Electrical Protection related - CT_EN_AP_D11[1]

Burdens & Current Transformer Requirements of MiCOM Relays

Application Notes

B&CT/EN AP/D11

Application Notes B&CT/EN AP/D11 Burdens & CT Req. of MiCOM Relays

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CONTENTS

1 INTRODUCTION TO CURRENT TRANSFORMERS 12 1.1 Current transformer magnetization 12 1.2 Limiting secondary voltage (Vk) 12 1.3 Rated accuracy limit factor 12 1.4 Primary winding current rating 12 1.5 Secondary winding current rating 13

2 CURRENT TRANSFORMER STANDARDS AND CLASSES 14 2.1 Types 14 2.2 IEC 60044-1 standard 14 2.2.1 Class P CTs 14 2.2.2 Class PX 15 2.3 IEC 60044-6 standard 15 2.3.1 Class TPS 15 2.3.2 Class TPX 15 2.3.3 Class TPY 16 2.3.4 Class TPZ 16

3 OVERCURRENT AND FEEDER MANAGEMENT PROTECTION 17 3.1 P111 17 3.1.1 Burdens 17 3.1.2 Current Transformer Requirements 17 3.1.3 Protection 17 3.2 P115 and P114D 18 3.2.1 Burdens 19 3.2.2 Current Transformer requirements 20 3.2.3 Protection 21 3.3 P120 - P123, P125 - P127 22 3.3.1 Burdens 22 3.3.2 Current Transformer requirements 23 3.3.3 Protection 23 3.3.3.1 High impedance REF protection 24 3.3.3.2 High impedance differential protection 24


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3.4 P124 25 3.4.1 Burdens 25 3.4.2 Current Transformer requirements 25 3.5 P130C, P132, P138, P139 25 3.5.1 Burdens 26 3.5.2 Current Transformer requirements 26 3.5.3 Protection 27 3.6 P141 - P145 28 3.6.1 Burdens 28 3.6.2 Current Transformer requirements 29 3.6.3 Protection 29 3.6.3.1 REF protection 30 3.6.3.2 High impedance differential protection 31

4 MOTOR PROTECTION RELAYS 32 4.1 P210, P211 32 4.1.1 Burdens 32 4.1.2 Current Transformer Requirements 32 4.2 P220, P225, P226C 33 4.2.1 Burdens 33 4.2.2 Current transformer requirements 34 4.3 P241 - P243 35 4.3.1 Burdens 35 4.3.2 Current Transformer requirements 35 4.3.3 Protection 36 4.3.3.1 Biased differential protection (P243 only) 37 4.3.3.2 High impedance differential protection (P243 only) 37

5 INTERCONNECTION AND GENERATOR PROTECTION RELAYS 38 5.1 P341 38 5.1.1 Burdens 38 5.1.2 Current Transformer requirements 38 5.1.3 Protection 39 5.2 P342 – P345 40 5.2.1 Burdens 40 5.2.2 Protection 41 5.2.2.1 Biased differential protection 41 5.2.2.2 High impedance differential protection 42 5.2.2.3 Voltage dependent overcurrent, field failure and negative phase sequence protection 42 5.2.2.4 Sensitive directional earth fault protection 42 5.2.2.5 Stator earth fault protection 43 5.2.2.6 Restricted earth fault protetion 43 5.2.2.7 Reverse and low forward power protection 44


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6 DISTANCE PROTECTION RELAYS 45 6.1 P430C, P432, P433, P435 – P439 45 6.2 Burdens 45 6.2.1 Current Transformer requirements 45 6.2.2 Protection 46 6.3 P441, P442, P444 47 6.3.1 Burdens 47 6.3.2 Current Transformer requirements 47 6.3.3 Protection 47 6.4 P443, P445 (MiCOMho) 48 6.4.1 Burdens 48 6.4.2 Current Transformer requirements 49 6.4.3 Protection 49

7 CURRENT DIFFERENTIAL PROTECTION RELAYS 50 7.1 P521 50 7.1.1 Burdens 50 7.1.2 Curent Transformer requirements 50 7.1.3 Protection 50 7.1.3.1 Current differential protection 50 7.1.3.2 SEF protection 51 7.2 P541 - P546 51 7.2.1 Burdens 51 7.2.2 Current Transformer requirements 52 7.2.3 Protection 52 7.2.3.1 Current differential protection 52 7.2.3.2 Distance protection 53 7.3 P547 54 7.3.1 Burdens 54 7.3.2 Current Transformer requirements 54 7.3.3 Protection 54 7.3.3.1 Phase comparison protection 54 7.3.3.2 Distance protection 55

8 TRANSFORMER DIFFERENTIAL PROTECTION RELAYS 56 8.1 P630C, P631 - P634, P638 56 8.1.1 Burdens 56 8.1.2 Current Transformer requirements 56 8.1.3 Protection 56 8.1.3.1 Differential protection 56 8.1.3.2 REF protection 58


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8.2 P642 – P645 59 8.2.1 Burdens 59 8.2.2 Current Transformer requirements 59 8.2.3 Protection 59 8.2.3.1 Differential protection 59 8.2.3.2 Low impedance REF protection 61 8.2.3.3 One breaker application 62 8.2.3.4 One and a half breaker application 62

9 BUSBAR PROTECTION RELAYS 63 9.1 P742 - P743 63 9.1.1 Burdens 63 9.1.2 Current Transformer requirements 63 9.1.3 Protection 64 9.1.3.1 Differential protection 64 9.2 P746 64 9.2.1 Burdens 64 9.2.2 Current Transformer requirements 64 9.2.3 Protection 65 9.2.3.1 The knee-point voltage of the CT should comply with the minimum requirements of the

formulae shown below. 65

10 CIRCUIT BREAKER FAIL PROTECTION RELAYS 66 10.1 P821 66 10.1.1 Burdens 66 10.1.2 Protection 66

11 VOLTAGE AND FREQUENCY PROTECTION RELAYS 67 11.1 P921 - P923 67 11.1.1 Burdens 67 11.2 P941 - P943 67 11.2.1 Burdens 67

APPENDIX A: CONVERTING TO A LIMITING SECONDARY VOLTAGE 69

APPENDIX B: CONVERTING TO IEEE STANDARD VOLTAGE RATING 70

APPENDIX C: USE OF METROSIL NON-LINEAR RESISTORS 71

APPENDIX D: FUSE RATING OF AUXILIARY SUPPLY 73


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FIGURES Figure 1: Dimensioning factor, for definite time overcurrent protection (fn = 50Hz) 27 Figure 2: Dimensioning factor, for definite time overcurrent protection (fn = 50Hz) 28 Figure 3: Dimensioning factor, k, for distance protection (fn = 50Hz) 46 Figure 4: REF operating times 58

TABLES Table 1: Errors v accuracy class for class P CTs 15 Table 2: Current circuit for P111 17 Table 3: Auxiliary supply for P111 17 Table 4: Additional burdens on auxiliary supply for P111 17 Table 5: CT Specification for P111 17 Table 6: Knee-point voltages for P111 18 Table 7: Current input resistance for P115/P114D 19 Table 8: Auxiliary supply for P115/P114D 19 Table 9: Additional burdens on auxiliary supply for P115/P114D 19 Table 10: Opto-inputs for P115/P114D 20 Table 11: Current circuit for P120 – P123, P125 – P127 22 Table 12: Voltage circuit for P120 – P123, P125 – P127 22 Table 13: Auxiliary supply for P120 – P123, P125 – P127 23 Table 14: Additional burdens on auxiliary supply for P120 – P123, P125 – P127 23 Table 15: CT Specification for P120 – P123, P125 – P127 23 Table 16: Knee-point voltages for P120 – P123, P125 – P127 24 Table 17: Current circuit for P124 25 Table 18: Auxiliary supply for P124 25 Table 19: Additional burdens on auxiliary supply for P124 25 Table 20: Opto-inputs for P124 25 Table 21: Current circuit for P130C, P132, P138, P139 26 Table 22: Voltage circuit for P130C, P132, P138, P139 26 Table 23: Auxiliary supply for P130C, P132, P138, P139 26 Table 24: Auxiliary supply for P130C, P132, P138, P139 26 Table 25: CT specification for P130C, P132, P138, P139 26 Table 26: Knee-point voltages for P130C, P132, P138, P139 27 Table 27: Current circuit for P141 - 145 28 Table 28: Voltage circuit for P141 - 145 28 Table 29: Auxiliary supply for P141 – P145 28 Table 30: Additional burden for P141 – P145 29 Table 31: Opto-inputs for P141 – P145 29 Table 32: CT specification for P141 – P145 29


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Table 33: Knee-point voltages for P141 – P145 30 Table 34: REF protection for P141 – P145 30 Table 35: Current circuit for P210, 211 32 Table 36: Auxiliary supply for P210, 211 32 Table 37: Additional burdens on auxiliary supply for P210, 211 32 Table 38: Recommended CT types for P210, P211 32 Table 39: IEC 60044-1 specifications for circuit breaking 33 Table 40: Current circuit for P220, P225, P226C 33 Table 41: Voltage circuit for P220, P225, P226C 33 Table 42: Auxiliary supply for P220, P225, P226C 33 Table 43: Additional burdens on auxiliary supply for P220, P225, P226C 34 Table 44: Recommended CT types for P220, P225, P226C 34 Table 45: IEC 60044-1 specifications for circuit breaking 34 Table 46: Current circuit for P241 - P243 35 Table 47: Voltage circuit for P241 - P243 35 Table 48: Voltage circuit for P241 - P243 35 Table 49: Additional burdens on auxiliary supply for P241 - P243 35 Table 50: Opto-inputs for P241 - P243 35 Table 51: CT specification for P241 - P243 36 Table 52: Knee-point voltages for P241 - P243 36 Table 53: Conditions for P243 knee-point voltage calculations 37 Table 54: Knee-point calculations for P243 – biased differential protection 37 Table 55: Current circuit for P341 38 Table 56: Voltage circuit for P341 38 Table 57: Auxiliary supply for P341 38 Table 58: Additional burdens on auxiliary supply for P341 38 Table 59: Opto-inputs for P341 38 Table 60: CT specification for P341 39 Table 61: Knee-point voltages for P341 – P345 40 Table 62: Current circuit for P342 – P345 40 Table 63: Voltage circuit for P342 – P345 40 Table 64: Auxiliary supply for P342 – P345 41 Table 65: Additional burdens on auxiliary supply for P342 – P345 41 Table 66: Opto-inputs for P342 – P345 41 Table 67: Conditions for P342 – P345 knee-point voltage calculations 41 Table 68: Knee-point calculations for P342-345 – biased differential protection 41 Table 69: Knee-point calculations for P342 – P345 – biased differential protection 42 Table 70: Stator earth fault protection for P342 – P345 43 Table 71: Low impedance REF protection for P342 – P345 43 Table 72: Power settings v. measuring class 44 Table 73: Current circuit for P430C, P432, P433, P435 – P439 45 Table 74: Voltage circuit for P430C, P432, P433, P435 – P439 45


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Table 75: Auxiliary supply for P430C, P432, P433, P435 – P439 45 Table 76: Additional burdens on auxiliary supply for P430C, P432, P433, P435 – P439 45 Table 77: Current circuit for P441, P442, P444 47 Table 78: Voltage circuit for P441, P442, P444 47 Table 79: Auxiliary supply for P441, P442, P444 47 Table 80: Additional burdens on auxiliary supply for P441, P442, P444 47 Table 81: Opto-inputs for P441, P442, P444 47 Table 82: Knee-points for distance protection P441, P442, P444 48 Table 83: Current circuit for P443, P445 48 Table 84: Voltage circuit for P443, P445 48 Table 85: Auxiliary supply for P443, P445 48 Table 86: Additional burdens on auxiliary supply for P441, P442, P444 48 Table 87: Opto-inputs for P443, P445 48 Table 88: Knee-point voltages for P443, P445 49 Table 89: Current circuit for P521 50 Table 90: Auxiliary supply for P521 50 Table 91: Additional burdens on auxiliary supply for P521 50 Table 92: Calculation of ks 51 Table 93: Calculation of Kt 51 Table 94: Current circuit for P541 - 546 51 Table 95: Voltage circuit for P541 - 546 52 Table 96: Auxiliary supply for P541 - 546 52 Table 97: Auxiliary burdens on auxiliary supply for P541 - 546 52 Table 98: Opto-inputs for P541 - 546 52 Table 99: Calculation of K 53 Table 100: Current circuit for P547 54 Table 101: Auxiliary supply for P547 54 Table 102: Additional burdens on auxiliary supply for P547 54 Table 103: Opto-inputs for P547 54 Table 104: Calculation of K 55 Table 105: Current circuit for P630C, P631 – P634, P638 56 Table 106: Voltage circuit for P630C, P631 – P634, P638 56 Table 107: Auxiliary supply for P630C, P631 – P634, P638 56 Table 108: Additional burdens on auxiliary supply for P630C, P631 – P634, P638 56 Table 109: Conditions for P630C, P631 – P634, P638 calculations 57 Table 110: Knee-point voltages for P630C, P631 – P634, P638 57 Table 111: Calculation of K for phase and earth faults 57 Table 112: Current circuit for P642 – P645 59 Table 113: Voltage circuit for P642 – P645 59 Table 114: Auxiliary supply for P642 – P645 59 Table 115: Additional burdens on auxiliary supply for P642 – P645 59 Table 116: Conditions for P642 – P645 calculations 60


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Table 117: K requirements for P642 – P645 61 Table 118: K requirements for P642 – P645 (one-breaker application) 62 Table 119: K requirements for P642 – P645 (one and a half breaker application) 62 Table 120: Current circuit for P742 – P743 63 Table 121: Auxiliary supply for P742 – P743 63 Table 122: Additional burdens on auxiliary supply for P742 – P743 63 Table 123: Opto-inputs for P742 – P743 63 Table 124: Current circuit for P746 64 Table 125: Auxiliary supply for P746 64 Table 126: Additional burdens on auxiliary supply for P746 64 Table 127: Opto-inputs for P746 64 Table 128: Current circuit for P821 66 Table 129: Auxiliary supply for P821 66 Table 130: Additional burdens on auxiliary supply for P821 66 Table 131: Current transformer requirements for P821 66 Table 132: Voltage circuit for P921 – P923 67 Table 133: Auxiliary supply for P921 – P923 67 Table 134: Additional burdens on auxiliary supply for P921 – P923 67 Table 135: Voltage circuit for P941 – P943 67 Table 136: Auxiliary supply for P941 – P943 67 Table 137: Additional burdens on auxiliary supply for P941 – P943 68 Table 138: Opto-inputs for P941 – P943 68 Table 139: C57.13 ratings 70 Table 140: Metrosil units 72 Table 141: Metrosil units for 5A CTs 72


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GLOSSARY OF TERMS The following abbreviations are used in this document:

Symbol Description ALF Accuracy Limit Factor

ANSI American National Standards Institute

C IEEE standard C57.13 "C" classification

CT Current Transformer

DIN Deutsches Institut für Normung (German standards body)

DT Definite Time

E/F Earth Fault

IDMT Inverse Definite Minimum Time

IEC International Electrotechnical Commission

IEEE Institute of Electrical and Electronics Engineers

MCB Mini Circuit Breaker

Metrosil Brand of non-linear resistor produced by M&I Materials Ltd.

