Application Notes forKCGG High Impedance Protection

2

Application Notes forKCGG High Impedance Protection

IntroductionThe application of the KCGGnumerical 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 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 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. 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 ofconjunctive scheme testing as beinga satisfactory basis for application.At one end the current transformercan be considered fully saturated,with its magnetising impedance ZMB

short 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 currenttransformer.

To achieve stability for externalfaults, the stability voltage for theprotection (Vs) must be determinedin accordance with formula 1.The 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).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 tolerated.For the KCGG relay I> element thevalue of K is 0.5 as shown informula 2.

CTA

ZMA

RCTA

RL

RL

CTB

ZMB

RCTB

RL

RL

Protectedunit

RRELAY CIRCUIT

Figure 1: Principle of high impedance protection

3

Vs > KIf(RCT + 2RL) (1)

Vs > 0.5If(RCT + 2RL) (2)

where RCT = current transformersecondary windingresistance

RL = maximum leadresistance from thecurrent transformer tothe relaying point

If = maximum secondaryexternal fault current

K = a constant affected bythe dynamic responseof the relay

Note: 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 current.

To 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 Stability.This describes an alternative methodwhereby lower values of Vs may beobtained.

Typical operating times for differentVK/Vs ratios are shown in thefollowing table:

VK/Vs 12 6 3 2

Typicaloperating 30 40 50 60time (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 time.

The 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 excitingcurrent.

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 the reactance ofthe current transformer in thecalculations for the voltage setting.Thus, 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 voltage.

In the case of the high impedancerelay, the operating current isadjustable in discrete steps.The 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. 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, 0.06Ω (1A) and0.006Ω (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 peak.

The following formulae should beused to estimate the peak transientvoltage that could be produced foran internal fault. 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 thegenerator.

Vp = 2 2VK (Vf – VK) (6)

Vf = I'f (RCT + 2RL + RST + Rr) (7)

where Vp = peak voltagedeveloped by the CTunder internal faultconditions.

Vk = current transformerknee-point voltage.

4

Vf = maximum voltage thatwould be produced ifCT saturation did notoccur.

I'f = maximum internalsecondary faultcurrent.

RCT= current transformersecondary windingresistance.

RL = maximum lead burdenfrom currenttransformer to relay.

RST = relay stabilisingresistor.

Rr = Relay ohmicimpedance at setting.

When 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 voltages.

These Metrosils are externallymounted and take the form ofannular discs, of 152mm diameterand approximately 10mm thickness.Their operating characteristicsfollow the expression:

V = CI0.25 (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 0.52x thepeak current. This current value canbe calculated as follows:

I(rms) = 0.52Vs(rms) x 2 4

C (9)

where Vs(rms) = rms value of thesinusoidal voltage applied acrossthe Metrosil.

This is due to the fact that the currentwaveform through the Metrosil is notsinusoidal but appreciably distorted.

For 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 5Acurrent transformers.

The metrosil units normallyrecommended for use with 1A CTsare as follows:

Stability voltage Recommended metrosil type

Vs (V) rms Single pole Triple pole

Up to 125V 600A/S1/S256 600A/S3/I/S802C = 450 C = 450

125-300V 600A/S1/S1088 600A/S3/I/S1195C = 900 C = 900

The metrosil units normallyrecommended for use with 5A CTs andsingle pole relays are as follows:

Secondary Recommended metrosil type

internal fault Relay stability voltage, Vs (V) rms

Current(A) rms Up to 200V 250V 275V 300V

50A 600A/S1/S1213 600A/S1/S1214 600A/S1/S1214 600A/S1/S1223C = 540/640 C = 670/800 C = 670/800 C = 740/870

100A 600A/S2/P/S1217 600A/S2/P/S1215 600A/S2/P/S1215 600A/S2/P/S1196C = 470/540 C = 570/670 C = 570/670 C = 620/740

150A 600A/S3/P/S1219 600A/S3/P/S1220 600A/S3/P/S1221 600A/S3/P/S1222C = 430/500 C = 520/620 C = 570/670 C = 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. A ‘triple pole’Metrosil type and the referenceshould be specified when ordering.Metrosil units for higher stabilityvoltage settings and fault currentscan be supplied if required.

5

The 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. 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 seconds.

The 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 dwell.

