Instrument transformers - Part 100: Guidance for application of current transformers in power system protection

IEC TR 61869-100:2017(E) is applicable to inductive protective current transformers meeting the requirements of the IEC 61869-2 standard.
It may help relay manufacturers, CT manufacturers and project engineers to understand how a CT responds to simplified or standardized short circuit signals. Therefore, it supplies advanced information to comprehend the definition of inductive current transformers as well as their requirements.
The document aims to provide information for the casual user as well as for the specialist. Where necessary, the level of abstraction is mentioned in the document. It also discusses the question about the responsibilities in the design process for current transformers.

General Information

Status
Published
Publication Date
19-Jan-2017
Current Stage
PPUB - Publication issued
Completion Date
20-Jan-2017
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IEC TR 61869-100
Edition 1.0 2017-01
TECHNICAL
REPORT
colour
inside
Instrument transformers –
Part 100: Guidance for application of current transformers in power system
protection
IEC TR 61869-100:2017-01(en)
---------------------- Page: 1 ----------------------
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IEC TR 61869-100
Edition 1.0 2017-01
TECHNICAL
REPORT
colour
inside
Instrument transformers –
Part 100: Guidance for application of current transformers in power system
protection
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.220.20 ISBN 978-2-8322-3808-0

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 61869-100:2017 © IEC 2017
CONTENTS

FOREWORD ........................................................................................................................... 7

INTRODUCTION ..................................................................................................................... 9

1 Scope ............................................................................................................................ 10

2 Normative references .................................................................................................... 10

3 Terms and definitions and abbreviations ........................................................................ 10

3.1 Terms and definitions ............................................................................................ 10

3.2 Index of abbreviations ........................................................................................... 12

4 Responsibilities in the current transformer design process............................................. 14

4.1 History .................................................................................................................. 14

4.2 Subdivision of the current transformer design process .......................................... 14

5 Basic theoretical equations for transient designing ........................................................ 15

5.1 Electrical circuit .................................................................................................... 15

5.1.1 General ......................................................................................................... 15

5.1.2 Current transformer ....................................................................................... 18

5.2 Transient behaviour .............................................................................................. 20

5.2.1 General ......................................................................................................... 20

5.2.2 Fault inception angle ..................................................................................... 22

5.2.3 Differential equation ...................................................................................... 23

6 Duty cycles .................................................................................................................... 25

6.1 Duty cycle C – O ................................................................................................... 25

6.1.1 General ......................................................................................................... 25

6.1.2 Fault inception angle ..................................................................................... 27

6.1.3 Transient factor K and transient dimensioning factor K ............................. 28

tf td
6.1.4 Reduction of asymmetry by definition of the minimum current inception

angle ............................................................................................................. 50

6.2 Duty cycle C – O – C – O ...................................................................................... 53

6.2.1 General ......................................................................................................... 53

6.2.2 Case A:No saturation occurs until t’ ............................................................... 54

6.2.3 Case B:Saturation occurs between t’ and t’ ................................................. 56

6.3 Summary .............................................................................................................. 58

7 Determination of the transient dimensioning factor K by numerical calculation ............ 61

7.1 General ................................................................................................................. 61

7.2 Basic circuit .......................................................................................................... 61

7.3 Algorithm .............................................................................................................. 62

7.4 Calculation method ............................................................................................... 63

7.5 Reference examples ............................................................................................. 64

8 Core saturation and remanence ..................................................................................... 69

8.1 Saturation definition for common practice ............................................................. 69

8.1.1 General ......................................................................................................... 69

8.1.2 Definition of the saturation flux in the preceding standard IEC 60044-1 ........ 69

8.1.3 Definition of the saturation flux in IEC 61869-2 .............................................. 71

8.1.4 Approach “5 % – Factor 5” ............................................................................. 72

8.2 Gapped cores versus non-gapped cores ............................................................... 73

8.3 Possible causes of remanence .............................................................................. 75

9 Practical recommendations ............................................................................................ 79

