IEC TR 61869-100:2017
(Main)Instrument transformers - Part 100: Guidance for application of current transformers in power system protection
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.
The contents of the corrigendum of October 2023 have been included in this copy.
General Information
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Standards Content (Sample)
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
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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
– 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 . 51
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
al
6.3 Summary . 58
7 Determination of the transient dimensioning factor K by numerical calculation . 61
td
7.1 General . 61
7.2 Basic circuit . 61
7.3 Algorithm . 63
7.4 Calculation method . 63
7.5 Reference examples . 64
8 Core saturation and remanence . 70
8.1 Saturation definition for common practice . 70
8.1.1 General . 70
8.1.2 Definition of the saturation flux in the preceding standard IEC 60044-1 . 70
8.1.3 Definition of the saturation flux in IEC 61869-2 . 72
8.1.4 Approach “5 % – Factor 5” . 73
8.2 Gapped cores versus non-gapped cores . 74
8.3 Possible causes of remanence . 76
9 Practical recommendations . 80
9.1 Accuracy hazard in case various PR class definitions for the same core . 80
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 . 80
s td
10 Relations between the various types of classes . 81
10.1 Overview. 81
10.2 Calculation of e.m.f. at limiting conditions . 81
10.3 Calculation of the exciting (or magnetizing) current at limiting conditions . 82
10.4 Examples . 82
10.5 Minimum requirements for class specification . 83
10.6 Replacing a non-gapped core by a gapped core . 83
11 Protection functions and correct CT specification . 84
11.1 General . 84
11.2 General application recommendations . 84
11.2.1 Protection functions and appropriate classes . 84
11.2.2 Correct CT designing in the past and today . 86
11.3 Overcurrent protection: ANSI code: (50/51/50N/51N/67/67N); IEC symbol: I> . 88
11.3.1 Exposition . 88
11.3.2 Recommendation . 90
11.3.3 Example . 90
11.4 Distance protection: ANSI codes: 21/21N, IEC code: Z< . 90
11.4.1 Exposition . 90
11.4.2 Recommendations . 92
11.4.3 Examples. 92
11.5 Differential protection . 99
11.5.1 Exposition . 99
11.5.2 General recommendations . 100
11.5.3 Transformer differential protection (87T) . 100
11.5.4 Busbar protection: Ansi codes (87B) . 105
11.5.5 Line differential protection: ANSI codes (87L) (Low impedance) . 108
11.5.6 High impedance differential protection . 111
Annex A (informative) Duty cycle C – O software code. 130
Annex B (informative) Software code for numerical calculation of K . 132
td
Bibliography . 137
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
ct
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
– 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
p,
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
tf
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 . 45
Figure 26 – Primary current for a short primary time constant . 45
Figure 27 – K values for a short primary time constant . 46
tf
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
tf
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
td
Figure 44 – K calculation for C-O cycle . 65
td
Figure 45 – K calculation for C-O-C-O cycle without core saturation in the first cycle . 66
td
Figure 46 – K calculation for C-O-C-O cycle considering core saturation in the first
td
cycle . 67
Figure 47 – K calculation for C-O-C-O cycle with reduced asymmetry . 68
td
Figure 48 – K calculation for C-O-C-O cycle with short t’ and t’’ . 69
td al al
Figure 49 – K calculation for C-O-C-O cycle for a non-gapped core . 70
td
Figure 50 – Comparison of the saturation definitions according to IEC 60044-1 and
according to IEC 61869-2 . 71
Figure 51 – Remanence factor K according to the previous definition IEC 60044-1 . 72
r
Figure 52 – Determination of saturation and remanence flux using the DC method for a
gapped core . 73
Figure 53 – Determination of saturation and remanence flux using DC method for a
non-gapped core . 73
Figure 54 – CT secondary currents as fault records of arc furnace transformer . 77
Figure 55 – 4-wire connection . 78
