Application Notes for KCGG High Impedance …

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. 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.

2 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. 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. 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).

3 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. 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.

4 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. 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.

5 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. 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). 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.

6 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. 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).Metrosils are used to limit the peakvoltage developed by the currenttransformers, under internal faultconditions, to a value below theinsulation level of the currenttransformers, relay andinterconnecting leads, which areable to withstand 3000V following formulae should beused to estimate the peak transientvoltage that could be produced foran internal fault.

7 This voltage is afunction of the current transformerkneepoint voltage and theprospective voltage that would beproduced for an internal fault ifcurrent transformer saturation didnot occur. Note, the internal faultlevel, I'f , can be significantly higherthan the external fault level, If , ongenerators where current can be fedfrom the supply system and = 2 2VK (Vf VK)(6)Vf = I'f (RCT + 2RL + RST + Rr) (7)where Vp= peak voltagedeveloped by the CTunder internal current transformerknee-point maximum voltage thatwould be produced ifCT saturation did 'f= maximum internalsecondary current transformersecondary maximum lead burdenfrom currenttransformer to relay Relay ohmicimpedance at the value of Vp is greaterthan 3000V peak, non-linearresistors (Metrosils) should beapplied. These Metrosils areeffectively connected across therelay circuit, or phase to neutral ofthe ac buswires, and serve thepurpose of shunting the secondarycurrent output of the currenttransformer from the relay circuit inorder to prevent very highsecondary Metrosils are externallymounted and take the form ofannular discs, of 152mm diameterand approximately 10mm operating characteristicsfollow the expression:V = (8)where V = Instantaneous voltageapplied to thenon-linear resistor(Metrosil)C = constant of the non-linear resistor(Metrosil)I= instantaneous currentthrough the non-linearresistor (Metrosil)With a sinusoidal voltage appliedacross the Metrosil, the RMS currentwould be approximately thepeak current.

8 This current value canbe calculated as follows:I(rms) = (rms) x 2 4C(9)where Vs(rms) = rms value of thesinusoidal voltage applied acrossthe is due to the fact that the currentwaveform through the Metrosil is notsinusoidal but appreciably satisfactory Application of anon-linear resistor (Metrosil), it scharacteristic should be such that itcomplies with the followingrequirements:At the relay voltage setting, the non-linear resistor (Metrosil) currentshould be as low as possible, but nogreater than approximately 30mArms for 1A current transformers andapproximately 100mA rms for 5 Acurrent metrosil units normallyrecommended for use with 1A CTsare as follows:Stability voltageRecommended metrosil typeVs (V) rmsSingle poleTriple poleUp to 125V600A/S1/S256600A/S3/I/S802C = 450C = 450125-300V600A/S1/S1088600A/S3/I/S1195C = 900C = 900 The metrosil units normallyrecommended for use with 5A CTs andsingle pole relays are as follows:SecondaryRecommended metrosil typeinternal faultRelay stability voltage, Vs (V) rmsCurrent(A) rmsUp to 200V250V275V300V50A600A/S1/S1213600A/S1/ S1214600A/S1/S1214600A/S1/S1223C = 540/640C = 670/800C = 670/800C = 740/870100A600A/S2/P/S1217 600A/S2/P/S1215 600A/S2/P/S1215 600A/S2/P/S1196C = 470/540C = 570/670C = 570/670C = 620/740150A600A/S3/P/S1219 600A/S3/P/S1220 600A/S3/P/S1221 600A/S3/P/S1222C = 430/500C = 520/620C = 570/670C = 620/740 The single pole Metrosil unitsrecommended for use with 5A CTscan also be used with triple polerelays and consist of three singlepole units mounted on the samecentral stud but electrically insulatedfrom each other.

9 A triple pole Metrosil type and the referenceshould be specified when units for higher stabilityvoltage settings and fault currentscan be supplied if KCGGThe KCGG142 is a numerical 3phase overcurrent and earth faultrelay with 3 stages of phase andearth fault protection , I>/Io>, I>>/Io>> and I>>>/Io>>> which canbe used for 3 phase differentialprotection or restricted earth fault(REF) protection . The KCGG122 is anumerical single phase overcurrentand earth fault relay with the same3 stages of phase and earth faultprotection, which can be used forREF protection only. It isrecommended that the I> element isused as the main protection elementfor 3 phase differential protectionand the Io> element for restrictedearth fault applications . This isbecause the I>/Io> elements haveincreased through fault stabilitycompared to the I>>/Io>> andI>>>/Io>>> elements. The I>/Io>elements operate when the Fouriervalue exceeds the threshold settingand the positive and negative peakvalues exceed 90% of the thresholdsetting.

10 The I>>/Io>> and I>>>/Io>>> elements operate when theFourier derived values exceeds thethreshold setting or where the peakof any half cycle exceeds twice theset threshold. Since the differentialspill current is likely to contain a dcoffset level, the positive andnegative peaks will have differentamplitudes and so the I>/Io>element is more stable. The timedelay characteristic should beselected to be definite time and witha setting of zero output relay that is to trip thecircuit breakers must be allocated inthe relay masks for t>A, t>B andt>C. Any relay allocated in theserelay masks will dwell in the closedstate for a minimum of 100milliseconds, even if fleetingoperation of the protection shouldoccur, ensuring positive operation ofthe circuit breaker, or trip relay. It isnot advised that the start outputsfrom I> are used because they donot have this in-built minimumcontact output relays may beallocated to each phase trip if it isrequired to have phase segregatedoutputs.