Outdoor Instrument Transformers Application Guide | Handbook of TU-TI ABB

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Contents
1. General principles of measuring current and voltage 7
1.1 Instrument transformers 8
1.2 Current transformers operating principles 9
1.2.1 Measuring errors 9
1.2.2 Calculation of errors 11
1.2.3 Variation of errors with current 13
1.2.4 Saturation factor 14
1.2.5 Core dimensions 16
1.3 Voltage transformers operating principles 17
1.3.1 Measuring errors 17
1.3.2 Determination of errors 19
1.3.3 Calculation of the short-circuit impedance Zk 21
1.3.4 Variation of errors with voltage 22
1.3.5 Winding dimensions 23
1.3.6 Accuracy and burden capability 24
2. How to specify current transformers 25
2.1 Rated insulation level 26
2.2 Rated primary current 29
2.3 Rated continuous thermal current 30
2.4 Rated secondary current 30
2.5 Short-time thermal current (Ith) and dynamic current (Idyn) 30
2.6 Burden and accuracy 32
2.6.1 Measurement of current 32
2.6.2 Metering cores 32
2.6.3 Relay cores 35
2.6.4 Magnetizing curves 36
2.6.5 Accuracy classes according to IEC 60044-1 38
2.6.6 Accuracy classes according to IEEE C57.13 40
2.7 Pollution levels 42
3. Measurement of current during transient conditions 45
3.1 Background 46
3.2 The network 46
3.3 Important parameters 49
3.3.1 Rated primary current (Ipn) 49
3.3.2 Rated secondary current (Isn) 49
3.3.3 Rated primary symmetrical short-circuit current (Ipsc) 49
3.3.4 Secondary burden (Rb) 49
3.3.5 Specified duty cycle (C-O and C-O-C-O) 50
3.3.6 Secondary-loop time constant (Ts) 50
3.3.7 Rated symmetrical short-circuit current factor (Kssc) 50
3.3.8 Rated transient dimensioning factor (Ktd) 51
3.3.9 Flux overcurrent factor (nf) 51
3.3.10 Maximum instantaneous error 51
3.4 Air gapped cores 51
3.4.1 Remanent flux (Øs) 51
3.4.2 Remanence factor (Kr) 52
3.5 Accuracy classes for transient cores according to IEEE 60044-6 53
3.5.1 Error limits for TPX, TPY and TPZ current transformers 53
3.5.2 Error limits for TPS current transformers 54
3.6 How to specify current transformers for transient performance 54
4. How to specify voltage transformers 57
4.1 Type of voltage transformer 58
4.2 Rated insulation level 59
4.2.1 Rated insulation levels according to IEC 59
4.2.2 Basic insulation levels according to IEEE/ANSI 60
4.3 Rated primary and secondary voltage 61
4.4 Rated voltage factor 61
4.5 Burdens and accuracy classes 62
4.6 Pollution levels 65
4.7 Transient response for capacitor voltage transformers 65
4.8 Transient response for inductive voltage transformers 66
4.9 Ferroresonance 66
4.10 Ferroresonance in capacitor voltage transformers 66
4.11 Ferroresonance in inductive voltage transformers 67
4.12 Fuses 68
4.13 Voltage drops in the secondary circuits 68
4.14 Coupling capacitors 69
4.15 CVTs as coupling capacitors 70
5. Design of current transformers 71
5.1 General 72
5.2 Hair-pin type (Tank type) 74
5.3 Eye-bolt type 74
5.4 Top-core type 75
5.5 Mechanical stress on current transformers 76
5.5.1 Forces in the primary terminals 76
5.5.2 Wind stress 76
5.5.3 Seismic withstand 77
6. Design of inductive voltage transformers 79
6.1 Mechanical stress on inductive voltage transformers 80
7. Design of capacitor voltage transformers 81
7.1 External disturbances on capacitor voltage transformers 83
7.2 Mechanical stress on capacitor voltage transformers 84
7.3 Seismic properties of ABB’s capacitor voltage transformers 86

8. Instrument transformers in the system 87
8.1 Terminal designations for current transformers 88
8.2 Secondary grounding of current transformers 89
8.3 Secondary grounding of voltage transformers 91
8.4 Connection to obtain the residual voltage 92
8.5 Fusing of voltage transformer secondary circuits 94
8.6 Location of current and voltage transformers in substations 95
8.6.1 Location of current transformers 97
8.6.2 Transformer and reactor bushing current transformers 98
8.6.3 Physical order of cores in a current transformer 98
8.6.4 Location of voltage transformers 99
9. Protective relays 101
9.1 General 102
9.2 Current transformer requirements for CTs according to the IEC 60044-6
standard 104
9.2.1 Distance protection REL 501-531 104
9.2.2 Line differential protection REL 551 105
9.2.3 Line differential protection REL 561 106
9.2.4 Pilot-wire differential relay RADHL 106
9.2.5 Transformer protection RET 521 and transformer differential protection RADSB 107
9.2.6 Busbar protection RED 521 108
9.2.7 Overcurrent protection 109
9.2.8 High impedance differential protection RADHA 110
9.3 Current transformer requirements for CTs according to other standards 110
9.3.1 Current transformers according to IEC 60044-1 110
9.3.2 Current transformers according to British Standard (BS) 111
9.3.3 Current transformers according to ANSI/IEEE 111
10. Optical current and voltage transducers 113
10.1 DOIT — Technical description 114
10.2 MOCT-EOVT — Technical description 115
10.3 FOCT — Technical description 118
11. Index 121

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