O/C Overcurrent

ph Phase

REF Restricted Earth Fault

rms Root mean square

SEF Sensitive Earth Fault

SIR Source Impedance Ratio

VA Current transformer rated burden

VT Voltage Transformer

The following terms are used in the equations in this document:

Symbol Description Units< As well as the usual “less than”, this symbol also depicts undercurrent N/A

> As well as the usual “greater than”, this symbol also depicts overcurrent N/A ω system angular frequency rad

minf Minimum required operating frequency Hz

maxf Minimum required operating frequency Hz

nf Nominal operating frequency Hz

f'I Maximum internal secondary fault current (may also be expressed as a multiple of In) A

>>I Second stage current setting of short circuit element (overcurrent) In

diffI Current setting of first stage biased differential A

fI Maximum secondary through fault current A

maxfI Maximum secondary fault current (same for all feeders) A

intmaxfI Maximum secondary contribution from a feeder to an internal fault A

1fZI Maximum secondary phase fault current at Zone 1 reach point A


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Symbol Description Units

feI Maximum secondary through fault earth current A

1feZI Maximum secondary earth fault current at Zone 1 reach point A

fnI Maximum prospective secondary earth fault current A

fpI Maximum prospective secondary phase fault current A

nI Current transformer nominal secondary current A

0I Earth fault current setting A

2RmI Second knee-point bias current threshold setting of P63x biased differential element A

refI Reference current of P63x calculated from the reference power and nominal voltage A

sI Value of stabilizing current A

1SI Differential current pick-up setting of biased differential element A

2SI Bias current threshold setting of biased differential element A

snI Rated secondary current (I secondary nominal) A

spI Stage 2 and 3 setting A

stI Motor start up current referred to CT secondary side A

K Dimensioning factor None

1K Lower bias slope setting of biased differential element %

2K Higher bias slope setting of biased differential element %

eK Dimensioning factor for earth fault None

maxK Maximum dimensioning factor None

rpaK Dimensioning factor for reach point accuracy None

sK Dimensioning factor dependent upon through fault current None

sscK Short circuit current coefficient or ALF None

tK Dimensioning factor dependent upon operating time None

1m Lower bias slope setting of P63x biased differential element None

2m Higher bias slope setting of P63x biased differential element None

nP Rotating plant rated single phase power W

ctR Secondary winding resistance Ω

lR Resistance of single lead from relay to current transformer Ω

rR Resistance of any other protective relays sharing the current transformer Ω

rnR Resistance of relay neutral current input at 30In Ω

rpR Resistance of relay phase current input at 30In Ω


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Symbol Description Units

sR Value of stabilizing resistor Ω

't Duration of first current flow during auto-reclose cycle s

1T Primary system time constant s

frt Auto-reclose dead time s

IDifft Current differential operating time s

sT Secondary system time constant s

fV Theoretical maximum voltage produced if CT saturation did not occur V

inV Input voltage e.g. to an opto-input V

nV Nominal voltage V

kV Required CT knee-point voltage V

sV Value of stabilizing voltage V

RX

Primary system reactance/resistance ratio None

e

e

RX

Primary system reactance/resistance ratio for earth loop None

tX Transformer reactance (per unit) p.u.


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1 INTRODUCTION TO CURRENT TRANSFORMERS The role of current transformers in the transmission and distribution of electrical energy is extremely important. They are used to enable accurate metering and effective protection of transmission and distribution circuits.

Current transformers isolate the secondary circuits, which are used for relaying and metering, from the primary circuit, which carries the power. They also provide quantities in the secondary which are proportional to those in the primary. The role of current transformers in protective relaying is not as specific as for metering and instrumentation. The role of a measuring transformer is to deliver from its secondary winding a voltage or current waveform that accurately represents that of the primary side. The role of a protecting transformer, however, varies depending on the type of protection it is designed to offer.

There is no major difference between a protective voltage transformer and a measuring voltage transformer, the difference being only in the nature of the voltage transformed. Normally the same transformer can serve both purposes. This cannot be said for current transformers, however, as the requirements for protective current transformers are often radically different from those of measuring current transformers. Occasionally the same transformer may serve both purposes but in modern practice this is rare. The main difference is that a measuring current transformer must be accurate over the normal range of load currents, whereas a protective current transformer must be capable of providing an adequate output over a wide range of fault conditions, from a fraction of full load to many times full load.

1.1 Current transformer magnetization The primary current contains two components:

1. The secondary current referred to the primary 2. The excitation current, which supplies the core magnetization, eddy currents and

hysteresis losses. The excitation current flows in the primary winding only and is therefore the cause of transformer errors. The amount of excitation current drawn by a current transformer depends on the core material and the amount of flux that must be developed in the core to satisfy the burden requirements of the current transformer.

It is, therefore, not sufficient to assume a value of secondary current and to work backwards to determine the value of primary current by using the constant ampere-turns rule, since this approach does not take into account the excitation current. When the core saturates, a disproportionate amount of primary current is required to magnetize the core and the secondary current does not accurately reflect the primary current.

1.2 Limiting secondary voltage (Vk) IEC 60044 defines the limiting secondary voltage of the excitation characteristic as the point at which a 10% increase in secondary voltage requires a 50% increase in excitation current. This is also commonly referred to as the knee-point voltage. The knee-point may be regarded as the practical limit beyond which a specified current ratio may not be maintained, as the current transformer enters saturation. Since the shunt admittance is not linear in this region, both the excitation and secondary currents depart from their sinusoidal form. ANSI and IEEE have a different definition of the knee-point voltage.

1.3 Rated accuracy limit factor A current transformer is designed to maintain its ratio within specified limits up to a certain value of primary current, expressed as a multiple of its rated primary current. This multiple is known as the current transformer’s rated accuracy limit factor (ALF).

1.4 Primary winding current rating The current transformer primary rating is usually chosen to be equal to or greater than the normal full load current of the protected circuit. This avoids overloading and consequent overheating of the CT. Standard primary ratings are specified in IEC 60044-1. The maximum ratio of current transformers is typically limited to 3000:1 for two reasons:


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1. Size limitation of the current transformer 3. Maximum safe open circuit voltage Some transformers, like large turbo alternators, have very high primary ratings (e.g. 5000A). In such cases it is standard practice to use a cascade arrangement. For example a 5000:20A in series with a 20:1A interposing auxiliary current transformer.

1.5 Secondary winding current rating The total secondary burden of a current transformer includes the internal impedance of the secondary winding, the impedance of the connected instruments and relays and the impedance of the secondary leads. A typical value of rated secondary current is 5A, provided that the length of the leads between the current transformers and the connected apparatus does not exceed 25 meters. Up to this length the lead resistance is small in relation to the total output of the transformer. In installations with longer lead lengths, transformers with 1A secondary windings are used to keep the lead losses within reasonable limits. Power losses vary as the square of the current, thus losses in 1A windings are 1/25th of losses in 5A windings.


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2 CURRENT TRANSFORMER STANDARDS AND CLASSES

2.1 Types Generally, there are three different types of CT:

1. High-remanence 2. Low-remanence 3. Non-remanence Remanence is the magnetization left behind in a medium after an external magnetic field is removed. In our case, the amount of residual magnetization left in the core.

High-remanence CT The high-remanence type has no given limit for the remanent flux. The CT has a magnetic core without any air gaps and the remanent flux is very high. The remanent flux can be up to 70-80% of the saturation flux. Examples of high-remanent type CTs are:

• IEC 60044-1 classes P, PX • IEC 60044-6 classes TPS and TPX

Low-remanence CT The low-remanence type has a specified limit for the remanent flux. The magnetic core is provided with small air gaps to reduce the remanent flux to a level that does not exceed 10% of the saturation flux. Examples of low-remanent type CTs are:

• IEC 60044-1 class PR • IEC 60044-6 class TPY

Non-remanence CT The non-remanence type has a negligible level of remanent flux. The magnetic core has relatively large air gaps, which reduce the remanent flux to practically zero level. An example of a non-remanent type CT is:

• IEC 60044-6 class TPZ IEC 60044-1 and IEEE C57.13 specify the behavior of inductive current transformers for steady state symmetrical AC currents. The more recent standard IEC 60044-6 specifies the performance of inductive current transformers (classes TPX, TPY and TPZ) for currents containing exponentially decaying DC components.

2.2 IEC 60044-1 standard The IEC 6044-1 standard defines class P and class PX current transformers.

2.2.1 Class P CTs Class P CTs are high remanence types, typically used for general applications, such as overcurrent protection. What happens after the relay has been tripped is of no consequence, so there is no point in having a secondary accuracy limit greatly in excess of the value needed to cause relay operation. A rated accuracy limit of 5 is therefore usually adequate (Secondary current accurately reflects primary current for primary currents of 5 times greater than its rating).

When relays (such as instantaneous ‘high set’ overcurrent relays) are set to operate at high values of overcurrent (say 5 to 15 times the rated current of the transformer), the accuracy limit factor must be at least as high as the value of the setting current used in order to ensure fast relay operation.

Rated output burdens higher than 15 VA and rated accuracy limit factors higher than 10 are not recommended for general purposes. It is possible, however, to combine a higher rated accuracy limit factor with a lower rated output and vice versa. When the product of these two exceeds 150, the resulting current transformer may be uneconomical or unduly large.

Class P CTs are defined such that the current, phase and composite errors do not exceed the values given in Table 1, at the rated frequency and burden.


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Accuracy Class

Current Error at Rated Primary

Current

Phase Displacement at Rated Primary Current

Composite Error at Rated Accuracy Limit

Primary Current 5P ±1% ±60 minutes (±0.018 radians) 5%

10P ±3% ±60 minutes (±0.018 radians) 10%

Table 1: Errors v accuracy class for class P CTs

2.2.2 Class PX Class PX CTs are typically used for high impedance circulating current protection. They are also suitable for most other protection applications.

Class PX CTs have high remanence and low leakage reactance. To assess their performance in relation to the protective relay system in which they are to be used, you only need knowledge of:

• the transformer secondary excitation characteristic • the secondary winding resistance • the secondary burden resistance • the turns ratio Class PX is the IEC 60044-1 definition for quasi-transient current transformers, formerly covered by BS 3938 class X.

2.3 IEC 60044-6 standard The IEC 60044-6 standard defines current transformers in the classes TPS, TPX, TPY and TPZ.

2.3.1 Class TPS Class TPS CTs are high remanence types. They are typically used for high impedance circulating current protection, and are generally applied to unit systems where the balancing of outputs from each end of the protected plant is vital.

This balance, or stability during through-fault conditions, is essentially of a transient nature and thus the extent of the unsaturated (or linear) zones is of paramount importance. Using heavy current test results, it is normal to derive a formula stating the lowest permissible value of Vk for stable operation to be guaranteed.

IEC 60044-6 defines the performance of low secondary reactance, TPS class current transformers for transient performance. They are specified in terms of each of the following characteristics:

• Rated primary current • Turns ratio (the error in turns ratio cannot exceed ±0.25%) • Secondary limiting voltage • Resistance of secondary winding

2.3.2 Class TPX The basic characteristics for class TPX current transformers are generally similar to those of class TPS current transformers except for the different error limits prescribed and possible influencing effects which may necessitate a physically larger construction. Class TPX CTs have no air gaps in the core and therefore a high remanence factor (70-80% remanent flux). The accuracy limit is defined by the peak instantaneous error during the specified transient duty cycle.

Class TPX CTs are typically used for line protection.


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2.3.3 Class TPY

Class TPY CTs have a specified limit for the remanent flux. The magnetic core is provided with small air gaps to reduce the remanent flux to a level that does not exceed 10% of the saturation flux. They have a higher error in current measurement than TPX during unsaturated operation and the accuracy limit is defined by peak instantaneous error during the specified transient duty cycle.

Class TPY CTs are typically used for line protection with auto-reclose function.

2.3.4 Class TPZ For class TPZ CTs the remanent flux is negligible due to large air gaps in the core. These air gaps also minimize the influence of the DC component from the primary fault current, but reduce the measuring accuracy in the unsaturated (linear) region of operation. The accuracy limit is defined by the peak instantaneous alternating current component error during a single excitation, with maximum DC offset at a specified secondary loop time constant.

Class TPZ CTs are typically used for special applications such as differential protection of large generators.


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3 OVERCURRENT AND FEEDER MANAGEMENT PROTECTION The range of Overcurrent and Feeder protection relays consists of the following devices: P111, P120 - P123, P124, P125 - P127, P130C, P132, P138, P139 and P141 - P145.