9.1 Accuracy hazard in case various PR class definitions for the same core ............... 79

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IEC TR 61869-100:2017 © IEC 2017 – 3 –
9.2 Limitation of the phase displacement ∆ϕ and of the secondary loop time

constant T by the transient dimensioning factor K for TPY cores ...................... 79

s td

10 Relations between the various types of classes ............................................................. 80

10.1 Overview............................................................................................................... 80

10.2 Calculation of e.m.f. at limiting conditions ............................................................. 80

10.3 Calculation of the exciting (or magnetizing) current at limiting conditions .............. 81

10.4 Examples .............................................................................................................. 81

10.5 Minimum requirements for class specification ....................................................... 82

10.6 Replacing a non-gapped core by a gapped core .................................................... 82

11 Protection functions and correct CT specification .......................................................... 83

11.1 General ................................................................................................................. 83

11.2 General application recommendations .................................................................. 83

11.2.1 Protection functions and appropriate classes ................................................. 83

11.2.2 Correct CT designing in the past and today ................................................... 85

11.3 Overcurrent protection: ANSI code: (50/51/50N/51N/67/67N); IEC symbol: I> ....... 87

11.3.1 Exposition ...................................................................................................... 87

11.3.2 Recommendation ........................................................................................... 89

11.3.3 Example ........................................................................................................ 89

11.4 Distance protection: ANSI codes: 21/21N, IEC code: Z< ...................................... 89

11.4.1 Exposition ...................................................................................................... 89

11.4.2 Recommendations ......................................................................................... 91

11.4.3 Examples....................................................................................................... 91

11.5 Differential protection ............................................................................................ 98

11.5.1 Exposition ...................................................................................................... 98

11.5.2 General recommendations ............................................................................. 99

11.5.3 Transformer differential protection (87T) ........................................................ 99

11.5.4 Busbar protection: Ansi codes (87B) ............................................................ 104

11.5.5 Line differential protection: ANSI codes (87L) (Low impedance) .................. 107

11.5.6 High impedance differential protection ......................................................... 109

Annex A (informative) Duty cycle C – O software code....................................................... 128

Annex B (informative) Software code for numerical calculation of K ................................ 130

Bibliography ........................................................................................................................ 135

Figure 1 – Definition of the fault inception angle γ ................................................................. 12

Figure 2 – Components of protection circuit .......................................................................... 16

Figure 3 – Entire electrical circuit .......................................................................................... 17

Figure 4 – Primary short circuit current ................................................................................. 18

Figure 5 – Non-linear flux of L ............................................................................................ 19

Figure 6 – Linearized magnetizing inductance of a current transformer ................................. 20

Figure 7 – Simulated short circuit behaviour with non-linear model ....................................... 21

Figure 8 – Three-phase short circuit behaviour ..................................................................... 23

Figure 9 – Composition of flux .............................................................................................. 24

Figure 10 – Short circuit current for two different fault inception angles ................................ 26

Figure 11 – ψ as the curve of the highest flux values ...................................................... 26

max

Figure 12 – Primary current curves for the 4 cases for 50 Hz and ϕ = 70° ............................. 27

Figure 13 – Four significant cases of short circuit currents with impact on magnetic

saturation of current transformers ......................................................................................... 28

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– 4 – IEC TR 61869-100:2017 © IEC 2017

Figure 14 – Relevant time ranges for calculation of transient factor ...................................... 31

Figure 15 – Occurrence of the first flux peak depending on T at 50 Hz ............................... 32

Figure 16 – Worst-case angle θ as function of T and t’ ......................................... 33

tf,ψmax p al

Figure 17 – Worst-case fault inception angle γ as function of T and t’ ................... 34

tf,ψmax p al

Figure 18 – K calculated with worst-case fault inception angle θ ...................... 34

tf,ψmax ψmax

Figure 19 – Polar diagram with K and γ ......................................................... 35

tf,ψmax tf,ψmax

Figure 20 – Determination of K in time range 1 ................................................................... 40

