 - 0.800A0.250 - 1.00A0.500 - 2.00A

1.00 - 4.00A

The relay has a fixed burden ofapproximately 1VA at setting currentand its impedance varies with thesetting current used. To comply withthe definition for a high impedancerelay, it is necessary, in mostapplications, to utilise an externallymounted stabilising resistor in serieswith the relay coil.

The standard ratings of the stabilising

resistors normally supplied with therelay are 470, 220and 47for0.5A, 1A and 5A current transformersecondary respectively. Inapplications such as busbarprotection, where higher values ofstabilising resistor are often requiredto obtain the desired relay voltagesetting, non-standard resistor valuescan be supplied. The standardresistors are wire wound, continuouslyadjustable and have a continuous

rating of 145W.

The MCTI14/34

The MCTI14/ 34 is an electronicinstantaneous overcurrent relaysuitable for high impedancecirculating current protection. TheMCTI14 single phase earth fault relayor MCTI34 three phase relay, whenused with a stabilising resistor isdesigned for applications where

sensitive settings with stability onheavy through faults are required. Thesetting ranges available are asfollows;

5% to 322.5%In in 2.5% steps (K=1)50% to 3225%In in 25% steps (K=10)

Applying the MCAG14/34 &

MCTI14/34

The recommended relay current settingfor restricted earth fault protection isusually determined by the minimumfault current available for operation ofthe relay and whenever possible itshould not be greater than 30% of theminimum fault level. For busbar

To determine the maximum relaycurrent setting to achieve a specificprimary operating current with agiven current transformermagnetising current

To express the protection primary

operating current for a particularrelay operating current and with aparticular level of magnetisingcurrent.

In order to achieve the requiredprimary operating current with thecurrent transformers that are used, acurrent setting (Ir) must be selectedfor the high impedance relay, asdetailed in the second expression

above. The setting of the stabilisingresistor (RST) must be calculated inthe following manner, where thesetting is a function of the relayohmic impedance at setting (Rr), the


4/124

Vp = 2 2 VK(Vr - VK)I(rms) = 0.52 (Vs(rms) x 2)

4

C

VSA =B

Ir

Rr =B

Ir2

RST =VS - RrIr

required stability voltage setting (VS)and the relay current setting (Ir).

Note: the MCAG14/ 34 is a fixedburden relay, therefore the ohmicimpedance of the relay will varywith setting. The ohmic impedance(Rr) of the MCAG14/ 34 can becalculated using the relay VAburden at current setting (B) and therelay current setting (Ir);

The ohmic impedance (Rr) of theMCTI14/ 34 over the whole settingrange is less than 0.25for 1Arelays and less than 0.02for 5A

relays i.e. independent of current.The stabilising resistor supplied iscontinuously adjustable up to itsmaximum declared resistance. Insome applications, such asgenerator winding differentialprotection, the through fault currentis low which results in a lowstability voltage setting. In manysuch cases, a negative stabilisingresistor value can be obtained fromthe above formula. This negative

result indicates that the relay will bemore than stable without astabilising resistor. When astabilising resistor is not required,the setting voltage(VSA) can becalculated using the followingformula and the current transformerkneepoint voltage should be at leasttwice this value.

Use of Metrosil non-

linear resistors -

MCAG14/ 34 & MCTI14/

34

When the maximum through faultcurrent is limited by the protectedcircuit impedance, such as in thecase of generator differential andpower transformer restricted earthfault protection, it is generally foundunnecessary to use non-linearvoltage limiting resistors (metrosils).However, when the maximumthrough fault current is high, such

as in busbar protection, it is alwaysadvisable to use a non-linear resistor(metrosil) across the relay circuit(relay and stabilising resistor).Metrosils are used to limit the peakvoltage developed by the currenttransformers under internal faultconditions, to a value below the

insulation level of the currenttransformers, relay andinterconnecting leads, which arenormally able to withstand 3000Vpeak.

The following formulae should beused to estimate the peak transientvoltage that could be produced foran internal fault. The peak voltageproduced during an internal faultwill be a function of the currenttransformer kneepoint voltage and

the prospective voltage that wouldbe produced for an internal fault ifcurrent transformer saturation didnot occur. This prospective voltagewill be a function of maximuminternal fault secondary current, thecurrent transformer ratio, the currenttransformer secondary windingresistance, the current transformerlead resistance to the commonpoint, the relay lead resistance, thestabilising resistor value and therelay VA burden at relay operatingcurrent.

where Vp = peak voltagedeveloped by the CTunder internal faultconditions.

Vk = current transformerknee-point voltage.

Vf = maximum voltage thatwouldbe produced ifCT saturation did notoccur.

If = maximum internalsecondary faultcurrent.

Rct = current transformersecondary windingresistance.

RL = maximum lead burdenfrom currenttransformer to relay.

