current transformer requirements for reyrolle protection relays

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TECHNICAL REPORT APPLICATION GUIDE

TITLE: Current Transformer Requirements for Reyrolle Protection Relays PREPARED BY:- A Allen .................................. APPROVED :- B Watson .................................. REPORT NO:- 990/TIR/005/02 DATE :- 24 Jan 2000

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Issue Date Modification Revision 1 24 Jan 2000 Original Issue Revision 2 Jan 2004 Reference to BS standard changed to IEC. Relays added. Minor

corrections. Revision 3 March 2006 Scope added. Modifications to relays listed and CT formulae. (PVS)

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Protection Relays Covered in This Report

1. Over Current and Earth Fault Protection

Argus 1 – Numerical Over current and Earth Fault, REF and SEF Protection Relays.

Argus 2 – Numerical Directional Over Current and Direction Earth Fault, REF and Directional SEF Protection Relays.

Argus 4 - Numerical One, three and four pole Over current and Earth Fault, REF and SEF Protection with five shot to lockout three pole auto-reclose.

Argus 6 - Numerical one, three and four pole Directional Over Current and Direction Earth Fault, REF and Directional SEF Protection ith five shot to lockout three pole auto-reclose.

2TJM – Electro-Mechanical Over current and Earth Fault Protection.

2. High Impedance Protection:

5B3 and B3 – Electro-mechanical High Impedance Differential and Restricted Earth Fault Protection.

DAD - Static High Impedance Differential Protection.

DAD-N – Numerical High Impedance Differential Protection Relays.

3. Duobias M – Numerical Transformer Differential.

4. Solkor Rf/R – Static Pilotwire Feeder Differential.

5. Solkor N – FO/Metallic Numerical Feeder Differential.

6. Solkor M – Vf Numerical Distribution Feeder Differential.

7. Microphase FM – Vf Numerical Transmission Feeder Differential. 8. Ohmega – Numerical Feeder Distance Protection.

9. Gamma – Numerical Generator Protection.

10. Rho 3 – Numerical Motor Protection.

11. Delta FM1 and FM0 – Numerical Feeder Management

12. MicroTAPP – Numerical On Load Tap Change Voltage Control

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CURRENT TRANFORMER REQUIREMENTS FOR REYROLLE PROTECTION RELAYS General Notes:

1. The impedance of numerical and static(analogue) relay current inputs, do not change with the level of current injected and the resistance to include in the CT calculation is independent of the setting selected. The relay burden can be considered to be a fixed resistance at all levels of current.

2. The relay input impedance of electro-mechanical measuring relays does change with setting. The general

formula is Relay burden = VA burden at rated current / Setting2. The burdens using this formula will over estimate the relay burden as some saturation occurs at high multiples of pickup selected.

3. Where the relay burden is not included in the CT formula, it is because the burden is insignificant compared

to the other secondary burdens such as leads and CT secondary winding resistance.

4. Where X/R must be found to calculate the minimum CT requirement for a relay, some assistance can be given with typical source and circuit parameters can be provided upon request.

5. The Technical Manual or other literature provides further information to supplement this report.

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1. ARGUS 1 to 6 (Numeric) & 2TJM10 (Electro-mechanical): I.D.M.T.L. , Definite Time and Instantaneous Overcurrent and Earth Fault Protection

1.1 Introduction

A protection 'P' class CT must be used, e.g. in accordance with IEC 60044:Pt1. Typically the CT is specified by means of an accuracy class and an overcurrent accuracy limit factor up to which the CT remains within accuracy error limits with the maximum specified load burden connected to its secondary circuit. eg.: 5VA class 5P 10

5P10 5VA

Accuracy Overcurrent Maximum burden Class Accuracy at nominal current Limit Factor rating