Their burdens and current transformer requirements are detailed in the following sections.

3.1 P111 The P111 is used in LV and MV networks to protect MV/LV transformers, incoming and outgoing lines.

3.1.1 Burdens

CT Input In CT Burden Phase 1A < 0.03VA at InEarth 1A < 0.05VA at 0.1InPhase 5A < 0.1VA at InEarth 5A < 0.15VA at 0.1In

Table 2: Current circuit for P111

Relay Case Size Nominal Burden*

P111 35mm DIN rail or flush mount 4.5VA

Table 3: Auxiliary supply for P111

* Typical minimum burden with no opto-inputs or output contacts energized.

Additional Burden Energizing Voltage Burden Energized opto-input 48V 0.7VA

Energized opto-input 230V 0.6VA

Table 4: Additional burdens on auxiliary supply for P111

3.1.2 Current Transformer Requirements The CT requirements are based on a maximum fault current of 50In and the relay having an instantaneous setting of 25In. These CT requirements are designed to provide operation of all protection elements.

Nominal Rating Nominal Output Accuracy Class Accuracy Limit Factor (ALF)

Limiting Lead Resistance

1A 2.5VA 10P 20 1.30Ω

5A 7.5VA 10P 20 0.11Ω

Table 5: CT Specification for P111

Where the criteria for a specific application are in excess of those detailed above, or the lead resistance exceeds the limiting values, the CT requirements may need to be increased according to the formulae in the following sections. For specific applications such as SEF protection, refer to the sections below for CT accuracy class and knee-point voltage requirements as appropriate.

3.1.3 Protection

Protection type Knee-point voltage Non-directional/directional DT/IDMT overcurrent and earth fault protection

Time-delayed phase overcurrent ( )rplctfp

k RRRI

V ++≥2


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Protection type Knee-point voltage

Time-delayed earth fault overcurrent ( )rnrplctfn

k RRRRI

V +++≥ 22

Non-directional instantaneous overcurrent and earth fault protection

Instantaneous phase overcurrent ( )rplctspk RRRIV ++≥

Instantaneous earth fault overcurrent ( )rnrplctsnk RRRRIV ++ +≥ 2

Directional instantaneous overcurrent and earth fault protection

Instantaneous phase overcurrent ( )rplctfp

k RRRI

V ++≥2

Instantaneous earth fault overcurrent ( )rnrplctfn

k RRRRI

V +++≥ 22

Non-directional/directional DT/IDMT SEF protection - residual CT connection

Non-directional time delayed SEF ( )rnrplctfn

k RRRRI

V +++≥ 22

Directional time delayed SEF ( )rnrplctfn

k RRRRI

V +++≥ 22

Non-directional instantaneous SEF ( )rnrplctsnk RRRRIV ++ +≥ 2

Directional instantaneous SEF ( )rnrplctfn

k RRRRI

V +++≥ 22

Non-directional/directional DT/IDMT SEF protection - core-balance CT connection Core-balance current transformers of metering class accuracy are required and should have a limiting secondary voltage satisfying the formulae given below:

Non-directional time delayed SEF ( )rnlctfn

k RRRI

V ++≥ 22

Directional time delayed SEF ( )rnlctfn

k RRRI

V ++≥ 22

Non-directional instantaneous SEF ( )rnlctsnk RRRIV + +≥ 2

Directional instantaneous SEF ( )rnlctfn

k RRRI

V ++≥ 22

Table 6: Knee-point voltages for P111

Note:: The phase error of the applied core balance current transformer must be less than 90 minutes at 10% of rated current and less than 150 minutes at 1% of rated current.

3.2 P115 and P114D MiCOM P115/P114D are numerical relays designed to offer overcurrent and earth fault protection without requiring any external auxiliary supply.

The relays are available in two hardware option:

1. Self-powered - powered by > 0.2In secondary current, or; 2. Dual-powered - powered by either > 0.2In secondary current or an auxiliary supply.


(AP) -19

3.2.1 Burdens

Rrp (Rrn) for a single current input

I In=1 A In=5 A (In=1 A)+WA25

In Ohms

0.2 28.9 1.0 21.9

0.3 11.5 0.36 16.7

0.4 6.6 0.27 6.8

0.5 3.8 0.22 3.8

1 0.63 0.12 2.5

10 0.28 0.056 2.2

20 0.28 0.056 2.2

40 0.28 0.056 2.2

Table 7: Current input resistance for P115/P114D

Auxiliary supply

Note: 1. Initial position: no output nor LED energized. 2. Active position: all outputs and LEDs energized.

Input range Vx Initial Position of S Active position of S

24-48Vac 24 3.1VA 5.5VA

24-48Vac 48 2.4VA 6.0VA

60-240Vac 48 2.6VA 5.5VA

60-240Vac 60 2.7VA 5.2VA

60-240Vac 100/110 3.1VA 5.7VA

60-240Vac 220/230 5.1VA 7.4VA

60-240Vac 264 6.1VA 8.4VA

24-48Vdc 24 1.5W 3.7W

24-48Vdc 48 1.5W 3.7W

Table 8: Auxiliary supply for P115/P114D

Additional Burden Relay Auxiliary Voltage Burden Per energized opto-input (for P115/P114D) 24 to 48 V dc/ac 5,5 kΩ ±5%

Per energized opto-input (for P115/P114D) 60 to 250 V dc 60 to 240 V ac 100 kΩ ±5%

Table 9: Additional burdens on auxiliary supply for P115/P114D

For example:

For 220 Vdc and a one binary input: (220 Vdc)2 x 100 kΩ±5% = 0.484 W±5%


(AP) -20

Nominal pick-up and reset thresholds

Energizing Voltage Nominal pick-up and reset thresholds

24 to 48 V dc Pick-up: approx. 12 Vdc

Reset: approx. 11 Vdc

60 to 240 V dc Pick-up: approx. 21 Vdc

Reset: approx. 20 Vdc

24 to 48 V ac Pick-up: approx. 16 Vac

Reset: approx. 11 Vac

60 to 240 V ac Pick-up: approx. 26 Vac

Reset: approx. 19 Vac

Table 10: Opto-inputs for P115/P114D

3.2.2 Current Transformer requirements It is not possible to recommend any CT without detailed information. The decision needs to be based on calculation.

The following parameters have to be considered:

• Type of CT (nominal power, nominal current and current ratio, internal resistance, nominal accuracy limit factor, class and construction),

• Resistance of wiring (length, cross section, specific resistance of material), • Resistance of P115/P114D current inputs (as per table 7).

Two critical cases have to be checked for different types of faults:

• The lowest set current threshold value at which the relay has to operate (minimum current).

• The highest possible short-circuit current, which depends on the maximum short-circuit power on the busbar of the substation (maximum current).

The following equation is used for dimensioning a current transformer:

( ) ( )bctn

pscbnctsnnsal RR

KI

RRInV +⋅≥+⋅⋅=

The current transformer can be dimensioned for the minimum required secondary accuracy limiting voltage acc. to IEC 60044-1, 2.3.4:

( )

( )bctsnpn

psc

bctn

pscsal

RRIII

RRKI

V

+⋅⋅≥

+⋅≥

( )bctsnsscsal RRIKV +⋅⋅≥

Alternatively, the current transformer can also be dimensioned for the minimum required rated accuracy limit factor acc. to IEC 60044-1, 2.3.3:


(AP) -21

( )( )

( )( )bnct

bct

pn

psc

bnct

bct

sn

npscn

RRRR

II

RRRR

IKI

n

++

⋅≥

++

⋅≥

( )( )

( )( )bnct

bctssc

bnct

bctsscn PP

PPK

RRRR

Kn++

⋅=++

⋅≥

The actual secondary connected burden Rb is given as follows:

• For phase-to-ground faults: rnrplb RRRR ++⋅= 2

• For phase-to-phase faults: rplb RRR +=

The relay’s burden Rrp and Rrn is per table 7. The lead resistance Rl is to be calculated from wire length, cross section and specific resistance.

The relation between secondary accuracy limiting voltage acc. to IEC 60044-1, 2.3.4 and rated accuracy limit factor acc. to IEC 60044-1, 2.3.3 is given as follows:

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅+⋅= ctsn

sn

bnnsal RI

IPnV

3.2.3 Protection

Sample calculation The following application data are given:

CT ratio 100/1 A

CT nominal power =2.5 VA (RbnS bn = 2.5 Ohm, assumption: bnbn SP = )

CT internal burden Rct = 0.5 Ohm

Lead resistance Rl = 0.01774 Ohm (2 m one way, 2.5 mm2 Cu)

Max. short-circuit current:

phase-ground pscI = 2 kA = 20 Inom: Rrel = 0.28 Ohm

phase-phase pefI = 10 kA = 100 Inom: Rrel = 0.28 Ohm

Relay minimum operating current:

IN> = 0.2 Inom: Rrel = 28.9 Ohm

I> = 1 Inom: Rrel = 0.63 Ohm

Phase-ground fault, minimum current:

2.02.0==

>=

n

n

snss I

II

INK

( )( ) 9.3

5.25.0)9.2801774.0(25.02.0 =

++⋅+

⋅=++

⋅≥bnct

bctssn RR

RRKn

Phase-ground fault, maximum current:


(AP) -22

201002

===A

kAII

Kpn

pefssc

( )( ) 3,7

5.25.0)28.001774.0(25.020 =

++⋅+

⋅=++

⋅≥bnct

bctsscn RR

RRKn

Phase-phase fault, minimum current:

0.10.1==

>=

n

n

snss I

IIIK

( )( ) 38.0

5.25.063.001774.05.00.1 =

+++

⋅=++

⋅≥bnct

bctssn RR

RRKn

Phase-phase fault, maximum current:

10010010

===A

kAII

Kpn

pscssc

( )( ) 6.26

5.25.028.001774.05.0100 =

+++

⋅=++

⋅≥bnct

bctsscn RR

RRKn

Overall, a minimum rated accuracy limit factor of 26.6 is required (maximum value for above cases). A typical (standard) value thus would be nn = 30.

3.3 P120 - P123, P125 - P127 The MiCOM P12x range of protection relays provides comprehensive phase/earth overcurrent protection, typically used in MV substations, to protect overhead lines, underground cables and small transformers. In addition the MiCOM P127 offers directional, voltage and frequency protection functions.

3.3.1 Burdens

CT Input In CT Burden Phase 1A < 0.025 VA at InEarth 1A < 0.008 VA at 0.1InPhase 5A < 0.3 VA at InEarth 5A < 0.01 VA at 0.1In

Table 11: Current circuit for P120 – P123, P125 – P127

Vn VT Burden 57 - 130 V 0.074 W at 57 V

57 - 130 V 0.38 W at 130 V

57 - 130 V 1.54 W at 260 V

220 - 480 V 0.1102 W at 220 V

220 - 480 V 0.525 W at 480 V

220 - 480 V 2.1 W at 960 V

Table 12: Voltage circuit for P120 – P123, P125 – P127


(AP) -23

Relay Case Size Nominal Burden* Maximum Burden

P120 - P123, P125 Size 4/20TE < 3 W or 8 VA < 6 W or 14 VA

P126, P127 Size 6/30TE < 3W or 8VA < 6W or 14VA

Table 13: Auxiliary supply for P120 – P123, P125 – P127


Additional Burden Relay Auxiliary Voltage Burden Per energized opto-input - 10 mA

Per energized output contact - 0.25 W or 0.4 VA

Table 14: Additional burdens on auxiliary supply for P120 – P123, P125 – P127

3.3.2 Current Transformer requirements The current transformer requirements are based on a maximum fault current of 50 x In and the relay having an instantaneous setting of 25 x In. These CT requirements are designed to provide operation of all protection elements.



1A 2.5 VA 10P 20 1.30 Ω

5A 7.5 VA 10P 20 0.11 Ω

Table 15: CT Specification for P120 – P123, P125 – P127

Where the criteria for a specific application are in excess of those detailed above, or the actual lead resistance exceeds the limiting values, the CT requirements may need to be increased according to the formulae in the following sections. For specific applications such as SEF and REF protection, refer to the sections below for CT accuracy class and knee-point voltage requirements as appropriate.

3.3.3 Protection



k RRRI

V ++≥2


k RRRRI

V +++≥ 22




k RRRRI

V +++≥ 22



k RRRI

V ++≥2


k RRRRI

V +++≥ 22



k RRRRI

V +++≥ 22


(AP) -24



k RRRRI

V +++≥ 22

Non-directional instantaneous SEF ( )nrplctsnk RRRRIV ++ +≥ 2


k RRRRI

V +++≥ 22


Non-directional time delayed SEF ( )rplctfn

k RRRI

V ++≥2

Directional time delayed SEF ( )rplctfn

k RRRI

V ++≥2


Directional instantaneous SEF ( )rplctfn

k RRRI

V ++≥2

Table 16: Knee-point voltages for P120 – P123, P125 – P127

3.3.3.1 High impedance REF protection The high impedance REF element should be stable for through-faults and operate in less than 40 ms for internal faults, provided the following conditions are met in determining the CT requirements and value of associated stabilizing resistor.

and ssk RIV 4≥ ( )lcts

fs RR

II

R 2+=

Note:: Class 5P or PX CTs should be used for high impedance REF applications.

3.3.3.2 High impedance differential protection The relay can be applied as a high impedance differential protection for 3 phase applications such as busbars, generators and motors. The high impedance differential protection should be stable for through faults and operate in less than 40 ms for internal faults, provided the following conditions are met in determining the CT requirements and value of associated stabilizing resistor.

ssk RIV 4≥ and ( )lcts

fs RR

II

R 24.1 +=

Where X/R ≤ 40 and through-fault stability with a transient DC offset in the fault current must be considered, the following equation can be used to determine the required stabilizing voltage.

( )lctfs RRRXIV 205.1007.0 +⎟

⎠⎞

⎜⎝⎛ +=

If the calculated stabilizing voltage is less than IsRs calculated above then it may be used instead.

