1 Argus 1 to 6 (Numeric); 2TJM10 (Electro-mechanical): I.D.M.T.L., Definite Time and Instantaneous Overcurrent and Earth FaultProtection

A protection class c.t. must be used, eg in accordance with IEC 185 or BS3938.

Typically the c.t. is specified by means of an accuracy factor and an overcurrent factor up towhich the c.t. remains accurate with the maximumspecified burden connected to its secondary, eg:

The accuracy limit is in percent, and althoughtermed a composite error (which takes account ofpolarity error and magnitude error) is effectivelythe maximum ratio error. The c.t. is more accurate at current values up to full load. Beyondfull load, up to the overcurrent factor, the error willnot exceed the specified value.

The overcurrent factor is a multiple of the c.t. rating up to which the c.t. remains within the accuracy limit.

Thus in the example of a 5P10, 5VA c.t., this willtransform primary current within the accuracy limitof 5% when a burden of 5VA (at rated current) isconnected to the c.t. secondary, for a primarycurrent up to 10x its rating.

If the load burden is less than the rated burden ahigher overcurrent factor can be tolerated, although not necessarily exactly in inverse proportion, i.e. half the burden, twice the overcurrent factor does not necessarily apply.

The internal burden of the c.t. (eg. Its secondaryresistance) must be taken into account if a trueequivalent overcurrent factor is to be establishedfor a lower burden of the load.

Typically, c.t. requirements vary dependent onthe project specific requirements. The followingconsiderations must be made.

A) C.T. Rating – should be chosen at least equal tothe maximum continuous load current of thecircuit. This includes any emergency rating, eg.of a power transformer where typically onehour) one hour or two hour overload ratings areoften provided.

B) Accuracy Factor – Typically standard values of5% or 10% are employed. 5% where currentgrading requirement are onerous eg. where thecircuits being graded have similar ratings andthere are several stages of grading. In thesecircumstances an accuracy of 5% assists inallowing small grading steps. Accuracy levels of10% are acceptable where large grading stepscan be tolerated and only a small number ofgrading steps are required.

C) Overcurrent Factor – The factor should be chosen to ensure:-

i I.d.m.t.l. relay performance is not undermined by the c.t’s inability to transformall the current into the secondary circuit.

ii That any a.c. saturation due to a very largeprimary current is not so sever as to result ininsufficient energy in the secondary waveform to prevent the relay from operating.

Provided the above criterion are met it is notnecessary to select an overcurrent factorwhich ensures the maximum fault currentcan be accurately transformed.

5P10 : 5VA

AccuracyLimit

Overcurrent Factor

Maximum burden(at c.t. secondary level)

CURRENT TRANSFORMER REQUIREMENTS FOR VA TECH REYROLLE RELAYS

d) Burden – this is the load burden in VA, at the ratedc.t. secondary current, of all equipment connectedto the c.t. secondary, including all pilot burden.Pilot burden can be significant, particularly for 5amp rated c.t’s. Most modern protection relays(static or numeric) represent a fixed burden nomatter what the setting. However, for electro-mechanical relays, the burden may be dependenton the setting of the relay.

Examples of typical applications are as follows.

• I.D.M.T.L. Overcurrent

a) For industrial systems with relatively low faultcurrent and no onerous grading requirements – aclass 10P10 with rated burden to suit the load.

b) For utility distribution networks with relatively highfault current and several grading stages - a class5P20, with rated burden to suit the load.

Note: Where the maximum fault level is considerablyhigher than the overcurrent factor and therefore the c.t.secondary current could be significantly lower thanthat equivalent to the primary current (due to a.c.saturation), it is necessary to consider any effect onthe protection system performance, eg. gradingmargins.

For i.d.m.t.l. applications, because the operating timeat high fault current is approaching a definite minimumvalue, partial saturation of the c.t. at values beyond theovercurrent factor has only a limited effect. However,this must be taken into account in establishing theappropriate setting to ensure proper grading.

• Definite Time and Instantaneous Overcurrent

a) For industrial systems with requirements as fori.d.m.t.l. relays item (a) above, a class 10P 10.

b) For utilites as for (b) above – a class 5P 10.

Note: Overcurrent factors do not need to be high fordefinite time protection because once the setting isexceeded accuracy is not important. Often, howeverthere is also the need to consider instantaneousovercurrent protection as part of the same protectionsystem and the settings would normally be of the orderof 10x the c.t. rating or higher. If they are higher than10x then the overcurrent factor must be raisedaccordingly, eg. to P20.