Figure 56 – CT secondary currents as fault records in the second fault of auto
reclosure . 79
Figure 57 – Application of instantaneous/time-delay overcurrent relay (ANSI codes
50/51) with definite time characteristic . 89
Figure 58 – Time-delay overcurrent relay, time characteristics . 89
Figure 59 – CT specification example, time overcurrent . 90
Figure 60 – Distance protection, principle (time distance diagram) . 91
Figure 61 – Distance protection, principle (R/X diagram) . 92
Figure 62 – CT Designing example, distance protection . 93
Figure 63 – Primary current with C-O-C-O duty cycle . 97
Figure 64 – Transient factor K with its envelope curve K . 97
tf tfp
Figure 65 – Transient factor K for CT class TPY with saturation in the first fault . 98
tf
Figure 66 – Transient factor K for CT class TPZ with saturation in the first fault . 98
tf
Figure 67 – Transient factor K for CT class TPX . 99
tf
Figure 68 – Differential protection, principle . 100
Figure 69 – Transformer differential protection, faults . 101
Figure 70 – Transformer differential protection . 102
Figure 71 – Busbar protection, external fault . 105
Figure 72 – Simulated currents of a current transformer for bus bar differential
protection . 108
Figure 73 – CT designing for a simple line with two ends . 109
Figure 74 – Differential protection realized with a simple electromechanical relay . 112
Figure 75 – High impedance protection principle . 113
Figure 76 – Phasor diagram for external faults . 114
Figure 77 – Phasor diagram for internal faults . 115
Figure 78 – Magnetizing curve of CT. 116
Figure 79 – Single-line diagram of busbar and high impedance differential protection . 120
Figure 80 – Currents at the fault location (primary values) . 122
Figure 81 – Primary currents through CTs, scaled to CT secondary side . 122
Figure 82 – CT secondary currents . 123
Figure 83 – Differential voltage . 123
Figure 84 – Differential current and r.m.s. filter signal . 124
Figure 85 – Currents at the fault location (primary values) . 124
Figure 86 – Primary currents through CTs, scaled to CT secondary side . 125
Figure 87 – CT secondary currents . 125
Figure 88 – Differential voltage . 126
Figure 89 – Differential current and r.m.s. filtered signal . 126
Figure 90 – Currents at the fault location (primary values) . 127
Figure 91 – Primary currents through CTs, scaled to CT secondary side . 127
– 6 – IEC TR 61869-100:2017 © IEC 2017
Figure 92 – CT secondary currents . 128
Figure 93 – Differential voltage . 128
Figure 94 – Differential current and r.m.s. filtered signal . 129
Figure 95 – Differential voltage without varistor limitation. 129
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 . 74
Table 4 – Measured remanence factors . 75
Table 5 – Various PR class definitions for the same core . 80
Table 6 – e.m.f. definitions . 81
Table 7 – Conversion of e.m.f. values . 81
Table 8 – Conversion of dimensioning factors . 82
Table 9 – Definitions of limiting current . 82
Table 10 – Minimum requirements for class specification . 83
Table 11 – Effect of gapped and non-gapped cores . 84
Table 12 – Application recommendations . 85
Table 13 – Calculation results of the overdimensioning of a TPY core . 104
Table 14 – Calculation results of overdimensioning as PX core . 104
Table 15 – Calculation scheme for line differential protection . 111
Table 16 – Busbar protection scheme with two incoming feeders . 119
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSTRUMENT TRANSFORMERS –
Part 100: Guidance for application of current
transformers in power system protection
FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
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data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 61869-100, which is a technical report, has been prepared by IEC technical
committee 38: Instrument transformers.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
38/469/DTR 38/475A/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
– 8 – IEC TR 61869-100:2017 © IEC 2017
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all the parts in the IEC 61869 series, published under the general title Instrument
transformers, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
The contents of the corrigendum 1 (2023-10) have been included in this copy.