RST= relay stabilising resistor.Rr = Relay ohmic impedance

at setting.

When the value given by theformulae is greater than 3000Vpeak, non-linear resistors (metrosils)should be applied. These non-linearresistors (metrosils) are effectivelyconnected across the relay circuit,or phase to neutral of the acbuswires, and serve the purpose of

shunting the secondary currentoutput of the current transformerfrom the relay in order to preventvery high secondary voltages.

These non-linear resistors (metrosils)are externally mounted and take theform of annular discs, of 152mmdiameter and approximately 10mmthickness. Their operatingcharacteristics follow the expression:

V = CI0.25

where V=Instantaneous voltageapplied to the non-linearresistor (metrosil)

C=constant of the non-linear resistor (metrosi)

I =instantaneous currentthrough the non-linearresistor (metrosil).

With a sinusoidal voltage appliedacross the metrosil, the RMS currentwould be approximately 0.52x thepeak current. This current value canbe calculated as follows;

where Vs(rms) = rms value of thesinusoidal voltage appliedacross the metrosil.

This is due to the fact that thecurrent waveform through the non-linear resistor (metrosil) is not

sinusoidal but appreciablydistorted.

For satisfactory application of anon-linear resistor (metrosil), itscharacteristic should be such that itcomplies with the followingrequirements:

At the relay voltage setting, the non-linear resistor (metrosil) currentshould be as low as possible, butno greater than approximately

30mA rms for 1A currenttransformers and approximately100mA rms for 5A currenttransformers.

Vf=If(RCT+ 2RL+ RST+ Rr)


5/125

Iop = (CT ratio) x (Ir + nIe + ISH)

ISH = VSRSH

At the maximum secondary current,the non-linear resistor (metrosil)should limit the voltage to 1500Vrms or 2120V peak for 0.25second. At higher relay voltagesettings, it is not always possible tolimit the fault voltage to 1500V rms,so higher fault voltages may have to

be tolerated.

Applying the MFAC14/ 34

As the MFAC14/ 34 is a voltagecalibrated relay with setting rangesof 25-175V in 25V steps, 25-325Vin 50V steps and 15-185V in 5Vsteps, it is inherently a highimpedance relay requiring noexternal resistors. Due to the relaycircuit impedance always beingrelatively high, significant voltagescan be produced across the currenttransformers and secondary wiringduring an internal fault. To limit thevoltage to a value below theinsulation level of the currenttransformers, relay andinterconnecting leads, a non-linearresistor (metrosil) is always requiredand should always be used byconnecting in parallel with the relay.Refer to metrosil publication forselection chart).

The operating current is virtuallyfixed at around 20mA, but there issome slight variation with relayvoltage setting as a result ofvariation in the current drawn by thenon-linear resistor (metrosil). Theoperating current, including the non-linear resistor (metrosil) current, forthe various voltage settings is statedto the right:

The relay effective current settingcan be calculated in the samemanner as described for theMCAG14/ 34. For busbarprotection, it is considered goodpractice by some utilities to set theminimum primary operating currentin excess of the rated load. Thus, ifone of the current transformersbecomes open circuit the MFAC14/34 does not maloperate. TheMVTP11/ 31 (busbar supervision

relay) should give an alarm for opencircuit conditions but will not stop amaloperation if the relay is setbelow rated load. Thus, if theresultant value of Iop is too low, itmay be increased by the addition of

a shunt resistor (RSH) to give acurrent of:

The increased primary operatingcurrent with the shunt resistorconnected is:

Figure 2:

Phase and earth fault differential protection for generators, motors or reactors

Figures 2 to 8 show how highimpedance relays can be appliedin a number of different situations.

Figure 3:

Restricted earth fault protection for a 3 phase, 3 wire system-applicable to star

connected generators or power transformer windings

Setting range: 25-175V

Setting Voltage 25 50 75 100 125 150 175

Ir (mA) 19 19 20 23 27 36 53

Setting range: 25-325V

Setting Voltage 25 75 125 175 225 275 325

Ir (mA) 19 19 20 22 24 31 44


6/126

Figure 4:

Balanced or restricted earth fault protection for delta winding of a power

transformer with supply system earthed

Figure 6:

Restricted earth fault protection for 3 phase 4 wire system applicable to star connected

generators or power transformer windings earthed directly at the star point

Figure 5:

Restricted earth fault protection for 3 phase, 4 wire system-applicable to star connected

generators or power transformer windings with neutral earthed at switchgear


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Figure 7:

Phase and earth fault differential protection for an auto-transformer with CTs at the neutral star point

Figure 8:

Busbar protection simple single zone phase and earth fault scheme


8/128

=1000 x 103

3 x 415

= 1391A

.Ie

@ 50V

mcag14 & mfac r6136b high imp rlys,0

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