• Earth Fault Protection

Considerations and requirements for earth faultprotection are the same as for overcurrent. Usuallythe relay employs the same c.t’s eg. three phasemounted c.t’s connected in residual to establish theearth fault current.

The accuracy and overcurrent factors are thereforealready fixed and for both these factors the earth faultprotection requirements are normally less onerous thanfor overcurrent.

A major consideration for an electro-mechanical relay isits burden at relay setting. Employing a low earth faultsetting usually results in a high burden. In thesecircumstances the rated burden of the c.t. must bechosen on the basis of the requirements of the earthfault element.

2. Duobias 4C21 (Electro-Mechanical): Transformer Differential Protection

All CT’s should be of the low reactance type and theknee point voltage (Vk) should be equal to, or exceedtwice the maximum steady state working voltage underany through fault condition. To assess the steady stateworking voltage the impedance’s of the Duobias relaysare ignored and only the c.t. winding andinterconnecting lead resistance’s considered.

As a general guide the knee point voltage Vk shouldequal or exceed:-

For Star connected CT’s - Vk equal or greater than2I(A+C)

For Delta connected CT’s -Vk equal or greater than2I(B+3D)

Where:I = Maximum through fault current referred to the

secondary winding of the star connected c.t’swith a three phase system fault.

A = Secondary winding resistance of each of the starconnected c.t’s.

B = Secondary winding resistance of each deltaconnected c.t.

C = Resistance of each lead between the star connected c.t. terminals and the relay terminals.

D = Resistance of each lead between the deltaconnected terminals and the relay terminals.

Class ‘X’ current transformers to BS 3938 (or class TPSto IEC 44-6) can be specified to meet the aboverequirements and this type are recommended.

3. B3/5B3 (Electro-mechanical);DAD (Static): HighImpedance Differential and Restricted EarthFault Circulating Current Protection

The basic requirements are:1 .All the current transformers should have identical

turns ratios.

2 The Knee point voltage of the current transformers should be at least twice the relaysetting voltage. The knee point voltage is expressed as the voltage at fundamental frequency applied to the secondary circuit of thecurrent transformer which when increased in magnitude by 10% causes the magnetising current to increase by 50%.

3 The current transformers should be of the lowleakage reactance type. Generally most moderncurrent transformers are of this type and thereshould be no difficulty in meeting this requirement. Low leakage reactance current transformers have a jointless core with the secondary winding evenly distributed along thewhole length of the magnetic circuit, and the primary conductor passes through the centre ofthe core.

Class ‘X’ current transformers to BS 3938 (or classTPS to IEC 44-6) can be specified to meet the aboverequirements and this type are recommended.

4. Duobias M (Numeric): Transformer Differential and Restricted Earth Fault

For high speed operation under all fault conditionsthe minimum current transformer knee point voltageshould equal or exceed: Vk = 4I(A+C).

Where:I = Either the maximum three phase through fault

current (as limited by the transformer impedance) or the high-set setting, whicheveris greater

A = The secondary winding resistance of eachstar connected c.t.

C = The c.t. secondary loop lead resistance forinternal earth faults

For restricted earth fault protection it isrecommended that all c.t’s should have an equalnumber of secondary turns. Line c.t’s are normallystar connected and standard ratios can be selectedaccording to the transformer rating, ratios need notbe exact provided they are within the range of theDuobias-M relay current setting ranges and do notcause the c.t. or relay thermal ratings to beexceeded.

Ideally the line c.t. ratios should be selected to allowDuobias-M relay settings for c.t. ratio correctionfactors to be employed in order to balance thesecondary current to normal relay current. Thisallows maximum sensitivity to be achieved forinternal faults.

Class ‘X’ current transformers to BS 3938 (or classTPS to IEC 44-6) can be specified to meet the aboverequirements and this type are recommended.

5. Solkor R/Rf (Electro-mechanical): Pilot Wire,Feeder Current Differential Protection

The minimum knee point voltage of the line currenttransformers is given by:

Vk = 50 + If (Rct + 2Rl) In N

WhereIn = Rated current of relay, ampsIf = Primary current under maximum steady state

through fault conditionsN = Current transformer ratioRct = Secondary resistance of the current transformer

in ohmsRl = Cable/wiring resistance between the current

transformers and the relay summation transformer, for each single wire (route length),in ohms

Generally it is not recommended that any otherequipment burdens should be included in the currenttransformer circuit in order to avoid any possible mal-operation due to through faults. However, in someinstances the protection design often requires theinclusion of starting relays for the Solkor protection andoccasionally the addition of i.d.m.t.l. protection to thesame c.t’s for backup protection. In such cases the extraburden should be carefully established and included inthe calculation. The additional burden on each phaseshould be reasonably balanced.