Separate output relays may beallocated to each phase trip if it isrequired to have phase segregatedoutputs. However, the three relay

masks, t>A, t>B and t>C must alsobe assigned to relay RLY3, for faultrecords to be generated. Phaseinformation will be included in thefault flags.

The Io>>/Io>>>/I>>/I>>>elements not being used should bedisabled by setting the phase andearth fault function links PF1, PF2,EF1 and EF2 to 0.

Setting ranges of I>/Io> elementsare:

I> 0.08 – 3.2In

Io> 0.005 – 0.8In

The ohmic impedance (Rr) of theauxiliary powered KCGG over thewhole setting range is 0.06Ω for 1Arelays and 0.006Ω for 5A relays ie.independent of current. To complywith the definition for a highimpedance relay, it is necessary, inmost applications, to utilise anexternally mounted stabilisingresistor in series with the relay.

The standard values of thestabilising resistors normallysupplied with the relay, on request,are 220Ω and 47Ω for 1A and 5Arelay ratings 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,continuously adjustable and have acontinuous rating of 145W.

Applying the KCGGThe recommended relay currentsetting for restricted earth faultprotection is usually determined bythe minimum fault current availablefor operation of the relay andwhenever possible it should not begreater than 30% of the minimumfault level. For busbar protection, itis considered good practice bysome utilities to set the minimumprimary operating current in excessof the rated load. Thus, if one of thecurrent transformers becomes opencircuit the high impedance relaydoes not maloperate.

The Io> earth fault element in theKCGG142 with it’s low current

settings can be used for busbarsupervision. When a CT or thebuswires become open circuited the3 phase currents will becomeunbalanced and residual current willflow. Hence, the Io> earth faultelement should give an alarm foropen circuit conditions but will notstop a maloperation of thedifferential element if the relay is setbelow rated load. Wheneverpossible the supervision primaryoperating current should not bemore than 25 amps or 10% of thesmallest circuit rating, whichever isthe greater. The earth fault element(Io>) should be connected at thestar point of the stabilising resistors,as shown in Figure 9. The timedelay setting for the supervisionelements (to>) should be at least 3seconds to ensure that spuriousoperation does not occur during anythrough fault. This earth faultelement will operate for an opencircuit CT on any one phase, or twophases, but not necessarily for afault on all three when the currentsmay sumate to zero. The supervisionmay be supplemented with a sparephase protection stage (I>>>) set tothe same setting as the Io> elementor its lowest setting, 0.08In, if theIo> supervision setting is less than0.08In. Note that the Io currentshould be checked when the busbaris under load. This can be viewed inthe Measurements 1 menu in therelay. It is important that the Io>threshold is set above any standingIo unbalance current.The supervision element should beused to energise an auxiliary relaywith hand reset contacts connectedto short circuit the buswires.This renders the busbar zoneprotection inoperative and preventsthermal damage to the Metrosil.Contacts may also be required forbusbar supervision alarm purposes.

It is recommended that the dualpowered KCEG242 relay is notused for differential protectionbecause of the start-up time delaywhen powered from the CTs alone,approximately 200ms. Also, theminimum setting of the phaseovercurrent elements, 0.4In, wouldlimit its application for differentialprotection.

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Figures 3 to 9 show how highimpedance relays can be applied ina number of different situations.

Advanced applicationrequirements for through faultstability

When Vs from formula 2 becomestoo restrictive for the application, thefollowing notes should beconsidered. The information isbased on the transient and steadystate stability limits derived fromconjunctive testing of the relay.Using this information will allow alower stability voltage to be appliedto the relay, but the calculationsbecome a little more involved.

There are two factors to beconsidered that affect the stability ofthe scheme. The first is saturation ofthe current transformers caused bythe dc transient component of thefault current and the second issteady state saturation caused bythe symmetrical ac component offault current only.

Transient stability limit

To ensure through fault stability witha transient offset in the fault currentthe required voltage setting is givenby:

Vs = 40 + 0.05RST +0.04If(RCT + 2RL) (10)

If this value is lower than that givenby formula 2 then it should be usedinstead.

Vs and RST are unknowns inequation (10). However, for a relaycurrent setting Ir, the value of RSTcan be calculated by substituting forVs using equation (5), Vs = Ir RST.