INTRODUCTION
Since the publication of IEC 60044-6:1992 , Requirements for protective current transformers
for transient performance, the area of application of this kind of current transformers has been
extended. As a consequence, the theoretical background for the dimensioning according to
electrical requirements has become much more complex. For IEC 61869-2 to remain as user-
friendly as possible, the explanation of the background information has been transferred to
this part of IEC 61869.
___________
Withdrawn and replaced by IEC 61869-2:2012.
– 10 – IEC TR 61869-100:2017 © IEC 2017
INSTRUMENT TRANSFORMERS –
Part 100: Guidance for application of current
transformers in power system protection
1 Scope
This part of IEC 61869 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.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60255 (all parts), Measuring relays and protection equipment
IEC 60909-0:2016, Short circuit currents in three-phase a.c. systems – Calculation of currents
IEC 61869-1:2007, Instrument transformers – General requirements
IEC 61869-2:2012, Instrument transformers – Additional requirements for current transformers
3 Terms and definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61869-1:2007 and
IEC 61869-2:2012 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
rated primary short circuit current
I
psc
r.m.s. value of the a.c. component of a transient primary short-circuit current on which the
accuracy performance of a current transformer is based
[SOURCE: IEC 61869-2:2012, 3.3.206]
3.1.2
rated short-time thermal current
I
th
maximum value of the primary current which a transformer will withstand for a specified short
time without suffering harmful effects, the secondary winding being short-circuited
[SOURCE: IEC 60050-321:1986, 321-02-22; IEC 61869-2:2012, 3.3.203]
3.1.3
initial symmetrical short circuit current
I”
k
r.m.s. value of the a.c. symmetrical component of a prospective (available) short-circuit
current, applicable at the instant of short circuit if the impedance remains at zero-time value
[SOURCE: IEC 60909-0:2001, 1.3.5]
Note 1 to entry: While I is a basic parameter of a plant and of its components, I is an accuracy requirement,
th psc
and has a determining influence on the saturation behaviour of a current transformer. The protection system will
ensure tripping at a current I” , which is usually lower than I . Depending on the protection requirement, a current
k th
transformer may saturate much before reaching I” . Therefore, in certain cases, I may be much lower than I” .
k psc k
3.1.4
primary current
I
p
current flowing through the primary winding of a current transformer
3.1.5
secondary current
I
s
current flowing through the secondary winding of a current transformer
3.1.6
angular frequency
ω
angular frequency of the primary current
3.1.7
time
t
time
3.1.8
phase angle of the system short circuit impedance
φ
phase angle of the system short circuit impedance
3.1.9
fault inception angle
γ
inception angle of the primary short circuit, being 180° at voltage maximum (see Figure 1)
– 12 – IEC TR 61869-100:2017 © IEC 2017
Key
u primary voltage
γ fault inception angle
Figure 1 – Definition of the fault inception angle γ
3.1.10
minimum fault inception angle
γ
m
lowest value of fault inception angle γ to be considered in the design of a current transformer
3.1.11
alternative definition of fault inception angle
θ
inception angle of the primary short circuit, defined as γ – φ
3.2 Index of abbreviations
This table comprises Table 3.7 of IEC 61869-2:2012, complemented with the terms and
definitions given in 3.1.1 to 3.1.11.
AIS Air-Insulated Switchgear
ALF Accuracy limit factor
CT Current Transformer
CVT Capacitive Voltage Transformer
E rated equivalent limiting secondary e.m.f.
al
E secondary limiting e.m.f. for class P and PR protective current transformers
ALF
E secondary limiting e.m.f. for measuring current transformers
FS
E rated knee point e.m.f.