The accuracy class '5P', defines the maximum composite error at the rated accuracy limit, with rated burden connected, i.e. 5%. This declared maximum error takes account of both phase and magnitude errors and is due to the excitation current and also any turns ratio compensation, expressed as a percent of the primary current. The overcurrent accuracy limit factor is the multiple of the CT rating up to which the ratio, phase angle and composite errors of the CT remain within the accuracy class limits. Thus in the example the 5VA, 5P10 CT will transform primary current of up to 10x its rating and remain within the composite accuracy limit of 5% with a burden of 5VA (at rated current) connected to the CT secondary. If the load burden is less than the rated burden a higher overcurrent accuracy limit factor can be tolerated although not necessarily exactly in inverse proportion, i.e. half the external burden, will not give twice the overcurrent factor. The internal burden of the CT (e.g. its secondary resistance) must be taken in to account if the true equivalent accuracy limit factor is to be established for a lower load burden.

Typically, CT requirements for this type of protection vary dependent on the project specific requirements. The following considerations must be made.

a) CT Rating - should be chosen at least equal to the maximum continuous load current of the circuit. This includes any

emergency rating, e.g. of a power transformer where typically one hour or two hour overload ratings are often provided. b) Accuracy Class - Typically standard values of 5P or 10P are employed. 5P where current grading requirements are onerous

e.g. where the circuits being graded have similar ratings and there are several stages of grading. In these circumstances an accuracy of 5P assists in allowing small grading steps to be applied. Accuracy levels of 10P are acceptable where large grading steps can be tolerated and only a small number of grading steps are required.

c) Overcurrent Accuracy Limit Factor - The factor should be chosen to ensure:-

i i.d.m.t.l. timing accuracy is not adversely affected by CT ratio errors at high fault levels. ii That any a.c. saturation effects and errors resulting from very large fault current are not so severe as to result in excessive

distortion of the secondary current which may cause the relay to slow down or not operate. d) Burden - the total CT load in VA, at rated secondary current. The calculation must include pilot-lead loop-resistance plus the

relay and any other burdens. Pilot burden can be high, particularly for 5 amp rated CTs, i.e. I2R is 25x compared to only 1x for a 1A CT.

Numeric or static protection relays have a very low fixed impedance independent of setting, see the table below for Argus relay burdens. However, for electromechanical relays the impedance is normally dependent on the setting of the relay.

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e) Examples of typical applications are as follows. 1.2 Phase Fault I.D.M.T.L. Overcurrent

a) For industrial systems with relatively low fault current and no onerous grading requirements - a class 10P10 with VA rating

to match the load. b) For utility distribution networks with relatively high fault current and several grading stages - a class 5P20, with VA rating to

match the load.

Note: if an accuracy limit factor is chosen which is much lower than the maximum fault current it will be necessary to consider any effect on the protection system performance and accuracy e.g. grading margins. For i.d.m.t.l. applications, because the operating time at high fault current is approaching a definite minimum value, partial saturation of the CT at values beyond the overcurrent factor has only a minimal effect. However, this must be taken into account in establishing the appropriate setting to ensure proper grading.

1.3 Definite Time and Instantaneous Overcurrent

a) For industrial systems with requirements as for i.d.m.t.l. relays item (a) above, a class 10P10 (or 20). b) For utilities as for (b) above - a class 5P10 (or 20), with rated burden to suit the load. Note: Overcurrent factors do not need to be high for definite time protection because once the setting is exceeded magnitude accuracy is not important. Often, however, there is also the need to consider instantaneous HighSet overcurrent protection as part of the same protection system and the settings would normally be of the order of 10x the CT rating or higher. Where higher settings are to be used then the overcurrent factor must be raised accordingly, e.g. to P20. 1.4 Earth Fault Protection Considerations and requirements for earth fault protection are the same as for Phase fault. Usually the relay employs the same CT's e.g. three phase CTs star connected to derive the residual earth fault current.

The accuracy class and overcurrent accuracy limit factors are therefore already determined and for both these factors the earth fault protection requirements are normally less onerous than for overcurrent.