RST Ir = 40 + 0.05RST +0.04If(RCT+ 2RL) (11)

Steady state stability limit

To ensure through fault stability withnon offset currents:

(RCT+ 2RL) must not exceed(VK + Vs)/If. (12)

Typical Setting Examples

Restricted earth faultprotection

The correct application of theKCGG as a high impedance relaycan best be illustrated by taking thecase of the 11000/415V,1000kVA, X = 5%, powertransformer shown in Figure 10, forwhich restricted earth faultprotection is required on the LVwinding. CT ratio is 100/5A.

Stability voltage

The power transformer full loadcurrent

= 1000 x 103

3 x 415= 1391A

Maximum through fault level(ignoring source impedance)

= 1005

x 1391

= 27820A

Required relay stability voltage(assuming one CT saturated)

= 0.5If (RCT + 2RL)

= 0.5 x 27820

x5

1500 (0.3 + 0.08)

= 17.6V

Stabilising resistor

Assuming that the relay effectivesetting for a solidly earthed powertransformer is approximately 30% offull load current, we can choose arelay current setting, Io> = 20% of5A ie. 1A. On this basis therequired value of stabilising resistoris:

VsIr

RST =

17.61

=

= 17.6 ohms

5A rated KCGG relays can besupplied, on request, with stabilisingresistors that are continuouslyadjustable between 0 and 47Ω.Thus, a stabilising resistance of17.6Ω can be set using thestandard supplied resistor.

Current transformerrequirements

To ensure that internal faults arecleared in the shortest possible timethe knee point voltage of the currenttransformers should be at least 5times the stability voltage, Vs.

VK = 5Vs

= 5 x 17.6

= 88V

The exciting current to be drawn bythe current transformers at the relaystability voltage, Vs, will be:

Ie < Is – Ir

n

where Is = relay effective setting

= 30100

x 1391 x 51500

= 1.4A

Ir (Io>) = relay setting

= 1A

n = number of currenttransformers in parallelwith the relay

= 4

∴ Ie @ 17.6V <1.4 – 1

4

< 0.1A

The time delay setting of the to>element should be set to 0s.

The Io>>/Io>>>/I>>/I>>>elements not used should bedisabled by setting the phase andearth fault function links PF1, PF2,EF1 and EF2 to 0. Note, the phaseovercurrent elements not used forrestricted earth fault protection couldbe used to provide normalovercurrent protection.

Metrosil non-linear resistorrequirements

If the peak voltage appearingacross the relay circuit undermaximum internal fault conditionsexceeds 3000V peak then asuitable non-linear resistor(Metrosil), externally mounted,should be connected across therelay and stabilising resistor, inorder to protect the insulation of thecurrent transformers, relay andinterconnecting leads. In the present

7

case the peak voltage can beestimated by the formula:

Vp = 2 2VK (Vf – VK)

where VK = 88V (In practice thisshould be the actual currenttransformer kneepoint voltage,obtained from the currenttransformer magnetisation curve).

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

= 27820 x 51500 x

(0.3 + 0.08 + 17.6)

= 92.7 x 17.98

= 1667V

Therefore substituting these valuesfor VK and Vf into the main formula,it can be seen that the peak voltagedeveloped by the currenttransformer is:

Vp = 2 2VK (Vf – VK)

= 2 2 x 88 x (1667 – 88)

= 1054V

This value is well below themaximum of 3000V peak andtherefore no Metrosils are requiredwith the relay. If, on the other hand,the peak voltage VP given by theformula had been greater than3000V peak, a non-linear resistor(Metrosil) would have to beconnected across the relay and thestabilising resistor.The recommended non-linearresistor type would have to bechosen in accordance with themaximum secondary internal faultcurrent and the voltage setting.

Busbar ProtectionA typical 132kV double busgenerating station is made up oftwo 100MVA generators andassociated step-up transformers,providing power to the high voltagesystem, by means of four overheadtransmission lines, shown inFigure 2. The main and reservebusbars are sectionalised with bussection circuit breakers.The application for a highimpedance circulating currentscheme having 4 zones and anoverall check feature, is as follows:

The switchgear rating is 3500MVA,the system voltage is 132kV solidlyearthed and the maximum loop leadresistance is 4 ohms. The currenttransformers are of ratio 500/1amp and have a secondaryresistance of 0.7 ohms.

Stability voltage

The stability level of the busbarprotection is governed by themaximum through fault level whichis assumed to be the switchgearrating. Using the switchgear ratingallows for any future systemexpansion.