k
f frequency
F mechanical load
F factor of construction
c
f rated frequency
R
F relative leakage rate
rel
FS instrument security factor
GIS Gas-Insulated Switchgear
I” Initial symmetrical short circuit current
k
Î peak value of the exciting secondary current at E
al al
I rated continuous thermal current
cth
I rated dynamic current
dyn
I exciting current
e
I rated instrument limit primary current
PL
I primary current
p
I rated primary current
pr
I rated primary short circuit current
psc
I secondary current
s
I rated secondary current
sr
IT Instrument Transformer
I rated short-time thermal current
th
i instantaneous error current
ε
k actual transformation ratio
k rated transformation ratio
r
K remanence factor
R
K rated symmetrical short circuit current factor
ssc
K transient dimensioning factor
td
K transient factor
tf
K dimensioning factor
x
L non-linear inductance of a current transformer
ct
L linearized magnetizing inductance of a current transformer
m
L1,L2,L3 designation of the phases in the electrical three-phase system
n number of secondary turns
s
R rated resistive burden
b
R secondary winding resistance
ct
R secondary loop resistance
s
S rated output
r
t time
t’ duration of the first fault
t’’ duration of the second fault
t’ specified time to accuracy limit in the first fault
al
t’’ specified time to accuracy limit in the second fault
al
t fault repetition time
fr
T specified primary time constant
p
T secondary loop time constant
s
U highest voltage for equipment
m
U highest voltage for system
sys
VT Voltage Transformer
Δφ phase displacement
ε ratio error
ε composite error
c
peak value of instananeous error
ˆ
ε
– 14 – IEC TR 61869-100:2017 © IEC 2017
peak value of alternating error component
ˆ
ε
ac
φ phase angle of the system short circuit impedance
γ fault inception angle
γ minimum fault inception angle
m
θ alternative definition of fault inception angle
ψ secondary linked magnetic flux in the current transformer core
ψ remanent flux
r
ψ saturation flux
sat
angular frequency
ω
4 Responsibilities in the current transformer design process
4.1 History
The IEC 60044-6 standard “Instrument transformers – Part 6: Requirements for protective
current transformers for transient performance" was introduced in 1992 (at that time as
IEC 44-6). It was the first standard that considered the transient performance of protective
CT.
In the well-known P-class, the usually indispensable over-dimensioning due to the primary DC
component has to be “hidden” in the accuracy limit factor or in the burden.
The definition of the new classes TPX, TPY, and TPZ was strongly “cycle-oriented”, defining
all necessary parameters of a C-O and of a C-O-C-O cycle. These classes allowed the
shifting of responsibility for the calculation of the over-dimensioning due to the primary DC
component (represented by the “transient factor” K , nowadays called “transient dimensioning
td
factor”) to the CT manufacturer.
The transient performance classes never became widely accepted by different reasons:
– Their specification by duty cycles (and time to accuracy limit if necessary) is much more
complex than the conventional classes 5P and 10P, which were originally foreseen for
electromechanical relays.
– The duty cycle definition does no longer reflect the actual criteria for defining the
overdimensioning factors.
Nowadays, it is common practice that the relay developers stipulate the required
overdimensioning of the protection current transformers, taking into account the wave form of
the primary signal as well as their own protection requirements.
When IEC 60044-6 was integrated in IEC 61869-2, it was taken into account that the cycle
definition plays a declining role, as the aspects explained above have to be considered.
Therefore, the definition of transient performance was extended by allowing the direct
definition of the transient dimensioning factor K instead of the cycle parameters. This new
td
way of specification is easy and similar to that of the well-known P-classes. One intention of
this technical report is to explain such possible alternative specifications for several critical
applications.
4.2 Subdivision of the current transformer design process
In modern digital relays, the decision-making time has decreased continuously as a result of
increasing sampling rates and by refining the protection algorithms. As a consequence, the
required saturation-free time of the current transformers has been reduced correspondingly.