Note: Class 5P or PX CTs should be used for high impedance differential applications.


(AP) -25

3.4 P124

This model is available as either:

1. Self-powered (P124S) - powered by > 0.2In secondary current, or; 2. Dual-powered (P124D) - powered by either > 0.2In secondary current or an auxiliary

supply. The MiCOM P124S is a self powered relay, which includes non-directional phase/earth overcurrent and thermal overload protection.

The MiCOM P124D is a dual powered relay, which includes all of the P124S features plus negative sequence overcurrent and autoreclose functionality.

3.4.1 Burdens

CT Input In CT Burden Phase 1A 2.5 VA

Earth 1A 2.5 VA

Phase 5A 2.5 VA

Earth 5A 2.5 VA


Auxiliary supply Relay Case Size Nominal Burden*

P124D Size 6/30TE 3 W or 6 VA



Additional Burden Relay Auxiliary Voltage Burden Per energized opto-input (for P124D) 24 to 60 V dc ≤ 9 mA

Per energized opto-input (for P124D) 48 to 150 V dc ≤ 4.7 mA

Per energized opto-input (for P124D) 130 to 250 V dc/ 100 to 250 V ac ≤ 2.68 mA

Per energized output contact - 0.25 W


Energizing Voltage Peak Current 0 to 300 V dc < 10 mA

Table 20: Opto-inputs for P124

3.4.2 Current Transformer requirements Assuming that the CT does not supply any circuits other than the MiCOM P124, the following CT types are recommended:

• 5 VA 10P20 for 1A secondary rating • 10 VA 10P20 for 5A secondary rating

3.5 P130C, P132, P138, P139 These units provide selective short-circuit protection, ground fault protection and overload protection for solidly-grounded, low-impedance-grounded, resonant-grounded or isolated-neutral medium and high voltage systems. The control functions can handle up to six (for the P139) electrically operated switchgear units.


(AP) -26

3.5.1 Burdens

CT Input In CT Burden Phase 1A < 0.1 VA

Earth 1A < 0.1 VA

Phase 5A < 0.1 VA

Earth 5A < 0.1 VA

Table 21: Current circuit for P130C, P132, P138, P139

Vn VT Burden 50 - 130V < 0.3VA rms at 130V

Table 22: Voltage circuit for P130C, P132, P138, P139

Relay Case size Nominal Burden* Maximum Burden P130C Compact 8 W 10 W

P132, P139 40TE 12.6 W 34.1 W

P138 40TE 13 W 32 W

P132, P139 ? 14.5 W 42.3 W

P138 ? 13 W 32 W

Table 23: Auxiliary supply for P130C, P132, P138, P139

* Typical minimum burden at 220 V dc with no opto-inputs or output contacts energized.

Additional Burden Energizing Voltage Burden Per energized opto-input 19 to 110 V dc 0.5 W ±30%

Per energized opto-input > 110 V dc Vin × 5 mA ±30%

Table 24: Auxiliary supply for P130C, P132, P138, P139

3.5.2 Current Transformer requirements



1A 2.5 VA 10P 20 1.30 Ω

5A 7.5 VA 10P 20 0.11 Ω

Table 25: CT specification for P130C, P132, P138, P139

Where the criteria for a specific application are in excess of those detailed above, or the actual lead resistance exceeds the limiting values, the CT requirements may need to be increased according to the formulae in the following sections.

Note: The P138 may be applied at low system frequencies of 16⅔ Hz or 25 Hz. Any VA or knee-point voltage quoted must apply at the chosen nominal frequency (fn).


(AP) -27

3.5.3 Protection

The knee-point voltage of the CT should comply with the minimum requirements of the formulae shown below.

Protection type Knee-point voltage DT/IDMT overcurrent and earth fault protection

Time-delayed phase overcurrent ( )rplctfpk RRRKIV ++≥

Time-delayed earth fault overcurrent ( )rnrplctfnk RRRRKIV +++≥ 2

Table 26: Knee-point voltages for P130C, P132, P138, P139

If the P13x is to be used for definite-time overcurrent protection, then the dimensioning factor, K must be selected as a function of the ratio of the maximum short-circuit current to the pick-up value and the system time constant, T1. The required value for K can be read from the empirically determined curves in Figure 1. When inverse time overcurrent protection is required, K can be determined from Figure 2.

Theoretically, the CT could be dimensioned to avoid saturation by using the maximum value of K, calculated as follows:

RXTK +=+≈ 11 1ω

However, this is not necessary. Instead, it is sufficient to select the dimensioning factor such that the correct operation of the required protection is guaranteed under the given conditions.

0.01

0.1

1

10

1 10 100

k

T1= 500 ms

T1= 250 ms

T1= 100 ms

T1= 50 ms

T1= 25 ms

T1= 10 ms

I'1,max

/ Ioperate

Figure 1: Dimensioning factor, for definite time overcurrent protection (fn = 50Hz)

Note:: nf

RX

RX

Tπω 21 == (in seconds)


(AP) -28

0

5

10

15

20

25

0 50 100 150 200 250

k

T1 / ms

Figure 2: Dimensioning factor, for definite time overcurrent protection (fn = 50 Hz)

3.6 P141 - P145 These feeder management relays provide a comprehensive solution for the complete protection, control and monitoring of overhead lines and underground cables for various voltage levels for distribution and transmission applications. They contain a range of integrated communication protocols.

3.6.1 Burdens

CT Input In CT Burden VA Burden 1A <0.04 VA at rated current

Impedance 1A <40 mΩ over 0 - 30 In

VA Burden 5A <0.01 VA at rated current


Table 27: Current circuit for P141 - 145

Vn VT Burden 100 - 120 V < 0.02 VA rms at 110 V

380 - 480 V < 0.02 VA rms at 440 V

Table 28: Voltage circuit for P141 - 145

Relay Case Size Nominal Burden* P141, P142 Size 8/40TE 11 W or 24 VA

P143 - P145 Size 12/60TE 11 W or 24 VA

Table 29: Auxiliary supply for P141 – P145



(AP) -29

Additional Burden Energizing Voltage Burden

Energized opto-input 24 to 54 V dc 0.09 W




With optional 2nd rear communications - 1.25 W

With optional 10 Mbps Ethernet card - 2.25 W


Table 30: Additional burden for P141 – P145

Energizing Voltage Peak Current 0 to 300 V dc 3.5 mA

Table 31: Opto-inputs for P141 – P145

3.6.2 Current Transformer requirements The current transformer requirements are based on a maximum prospective fault current of 50 x In and the relay having an instantaneous setting of 25 x In. These CT requirements are designed to provide operation of all protection elements.



1A 2.5 VA 10P 20 1.30 Ω

5A 7.5 VA 10P 20 0.11 Ω

Table 32: CT specification for P141 – P145


3.6.3 Protection

Item Knee-point voltage Non-directional/directional DT/IDMT overcurrent and earth fault protection


k RRRI

V ++≥2


k RRRRI

V +++≥ 22



Instantaneous earth fault overcurrent ( )rnrplctsnk RRRRIV +++≥ 2



k RRRI

V ++≥2


k RRRRI

V +++≥ 22



(AP) -30

Item Knee-point voltage


k RRRRI

V +++≥ 22


k RRRRI

V +++≥ 22

Non-directional instantaneous SEF ( )rnrplctsnk RRRRIV ++ +≥ 2


k RRRRI

V +++≥ 22



k RRRI

V ++≥ 22


k RRRI

V ++≥ 22



k RRRI

V ++≥ 22

Table 33: Knee-point voltages for P141 – P145


3.6.3.1 REF protection

Low impedance Condition Equation

40<RX

and nf II 15< ( )lctnk RRIV 224 +≥

40≤RX

and nfn III 4015 ≤<

or

12040 ≤<RX

and nf II 15<

( )lctnk RRIV 248 +≥

120<RX

and nf II 10< ( )rlctnk RRRIV ++≥ 40

Table 34: REF protection for P141 – P145

Note:: Class 5P or better CTs should be used for low impedance REF applications.

High impedance The high impedance REF element maintains stability for through faults and operate in less than 40 ms for internal faults provided the following conditions are met in determining the CT requirements and value of associated stabilizing resistor.


(AP) -31


fs RR

II

R 2+=

Where X/R ≤ 25 and through fault stability with a transient dc offset in the fault current must be considered, the following equation can be used to determine the required stability voltage.

( )lctfs RRRXIV 268.00123.0 +⎟

⎠⎞

⎜⎝⎛ +=

The standard equation sss RIV = should be used for X/R ratios greater than 25.

If the calculated stability voltage is less than the voltage calculated by the standard equation, then the standard equation must be used instead.

Note:: Class 5P or PX CTs should be used for high impedance REF applications.

3.6.3.2 High impedance differential protection The relay can be applied as a high impedance differential protection to 3 phase applications such as busbars, generators, motors etc. The high impedance differential protection shall maintain stability for through faults and operate in less than 40 ms for internal faults provided the following conditions are met in determining the CT requirements and value of associated stabilizing resistor.


fs RR

II

R 24.1 +=

Where X/R ≤ 80 and through-fault stability with a transient dc offset in the fault current must be considered, the following equation can be used to determine the required stability voltage.

( )lctfs RRRXIV 278.0005.0 +⎟

⎠⎞

⎜⎝⎛ +=

The standard equation sss RIV = should be used for X/R ratios greater than 80.

If the calculated stability voltage is less than the voltage calculated by the standard equation, then the standard equation must be used instead.

Note: Class 5P or PX CTs should be used for high impedance differential applications.


(AP) -32

4 MOTOR PROTECTION RELAYS The range of motor protection relays consists of the following devices: P210, P211, P220, P225 P226C, P241-P243.


4.1 P210, P211 P210 and P211 motor protection relays are suitable for low power, low voltage induction motors.

4.1.1 Burdens

CT Input In CT Burden Phase 1A < 0.03 VA at InEarth 1A < 0.05 VA at 1n

Phase 5A < 0.1 VA at InEarth 5A < 0.15 VA at 1n

Table 35: Current circuit for P210, 211

Relay Case Size Nominal Burden* P210 35 mm DIN rail mount ≤ 3.5 VA

P211 35 mm DIN rail or flush mount 4.5 VA

Table 36: Auxiliary supply for P210, 211

*Typical minimum burden with no opto-inputs or output contacts energized.

Additional Burden Energizing Voltage Burden Energized opto-input 48 V 0.7 VA

Energized opto-input 230 V 0.6 VA

Table 37: Additional burdens on auxiliary supply for P210, 211

4.1.2 Current Transformer Requirements Zero sequence current, a characteristic of earth faults, can be detected by either a residual connection of the three-phase CT or by the use of a core-balance CT. If the motor neutral is earthed through an impedance, or is isolated from earth, as in the case of an insulated network, a core-balance CT is preferred. This avoids possible problems with false zero sequence current detection arising from asymmetrical saturation during the motor start-up. Starting currents can reach values up to several times (typically 5 – 6 times) the rated motor current. This phenomenon can be aggravated by the magnetization of the CT when there are opposing residual fluxes in the CT.

These issues may be overcome by using suitable earth fault settings and by careful selection of the CT, but the use of a core-balance transformer is recommended.

Motor Earthing Recommended Alternative

Solidly earthed 3 ph CT (and stabilizing resistance*) 3 ph CT and core-balance CT

Impedance earthed 3 ph CT and core-balance CT 3 ph CT (and stabilizing resistance*) or 2

ph CT and core-balance CT

Insulated 3 ph CT and core-balance CT 2 ph CT and core-balance CT

Table 38: Recommended CT types for P210, P211

* Where a residual CT connection is used, the value of stabilizing resistance can be calculated from:


(AP) -33

( )rnlctst

s RnRRII

R ++=0

4.1

Where:

n = 1, for 4 wire CT connection (star point at CT)

n = 2, for 6 wire CT connection (star point at relay)

The short circuit current setting should be set to less than 90% of the CT accuracy limit factor. Under these conditions tripping is guaranteed for fault currents up to 50 times the value of saturation current for symmetrical CT current output.

Breaking Device In Rated Output Burden (VA) Accuracy Class

Accuracy Limit Factor (ALF)

( )lrn RRI 2025.0 2 ++≥ 5P 10 Fused contactor 1A

Circuit breaker 1A ( )lrn RRI 2025.0 2 ++≥ 5P n

fp

II

50≥

Fused contactor 5A ( )lrn RRI 23.0 2 ++≥ 5P 10


fp

II

50≥

Table 39: IEC 60044-1 specifications for circuit breaking

Note:: A CT with accuracy class 10P may be used instead of 5P, however the thresholds of thermal overload and unbalance protection functions will be less precise. This may be acceptable where the motor has been oversized in relation to its purpose or is not used for heavy duty services

4.2 P220, P225, P226C The P220, P225 and P226C motor protection relays are suitable for high power, low voltage induction motors and for low power, medium voltage induction motors. The P226C is a compact sized solution.

4.2.1 Burdens

CT Input In CT Burden Phase 1A < 0.025 VA at In for P225 and < 0.3 VA at Vn for P226C

Earth 1A < 0.004 VA at 0.1In for P225

Phase 5A < 0.3 VA at InEarth 5A < 0.01 VA at 0.1In

Table 40: Current circuit for P220, P225, P226C

Vn VT Burden 57 - 130V < 0.1 VA at Vn

220 - 480V < 0.1 VA at Vn

Table 41: Voltage circuit for P220, P225, P226C

Relay Case Size Nominal Burden* P220, P225, P226C Size 6/30TE < 3 W or 6 VA

Table 42: Auxiliary supply for P220, P225, P226C



(AP) -34

Additional Burden Burden

Per energized opto-input < 10 mA

Per energized output contact 0.25 W

Table 43: Additional burdens on auxiliary supply for P220, P225, P226C

4.2.2 Current transformer requirements Zero sequence current, a characteristic of earth faults, can be detected by either a residual connection of the three phase CT or by the use of a core-balance CT. If the neutral of the motor is earthed through an impedance or isolated from earth in the case of an insulated network, a core-balance CT is preferred as it avoids possible problems with false zero sequence current detection arising from asymmetrical saturation of phase CT during motor start-up. Starting currents can reach values up to several times (typically 5 – 6 times) the motor rated current. This phenomenon can be aggravated by the magnetization of CT when opposing residual fluxes exist in the CT.