The secondary magnetising currents of the currenttransformer at opposite ends of the feeder should notdiffer by more than In/20 amperes for output voltage up to50/In volts.

To ensure good balance of the protection the currenttransformers at the two ends should have equal ratios.Close balance of ratio is provided by current transformersto IEC44-6, Class TPS (BS3938, Class X), where ratioerror is limited to ±0.25% and these are recommended.

6 Solkor-M (Numeric): Feeder Current DifferentialProtection

The current transformers at the two ends should havesimilar design parameters and performancecharacteristics. In addition the secondary burden of thetwo current transformers should be kept similar. This willthen allow a low value of stability factor to be used, hencereducing the knee point voltage requirements of thecurrent transformers.

The minimum knee point voltage for the line currenttransformer is given by:

Vk = k x X/R x If/N x (Rct + 2Rl + Rb)

where:k = stabilityX/R = the X/R ratio for the maximum through

fault conditions. The value of this transient factor depends upon the sum of the source and transmission circuit impedance’s.

If = primary current under maximum through fault conditions (amps)

N = current transformer ratioRct = secondary resistance of the current

transformer (ohms)Re = lead resistance between the current

transformers and the relay ohms)Rb = burden of relay (ohms)The ac burden

of the relay per phase is0.05V at 1A for 1A tap = 0.05 ohm0.3VA at 5A for %a tap = 0.012 ohm

It is not recommended that any other burden shouldbe included in the current transformer circuit, butwhere this cannot be avoided the additional burdenshould be added to those listed when determining thecurrent transformer output voltage required.

In addition to the above, the secondary magnetisingcurrents of the current transformers at opposite endsof the feeder should not differ by more than In/20A foroutput voltages up to 50/InV.

For example, consider a 33kV feeder with a worstcase through fault of 8kA with a X/R of 10. Theminimum current transformer knee point required,given a turns ratio of 1/400, secondary CT resistanceof 2Ω, and lead burden of 1Ω, is :-

Vk ≥ 0.3 x 10 x 8000/400 x (2 + 2x1 + 0.03)

Vk ≥ 242volts

7 Microphase-FM (Numeric): Current Differential Telephase Protection

The minimum knee point voltage for the line currenttransformers is given by:

Vk = k x X x If x (Rct + 2Rl + Rb) R N

Where:k = Stability factor = 0.8X/R = The X/R ratio for the maximum through

fault condition. The value of this transient factor depends upon the sum of the source and transmission circuit impedance’s.

If = Primary current under maximum steady state through fault conditions (amps).

N = Current transformer ratioRct = Secondary resistance of the current

transformer (ohms)

Rl = Lead resistance between the current transformers and the relay (ohms)

Rb = Burden of relay (ohms)

It is not recommended that any other burdens should beincluded in the current transformer circuit, but where thiscannot be avoided the additional burden should beadded to those listed when determining the currenttransformer output voltage required. In addition to theabove, the secondary magnetising currents of thecurrent transformers at opposite ends of the feedershould not differ by more than In/20 amperes for outputvoltages up to 50/In volts.

To ensure good balance of the protection the currenttransformers at the two ends should have equal ratios.Close balance of ratios is provided by currenttransformers to IEC44-6, Class TPS, ratio error limitedto ±0.25%, and these are recommended.

• Relay AC Burden per phase0.05VA at 1A for 1A tap = 0.05 ohm0.3VA at 5A for %a tap = 0.012 ohm

• StabilityUnder through fault conditions the relay will bestable with fault current equivalent to 50 times thenormal current rating in use.

8 THR (Static): Distance (Impedance) Protectionfor Transmission Circuits

For high speed operation and accurate impedancemeasurement the c.t’s should be Class TPS to IEC44-6(Class X to BS3938) and have a knee point voltage (Vk)equal to or greater than the following:-

Vk ≥ If [R1 + R2 + X (R3 + R2)] R

Where:If = Secondary fault current for fault at end of Zone

1R1 = Resistive burden of THR relay (See table

below)R2 = Resistance of connecting loads plus resistance

of the C.T. secondary windingX = Ratio of reactance to resistance of the system

for R a fault at the end of Zone 1R3 = Constant depending on impedance setting of

Zone 1

To calculate values of R1 and R3 for 2A or 5A relays,divide values in the following table by 4 and 25respectively.