This leads to smaller CT cores, what is in line with the development of modern compact gas-
insulated switchgears. Furthermore, some of the algorithms apply Fourier and r.m.s. filtering
of the dynamic CT current signal. As a consequence, the relays react more ‘smoothly’ to high
currents with possible saturation. For these reasons, the required CT performance and final
core size cannot be expressed solely by the initial saturation time and one simple analytical
formula.
Therefore, it is recommended that the relay developers first use analytical formulae for an
initial consideration. In a second step, they may test their algorithms with a simulation model
of the CT core and publish the requirements in terms of simple parameters (e.g. over-
dimensioning factor, transient dimensioning factor, etc.) for significant worst case fault
scenarios in the protection relay manual (or other documents). During the project engineering
phase, the published requirements are applied to concrete cases and the hereby verified
class parameters are communicated to the CT manufacturer.
For backward compatibility to IEC 60044-6, the transient factor K – analytically calculated
tf
from parameters such as time to accuracy limit t’ , duty cycle, etc. – may be applied directly
al
as transient dimensioning factor K without further protection relay test. This way is not
td
recommended, but may be chosen in situations where the design responsibility is not
delegated entirely to the relay manufacturer.
The procedures used to determine the transient dimensioning factor K on the basis of
td
protection rules can be considered to be the specific know-how of relay specialists, and are
not part of this technical report.
In a complementary manner, this report highlights the path from the cycle definition, which still
has certain significance, to the definition of a transient dimensioning factor. The analytical
formulae in this technical report are ext
...
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
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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
– 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
al
6.3 Summary . 58
7 Determination of the transient dimensioning factor K by numerical calculation . 61
td
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
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
td
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
ct
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
– 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
p,
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
tf
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
tf
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
tf
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
td
Figure 44 – K calculation for C-O cycle . 64
td
Figure 45 – K calculation for C-O-C-O cycle without core saturation in the first cycle . 65
td
Figure 46 – K calculation for C-O-C-O cycle considering core saturation in the first
td
cycle . 66
Figure 47 – K calculation for C-O-C-O cycle with reduced asymmetry . 67
td
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
td
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
r
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
tf
Figure 66 – Transient factor K for CT class TPZ with saturation in the first fault . 97
tf
Figure 67 – Transient factor K for CT class TPX . 98
tf
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
– 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
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSTRUMENT TRANSFORMERS –
Part 100: Guidance for application of current
transformers in power system protection
FOREWORD
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 61869-100, which is a technical report, has been prepared by IEC technical
committee 38: Instrument transformers.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
38/469/DTR 38/475A/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
– 8 – IEC TR 61869-100:2017 © IEC 2017
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all the parts in the IEC 61869 series, published under the general title Instrument
transformers, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
INTRODUCTION
Since the publication of IEC 60044-6:1992 , Requirements for protective current transformers
for transient performance, the area of application of this kind of current transformers has been
extended. As a consequence, the theoretical background for the dimensioning according to
electrical requirements has become much more complex. For IEC 61869-2 to remain as user-
friendly as possible, the explanation of the background information has been transferred to
this part of IEC 61869.
___________
Withdrawn and replaced by IEC 61869-2:2012.
– 10 – IEC TR 61869-100:2017 © IEC 2017
INSTRUMENT TRANSFORMERS –
Part 100: Guidance for application of current
transformers in power system protection
1 Scope
This part of IEC 61869 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.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60255 (all parts), Measuring relays and protection equipment
IEC 60909-0:2016, Short circuit currents in three-phase a.c. systems – Calculation of currents
IEC 61869-1:2007, Instrument transformers – General requirements
IEC 61869-2:2012, Instrument transformers – Additional requirements for current transformers
3 Terms and definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61869-1:2007 and
IEC 61869-2:2012 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
rated primary short circuit current
I
psc
r.m.s. value of the a.c. component of a transient primary short-circuit current on which the
accuracy performance of a current transformer is based
[SOURCE: IEC 61869-2:2012, 3.3.206]
3.1.2
rated short-time thermal current
I
th
maximum value of the primary current which a transformer will withstand for a specified short
time without suffering harmful effects, the secondary winding being short-circuited
[SOURCE: IEC 60050-321:1986, 321-02-22; IEC 61869-2:2012, 3.3.203]
3.1.3
initial symmetrical short circuit current
I”
k
r.m.s. value of the a.c. symmetrical component of a prospective (available) short-circuit
current, applicable at the instant of short circuit if the impedance remains at zero-time value
[SOURCE: IEC 60909-0:2001, 1.3.5]
Note 1 to entry: While I is a basic parameter of a plant and of its components, I is an accuracy requirement,
th psc
and has a determining influence on the saturation behaviour of a current transformer. The protection system will
ensure tripping at a current I” , which is usually lower than I . Depending on the protection requirement, a current
k th
transformer may saturate much before reaching I” . Therefore, in certain cases, I may be much lower than I” .