= 3500 x 106

3 x 132 x 103 = 15300A

Required relay stability voltage(assuming one CT is saturated)

= 0.5 If (RCT + 2RL)

= 0.5 x 15300500

(0.7 + 4)

= 72V

Current setting

The primary operating current ofbusbar protection is normally set toless than 30% of the minimum faultlevel. It is also considered goodpractice by some utilities to set theminimum primary operating currentin excess of the rated load. Thus, ifone of the CTs becomes open circuitthe high impedance relay does notmaloperate.

The primary operating currentshould be made less than 30% ofthe minimum fault current and morethan the full load current of one ofthe incomers. Thus, if one of theincomer CTs becomes open circuit

the differential protection will notmaloperate. It is assumed that 30%of the minimum fault current is morethan the full load current of thelargest circuit.

Full load current

= 100 x 103

3 x 132= 438A

Discriminating zone

Magnetising current taken by eachCT at 72V = 0.072A

Maximum number of CTs perzone = 5

Relay current setting,Ir(I>) = 400A = 0.8In

Relay primary operating current,Iop = CT ratio x (Ir + nIe)

= 500 x (0.8 + (5 x 0.072))

= 500 x 1.16

= 580A (132% full loadcurrent)

Check zone

Magnetising current taken by eachCT at 72V = 0.072A

Maximum number of circuits = 6

Relay current setting, Ir (I>)= 0.8A

Relay primary operating current,Iop = 500 x (0.8 + (6 x 0.072))

= 500 x 1.232

= 616A(141% full load current)

Therefore, by setting Ir (I>) = 0.8A,the primary operating current of thebusbar protection meets therequirements stated earlier.

Stabilising resistor

The required value of the stabilisingresistor is:

RST = VsIr

= 720.8

= 90Ω

Therefore the standard 220Ωvariable resistor can be used.

8

Current transformerrequirements

To ensure that internal faults arecleared in the shortest possible timethe knee point voltage of the currenttransformers should be at least 5times the stability voltage, Vs.

Vk/Vs = 5

Vk = 360V

Metrosil non-linear resistorrequirements

If the peak voltage appearingacross the relay circuit undermaximum internal fault conditionsexceeds 3000V peak then asuitable non-linear resistor(Metrosil), externally mounted,should be connected across therelay and stabilising resistor, inorder to protect the insulation of thecurrent transformers, relay andinterconnecting leads. In the presentcase the peak voltage can beestimated by the formula:

Vp = 2 2VK (Vf – VK)

where VK = 360V (In practice thisshould be the actual currenttransformer kneepoint voltage,obtained from the currenttransformer magnetisation curve).

Vr = I'f(RCT + 2RL + RST + Rr)

= 15300 x 1

500 x (0.7 + 4 + 90)

= 30.6 x 94.7

= 2898V

Therefore substituting these valuesfor VK and Vf into the main formula,it can be seen that the peak voltagedeveloped by the currenttransformer is:

Vp = 2 2VK (Vf – VK)

= 2 2 x 360 x (2898 – 360)

= 2704V

This value is below the maximum of3000V peak and therefore noMetrosils are required with therelay. If, on the other hand, the peakvoltage VP given by the formula hadbeen greater than 3000V peak, anon-linear resistor (Metrosil) wouldhave to be connected across the

relay and the stabilising resistor.The recommended non-linearresistor type would have to bechosen in accordance with themaximum internal fault current andthe voltage setting.

Busbar supervision

Whenever possible the supervisionprimary operating current shouldnot be more than 25 amps or 10%of the smallest circuit, whichever isthe greater.

The Io> earth fault element in theKCGG142 with its low currentsettings can be used for busbarsupervision.

Assuming that 25A is greater than10% of the smallest circuit current.

Io> = 25/500 = 0.05In

Using the I>>> element for 3 phasebusbar supervision

I>>> = 0.08In (minimum setting)

The time delay setting of the to> andt>>> elements, used for busbarsupervision, is 3s.

The Io>>/Io>>>/I>> elements notused should be disabled by settingthe phase and earth fault functionlinks PF1, EF1 and EF2 to 0.

Advanced applicationrequirements for through faultstability

The previous busbar protectionexample is used here to demonstratethe use of the advanced applicationrequirements for through stability.