These issues may be overcome by employing suitable earth fault settings and by careful selection of the CT, but the use of a core-balance transformer is recommended.

Motor Earthing Recommended Alternative

Solidly earthed 3 ph CT (and stabilizing resistance*) 3 ph CT and core-balance CT

Impedance earthed 3 ph CT and core-balance CT 3 ph CT (and stabilizing resistance*) or 2

ph CT and core-balance CT

Insulated 3 ph CT and core-balance CT 2 ph CT and core-balance CT

Table 44: Recommended CT types for P220, P225, P226C

* Where a residual CT connection is used, the value of stabilizing resistance is calculated from:

Note::

( )rnlctst

s RnRRII

R ++=0

Where:

n = 1, for 4 wire CT connection (star point at CT)

n = 2, for 6 wire CT connection (star point at relay)

The short circuit (overcurrent) current setting I>> should be set less than 90% of the CT accuracy limit factor. Under these conditions tripping is guaranteed for fault currents up to 50 times the value of saturation current for symmetrical CT current output.

Breaking Device In Rated Output Burden (VA) Accuracy Class

Accuracy Limit Factor (ALF)



fp

II

50≥



fp

II

50≥

Table 45: IEC 60044-1 specifications for circuit breaking


(AP) -35

Note:: A CT with accuracy class 10P may be used instead of class 5P CT, however

the thresholds of thermal overload and imbalance protection functions will be less precise. This may be acceptable where the motor is oversized in relation to its purpose, or is not used for heavy duty services.

4.3 P241 - P243 P241, P242 and P243 motor manager relays are suitable for high power medium voltage induction and synchronous motors.

4.3.1 Burdens

In CT Burden VA Burden 1A <0.04 VA at rated current




Table 46: Current circuit for P241 - P243

Vn VT Burden 100 - 120V < 0.06 VA rms at 110 V

Table 47: Voltage circuit for P241 - P243

Relay Case Size Nominal Burden* P241 Size 8/40TE 11 W or 24 VA

P242 Size 12/60TE 11 W or 24 VA

P243 Size 16/80TE 11 W or 24 VA

Table 48: Voltage circuit for P241 - P243

*Typical minimum burden with no opto-inputs or output contacts energized.

Additional burden Energizing Voltage Burden Per energized opto-input 24 to 54 V dc 0.09 W

Per energized opto-input 110 to 125 V dc 0.12 W






Table 49: Additional burdens on auxiliary supply for P241 - P243


Table 50: Opto-inputs for P241 - P243

4.3.2 Current Transformer requirements The current transformer requirements are based on a maximum prospective fault current of 50 x In and the relay having an instantaneous setting of 25 x In. These CT requirements are designed to provide operation of all protection elements.


(AP) -36



1A 2.5 VA 10P 20 1.30 Ω

5A 7.5 VA 10P 20 0.11 Ω

Table 51: CT specification for P241 - P243

4.3.3 Protection

Protection type Knee-point voltage Non-directional DT Short Circuit and DT/IDMT Derived Earth Fault protection

DT-delayed Short Circuit elements ( )rplctfp

k RRRI

V ++≥2

DT-delayed/IDMT Derived Earth Fault elements ( )rnrplctcn

k RRRRI

V +++≥ 22

Non-directional instantaneous Short Circuit and Derived Earth Fault protection

Instantaneous Short Circuit elements ( )rplctspk RRRIV ++≥

Instantaneous Derived Earth Fault elements ( )rnrplctsnk RRRRIV ++ +≥ 2

Directional definite time/IDMTdelay Derived Earth Fault protection

Directional time-delayed Derived Earth Fault protection ( )rnrplct

cnk RRRR

IV +++≥ 2

2

Directional instantaneous Derived Earth Fault protection ( )rnrplct

fnk RRRR

IV +++≥ 2

2

Non-directional/directional definite time/IDMT sensitive earth fault (SEF) protection – residually connected

Non-directional time delayed SEF protection ( )rnrplctcn

k RRRRI

V +++≥ 22

Non-directional instantaneous SEF protection ( )rnrplctsn

k RRRRIV +++≥ 22

Directional time delayed SEF protection ( )rnrplctcn

k RRRRI

V +++≥ 22

Directional instantaneous SEF protection ( )rnrplctfn

k RRRRI

V +++≥ 22

SEF protection – as fed from a core-balanced CT

Directional / non-directional time delayed element ( )rnlctcn

k RRRIV ++≥ 22

Directional instantaneous element ( )rnlctfn

k RRRI

V ++≥ 22

Non-directional instantaneous element ( )rnlctsn

k RRRIV ++≥ 22

Table 52: Knee-point voltages for P241 - P243

Note:: The phase error of the applied core-balanced CT must be les than 90 minutes at 10% of the rated current and less than 150 minutes at 1% of the rated current.


(AP) -37

4.3.3.1 Biased differential protection (P243 only)

For IEC-compliant protection, class 5P CTs must be used.

The calculations in this section are based on the following conditions:

Parameter Description Value Is1 Current setting of differential element 1 0.05 x InIs2 Current setting of differential element 2 1.2 x InK1 Slope setting of first slope in double knee characteristic 0%

K2 Slope setting of second slope in double-knee characteristic 150%

Ist Boundary condition of starting current ≤ 10 x In

Table 53: Conditions for P243 knee-point voltage calculations

Condition Knee-point Equation Minimum

Motor not earthed or resistance earthed at motor neutral point ( )rlctnk RRRIV 230 ++≥

nI60

Motor is solidly earthed at the motor neutral point ( )rlctnk RRRIV 2240 ++≥

nI60

Table 54: Knee-point calculations for P243 – biased differential protection

For Class-X current transformers, the excitation current at the calculated knee-point voltage requirement should be less than 2.5 x Ιn (<5% of the maximum fault current, on which these CT requirements are based).

Note:: For IEC standard protection class 5P CTs should be used.

4.3.3.2 High impedance differential protection (P243 only) If the motor differential protection function is to be used to implement high impedance differential protection, then the CT knee-point voltage is:

ssk RIV 12≥

and the value of associated stabilizing resistor is:

( )1

25.1

s

lctsts I

RRIR +=


(AP) -38

5 INTERCONNECTION AND GENERATOR PROTECTION RELAYS The range of interconnection and generator protection relays consists of the following devices: P341-P345.


5.1 P341 The P341 provides overcurrent and ground fault, undervoltage and overvoltage, underfrequency and overfrequency, thermal overload, negative phase sequence overcurrent and overvoltage and power protection for generators.

5.1.1 Burdens







380 - 480V < 0.06 VA rms at 440 V

Table 56: Voltage circuit for P341


P342, P343 Size 12/60TE 11 W or 24 VA

P343, P344, P345 Size 16/80TE 11 W or 24 VA



Additional burdens on auxiliary supply Additional Burden Energizing Voltage Burden


Per energized opto 110 to 125 V dc 0.12 W





Energizing Voltage Peak Current 0 to 300V dc 3.5 mA


5.1.2 Current Transformer requirements The current transformer requirements for each current input depend on the protection function with which they are related and whether the line current transformers are being shared with other current inputs. Where current transformers are being shared by multiple current inputs, the knee-point voltage requirements should be calculated for each input and the highest calculated value used.


(AP) -39

The P34x can carry out all protection functions over a wide range of operating frequency due to its frequency tracking system (5–70 Hz).

When the P34x protection functions have to operate accurately at low frequency, it is necessary to use CTs with larger cores. The CT requirements need to be multiplied by fn/fmin.

The current transformer requirements are based on a maximum prospective fault current of 50 x In and the relay having an instantaneous setting of 25 x In. These CT requirements are designed to provide operation of all protection elements.



1A 2.5 VA 10P 20 1.30 Ω

5A 7.5 VA 10P 20 0.11 Ω

Table 60: CT specification for P341


5.1.3 Protection



k RRRI

V ++≥2


k RRRRI

V +++≥ 22



Instantaneous earth fault overcurrent ( )rnrplctsnk RRRRIV +++≥ 2



k RRRI

V ++≥2


k RRRRI

V +++≥ 22



k RRRRI

V +++≥ 22


k RRRRI

V +++≥ 22

Non-directional instantaneous SEF ( )rnrplctsnk RRRRIV +++≥ 2


k RRRRI

V +++≥ 22


(AP) -40




k RRRI

V ++≥ 22


k RRRI

V ++≥ 22



k RRRI

V ++≥ 22

Table 61: Knee-point voltages for P341 – P345


5.2 P342 – P345 The MiCOM P342 is suitable for protection of generators which require cost effective high quality protection. Protection includes overcurrent, ground fault, neutral voltage displacement, sensitive or restricted ground fault, voltage dependent overcurrent or underimpedance, under and overvoltage, under and overfrequency, reverse, low forward or overpower, field failure, negative phase sequence thermal, negative phase sequence overcurrent and overvoltage, turbine abnormal frequency, thermal and overfluxing, rotor ground fault as well as VT and CT supervision.

The MiCOM P343 is suitable for protection of larger or more important generators, providing generator differential, 100% stator ground fault via a 3rd harmonic measuring technique, pole slipping and unintentional energization at standstill protection in addition to the features of the P342.

The P344 is similar to the P343 but includes a second neutral voltage input for ground fault / inter-turn protection.

The P345 is similar to the P344 but includes 100% stator ground fault protection via a low frequency injection technique.

5.2.1 Burdens





Table 62: Current circuit for P342 – P345


380 - 480 V < 0.06 VA rms at 440 V

Table 63: Voltage circuit for P342 – P345


(AP) -41


P341, P342 Size 8/40TE 11 W or 24 VA

P342, P343 Size 12/60TE 11 W or 24 VA

P343, P344, P345 Size 16/80TE 11W or 24VA









Table 65: Additional burdens on auxiliary supply for P342 – P345



5.2.2 Protection

5.2.2.1 Biased differential protection The calculations in this section are based on the following conditions:

Parameter Description Value Is1 Current setting of differential element 1 0.05 x InIs2 Current setting of differential element 2 1.2 x In

K1Slope setting of first slope in double knee characteristic 0%

K2Slope setting of second slope in double-knee characteristic 150%

Ist Boundary condition of starting current ≤ 10 x In

Table 67: Conditions for P342 – P345 knee-point voltage calculations

Equation Minimum Condition

120<RX

and nf II 10< ( )rlctnk RRRIV ++≥ 250 nI

60

40<RX

and nf II 10< ( )rlctnk RRRIV 230 ++≥ nI

60

Table 68: Knee-point calculations for P342-345 – biased differential protection

Where the generator is impedance earthed and the maximum secondary earth fault current is less than Ιn then the CT knee point voltage requirements are:


(AP) -42

Condition Equation Minimum

160<RX


60

100<RX

and nf II 10<

or

1200<RX

and nf II 5<

( )rlctnk RRRIV ++≥ 30 nI

60

120<RX


60

Table 69: Knee-point calculations for P342 – P345 – biased differential protection

For Class-X current transformers, the excitation current at the calculated knee-point voltage requirement should be less than 2.5 x In (5% of the maximum prospective fault current, on which these CT requirements are based). For IEC standard protection, class 5P CTs should be used.

5.2.2.2 High impedance differential protection If the generator differential protection function is used to implement high impedance differential protection, then the CT knee-point voltage is:

s1Sk RI2V ≥

and value of associated stabilizing resistor is:

( )1S

lctsts I

R2RI5.1R +=

5.2.2.3 Voltage dependent overcurrent, field failure and negative phase sequence protection When determining the CT requirements for an input that supplies several protection functions, the most onerous condition must be met. This has been taken into account in the formula given below. The formula is equally applicable for CTs mounted at either the neutral-tail end or terminal end of the generator.

( )rlctnk RRRIV ++≥ 220

For class PX CTs, the excitation current at the calculated knee-point voltage requirement should be less than 1.0 x In. For IEC standard protection, class 5P CTs must be used.

5.2.2.4 Sensitive directional earth fault protection

Line current transformers It is assumed that the directional SEF protection function is only applied when the stator earth fault current is limited to the stator winding rated current or less. It is also assumed that the maximum X/R ratio for the impedance to a bus earth fault is no greater than 10. The required minimum knee-point voltage is therefore:

( )rlctnk RRRIV ++≥ 26

For class PX CTs, the excitation current at the calculated knee-point should be less than 0.3In (i.e. less than 5% of the maximum prospective fault current, 20In, on which these CT requirements are based). For IEC standard protection, class 5P CTs must be used.


(AP) -43

Core-balance CT connection Unlike a line CT, the rated primary current for a core-balanced CT may not be equal to the stator winding rated current. This has been taken into account in the formula:

( )rlctnk RRRNIV ++≥ 26

Note:: The maximum earth fault current should not be greater than 2In. i.e. N ≤ 2. The core-balance CT must be selected accordingly.

5.2.2.5 Stator earth fault protection The earth fault current input is used by the stator earth fault protection function.

Item Equation Non-directional DT/IDMT earth fault protection

Time-delayed earth fault overcurrent ( )rnlctfn

k RRRI

V ++≥ 22

Non-directional instantaneous earth fault protection

Instantaneous earth fault overcurrent ( )rnlctsnk RRRIV ++≥ 2

Table 70: Stator earth fault protection for P342 – P345

5.2.2.6 Restricted earth fault protetion

Low impedance REF protection Condition Equation

40<RX

and nf II 15< ( )lctnk RRIV 224 +≥

40≤RX

and nfn III 4015 ≤<

or

12040 ≤<RX

and nf II 15<

( )lctnk RRIV 248 +≥

120<RX

and nf II 10< ( )rlctnk RRRIV ++≥ 40

Table 71: Low impedance REF protection for P342 – P345

Note:: Class PX or 5P CTs should be used for low impedance REF applications.