k psc k
3.1.4
primary current
I
p
current flowing through the primary winding of a current transformer
3.1.5
secondary current
I
s
current flowing through the secondary winding of a current transformer
3.1.6
angular frequency
ω
angular frequency of the primary current
3.1.7
time
t
time
3.1.8
phase angle of the system short circuit impedance
φ
phase angle of the system short circuit impedance
3.1.9
fault inception angle
γ
inception angle of the primary short circuit, being 180° at voltage maximum (see Figure 1)
– 12 – IEC TR 61869-100:2017 © IEC 2017
Key
u primary voltage
γ fault inception angle
Figure 1 – Definition of the fault inception angle γ
3.1.10
minimum fault inception angle
γ
m
lowest value of fault inception angle γ to be considered in the design of a current transformer
3.1.11
alternative definition of fault inception angle
θ
inception angle of the primary short circuit, defined as γ – φ
3.2 Index of abbreviations
This table comprises Table 3.7 of IEC 61869-2:2012, complemented with the terms and
definitions given in 3.1.1 to 3.1.11.
AIS Air-Insulated Switchgear
ALF Accuracy limit factor
CT Current Transformer
CVT Capacitive Voltage Transformer
E rated equivalent limiting secondary e.m.f.
al
E secondary limiting e.m.f. for class P and PR protective current transformers
ALF
E secondary limiting e.m.f. for measuring current transformers
FS
E rated knee point e.m.f.
k
f frequency
F mechanical load
F factor of construction
c
f rated frequency
R
F relative leakage rate
rel
FS instrument security factor
GIS Gas-Insulated Switchgear
I” Initial symmetrical short circuit current
k
Î peak value of the exciting secondary current at E
al al
I rated continuous thermal current
cth
I rated dynamic current
dyn
I exciting current
e
I rated instrument limit primary current
PL
I primary current
p
I rated primary current
pr
I rated primary short circuit current
psc
I secondary current
s
I rated secondary current
sr
IT Instrument Transformer
I rated short-time thermal current
th
i instantaneous error current
ε
k actual transformation ratio
k rated transformation ratio
r
K remanence factor
R
K rated symmetrical short circuit current factor
ssc
K transient dimensioning factor
td
K transient factor
tf
K dimensioning factor
x
L non-linear inductance of a current transformer
ct
L linearized magnetizing inductance of a current transformer
m
L1,L2,L3 designation of the phases in the electrical three-phase system
n number of secondary turns
s
R rated resistive burden
b
R secondary winding resistance
ct
R secondary loop resistance
s
S rated output
r
t time
t’ duration of the first fault
t’’ duration of the second fault
t’ specified time to accuracy limit in the first fault
al
t’’ specified time to accuracy limit in the second fault
al
t fault repetition time
fr
T specified primary time constant
p
T secondary loop time constant
s
U highest voltage for equipment
m
U highest voltage for system
sys
VT Voltage Transformer
Δφ phase displacement
ε ratio error
ε composite error
c
peak value of instananeous error
εˆ
– 14 – IEC TR 61869-100:2017 © IEC 2017
peak value of alternating error component
ˆ
ε
ac
φ phase angle of the system short circuit impedance
γ fault inception angle
γ minimum fault inception angle
m
θ alternative definition of fault inception angle
ψ secondary linked magnetic flux in the current transformer core
ψ remanent flux
r
ψ saturation flux
sat
angular frequency
ω
4 Responsibilities in the current transformer design process
4.1 History
The IEC 60044-6 standard “Instrument transformers – Part 6: Requirements for protective
current transformers for transient performance" was introduced in 1992 (at that time as
IEC 44-6). It was the first standard that considered the transient performance of protective
CT.