To ensure through fault stability witha transient offset in the fault currentthe required voltage setting is givenby:

Vs = 40 + 0.05RST +0.04IF(RCT+ 2RL)

If this value is lower than that givenby formula 2 then it should be usedinstead.

To ensure through fault stability withnon offset currents:

(RCT+ 2RL) must not exceed(VK + Vs)/If.

Transient stability limit

Vs = 40 + 0.05 RST + 0.04 x15300/500 (0.7 + 4)

Vs = 45.753 + 0.05 RST

Vs = Ir RST

The relay current setting, Ir = 0.8In

0.8 RST = 45.753 + 0.05 RST

RST = 61Ω

Vs = 0.8 x 61 = 48.8V

Steady state stability limit

(RCT + 2RL) < (VK + Vs)/IF.

Assuming VK = 5 Vs

(0.7 + 4) < (6 x 48.8)

(15300/500)

4.7 < 9.57

Thus, the steady state stabilityrequirement is met.

VK = 5 Vs = 244V

Using the advanced applicationmethod the knee point voltagerequirement has been reduced to244V compared to the conventionalmethod where the knee pointvoltage was calculated to be 360V.

9

Figure 2: Double busbar generating station.

21 RA

RST22 v

23 RB

RST24 v

25 RC

RST26 v

Protectiverelays

Protectedplant

A

B

C

A

B

C

P1 P2

S1 S2

P1 P2

S1 S2

100MVA 15kV

100MVA 132/15kV

Mainreserve

132kV

A

B

C

P1 P2

S1 S2

27

R RST

28

v

P1

P2

S1

S2

Figure 3: Phase and earth fault differentialprotection for generators, motors orreactors.

Figure 4: Restricted earth fault protection for3 phase, 3 wire system-applicable tostar connected generators or powertransformer windings.

10

A

B

C

P1 P2

S1 S2

27

R RST

28

v

A

B

C

P2 P1

S2 S1

27

R RST

28

v

P2 P1

S2 S1

N

A

B

C

P2 P1

S2 S1

27

R RST

28

vP1

P2

S1

S2

P2 P1

S2 S1

N

Figure 5: Balanced or restricted earth faultprotection for delta winding of apower transformer with supplysystem earthed.

Figure 6: Restricted earth fault protection for3 phase, 4 wire system-applicable tostar connected generators or powertransformer windings with neutralearthed at switchgear.

Figure 7: Restricted earth fault protection for3 phase, 4 wire system-applicable tostar connected generators or powertransformer windings earthed directlyat the star point.

11

21 RA

RST22 v

23 RB

RST24 v

25 RC

RST26 v

Protectiverelays

A

B

C

A

B

C

P1 P2

S1 S2

P2 P1

S2 S1 P2 P1

S2 S1

21 RA

RST22 v

23 RB

RST24 v

25 RC

RST26 vProtective

relays

ABC

ABC

P2

P1

S2

S1

P2

P1

S2

S1

P1

P2

S1

S2

RN27

28

Buswiresupervision

Contacts frombuswiresupervisionauxiliary relay

A

B

C

RL RL

RCT

RL

RCT

Restrictedearth faultprotection

DataProtection: RL = 0.04Ω

RLC = 0.3Ω

Transformer: X = 5% RL

11kV415V 1500/5A

Figure 8: Phase and earth fault differentialprotection for an auto-transformerwith CTs at the neutral star point.

Figure 9: Busbar protection – simple singlezone phase and earth fault scheme.

Figure 10: Restricted earth fault protection on apower transformer LV winding.

St Leonards Works, Stafford, ST17 4LX EnglandTel: 44 (0) 1785 223251 Fax: 44 (0) 1785 212232 Email: pcs.enquiries@tde.alstom.com Internet: www.alstom.com

©2000 ALSTOM T&D Protection & Control Ltd

Our policy is one of continuous development. Accordingly the design of our products may change at any time. Whilst every effort is made to produce up to date literature, this brochure shouldonly be regarded as a guide and is intended for information purposes only. Its contents do not constitute an offer for sale or advice on the application of any product referred to in it.

ALSTOM T&D Protection & Control Ltd cannot be held responsible for any reliance on any decisions taken on its contents without specific advice.

Publication R6142B Printed in England.

ALSTOM T&D Protection & Control Ltd