1.5 ARGUS Burdens AC rated Impedance 5A Phase/Earth 1A Phase/Earth 5A SEF/REF 1A SEF/REF

< 0.2 VA < 0.05 VA < 0.4 VA < 0.2 VA

< 0.01Ω < 0.05Ω < 0.02Ω < 0.2Ω

Minimum CT recommended rated Burdens are as follows:

For 1A relays = 2.5VA For 5A relays = 7.5VA

This allows for series connection of Argus phase /EF/SEF inputs (0.3VA at 1A and 0.8VA at 5A) and a maximum CT wiring loop burden of 2.2VA for 1A CT (e.g. up to 200 metres of 2.5sq mm copper wire or 6.7VA for 5A CT (e.g. up to 30 metres of 2.5 sq mm copper wire) 1.6 2TJM Burdens < 3VA at setting, for burden impedance characteristics see the 2TJM Manual. A major consideration for an electro-mechanical relay is its burden at nominal current. Selecting a low earth fault setting results in a high nominal burden (VA burden≈ratedVA/setting2). Note the rated burden will reduce when the relay is subjected

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to very high currents, with 1.5VA being more appropriate. In these circumstances the rated burden of the CT must be chosen on the basis of the requirements of the earth fault element.

2. High Impedance Differential and Restricted Earth Fault and Circulating Current Protection:

B3/5B3 (Electro-mechanical); DAD (Static);

Argus 1/2 (Numeric High Impedance/REF); DAD-N(Numeric):

The basic requirements are: a) All the current transformers should have identical number of turns. b) The knee point voltage of the current transformers should be at least twice the relay voltage setting. The knee point

voltage is expressed as the voltage at fundamental frequency applied to the secondary circuit of the current transformer which when increased in magnitude by 10% causes the magnetising current to increase by 50%. Vk > 2 x Vs

There are separate report for setting high impedance unit and restricted earth fault protections, that can be used to calculate the minimum setting voltage for a particular application.

c) The current transformers should be of the low leakage reactance type. Low leakage reactance CTs have a jointless core with the secondary winding evenly distributed along the whole length of the magnetic circuit, and the primary conductor passes through the centre of the core, alternatively a uniformly wound primary.

Class 'PX' current transformers to IEC 60044: pt1 are recommended to meet the above requirements.

3. DUOBIAS M (Numeric): Transformer Differential Relay

For high-speed operation under all fault conditions and for through fault stability the minimum current transformer knee point voltage should equal or exceed:

Vk = 4 x I x Ib (Rct+Rl) – for applications without REF on all windings. Vk = 2 x I x Ib (Rct+Rl) – for applications with REF used to protect on all windings.

Where:

I = 87HS Differential Highset Settings = 1 / transformer % impedance. Ib = secondary current measured with transformer at full MVA rating. Ib = TX MVA / (√3 x Vk x CT ratio) Rct = Secondary winding resistance of the current transformer in ohms at 25ºC. Rl = The maximum CT secondary loop lead resistance in ohms at 25ºC. For restricted earth fault protection it is recommended that all CTs should have an equal number of secondary turns. Line CTs are normally star connected and standard ratios can be selected according to the transformer rating, ratios need not be exact provided they are within the range of the Duobias-M relay current settings ranges and do not cause the CT or relay thermal ratings to be exceeded.

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Ideally the line CT ratios should be selected to allow Duobias-M relay settings for CT ratio correction factors to be employed in order to balance the secondary current to nominal relay current. This allows maximum sensitivity to be achieved for internal faults. The CT ratio adjustment can be used to allow selection of a CT with an increased turns ratio and hence reduce the CT Uk requirement.

Class 'PX' current transformers to IEC 60044: pt1 are recommended, to meet the above requirements.

4. SOLKOR R/RF (Electro-mechanical): Pilot Wire, Feeder Current Differential Protection

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

Vk = 50 + If (Rct + Rl)

In N

Where: In = Nominal rated current of relay in amps If = Primary current under maximum steady state through fault conditions N = Current transformer ratio Rct = Current transformer secondary resistance in ohms Rl = The maximum CT secondary wiring loop lead resistance in ohms

Generally it is not recommended that any other equipment burdens should be included in the current transformer circuit in order to avoid any possible mal-operation due to through-faults. However, in some instances the protection design often requires the inclusion of starting relays for the Solkor protection and occasionally the addition of i.d.m.t.l. protection to the same CTs, for backup protection. In such cases the extra burden should be carefully established and included in the calculation. The additional burden on each phase should be reasonably balanced.