9 Ohmega (Numeric): Distance (Impedance)Protection for Transmission Circuits

For high speed operation and accurate impedancemeasurement the c.t’s should be Class TPS to IEC44-6 (Class X to BS3928) and have a knee point voltage(Vk) equal or greater than the higher of the followingtwo expressions:

1) Vk ≥ K • Ip (1+ Xp) (0.03 + Rct + Rl) N RpFor phase-phase faults

2) Vk ≥ K • Ie (1+ Xe) (0.06 + Rct + Rl) N Re For phase-earth faults

Where:Ip =Phase fault current calculated for Xp/Rp

ratio at the end of zone 1le =earth fault current calculated for Xe/ Re

ratio at the end of zone 1N =C.T. ratioXp/Rp =power system reactance to resistance ratio

for the total plant including the feeder line parameters calculated for a phase fault at

the end of zone 1Xe/Re =similar ratio to above but calculated for an

earth fault at the end of zone 1Rct =C.T. internal resistanceRI =lead burden, C.T. to Ohmega terminalsK =Factor chosen to ensure adequate

operating speed and is <1. K is usually 0.5 for distribution systems, a higher value is chosen for primary transmission systems. Reyrolle Protection should be consulted.

Both Vk values should be calculated and the highervalue chosen for the c.t. to be used.

10 GAMMA (Numeric): Generator Protection

a) Two off 3 phase Inputs (Line end and NeutralEnd):

The current transformer minimum requirementsdepend on the protection application, the functionsemployed and the primary circuit configuration.

For satisfactory operation of all functions except thelow impedance biased differential function, the use ofa class 5P20 to IEC60185, or any equivalent, would besatisfactory for any application since the fault levelsnever exceed 20 x the c.t. rating. The VA rating ischosen to allow for all the circuit burden (eg. c.t.secondary cabling and relay burden).

For stability of the low impedance biased differentialfunction it may be necessary to provide a design whichensures neither of the two 3-phase sets of c.t’s areoverfluxed in the event of re-occurring high magnitudefaults with high X/R ratio source impedance. In thesecircumstances, where high levels of d.c. componentcurrent, long time constants and a long operating timefor the network protection may occur, the c.t’s can beleft with a high level of remnant flux. Any subsequentfaults may then cause one of the c.t’s to fully saturateand the differential function mal-operate.

If this is possible, e.g. for a directly connectedgenerator (no generator transformer), where the twosets of c.t’s may be supplied by differentmanufacturers, where there is a multi-shot delayedauto-reclose scheme on feeders local to the gridconnection, and the differential setting chosen is verysensitive, it is recommended that any low reactancec.t’s (ie with high remanance factor) should have kneepoint voltages compliant with the following formula:-

Vk = 50ln (RCT + 2RL + RR) where maximum throughfault current = 10 x ln with maximum X/R = 120.Minimum Vk to be 60 Volts.

Vk = 30ln (RCT + 2RL + RR) where maximum throughfault current = 10 x ln with maximum X/R = 60.Minimum Vk to be 60 Volts.

Where:Vk = Knee point voltageln = Rated currentX/R = The X/R ratio for the maximum through fault

condition.RCT = Secondary resistance of the current

transformer (ohms)RL = Lead resistance between the current

transformers and the relay (ohms)RR = Resistance of any other protection functions

sharing the current transformer (ohms)

Where all the onerous conditions described above arenot required to be met and the c.t. accommodationfacility is limited, the requirements can be reduced, inthese circumstances contact VA Tech Reyrolle -Protection for advice.

b) Neutral Earth C.T. Inputs:

For solid earthed (eg. direct connected generator), use the same as (a) above

Relaynominalrating

(A)

Settingof

Zone 1

R1 R3Earth Phase

Fault Fault

1 0.8 to 44 to 88 to 1616 to 2424 to 48

0.30.40.81.53.8

0.30.40.60.91.2

0.20.30.50.50.9

For impedance earthed neutral, a lower specificationcan be employed eg 5P5.

Use of C.T’s Common to More Than One Relay

Generally the c.t’s employed for generator protectionshould be dedicated to that one duty, for security ofthe protection.

Technically however there is no reason why otherequipment may not share the same c.t’s, except thatthe additional burden should be taken into accountand also that the c.t’s for the three phase inputsshould have a reasonably balanced burden on eachphase. This ensures no possibility of mal-operationof the differential function.

For the requirement of redundancy, there is noproblem with the performance of either relay whenconnecting two Gamma relays in series. However,we recommend that the redundancy philosophy beextended to include the c.t’s, ie. use separate c.t.secondaries.

Use of duplicate Gamma relays, particularly on highrated generator units (eg over 15 MW) provides ahigh level of security and integrity which is still costeffective.