Figure 21 – Primary current curves for 50Hz, T = 1 ms, γ = 166° for t’ = 2 ms ......... 41

p ψmax al

Figure 22 – worst-case fault inception angles for 50Hz, T = 50 ms and T = 61 ms ............. 42

p s

Figure 23 – transient factor for different time ranges ............................................................. 43

Figure 24 – K in all time ranges for T = 61 ms at 50 Hz with t’ as parameter .................. 44

tf s al

Figure 25 – Zoom of Figure 24 .............................................................................................. 44

Figure 26 – Primary current for a short primary time constant ............................................... 45

Figure 27 – K values for a short primary time constant ........................................................ 46

Figure 28 – Short circuit currents for various fault inception angles ....................................... 47

Figure 29 – Transient factors for various fault inception angles (example) ............................ 48

Figure 30 – Worst-case fault inception angles for each time step (example for 50 Hz) .......... 48

Figure 31 – Primary current for two different fault inception angles (example for

16,67 Hz) .............................................................................................................................. 49

Figure 32 – Transient factors for various fault inception angles (example for 16,67 Hz) ........ 50

Figure 33 – Worst-case fault inception angles for every time step (example for

16,67 Hz) .............................................................................................................................. 50

Figure 34 – Fault occurrence according to Warrington .......................................................... 51

Figure 35 – estimated distribution of faults over several years .............................................. 52

Figure 36 – Transient factor K calculated with various fault inception angles γ .................... 53

Figure 37 – Flux course in a C-O-C-O cycle of a non-gapped core ........................................ 54

Figure 38 – Typical flux curve in a C-O-C-O cycle of a gapped core, with higher flux in

the second energization ........................................................................................................ 55

Figure 39 – Flux curve in a C-O-C-O cycle of a gapped core, with higher flux in the

first energization ................................................................................................................... 56

Figure 40 – Flux curve in a C-O-C-O cycle with saturation allowed ....................................... 57

Figure 41 – Core saturation used to reduce the peak flux value ............................................ 58

Figure 42 – Curves overview for transient designing ............................................................. 59

Figure 43 – Basic circuit diagram for numerical calculation of K ......................................... 62

Figure 44 – K calculation for C-O cycle .............................................................................. 64

Figure 45 – K calculation for C-O-C-O cycle without core saturation in the first cycle ......... 65

Figure 46 – K calculation for C-O-C-O cycle considering core saturation in the first

cycle ..................................................................................................................................... 66

Figure 47 – K calculation for C-O-C-O cycle with reduced asymmetry ................................ 67

Figure 48 – K calculation for C-O-C-O cycle with short t’ and t’’ .................................... 68

td al al

Figure 49 – K calculation for C-O-C-O cycle for a non-gapped core ................................... 69

Figure 50 – Comparison of the saturation definitions according to IEC 60044-1 and

according to IEC 61869-2 ..................................................................................................... 70

Figure 51 – Remanence factor K according to the previous definition IEC 60044-1 .............. 71

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IEC TR 61869-100:2017 © IEC 2017 – 5 –

Figure 52 – Determination of saturation and remanence flux using the DC method for a

gapped core .......................................................................................................................... 72

Figure 53 – Determination of saturation and remanence flux using DC method for a

non-gapped core ................................................................................................................... 72

Figure 54 – CT secondary currents as fault records of arc furnace transformer ..................... 76

Figure 55 – 4-wire connection ............................................................................................... 77

Figure 56 – CT secondary currents as fault records in the second fault of auto

reclosure .............................................................................................................................. 78

Figure 57 – Application of instantaneous/time-delay overcurrent relay (ANSI codes

50/51) with definite time characteristic .................................................................................. 88

Figure 58 – Time-delay overcurrent relay, time characteristics .............................................. 88

Figure 59 – CT specification example, time overcurrent ........................................................ 89

Figure 60 – Distance protection, principle (time distance diagram) ........................................ 90