The secondary magnetising currents of the current transformer at opposite ends of the feeder should not differ by more than

In/20 amperes for output voltage up to 50/In volts.

To ensure good balance of the protection the current transformers at the two ends should have equal turns ratios. Close balance of ratio is provided by current transformers to IEC60044: pt1, class PX, whose ratio error is limited to ± 0.25% and these CTs are recommended to meet the above requirements.

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5. SOLKOR N (Numeric): Feeder Current Differential Protection- using RS485 cable, Pilotwires or Fibre optic.

The minimum kneepoint voltage Vk required from the CT design is dependant on the settings used:

With Bias Slope 2 = 150% and Bias Break Point = 2 x IN

With Bias Slope 2 = 150% and Bias Break Point = 1 x IN

With Bias Slope 2 = 150% and Bias Break Point = 0.5 x IN Vk - is the knee point voltage of the CT defined as the point where a 10% increase in excitation voltage produces a 50% increase in magnetising or excitation current. X/R - is the system reactance to resistance ratio for a three phase through fault on the protected feeder. IFM - is the feeder maximum primary three phase through fault current referred to the secondary side. RS - is the total resistive burden of the secondary circuit, including CT secondary winding, relay phase input and lead loop resistance. The above formulae include a minimum safety margin in excess of 120%. This may be utilised if the CT’s calculated above are loo large to fit in the Circuit Breaker chamber. Therefore a 120% reduction may be made to the above minimum kneepoint requirements. This margin is present, as the above expressions where based on tests using the saturation e.m.f (Esat) level of the CT. As the kneepoint voltage (Vk) of the CT is a measurable constant, this was instead of Esat in the expression above. Esat is always at between 120% and 160% of the kneepoint voltage Vk and therefore reducing the Vk calculated above by up to 20% is valid.

For higher values of X/R, VA TECH Reyrolle should be contacted for advice.

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6. SOLKOR M (Numeric): Feeder Current Differential Protection

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

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

where: k = stability factor

= 0.3 for system voltages up to 33kV = 0.5 for system voltages of 66 - 132kV = 0.8 for system voltages 132kV and above

X/R = X/R ratio for the maximum through fault conditions. (Includes both source and line impedance)

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

N = Current Transformer ratio

Rct = Current transformer secondary resistance in ohms

Rl = The maximum CT secondary wiring loop lead resistance in ohms

Rb = burden of relay (ohms). The ac burden of the relay per phase is

0.05V at 1A for 1A tap = 0.05 ohm 0.3VA at 5A for 5A tap = 0.012 ohm

It is not recommended that any other burden should be included in the current transformer circuit, but where this cannot be avoided the additional burden should be added to those listed when determining the current transformer output voltage required. In addition to the above, the secondary magnetising currents of the current transformers at opposite ends of the feeder should not differ by more than In/20 amperes for output voltages up to 50/In volts. For example, consider a 33kV feeder with a worst case through-fault of 8kA with a X/R of 10. The minimum current transformer knee point required, given a turns ratio of 400:1, secondary CT resistance of 2Ω and max. lead loop burden of 2Ω is :-

Vk ≥ 0.3 x 10 x 8000/400 x (2+2+0.05) = ≥ 270volts The secondary burden of the two current transformers should be kept similar. This will then allow a low value of stability factor to be used, hence reducing the knee point voltage requirements of the current transformers. To ensure good balance between relays at each end of the circuit, the current transformers at the two ends shall have equal turns ratios. Close balance of ratios is provided by current transformers to IEC60044: pt1: class PX, the ratio error is limited to ±0.25%, and these are recommended