High impedance REF protection The high impedance REF element shall maintain stability for through faults and operate in less than 40 ms for internal faults provided the following conditions are met in determining the CT requirements and value of associated stabilizing resistor.

Condition Equation

Knee-point voltage s1Sk RI4V ≥

Stabilizing resistor value ( )lct1S

fs R2R

II

R +=


(AP) -44

5.2.2.7 Reverse and low forward power protection

For settings greater than 3% Pn, the phase angle errors of suitable protection class current transformers will not result in any risk of malfunction or failure to operate. However, for settings less than 3%, we recommend that the current input is driven by a correctly loaded metering class current transformer.

Protection class current transformers For less sensitive power function settings (>3% Pn), the phase current input of the P34x should be driven by a correctly loaded class 5P protection current transformer.

To load the current transformer correctly, its VA rating should match the VA burden (at rated current) of the external secondary circuit through which it is required to drive current.

Metering class current transformers For low power settings (<3% Pn), the In sensitive current input of the P34x should be driven by a correctly loaded metering class current transformer. The current transformer accuracy class is dependent on the reverse power and low forward power sensitivity required. The table below indicates the metering class current transformer required for various power settings below 3% Pn.

To load the current transformer correctly, its VA rating should match the VA burden (at rated current) of the external secondary circuit through which it is required to drive current. Use of the P34x sensitive power phase shift compensation feature will help in this situation.

Reverse and Low Forward Power Settings % Pn

Measuring CT Class

0.5 0.1

0.6 0.1

0.8 0.2

1.0 0.2

1.2 0.2

1.4 0.2

1.6 0.5

1.8 0.5

2.0 0.5

2.2 0.5

2.4 0.5

2.6 0.5

2.8 0.5

3.0 1.0

Table 72: Power settings v. measuring class


(AP) -45

6 DISTANCE PROTECTION RELAYS The range of distance protection relays consists of the following devices: P430C, P432, P433, P435 - P439, P441, P442, P444, P443, P445 (MiCOMho).


6.1 P430C, P432, P433, P435 – P439 The MiCOM P43x devices are cost effective one-box solutions for distance protection and control. They allow the user to cover a wide range of applications in the area of selective short-circuit protection, ground fault protection and overload protection. They can be applied in medium-, high- and extra-high-voltage systems.

6.2 Burdens CT Input In CT Burden

Phase 1A < 0.1 VA

Earth 1A < 0.1 VA

Phase 5A < 0.1 VA

Earth 5A < 0.1 VA

Table 73: Current circuit for P430C, P432, P433, P435 – P439


Table 74: Voltage circuit for P430C, P432, P433, P435 – P439

Case Size Relay Nominal Burden* Maximum Burden

Compact P430C 4 W 8 W

40TE P433, P435, P439 13 W 29 W

40TE P436 13 W 37 W

84TE P433, P435, P437, P438, P439 13 W 37 W

84TE P432 13 W 40 W

Table 75: Auxiliary supply for P430C, P432, P433, P435 – P439

* Typical minimum burden at 220V dc with no opto-inputs or output contacts energized.

Additional Burden Energizing Voltage Burden Per energized opto-input 19 to 110 V dc 0.5W ±30%


Table 76: Additional burdens on auxiliary supply for P430C, P432, P433, P435 – P439

6.2.1 Current Transformer requirements The current transformers should comply with the IEC 60044-1 class 5P fault limit values. If auto-reclosing is used, it is best to use class TPY current transformers conforming to IEC 60044-6 Part 6, as these have anti-remanence cores.

Note:: The P436 and P438 may be applied at low system frequencies of 16⅔ Hz or 25 Hz. Any VA or knee-point voltage quoted must apply at the chosen nominal frequency (fn).


(AP) -46

6.2.2 Protection

The knee-point voltage of the CT should comply with the minimum requirements of the formulas shown below:


Phase fault distance protection ( )rplctfpk RRRkIV + +≥

Earth fault distance protection ( )rnrplctfnk RRRRkIV ++ +≥ 2

Theoretically, the CT could be dimensioned to avoid saturation by using the maximum value of k, calculated as follows:

RXTk +=+≈ 11 1ω

However, this is not necessary; it is sufficient to select the dimensioning factor k, such that correct operation of the required protection is guaranteed under the given conditions.

If auto-reclosing is not used, k can be obtained from Figure 3. The dotted line represents the theoretical characteristic maximum k = 1+ X/R.

Figure 3: Dimensioning factor, k, for distance protection (fn = 50 Hz)

Note:: nf

RX

RX

Tπω 21 == (in seconds)

This CT requirement ensures tripping of distance element within 120 ms at 95% of the set zone reach.

If auto-reclosing is used, the dimensioning factor k for the CT is increased as follows:

s

fr

Tt

Tt

eeTk −−⎥⎦

⎤⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛−++ 1

'

1 11 ω


(AP) -47

6.3 P441, P442, P444

These are full scheme distance protection devices providing flexible, reliable, protection, control and monitoring of transmission lines. The MiCOM distance protection relays can be applied for a wide range of overhead lines and underground cables in high and extra high voltage systems.

6.3.1 Burdens





Table 77: Current circuit for P441, P442, P444


Table 78: Voltage circuit for P441, P442, P444

Relay Case Size Nominal Burden* Maximum BurdenP441 Size 8/40TE 15 W or 16 VA 20 W or 20 VA

P442 Size 12/60TE 18 W or 19 VA 26 W or 26 VA

P444 Size 16/80TE 21 W or 22 VA

Table 79: Auxiliary supply for P441, P442, P444

* Typical burden with half of the opto-inputs and one output contact per board energized.







With optional 10Mbps Ethernet card - 2.25 W


Table 80: Additional burdens on auxiliary supply for P441, P442, P444

Energizing Voltage Peak Current 0 to 300 V dc > 3.5 mA

Table 81: Opto-inputs for P441, P442, P444

6.3.2 Current Transformer requirements For accuracy, class PX or 5P CT are recommended.

6.3.3 Protection The knee-point voltage of the CT should comply with the minimum requirements of the formulae shown below.


(AP) -48

Item Equation

Phase fault distance protection ( )lctfZk RRRXIV +⎟

⎠⎞

⎜⎝⎛ +≥ 16.0 1

Earth fault distance protection ( )lcte

e1feZk R2R

RX1I6.0V +⎟⎟

⎠

⎞⎜⎜⎝

⎛+≥

Table 82: Knee-points for distance protection P441, P442, P444

Note:: The required knee-point voltage must be calculated for the three phase fault current at the Zone 1 reach and also for the earth fault current at the Zone 1 reach. The higher of the two calculated knee-point voltages is used.

6.4 P443, P445 (MiCOMho)

6.4.1 Burdens





Table 83: Current circuit for P443, P445

VT Input Vn VT Burden

Phase 100 – 120 V < 0.02 VA rms at V3

110⎟⎠

⎞⎜⎝

⎛

Table 84: Voltage circuit for P443, P445


P445 Size 12/60TE 12 W or 24 VA

P443 Size 16/80TE 12 W or 24 VA

Table 85: Auxiliary supply for P443, P445


Additional Burden Energizing voltage Burden Per energized opto-input 24 to 54 V dc 0.09 W





With optional Inter MiCOM communications - 1.25 W

Table 86: Additional burdens on auxiliary supply for P441, P442, P444

Energizing Voltage Peak current 0 to 300 V dc 3.5 mA

Table 87: Opto-inputs for P443, P445


(AP) -49

6.4.2 Current Transformer requirements

For accuracy, class PX or 5P CT are recommended.

6.4.3 Protection The knee-point voltage of the CT should comply with the minimum requirements of the formulae shown below.

Item Equation

Zone 1 reach-point accuracy ( )lctfZk RRRXIV +⎟

⎠⎞

⎜⎝⎛ +≥ 16.0 1

Zone 1 close-up fault operation ( )lctfk RRIV +≥ max4.1

Table 88: Knee-point voltages for P443, P445

The higher of the two calculated knee-point voltages is used. It is not necessary to repeat the calculation for earth faults, as the phase reach (3∅) calculation is the worst case for CT dimensioning.


(AP) -50

7 CURRENT DIFFERENTIAL PROTECTION RELAYS The range of current differential protection relays consists of the following devices: P521, P541 - P546, P547, P591 – P595.

Selection of X/R ratio and fault level The value of X/R ratio and fault level will vary from one system to another, but selecting the correct value for the CT requirements is critical. In the case of single end-fed (radial) systems the through fault level and X/R ratio should be calculated assuming the fault occurs at the location of the remote CT. For systems where the current can feed through the protected feeder in both directions, such as parallel feeders and ring main circuits, further consideration is required. In this case the fault level and X/R ratio should be calculated at both the local and remote CT. In doing so the X/R ratio and fault level will be evaluated for both fault directions. The CT requirements, however, should be based upon the fault direction that gives the highest knee-point voltage. Under no circumstances should the X/R ratio from one fault direction and the fault level from the other be used to calculate the knee-point. Doing so may result in exaggerated and unrealistic CT requirements.


7.1 P521 The MiCOM P521 relay provides high-speed two-ended current differential unit protection of overhead lines and underground cables in applications such as ring mains and parallel feeders.

7.1.1 Burdens



Case Size Relay Nominal Burden* Size 6/30TE P521 3 W or 6 VA



Additional Burden Energizing Voltage Burden Per energized opto-input - 10 mA



7.1.2 Curent Transformer requirements For accuracy, class X or 5P CT are recommended.

7.1.3 Protection

7.1.3.1 Current differential protection The knee-point voltage of the CT should comply with the minimum requirements of the formulae shown below.

( )lctntsk RRIKKV 2+≥


(AP) -51

Ks is a constant depending on the maximum value of through fault current If (as a multiple of In) and the primary system X/R ratio. Ks is determined as follows:

Condition Value of ks

40RX

< ⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ += 269.055023.0

RX

RXIK fs

40≥RX

⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ += 72506.044024.0

RX

RXIK fs

Table 92: Calculation of ks

Kt is a constant depending on the current differential operating time (tIDiff) and the primary system X/R ratio.

For applications where the CT knee-point voltage is fixed (e.g. a retrofit application where the CT are already installed), it may be possible to reduce the CT requirements by adding a small time delay to the relay The tIDiff setting allows the user to increase the relay operating time thus making the relay more stable. For some applications a time setting of 50ms may reduce the required CT knee-point voltage by as much as 30%. Further reductions in CT knee-point are possible with longer time delays.

For applications where the relay is set for instantaneous operation, i.e. tIDiff =0s, Kt =1. When a time delay is applied, Kt is determined as follows:

Condition Value of Kt

Idifft t2.61K = − (for tIDiff ≤ 0.15s)

07.0Kt = (for tIDiff > 0.15s) 40

RX

<

40RX

≥ Idifft t5.21K −= (for tIDiff ≤ 0.25s)

Table 93: Calculation of Kt

7.1.3.2 SEF protection Core-balance CT of metering class accuracy is required and should have a knee-point voltage satisfying the following formula:

( )rnlctfnk RRRIV ++≥ 2

Where: Rrn = Impedance of neutral current input

7.2 P541 - P546 The MiCOM P541-P546 series provides high-speed current differential unit protection. The P54x is designed for all overhead line and cable applications, as it interfaces readily with the longitudinal (end-end) communications channel between line terminals. The interface options support direct fiber optic, or multiplexed digital links.

7.2.1 Burdens





Table 94: Current circuit for P541 - 546


(AP) -52

Vn VT Burden

100 - 120 V < 0.02 VA rms at 110 V

Table 95: Voltage circuit for P541 - 546


P542 - P544 Size 12/60TE 11 W or 24 VA

P545, P546 Size 16/80TE 11 W or 24 VA

Table 96: Auxiliary supply for P541 - 546


Additional Burden Energizing Voltage Burden Per energized opto-input 24 to 54 V dc 0.09 W







Table 97: Auxiliary burdens on auxiliary supply for P541 - 546

Opto-inputs Energizing Voltage Peak Current

0 to 300V dc 3.5 mA

Table 98: Opto-inputs for P541 - 546


7.2.3 Protection

7.2.3.1 Current differential protection The knee point voltage of the CT should comply with the minimum requirements of the formulae shown below.

( )lctnk RRKIV 2+≥

K is a constant dimensioning factor and is defined as follows:

Condition Value of K Minimum value allowed For relays set at: IS1 = 20%, IS2 = 2 In, K1 = 30%, K2 = 150%

1000RX

≤ ⎟⎠⎞

⎜⎝⎛+=

RXI07.040K f 65

1600RX1000 ≤< 107K = 107

For relays set at: IS1 = 20%, IS2 = 2 In, K1 = 30%, K2 = 100%

600RX

≤ ⎟⎠⎞

⎜⎝⎛+=

RXI35.040K f 65


(AP) -53

Condition Value of K Minimum value allowed

For relays set at: IS1 = 20%, IS2 = 2 In, K1 = 30%, K2 = 150%

1600RX600 ≤< 256K = 256

Table 99: Calculation of K

7.2.3.2 Distance protection

Zone 1 reach point accuracy (RPA)

( )lct1fZrpak RRRX1IKV +⎟

⎠⎞

⎜⎝⎛ +≥

Where:

Krpa = Dimensioning factor – fixed at 0.6

IfZ1 = Maximum secondary phase fault current at Zone 1 reach point (A)

Zone 1 close-up fault operation An additional calculation must be performed for all cables, and any lines where the source impedance ratio might be less than SIR = 2.

( )lctmaxfmaxk RRIKV +≥

Where:

Kmax = Dimensioning factor - fixed at 1.4

Ifmax = Maximum secondary phase fault current (A).

The highest of the two calculated knee-points must be used.