Figure 61 – Distance protection, principle (R/X diagram) ....................................................... 91

Figure 62 – CT Designing example, distance protection ........................................................ 92

Figure 63 – Primary current with C-O-C-O duty cycle ............................................................ 96

Figure 64 – Transient factor K with its envelope curve K ................................................. 96

tf tfp

Figure 65 – Transient factor K for CT class TPY with saturation in the first fault ................. 97

Figure 66 – Transient factor K for CT class TPZ with saturation in the first fault .................. 97

Figure 67 – Transient factor K for CT class TPX ................................................................. 98

Figure 68 – Differential protection, principle .......................................................................... 99

Figure 69 – Transformer differential protection, faults ......................................................... 100

Figure 70 – Transformer differential protection .................................................................... 101

Figure 71 – Busbar protection, external fault ....................................................................... 104

Figure 72 – Simulated currents of a current transformer for bus bar differential

protection ........................................................................................................................... 107

Figure 73 – CT designing for a simple line with two ends .................................................... 108

Figure 74 – Differential protection realized with a simple electromechanical relay ............... 110

Figure 75 – High impedance protection principle ................................................................. 111

Figure 76 – Phasor diagram for external faults .................................................................... 112

Figure 77 – Phasor diagram for internal faults ..................................................................... 113

Figure 78 – Magnetizing curve of CT................................................................................... 114

Figure 79 – Single-line diagram of busbar and high impedance differential protection ......... 117

Figure 80 – Currents at the fault location (primary values) .................................................. 119

Figure 81 – Primary currents through CTs, scaled to CT secondary side ............................. 120

Figure 82 – CT secondary currents ..................................................................................... 120

Figure 83 – Differential voltage ........................................................................................... 121

Figure 84 – Differential current and r.m.s. filter signal ......................................................... 121

Figure 85 – Currents at the fault location (primary values) .................................................. 122

Figure 86 – Primary currents through CTs, scaled to CT secondary side ............................. 122

Figure 87 – CT secondary currents ..................................................................................... 123

Figure 88 – Differential voltage ........................................................................................... 123

Figure 89 – Differential current and r.m.s. filtered signal ..................................................... 124

Figure 90 – Currents at the fault location (primary values) .................................................. 124

Figure 91 – Primary currents through CTs, scaled to CT secondary side ............................. 125

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– 6 – IEC TR 61869-100:2017 © IEC 2017

Figure 92 – CT secondary currents ..................................................................................... 125

Figure 93 – Differential voltage ........................................................................................... 126

Figure 94 – Differential current and r.m.s. filtered signal ..................................................... 126

Figure 95 – Differential voltage without varistor limitation.................................................... 127

Table 1 – Four significant cases of short circuit current inception angles .............................. 27

Table 2 – Equation overview for transient designing ............................................................. 60

Table 3 – Comparison of saturation point definitions ............................................................. 73

Table 4 – Measured remanence factors ................................................................................ 74

Table 5 – Various PR class definitions for the same core ...................................................... 79

Table 6 – e.m.f. definitions .................................................................................................... 80

Table 7 – Conversion of e.m.f. values ................................................................................... 80

Table 8 – Conversion of dimensioning factors ....................................................................... 81

Table 9 – Definitions of limiting current ................................................................................. 81

Table 10 – Minimum requirements for class specification ...................................................... 82

Table 11 – Effect of gapped and non-gapped cores .............................................................. 83

Table 12 – Application recommendations .............................................................................. 84

Table 13 – Calculation results of the overdimensioning of a TPY core ................................ 103

Table 14 – Calculation results of overdimensioning as PX core ........................................... 103

Table 15 – Calculation scheme for line differential protection .............................................. 109

Table 16 – Busbar protection scheme with two incoming feeders ........................................ 117

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IEC TR 61869-100:2017 © IEC 2017 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSTRUMENT TRANSFORMERS –
Part 100: Guidance for application of current
transformers in power system protection
FOREWORD

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