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7. MicroPHASE-FM (Numeric): Current Differential The minimum knee point voltage for the line current transformers is given by: Vk = k x X/R x If/N x (Rct + Rl + Rb) Where: K = stability factor = 0.8 X/R = X/R ratio for the maximum through-fault condition. (Includes both source line impedance's)

If = Primary current under maximum steady state through fault conditions (amps) N = Current transformer ratio Rct = Current transformer secondary resistance in ohms Rl = The maximum CT secondary wiring loop lead resistance in ohms Rb = Burden of relay in ohms

It is not recommended that any other burden should be included in the current transformer circuit, but where this cannot be avoided the additional burden should be added to those listed when determining the current transformer output voltage required. In addition to the above, the secondary magnetising currents of the current transformers at opposite ends of the feeder should not differ by more than In/20 amperes for output voltages up to 50/In volts. To ensure good balance between relays at each end of the protected circuit, the current transformers at the two ends shall have equal turns ratios. Close balance of ratios is provided by current transformers to IEC60044: pt1: class PX, the ratio error is limited to ±0.25%, and these are recommended

• Relay AC Burden per phase

0.05VA at 1A for 1A tap = 0.05 ohm 0.3VA at 5A for 5A tap = 0.012 ohm

• Stability Under through-fault conditions the relay will be stable with fault current equivalent to 50 times the nominal current rating.

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8. Ohmega (Numeric): Distance (Impedance) Protection

For high speed operation and accurate impedance measurement the CTs should be class PX to EC60044; pt1 and have a knee point voltage (Vk) equal or greater than the higher of the following two expressions: a) For phase-phase faults :- Vk ≥ k . Ip (1 + Xp) (0.03 + Rct + Rl) N Rp b) For phase-earth faults :- Vk ≥ k . Ie (1 + Xe) (0.06 + Rct + Rl) N Re Where: Ip = Phase fault current calculated for Xp/Rp ratio at the end of zone 1 Ie = earth fault current calculated for Xe/Re ratio at the end of zone 1 N = CT ratio Xp/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 1 Xe/Re = similar ratio to above but calculated for an earth fault at the end of zone 1 Rct = CT internal resistance Rl = lead burden, CT to Ohmega terminals K = 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 ACP should be consulted.

Both Vk values should be calculated, and the highest calculated value defines the CT to be used.

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9. GAMMA (Numeric): Generator Protection a) Two off, 3 phase inputs (line end and neutral end) The current transformer minimum requirements depend on the protection application, the functions employed and the primary circuit configuration.

For satisfactory operation of all functions, except the low impedance biased differential function, the use of a class 5P20 to IEC60044: pt1 or any equivalent, would be satisfactory for any application since the fault levels never exceed 20 x the CT rating. The VA rating is chosen to allow for all the circuit burden (e.g. CT secondary cabling and relay burden).

For stability of the low impedance biased differential function it may be necessary to provide a design of CT which ensures neither of the two 3-phase sets of CTs are overfluxed, e.g. in the event of re-occurring, high magnitude fault current on circuits with a high X/R ratio source impedance. In these circumstances, the CTs can be left with a high level of remnant flux. Any subsequent faults may then cause one of the CTs to fully saturate and the differential function may mal-operate.

If this is possible, e.g. for a directly connected generator (no generator transformer), where the two sets of CTs may be supplied by different manufacturers and with slightly different characteristics, and where there is a multi-shot delayed auto-reclose scheme on feeders local to the grid connection, or where the differential setting chosen is very sensitive, in these circumstances it is recommended that any low reactance CTs (i.e. with high remnance factor) should have a knee point voltage compliant with the following formula:-

Vk > 50In (Rct + Rl + Rb) Where maximum through fault current = 10 x In with maximum X/R = 120. Minimum Uk must be 60 volts.