Note:: It is not necessary to repeat the calculation for earth faults, as the phase reach calculation (3ϕ) is the worst-case for CT dimensioning.

Time delayed distance zones When a time delayed distance zone is being used, there is no need to calculate the required Vk separately. This is due to the employed time delay (usually more than 3 times the primary time constant), which overrides the transient conditions. When it is insisted to do some calculations for the time delayed distance zone, then we should use the following equation.

( )lctfk RRIV +>

Where:

If is the current for a fault at the remote bus of the protected feeder (the through fault current for the current differential function).


(AP) -54

P547

The MiCOM P547 Phase Comparison Protection Relay provides unit protection of EHV / HV lines. Phase comparison is an established mode of protection for medium and high voltage lines.

7.2.4 Burdens







P547 Size 8/40TE 15 W












7.2.6 Protection

7.2.6.1 Phase comparison protection The knee point voltage of the CT should comply with the minimum requirements of the formula shown below:

( )rlctnk RKIV 2+≥

K is a constant dimensioning factor and is defined as follows:


(AP) -55

Condition Value of K

If ≤ 20

1600RX

≤ 125K =

2400RX1600 ≤< ⎟

⎠⎞

⎜⎝⎛+−=

RXI13.090K f

2400RX

≤ 230K =

If > 20

1600RX

≤ 230K =

2400RX1600 ≤< 230K =

2400RX

≤ 230K =

Table 104: Calculation of K

7.2.6.2 Distance protection

( )lct1fZrpak RRRX1IKV +⎟

⎠⎞

⎜⎝⎛ +≥

Where:

Krpa = Dimensioning factor – fixed at 0.6

IF Z1 = Maximum secondary phase fault current at Zone 1 reach point (A)

Zone 1 close-up fault operation An additional calculation must be performed for all cables, and any lines where the source impedance ratio might be less than SIR = 2.

( )lctmaxfmaxk RRIKV +≥

Where:

Kmax = Dimensioning factor - fixed at 1.4

Ifmax = Maximum secondary phase fault current (A).

The highest of the two calculated knee-points must be used.

Note:: It is not necessary to repeat the calculation for earth faults, as the phase reach calculation (3ϕ) is the worst-case for CT dimensioning.


(AP) -56

8 TRANSFORMER DIFFERENTIAL PROTECTION RELAYS The range of transformer differential protection relays consists of the following devices: P630C, P631 - P634, P638, P642 – P645.


8.1 P630C, P631 - P634, P638 The P63x is a differential protection relay that provides fast and selective short circuit protection of transformers, reactors, generators, motors and any installation with two, three, or four windings.

8.1.1 Burdens


Earth 1A < 0.1 VA

Phase 5A < 0.1 VA

Earth 5A < 0.1 VA

Table 105: Current circuit for P630C, P631 – P634, P638


Table 106: Voltage circuit for P630C, P631 – P634, P638

Relay Case Size Nominal Burden* Maximum BurdenP630C Compact 8 W 10 W

P631 - P633 40TE 12.6 W 34.1 W

P632 - P634 84TE 14.5 W 42.3 W

P638 84TE 13 W 32 W

Table 107: Auxiliary supply for P630C, P631 – P634, P638


Additional Burden Energizing Voltage Burden Per energized opto-input 19 to 110 V dc 0.5 W ±30%


Table 108: Additional burdens on auxiliary supply for P630C, P631 – P634, P638

8.1.2 Current Transformer requirements IEC 60044-1 accuracy class 5P or equivalent

Note:: The P638 may be applied at low system frequencies of 16⅔ Hz or 25 Hz. Any VA or knee-point voltage quoted must apply at the chosen nominal frequency (fn).

8.1.3 Protection

8.1.3.1 Differential protection The required knee-point voltage must be calculated for phase fault current and also for the earth fault current. The higher of the two calculated knee-point voltages is used.

The CT requirements are based on the default settings.


(AP) -57

Parameter Description Value

Transformer differential protection

Idiff> Current setting of first stage biased differential 0.2 x Iref

IRm2>Setting that defines the second knee of the tripping characteristic 4 x Iref

m1 Lower bias slope setting of biased differential element 0.3

m2 Higher bias slope setting of biased differential element 0.7

Busbar differential protection

Idiff> Current setting of first stage biased differential 1.2 x Iref

IRm2Setting that defines the second knee of the tripping characteristic 1.8 x Iref

m1 Lower bias slope setting of biased differential element 0.2

m2 Higher bias slope setting of biased differential element 0.8

Table 109: Conditions for P630C, P631 – P634, P638 calculations


Phase fault differential protection ( )lctk RRKV +≥

Earth fault differential protection ( )lctek RRKV 2+≥

Table 110: Knee-point voltages for P630C, P631 – P634, P638

K is a constant depending on the maximum value of through fault current (as a multiple of In) and the primary system X/R ratio. K is determined as follows:

Condition Value of K For phase faults, K is determined as follows

nf IRXI 500≤

RXIK f14.0=

1200500 <<RXII fn 70=K

For earth faults, Ke is determined as follows

nfe IRXI 500≤

RXIfeKe 14.0=

( ) nfen IRXII 1200500 << 70=eK

Table 111: Calculation of K for phase and earth faults

Typical knee-point voltage requirement for transformer differential protection The through fault stability required for most transformer applications is determined by the external through fault current and transformer X/R ratio. The through fault current in all but ring bus or mesh fed transformers is given by the inverse of the per unit reactance of the transformer. For most transformers, the reactance varies between 0.05 to 0.1pu, therefore typical through fault current is given by 10 to 20In.

For conventional transformers (non-autotransformer), the X/R ratio is typically 7. This cancels out the 0.14 multiplier, leaving only the maximum secondary through fault current (If) to multiply with the loop resistance, giving:


(AP) -58

( )lctk RRIVf

2+≥

Alternatively, as a conservative estimate:

( )t

lctk X

RRV 2+≥

8.1.3.2 REF protection

Low impedance The CT requirements for low impedance REF protection are generally lower than those for differential protection. As the line CT for low impedance REF protection are the same as those used for differential protection the differential CT requirements cover both differential and low impedance REF applications.

High impedance The high impedance REF element shall maintain stability for through faults and operate in less than 40ms for internal faults provided the following conditions are met in determining the CT requirements and value of associated stabilizing resistor:

sdiffk RIV 2>

( )( )rlctdiff

fs R

II

R 21.1 +=

For faster operation of the REF element, a larger knee-point voltage will provide reduced operating times. Refer to the graph below showing the operating time of the REF element for differing ratios.

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20

Vk / (Idiff> × Rs)

Ope

ratin

g tim

e (m

s)

Figure 4: REF operating times


(AP) -59

Note:: The diagram is the result of investigations which were carried out for

impedance ratios in the range of 5 to 120 and for fault currents in the range of 0.5 to 40 In.

8.2 P642 – P645 The P64x incorporates differential, REF, thermal, and overfluxing protection, plus backup protection for un-cleared external faults. It offers fast and reliable protection for transformers, reactors, motors and short interconnectors.

8.2.1 Burdens


Earth 1A < 0.2 VA

Phase 5A < 0.2 VA

Earth 5A < 0.2 VA


Vn VT Burden 100 - 120V < 0.06 VA at 110 V


Relay Case Size Minimum* P642 40TE 11 W or 24 VA

P643/5 60TE NA

P645 80TE NA



Additional Burden Energizing Voltage Burden Per energized opto-input 24 to 54 V 0.09 W

Per energized opto-input 110 to 125 V 0.12 W

Per energized opto-input 220 to 250 V 0.19 W

Per energized output relay - 0.13 W


8.2.2 Current Transformer requirements IEC 60044-1 accuracy class 5P or equivalent

8.2.3 Protection

8.2.3.1 Differential protection For accuracy, class X or class 5P current transformers (CT) are strongly recommended.

The current transformer knee-point voltage requirements are based on the following settings:

Parameter Description Value Transformer differential protection

Is1Minimum differential threshold of the low set differential characteristic 0.2 pu

Is2Bias current threshold for the second slope of the low set differential characteristic 1 pu


(AP) -60

Parameter Description Value

K1 First slope setting of the low set differential characteristic 20%

K2 Second slope setting of the low set differential characteristic 80%

Is-HS1 High set element one setting 10 pu

Is-HS2 High set element two setting 32 pu

Ih(2)%> Second harmonic blocking threshold 15%

Cross blocking If enabled the low set element trip is blocked in all three phases when the ratio of second harmonic to fundamental is above Ih(2)%> in any of the phases.

Disabled

5th harmonic blocking Fifth harmonic blocking threshold Disabled

Table 116: Conditions for P642 – P645 calculations

A series of internal and external faults were performed to determine the CT requirements for the differential function. These tests were performed under different X/R ratios, CT burdens, fault currents, fault types and point on wave. The CT requirements are determined either by the operating time for internal faults (≤35msec in the P64x) or by through fault stability, whichever requires the highest K dimensioning factor. The K factors were calculated for internal and external faults.

The following equation shows that the knee point voltage (VK) is directly proportional to the

CT burden (RCT) and to the loop burden (2RL). K is the CT dimensioning factor calculated using the equation below and the CT requirements test results.

( )rlctnk RRRkIV ++≥ 2

To achieve through fault stability and operating times ≤35msec, the K dimensioning factor must comply with the following expressions:

System conditions K (CT dimensioning factor)

5≤fI

405 <≤RX

⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ += 17706.025.2404.0

RX

RXIK f

5≤fI

12040 <≤RX

1612 += fIK

105 ≤< fI

405 <≤RX

⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ −−= 5.20244.01514.0

RX

RXIK f

105 ≤< fI

12040 ≤≤RX

11238.7 +−= fIK


(AP) -61


205 ≤< fI

405 <≤RX

⎟⎠⎞

⎜⎝⎛ +−+⎟

⎠⎞

⎜⎝⎛ −= 9.741.15.5206.0

RX

RXIK f

2010 <≤ fI

12040 ≤≤RX

4581.0 +−= fIK

Table 117: K requirements for P642 – P645

The Knee-point voltage must be calculated for both phase fault current and earth fault current. The higher of the two calculated voltages must be used.

To calculate the knee point voltage required for the maximum phase fault used the following equation:

( )lctk RRKV +≥

To calculate the knee point voltage required for the maximum earth fault used the following equation:

( )lctek RRKV 2+≥

8.2.3.2 Low impedance REF protection For accuracy, class X or class 5P current transformers (CTs) are strongly recommended.

The CT requirements for low impedance REF protection are generally lower than those for differential protection. As the line CTs for low impedance REF protection are the same as those used for differential protection the differential CT requirements cover both differential and low impedance REF applications.

The current transformer knee-point voltage requirements are based on the following settings for transformer REF protection:

I = 0.09In, I = 0.9In, K = 0%, K = 150%. S1 S2 1 2

A series of internal and external faults were performed to determine the CT requirements for the REF function. These tests were performed under different X/R ratios, CT burdens, fault currents, fault types and point on wave. The CT requirements are determined either by the operating time for internal faults (≤45msec in the P64x) or by through fault stability, whichever requires the highest K dimensioning factor. The K factor was calculated for internal and external faults.

The knee point voltage (VK) is directly proportional to the CT burden (Rct) and to the loop burden (2Rl). K is the CT dimensioning factor, and it was calculated using the equation below and the CT requirements test results.

( )rlctnk RRRKIV ++≥ 2

For the REF protection, the K dimensioning factor is smaller than for transformer differential protection. The higher of the two must be considered when applying both types of protection.


(AP) -62

8.2.3.3 One breaker application

According to the CT requirements test results, to achieve through fault stability and operating times ≤ 45 msec, the dimensioning factor K must comply with the following formula:


20I2 f ≤≤

40RX5 <≤

⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ +−= 28

RX15.07.17

RXI013.0K f

20I2 f ≤≤

120RX40 <≤

⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ −= 215

RX04.0934

RXI0003.0K f

Table 118: K requirements for P642 – P645 (one-breaker application)

Note:: K must always be greater or equal to 6.5.

8.2.3.4 One and a half breaker application According to the CT requirements test results, to achieve through fault stability and operating times ≤ 45 msec, the K dimensioning factor must comply with the following expressions:


20I2 f ≤≤

20RX5 <≤

⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ +−= 18.0

RX95.05

RXI04.0K f

20I2 f ≤≤

120RX20 <≤

⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ += 7.115

RX14.01.97

RXI007.0K f

Table 119: K requirements for P642 – P645 (one and a half breaker application)

Note:: K must always be greater or equal to 5.


(AP) -63

9 BUSBAR PROTECTION RELAYS The range of Busbar protection relays consists of the following devices: P742-P743, P746.


9.1 P742 - P743 The P742 and P743 differential busbar protection relays offer integral biased differential distributed busbar, breaker failure, dead zone, overcurrent and earth-fault protection.

9.1.1 Burdens






Relay Case Size Nominal Burden* P742 Size 8/40TE 16 - 23 W

P743 Size 12/60TE 22 - 32 W

P741 (with all communications boards) Size 16/80TE 37 - 41 W










9.1.2 Current Transformer requirements The characteristics of the CT are set in each peripheral unit (P742/P743), therefore allowing different classes of CT to be used in the same scheme. The following CT specifications may be used:

• IEC 60044-1 class 5P or PX (equivalent to BS 3938 class X) • IEC 60044-6 class TPX, TPY or TPZ • IEEE C57.13 class C.

Note:: The following knee-point requirements can be converted to an equivalent C voltage classification as per Appendix B.


(AP) -64

9.1.3 Protection

9.1.3.1 Differential protection The knee-point voltage of the CT should comply with the minimum requirements of the formulae shown below.

( )lctfk RRIV 25.0 max +≥

And for each CT:

( )lctfk RRIV 2intmax +≥

The recommended specification makes it possible to guarantee a saturation time greater than 1.4 ms with a remanent flux of 80% of maximum flux (class TPX). This provides a sufficient margin of security for CT saturation detection.