In the well-known P-class, the usually indispensable over-dimensioning due to the primary DC
component has to be “hidden” in the accuracy limit factor or in the burden.
The definition of the new classes TPX, TPY, and TPZ was strongly “cycle-oriented”, defining
all necessary parameters of a C-O and of a C-O-C-O cycle. These classes allowed the
shifting of responsibility for the calculation of the over-dimensioning due to the primary DC
component (represented by the “transient factor” K , nowadays called “transient dimensioning
td
factor”) to the CT manufacturer.
The transient performance classes never became widely accepted by different reasons:
– Their specification by duty cycles (and time to accuracy limit if necessary) is much more
complex than the conventional classes 5P and 10P, which were originally foreseen for
electromechanical relays.
– The duty cycle definition does no longer reflect the actual criteria for defining the
overdimensioning factors.
Nowadays, it is common practice that the relay developers stipulate the required
overdimensioning of the protection current transformers, taking into account the wave form of
the primary signal as well as their own protection requirements.
When IEC 60044-6 was integrated in IEC 61869-2, it was taken into account that the cycle
definition plays a declining role, as the aspects explained above have to be considered.
Therefore, the definition of transient performance was extended by allowing the direct
definition of the transient dimensioning factor K instead of the cycle parameters. This new
td
way of specification is easy and similar to that of the well-known P-classes. One intention of
this technical report is to explain such possible alternative specifications for several critical
applications.
4.2 Subdivision of the current transformer design process
In modern digital relays, the decision-making time has decreased continuously as a result of
increasing sampling rates and by refining the protection algorithms. As a consequence, the
required saturation-free time of the current transformers has been reduced correspondingly.
This leads to smaller CT cores, what is in line with the development of modern compact gas-
insulated switchgears. Furthermore, some of the algorithms apply Fourier and r.m.s. filtering
of the dynamic CT current signal. As a consequence, the relays react more ‘smoothly’ to high
currents with possible saturation. For these reasons, the required CT performance and final
core size cannot be expressed solely by the initial saturation time and one simple analytical
formula.
Therefore, it is recommended that the relay developers first use analytical formulae for an
initial consideration. In a second step, they may test their algorithms with a simulation model
of the CT core and publish the requirements in terms of simple parameters (e.g. over-
dimensioning factor, transient dimensioning factor, etc.) for significant worst case fault
scenarios in the protection relay manual (or other documents). During the project engineering
phase, the published requirements are applied to concrete cases and the hereby verified
class parameters are communicated to the CT manufacturer.
For backward compatibility to IEC 60044-6, the transient factor K – analytically calculated
tf
from parameters such as time to accuracy limit t’ , duty cycle, etc. – may be applied directly
al
as transient dimensioning factor K without further protection relay test. This way is not
td
recommended, but may be chosen in situations where the design responsibility is not
delegated entirely to the relay manufacturer.
The procedures used to determine the transient dimensioning factor K on the basis of
td
protection rules can be considered to be the specific know-how of relay specialists, and are
not part of this technical report.
In a complementary manner, this report highlights the path from the cycle definition, which still
has certain significance, to the definition of a transient dimensioning factor. The analytical
formulae in this technical report are extended and d
...
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