Uk > 30In (Rct + Re + Rb) Where maximum through fault current = 10 x In with maximum X/R = 60. Minimum Uk must be 60 volts

Where: Vk = Knee point voltage In = Rated current X/R = X/R ratio for the maximum through-fault condition Rct = Secondary resistance of the current transformer in ohms Rl = Lead loop resistance between the current transformers and the relay in ohms Rb = Resistance of any other protection functions sharing the current transformer in ohms

Where all the onerous conditions described above are not required to be met or the CT accommodation facility is limited, the requirements can be reduced. In these circumstances contact VA TECH Reyrolle ACP Ltd for advice.

b) Neutral Earth C.T. Inputs:

For solid earthed (e.g. direct connected generator), use the same specification for a class X c.t. as (a) above. For impedance earthed neutrals, a lower specification can be employed, e.g. 5P5.

c) Use of CTs Common to More Than One GAMMA Relay

Generally the CTs employed for generator protection should be dedicated to that one duty, for security of the protection.

Technically however there is no reason why other equipment may not share the same CTs, provided that the additional burden is taken into account and also that the CTs for the three phase inputs should have a reasonably balanced burden on each phase. This ensures no possibility of mal-operation of the differential function.

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For the requirement of redundancy, there is no problem with the performance of either relay when connecting two Gamma relays is series. However, we recommend that the redundancy philosophy be extended and separate CTs used for each relay. Use of duplicate Gamma relays, particularly on high rated generator units e.g. over 15MW, provides a high level of security and integrity which is still cost effective.

10. RHO 3 motor protection for 3 phase induction motors and electrical plant

The CT class recommended is 5P10. This provides accurate measurement for overloads and also for high current magnitudes beyond typical motor stall current e.g. 6 x full load current.

The rated burden is established by selecting a value in excess of the CT secondary circuit loading e.g.-

RHO 3 Burdens:-

AC Burden Impedance 5A Phase 1A Phase 5A Earth 1A Earth

0.2 VA 0.05 VA 0.2 VA 0.05 VA

0.01Ω 0.05Ω 0.01Ω 0.05Ω

For earth fault detection RHO 3 has a separate input which can be employed either from a residual connection of the phase inputs or a separate core balance CT (the preferred option). There is no selection made within the relay, the primary current setting is determined by the relay current setting and the CT ratio. If a residual connection is employed and the earth fault setting chosen is both sensitive (e.g. less than 0.5In) and instantaneous, it is recommended that a stabilising resistor be employed in the earth fault input circuit. This must then be taken into account in establishing R1. For residual connection arrangement, Rb will be a summation of the phase fault and earth fault input burdens.

11. DELTA FM1 and FM 0 Feeder Management Relays

The CT requirements for Accuracy Class and Overcurrent Accuracy Limit Factor, for Phase and Earth Fault circuits, are identical to those of the ARGUS Overcurrent relays. Generally, where high accuracy is needed, 5P20 CTs should be specified with VA rating to exceed the total secondary resistive load.

Delta Burdens:-

AC Burden Impedance 1A Phase/Earth 5A Phase/Earth 1A SEF/REF 5A SEF/REF

< 0.1 VA < 0.3 VA < 0.1 VA < 0.3 VA

< 0.10Ω < 0.02Ω < 0.10Ω < 0.02Ω

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12. MICROTAPP Automatic Voltage Control of On Load Tap Changers

The MicroTAPP uses a single current transformer inputs for monitoring and control rather than protection. The class of CT is therefore not important and either instrument or protection class CTs can be employed i.e. any CTs already employed for another function. It is only necessary to ensure the additional small burden of the MicroTAPP relay is included in the requirements of the CTs being employed. Normally a Class 1 metering CT is specified with a rated VA slightly in excess (>150%) of the total connected secondary burden to allow sufficient output to operate the transformer overload tap change blocking.

MicroTAPP BURDENS:-

AC Burden Impedance 1 Amp 5 Amp

< 0.1 VA < 0.3 VA

< 0.10Ω < 0.02Ω

CT Rated Burden>1.5(Rct+Rl+Relay) Protection Class: 10P5 Metering Class: Class1

current transformer requirements for reyrolle protection relays

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