Application Notes for KCGG High Impedance Protection

1 Application Notes forKCGG high Impedance Protection2 Application Notes forKCGG high Impedance ProtectionIntroductionThe Application of the kcgg numerical overcurrent relay asdifferential Protection for machines,power transformers and busbarinstallations is based on the highimpedance differential principle,offering stability for any type of faultoccurring outside the protected zoneand satisfactory operation for faultswithin the 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 can be seen from Figure 1 thatduring an external fault the throughfault current should circulatebetween the current transformersecondaries.

2 The only current thatcan flow through the relay circuit isthat due to any difference in thecurrent transformer outputs for thesame primary current. Magneticsaturation will reduce the output of acurrent 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 ofthrough fault level by the protectedunit s Impedance , and the fact thatthe comparison is made between alimited number of currenttransformers.

3 Differences in currenttransformer remanent flux can,however, result in asymmetriccurrent transformer saturation withall based on the aboveextreme case for stability havebecome accepted in lieu ofconjunctive scheme testing as beinga satisfactory basis for one end the current transformercan be considered fully saturated,with its magnetising Impedance ZMBshort circuited while the currenttransformer at the other end, beingunaffected, delivers its full currentoutput. This current will then dividebetween the relay and the saturatedcurrent transformer.

4 This division willbe in the inverse ratio ofRRELAY CIRCUIT to (RCTB + 2RL) and, ifRRELAY CIRCUIT is high compared withRCTB + 2RL, the relay will beprevented from undesirableoperation, as most of the current willpass through the saturated achieve stability for externalfaults, the stability voltage for theprotection (Vs) must be determinedin accordance with formula setting will be dependent uponthe maximum current transformersecondary current for an externalfault (If) and also on the highestloop resistance value from therelaying point (RCT + 2RL).

5 The stability of the scheme is alsoaffected by the characteristics of thedifferential relay and the value of Kin the expression takes account ofthis. One particular characteristicthat affects the stability of thescheme is the operating time of thedifferential relay. The slower therelay operates the longer the spillcurrent can exceed its setting beforeoperation occurs and the higher thespill current that can be the kcgg relay I> element thevalue of K is as shown informula CIRCUITF igure 1: Principle of high Impedance protection3Vs > KIf(RCT + 2RL)(1)Vs > (RCT + 2RL)(2)where RCT= current transformersecondary windingresistanceRL= maximum leadresistance from thecurrent transformer tothe relaying pointIf= maximum secondaryexternal fault currentK= a constant affected bythe dynamic responseof the relayNote: When high impedancedifferential Protection isapplied to motors orreactors, there is no externalfault current.

6 Therefore, thelocked rotor current orstarting current of the motor,or reactor inrush current,should be used in place ofthe external fault obtain high speed operation forinternal faults, the knee pointvoltage, VK, of the CTs must besignificantly higher than the stabilityvoltage, Vs. This is essential so thatthe operating current through therelay is a sufficient multiple of theapplied current setting. Ideally aratio of VK 5Vs would beappropriate, but where this is notpossible refer to the AdvancedApplication Requirements forThrough Fault describes an alternative methodwhereby lower values of Vs may operating times for differentVK/Vs ratios are shown in thefollowing table:VK/Vs12632 Typicaloperating30 405060time (ms)These times are representative of asystem X/R ratio of 40 and a faultlevel of 5Is to 10Is.

7 Lower values ofX/R and higher fault currents willtend to reduce the operating kneepoint voltage of a currenttransformer marks the upper limit ofthe roughly linear portion of thesecondary winding excitationcharacteristic. This is defined exactlyin British practice as that point onthe excitation curve where a 10%increase in exciting voltageproduces a 50% increase in 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 the reactance ofthe current transformer in thecalculations for the voltage , the current transformerimpedance is expressed as acomplex number in the formRCT + jXCT.

8 It is also necessary toensure that the exciting impedanceof the current transformer is large incomparison with its secondaryohmic Impedance at the relaysetting the case of the high impedancerelay, the operating current isadjustable in discrete primary operating current (Iop)will be a function of the currenttransformer ratio, the relayoperating current (Ir), the number ofcurrent transformers in parallel witha relay element (n) and themagnetising current of each currenttransformer (Ie) at the stabilityvoltage (Vs).

9 This relationship can beexpressed as follows:Iop = (CT ratio) x (Ir + nIe)(3)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 above. The setting of thestabilising resistor (RST) must becalculated in the following manner,where the setting is a function of therelay ohmic Impedance at setting(Rr), the required stability voltagesetting (Vs) and the relay currentsetting (Ir).RST =VsIr Rr(4)Note: The auxiliary poweredKCGG ohmic impedanceover the whole setting rangeis small, (1A) (5A) and so can beignored.

10 Therefore:RST =VsIr(5)Use of MetrosilNon-linear ResistorsWhen 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 asin busbar Protection , it is morecommon to use a non-linear resistor(Metrosil) across the relay circuit(relay and stabilising resistor).