9.2 P746 The P746 differential centralized busbar protection relay protects substation busbars from distribution to transmission voltage levels. It offers integral biased differential busbar, breaker failure, dead zone and overcurrent protection.

9.2.1 Burdens






Relay Case Size Nominal Burden* P746 (with all comms boards) Size 16/80TE 12 W










9.2.2 Current Transformer requirements The characteristics of the CT are set in each peripheral unit (P742/P743), therefore allowing different classes of CT to be used in the same scheme. The following CT specifications may be used:

• IEC 60044-1 class 5P or PX (equivalent to BS 3938 class X) • IEC 60044-6 class TPX, TPY or TPZ • IEEE C57.13 class C.


(AP) -65

Note:: The following knee-point requirements can be converted to an equivalent

voltage classification as per Appendix B.

9.2.3 Protection Differential protection

9.2.3.1 The knee-point voltage of the CT should comply with the minimum requirements of the formulae shown below.

( )lctfk RRIV 25.0 max +≥

And for each CT:

( )lctfk RRIV 2intmax +≥

The recommended specification makes it possible to guarantee a saturation time greater than 1.4 ms with a remnant flux of 80% of maximum flux (class TPX). This provides a sufficient margin of security for CT saturation detection.


(AP) -66

10 CIRCUIT BREAKER FAIL PROTECTION RELAYS The range of Circuit Breaker fail differential protection relays consists of the following devices: P821.


10.1 P821 The MiCOM P821 is an Advanced Breaker Failure relay based on the latest numerical technology.

10.1.1 Burdens






Additional Burden Energizing Voltage Burden Per energized opto-input - < 10 mA



10.1.2 Protection The current transformer requirements are based on a maximum prospective fault current of 50 times the relay rated current (In) and the relay having an instantaneous setting of 25 times rated current (In). The current transformer requirements are designed to provide operation of all protection elements.

Where the criteria for a specific application are in excess of those detailed above, or the actual lead resistance exceeds the limiting value quoted, the CT requirements may need to be increased according to the formulae in the following sections.

Nominal Rating Nominal Output Accuracy Class Accuracy Limit

Factor Limiting lead

resistance 1A 2.5 VA 10P 20 1.3 ohms

5A 7.5 VA 10P 20 0.11 ohms

Table 131: Current transformer requirements for P821

The knee-point voltage of the CT should comply with the minimum requirements of the formulae shown below:

( )rnrplctfpk RRR2RIV +++≥


(AP) -67

11 VOLTAGE AND FREQUENCY PROTECTION RELAYS The range of Voltage and Frequency protection relays consists of the following devices: P921 – P923.


11.1 P921 - P923 The MiCOM P92x relays are suitable for all applications where voltage and/or frequency protection is required to provide load or generation shedding to ensure system stability.

11.1.1 Burdens

Vn VT Burden 57 - 130 V < 0.25 VA

220 - 480 V < 0.36 VA


Relay Case Size Nominal Burden* P921 - P923 Size 4/20TE 3 W


* Nominal is with 50% of the opto-inputs energized and one output contact per card energized.

Additional Burden Relay Auxiliary Voltage Burden Per energized opto-input 24 to 60 V dc 10 mA

Per energized opto-input 48 to 125 V dc 5 mA

Per energized opto-input 130 to 250 V dc 2.5 mA



11.2 P941 - P943 The MiCOM P94x relays are suitable for all applications where voltage and/or frequency protection is required to provide load or generation shedding to ensure system stability.

11.2.1 Burdens


380 - 480V < 0.15 VA r.m.s at 440 V



P943 Size 12/60TE 11 W or 24 VA






(AP) -68

Additional Burden Energizing Voltage Burden


Per energized output contact - 0.13W


Energizing Voltage Peak Current 0 to 300V dc 3.5 mA



(AP) -69

APPENDIX A: CONVERTING TO A LIMITING SECONDARY VOLTAGE The suitability of a standard protection current transformer can be checked against the limiting secondary voltage requirements, specified in this document.

An estimated limiting secondary voltage can be obtained as follows:

( )ctnn

k RIALFIALFVAV ×+

×≥

If Rct is not available, then the second term in the above equation can be ignored as it typically only adds a small amount to the estimated secondary limiting voltage.

To ensure that the current transformer has a high enough rating for the relay’s burden it is necessary to work out the current transformer’s continuous VA rating using the following formula:

( )rlnct RRIVA +> 2

Example 1: An estimate of the secondary limiting voltage of a 400/5A current transformer of class 5P 10 with a rated output burden of 15 VA and a secondary winding resistance of 0.2 Ω will be:

2.05105

1015××+

×≈kV = V40

Example 2: For a particular application of a 1A MiCOM overcurrent relay it is required to determine the most appropriate class P current transformer to be used. The secondary limiting voltage required has been calculated at 87.3 V using a current transformer secondary winding resistance of 2 Ω.

The current transformer rated output burden must be:

( )rlnct RRIVA 2>

( 025.0112 +×≥ctVA ) = 1.025 VA

The nearest rating above this will be 2.5 VA.

The accuracy limit factor required can be determined by:

ctnn

k RIALFIALFVAV ×+

×=

ALFALFALF×=××+

×= 5.421

15.23.87

4.19=∴ ALF

The nearest accuracy limit factor above 19.4 is 20.

Therefore the current transformer required to supply the MiCOM overcurrent relay will be a 2.5 VA 10P 20. (i.e. 2.5VA is the rated burden, 10 (%) is the nominal accuracy class, 20 is the ALF).


(AP) -70

APPENDIX B: CONVERTING TO IEEE STANDARD VOLTAGE RATING The MiCOM series protection relays are compatible with ANSI/IEEE CTs as specified in the IEEE C57.13 standard. The applicable class for protection is class "C", which specifies a non air-gapped core. The CT design is identical to IEC class P but the rating is specified differently.

The IEEE C class standard voltage rating required will be lower than an IEC knee-point voltage. This is because the IEEE voltage rating is defined in terms of useful output voltage at the terminals of the CT, whereas the IEC knee-point voltage includes the voltage drop across the internal resistance of the CT secondary winding added to the useful output. The IEC knee-point is also typically 5% higher than the IEEE knee-point.

Where IEEE standards are used to specify CTs, the C class voltage rating can be checked to determine the equivalent knee-point voltage (Vk) according to IEC. The equivalence formula is:

ctnssck RIKCV += 05.1

ctk RCV 10005.1 +=

Note:: IEEE CTs are always 5A secondary rated, i.e. In =5A, and are defined with an accuracy limit factor of 20, i.e. Kssc =20.

The following table allows C57.13 ratings to be converted to a typical IEC knee-point voltage:

CT Ratio Rct* Vk - C50 Vk - C100 Vk - C200 Vk-- C400 Vk - C800 100/5 0.04 Ω 56.5 V 109 V 214 V 424 V 844 V

200/5 0.08 Ω 60.5 V 113 V 218 V 428 V 848 V

400/5 0.16 Ω 68.5 V 121 V 226 V 436 V 856 V

800/5 0.32 Ω 84.5 V 137 V 242 V 452 V 872 V

1000/5 0.40 Ω 92.5 V 145 V 250 V 460 V 880 V

1500/5 0.60 Ω 112.5 V 165V 270 V 480 V 900 V

2000/5 0.80 Ω 132.5 V 185 V 290 V 500 V 920 V

3000/5 1.20 Ω 172.5 V 225 V 330 V 540 V 960 V

Table 139: C57.13 ratings

* Assuming 0.002 ohms per turn typical secondary winding resistance for 5A CTs.


(AP) -71

APPENDIX C: USE OF METROSIL NON-LINEAR RESISTORS “Metrosils” (non-linear resistors) are used to limit the peak voltage developed by the current transformers under internal fault conditions, to a value below the insulation level of the current transformers, relay and interconnecting leads, which are normally able to withstand 3000 V peak.

The following formulae should be used to estimate the peak transient voltage (Vp) that could be produced for an internal fault. The peak voltage produced during an internal fault will be a function of the current transformer knee-point voltage and the prospective voltage (Vf) that would be produced for an internal fault if current transformer saturation did not occur.

( )kfkp VVVV −= 22

( )slctff RRRIV ++= 2'

When the value given by the formulae is greater than 3000 V peak, Metrosils should be applied. They are connected across the relay circuit and serve the purpose of shunting the secondary current output of the current transformer from the relay in order to prevent very high secondary voltages.

Metrosils are externally mounted and take the form of annular discs. Their operating characteristics follow the expression:

25.0CIV =

where :

V = Instantaneous voltage applied to the Metrosil

C = Characteristic constant of the Metrosil

I = Instantaneous current through the Metrosil

With a sinusoidal voltage applied across the Metrosil, the rms current would be approximately 0.52 times the peak current. This current value can be calculated as follows:

4

)sin(252.0 ⎟

⎟⎠

⎞⎜⎜⎝

⎛=

CV

I rmsrms

Where

Vsin(rms) = rms value of the sinusoidal voltage applied across the Metrosil.

This is due to the fact that the current waveform through the metrosil is not sinusoidal but appreciably distorted.

For satisfactory application of a Metrosil, it's characteristic should be such that it complies with the following requirements:

At the relay voltage setting, the Metrosil current should be as low as possible, and no greater than ≈ 30 mA rms for 1A CTs and ≈ 100 mA rms for 5A CTs.

At the maximum secondary current, the Metrosil should limit the voltage to 1500 V rms or 2120 V peak for 0.25 s. At higher relay voltage settings, it is not always possible to limit the fault voltage to 1500 V rms, so higher fault voltages may have to be tolerated.

The following tables show the typical Metrosil types that will be required, depending on relay current rating, REF voltage setting etc.

Metrosil units for relays using 1A CTs


(AP) -72

The Metrosil units for 1A CTs have been designed to comply with the following restrictions:

At the relay voltage setting, the Metrosil current should be less than 30 mA rms.

At the maximum secondary internal fault current the Metrosil should limit the voltage to 1500 V rms if possible.

The Metrosil units normally recommended for use with 1A CTs are as shown in the following table:

Nominal Characteristic Recommended Metrosil Type

Relay Voltage Setting C β Single Pole Relay Triple Pole Relay

Up to 125 V rms 450 0.25 600A/S1/S256 600A/S3/1/S802

125 - 300 V rms 900 0.25 600A/S1/S1088 600A/S3/1/S1195

Table 140: Metrosil units

Note:: Single pole Metrosil units are normally supplied without mounting brackets unless otherwise specified by the customer.

Metrosil units for relays using 5A CTs

These Metrosil units have been designed to comply with the following requirements:

At the relay voltage setting, the Metrosil current should be less than 100 mA rms (the actual maximum currents passed by the units is shown below their type description).

At the maximum secondary internal fault current the Metrosil unit should limit the voltage to 1500 V rms for 0.25 s. At the higher relay settings, it is not possible to limit the fault voltage to 1500 V rms hence higher fault voltages have to be tolerated (indicated by *, **, ***).

The Metrosil units normally recommended for use with 5A CTs and single pole relays are as shown in the following table:

Recommended Metrosil Type Relay Voltage Setting

Secondary Internal Fault

Current Up to 200V rms 250V rms 275V rms 300V rms

50A rms 600A/S1/S1213 C = 540/640 35 mA rms

600A/S1/S1214 C = 670/800 40 mA rms

600A/S1/S1214 C = 670/800 50 mA rms

600A/S1/S1223 C = 740/870* 50 mA rms

100A rms 600A/S2/P/S1217 C = 470/540 70 mA rms

600A/S2/P/S1215 C = 570/670 75 mA rms

600A/S2/P/S1215 C = 570/670 100 mA rms

600A/S2/P/S1196 C = 620/740* 100 mA rms

150A rms 600A/S3/P/S1219 C = 430/500 100 mA rms

600A/S3/P/S1220 C = 520/620 100 mA rms

600A/S3/P/S1221C = 570/670** 100 mA rms

600A/S3/P/S1222C = 620/740*** 100 mA rms

Table 141: Metrosil units for 5A CTs

* 2400 V peak, ** 2200 V peak, *** 2600 V peak

In some situations single disc assemblies may be acceptable. Metrosil units for higher relay voltage settings and fault currents can also be supplied if required. Contact AREVA T&D for detailed applications.

Note:: The Metrosil unit recommended for use with 5A CTs can also be applied for use with triple pole relays and consist of three single pole units mounted on the same central stud but electrically insulated from each other. To order these units please specify "Triple pole Metrosil type", followed by the single pole type reference.


(AP) -73

APPENDIX D: FUSE RATING OF AUXILIARY SUPPLY Use fuses with standard ratings between 6A and 16A. Low voltage fuse-links rated for 250 V minimum and compliant with IEC 60269-1 general application type gG with high rupturing capacity are acceptable. These have gives equivalent characteristics to HRC "Red Spot" fuse types NIT/TIA, often specified historically.

Where only one or two relays are wired as a fused spur, it is acceptable to use a 6A rated fuse. Generally, five relays would be connected on a spur protected by a 10A fuse and ten relays would have a 15 or 16A fuse.

Note:: This applies to MiCOM Px10, Px20, Px30 series devices. It also applies to Px40 series devices with hardware suffix C the later of these have inrush current limitation on switch-on to conserve the fuse-link.

The recommended external protective fuse for the auxiliary DC supply of the P59x series interface units is a 2A HRC (high rupture capacity) GE Red Spot type NIT or TIA, or if a UL recognized fuse is required, 2A time delay Gould type AJT2.

Alternatively, miniature circuit breakers (MCBs) may be used to protect the auxiliary supply circuits.


(AP) -74

AREVA T&D's Automation & Information Systems Business www.areva-td.com T&D Worldwide Contact Centre online 24 hours a day: +44 (0) 1785 25 00 70 http://www.areva-td.com/contactcentre/

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Electrical Protection related - CT_EN_AP_D11[1]