11 RHO (Numeric): Motor Protection and Electrical Plant Thermal Overload

11.1 RHO for low voltage 3 phase A.C. motors

The RHO 0 relay is compatible with CT’s havingeither 25mA or 5A secondaries.

25mA rated output c.t’s are recommended for motorcurrents up to 100A and 5A up to 3000A

For 5 amp relay input rating some motor controlsystems result in high multiples of rated currentflowing during the start up period, and for somemotors the run-up time may be very long (eg 60seconds). Because of this there are some limitationson the motor rating that can be utilised with aparticular c.t. rating due to the thermal capacity of therelay c.t. input terminals. If the motor rating is limitedto less than 75% of the c.t. rating there is unlikely tobe any problem with overheating no matter howonerous the starting current, starting time or numberof starts in a period of time for practicalconsiderations.

The c.t. classification should be 10P10 or better (eg.5P10 for improved accuracy) and have a rating to suitthe c.t. secondary total burdens, ie. C.t. leads, RHO0 relay and any equipment connected in series withthe relay.

The RHO c.t. input circuit burden is fixed and is nogreater than 0.25VA, eg 0.01 ohm at 5 amp and the ratedburden is therefore established by selecting a value inexcess of the c.t. secondary circuit loading eg. for phaseinputs:-

Rated VA ≥ (In)² (2Rl + Rb + R1)

Where:In = Rated secondary currentRl = Secondary lead burdens per phase (ohm)Rb = Relay circuit burden ≤ 0.015 ohm for 5 amp

≤ 11 ohm for 25 MaR1 = Other equipment burden (ohm) per phase

For earth fault detection RHO 0 has a separate inputwhich can be employed either from a residual connectionof the phase inputs or a separate core balance c.t. (thepreferred option). The relay is set to the selected modeeg. Residual Connection or CBCT. In residualconnection the trip setting and primary c.t. rating settingestablish the pick-up level.

In CBCT mode the setting range is 30mA to 3000mA,and the CBCT ratio is chosen with this in mind toestablish a required primary trip current. The actualcurrent input range at the relay terminals is 0.06 to 6.0MA.

The class and rating are selected as per the phase inputc.t’s. The rated burden is establish from:-

VA ≤ (In)² (2Rl + R1 ) [References as before]

For the earth fault c.t. input Rb ≤ 300 ohm

If the residual connection mode is employed the e.f. c.t.input burden should be added to the phase c.t. burdensince this is a significant value. For this form ofconnection a stabilising resistor may be required in theearth fault c.t. input, e.g. if the setting is instantaneous.This resistance must be included in the value for R1.However residual connection is not recommended.

11.2 RHO 3 for high voltage 3 phase A.C. motorsand electrical plant

Thermal Overload

The c.t. class recommended is 5P10. This providesaccurate measurement (maximum error of 5%) foroverloads and also for high current magnitudes beyondtypical motor stall current (eg. 6 x full load current).

The rated burden is established by selecting a value inexcess of the c.t. secondary circuit loading, eg:-

Rated VA ≥ (In)² (2Rl+ Rb + R1)

Where:In = Rated secondary currentRl = Secondary lead burdens per phaseRb = Relay circuit burden (See table

below)R1 = Other equipment burden per phase

RHO 3 C.T. Input Burdens

For earth fault detection RHO3 has a separate inputwhich can be employed either from a residualconnection of the phase inputs or a separate corebalance c.t. (the preferred option). There is noselection made within the relay, the primary currentsetting is a function only of the relay current settingand the c.t. ratio.

If a residual connection is employed and the earthfault setting chosen is both sensitive (e.g. less than0.5In) and instantaneous, it is recommended that astabilising resistor be employed in the earth faultinput circuit. This must then be taken into account inestablishing R1.

For residual connection arrangement, Rb will be asummation of the phase fault and earth fault inputburdens.

12 PHI (Numeric): Point-on-Wave Circuit Breaker Controller

The current transformer inputs do not need to beemployed for the point-on-wave function and can beleft unconnected. If connected to the c.t’s the PHIunit enables the current profile to be monitored forcontinuous load and switching conditions. The classof c.t. is not important and either instrument orprotection class c.t’s can be employed, eg any c.t’salready employed for another function. Dedicatedct’s are not necessary. It is only necessary toensure the additional small burden of the PHI relayis included in the requirements of the c.t’s beingemployed.

ACBurden

Impedance

5A Phase £ 0.2VA £ 0.01 W1A Phase £ 0.05VA £ 0.05 W5A Earth £ 0.4VA £ 0.02 W1A Earth £ 0.2VA £ 0.2 W