EN IEC 60255-187-1:2021

Measuring relays and protection equipment - Part 187-1: Functional requirements for differential protection - Restrained and unrestrained differential protection of motors, generators and transformers

IEC 60255-187-1:2021 specifies the minimum requirements for functional and performance evaluation of (longitudinal) differential protection designed for the detection of faults in ac motors, generators and transformers. This document also defines how to document and publish performance test results. This document defines the influencing factors that affect the accuracy under steady state conditions and performance characteristics during dynamic conditions. The test methodologies for verifying performance characteristics and accuracy are also included in this document. This document, together with IEC 60255-187-2 and IEC 60255-187-3, cancels and replaces IEC 60255-13. This document includes the following significant technical changes with respect to IEC 60255-13: a) IEC 60255-13 has been significantly revised to follow the common structure of the functional standards for protection relays (IEC 60255-1xx series). IEC 60255-187-1 has been developed to address the restrained and unrestrained differential protection of motors, generators and transformers. The revisions include detailed description of the functions including the performance specification, testing and documentation requirements.

Messrelais und Schutzeinrichtungen - Teil 187-1: Funktionsanforderungen für den stabilisierten und nicht stabilisierten Differentialschutz von Motoren, Generatoren und Transformatoren

Relais de mesure et dispositifs de protection - Partie 187-1: Exigences fonctionnelles pour la protection différentielle - Protection différentielle avec et sans caractéristique de retenue des moteurs, générateurs et transformateurs

IEC 60255-187-1:2021 spécifie les exigences minimales relatives à l'évaluation fonctionnelle et à l’évaluation de la performance de la protection différentielle (longitudinale) conçue pour la détection des défauts dans les moteurs à courant alternatif, les générateurs et les transformateurs. Le présent document définit également comment documenter et publier les résultats des essais de performance.Le présent document définit les facteurs d'influence qui affectent la précision en état stable et les caractéristiques de performance dans des conditions dynamiques. Les méthodologies d’essai permettant de vérifier les caractéristiques de performance et la précision sont également incluses dans le présent document. Le présent document, ainsi que l’IEC 60255-187-2 et l’IEC 60255-187-3, annule et remplace l’IEC 60255-13. Le présent document inclut les modifications techniques majeures suivantes par rapport à l'IEC 60255‑13: a. L’IEC 60255-13 a été considérablement révisée pour suivre la structure commune des normes fonctionnelles pour les relais de protection (série IEC 60255-1xx). L’IEC 60255-187-1 a été développée pour traiter la protection différentielle avec et sans caractéristique de retenue des moteurs, générateurs et transformateurs. Les révisions comprennent une description détaillée des fonctions, y compris les spécifications de performance, les essais et les exigences en matière de documentation.

Merilni releji in zaščitna oprema - 187-1. del: Funkcijske zahteve za diferenčno zaščito - Omejena in neomejena diferenčna zaščita motorjev, generatorjev in transformatorjev

General Information

• Status: Published
• Publication Date: 16-Sep-2021
• ICS: 29.120.70 - Relays
• Technical Committee: CLC/TC 95X - Measuring relays and protection equipment
• Drafting Committee: IEC/TC 95 - IEC_TC_95
• Current Stage: 6060 - Document made available - Publishing
• Start Date: 17-Sep-2021
• Completion Date: 17-Sep-2021
• Ref Project: SIST EN IEC 60255-187-1:2021 - Measuring relays and protection equipment - Part 187-1: Functional requirements for differential protection - Restrained and unrestrained differential protection of motors, generators and transformers

Relations

• Corrected By: EN IEC 60255-187-1:2021/AC:2023-04 - Measuring relays and protection equipment - Part 187-1: Functional requirements for differential protection - Restrained and unrestrained differential protection of motors, generators and transformers
• Effective Date: 02-May-2023

Overview

EN IEC 60255-187-1:2021 (IEC 60255-187-1:2021) specifies minimum functional and performance requirements for longitudinal (phase) differential protection used on AC motors, generators and transformers. The standard defines how to evaluate, test and document both restrained (biased) and unrestrained differential protection functions, describing the influencing factors that affect accuracy under steady-state and dynamic conditions and the test methods to verify performance. EN IEC 60255-187-1:2021 replaces IEC 60255-13 and aligns differential protection requirements with the IEC 60255-1xx functional standard structure.

Key topics and requirements

• Function specification
  • Inputs: energizing quantities, CT connections and digital interfaces
  • Binary inputs and functional logic for start (pick-up), operate (trip) and other signals
• Differential schemes
  • Phase-biased (restrained) differential protection
  • Unrestrained differential and biased restricted earth-fault protection
  • Compensation: ratio (magnitude) compensation, phase (vector) compensation and zero-sequence compensation
• Performance and accuracy
  • Effective and operating ranges; steady-state accuracy tests (basic characteristic, ratio/phase compensation, harmonic restraint)
  • Dynamic performance: typical operate times for double-infeed and radial single-infeed network models (restrained/unrestrained)
  • Stability requirements for CT saturation, magnetizing inrush, overexcitation, switch-on-to-fault, load harmonics and off‑nominal frequency
• Testing and documentation
  • Detailed functional and dynamic test methods, reporting of typical operate times and type test reports
  • Requirements for publishing performance test results and user documentation

Practical applications

EN IEC 60255-187-1:2021 is used to:

• Specify performance and test criteria in procurement and technical specifications for protection relays
• Guide factory and type testing of differential relays (manufacturers and testing laboratories)
• Support protection engineers during relay selection, commissioning and relay setting studies
• Ensure consistent testing and reporting for reliability on motors, generators and transformers in power plants, utilities and industrial installations

Who should use this standard

• Relay manufacturers and R&D teams developing differential protection functions
• Utility and industrial protection engineers and relay settings engineers
• Third‑party testing laboratories and certification bodies
• OEMs, consultants and procurement specialists responsible for protection system compliance

Related standards

• IEC 60255-187-2 and IEC 60255-187-3 (companion parts)
• IEC 60255-1 (common relay functional requirements)
• References to IEC 61850 series and IEC 61869 instrument transformer standards for digital/CT interfacing

Keywords: differential protection, restrained differential, unrestrained differential, motors generators transformers, EN IEC 60255-187-1:2021, protection relays, CT saturation, inrush, harmonic restraint, phase compensation, performance testing.

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN 60255-187-1:2021
01-november-2021
Merilni releji in zaščitna oprema - 187-1. del: Funkcijske zahteve za diferenčno
zaščito - Omejena in neomejena diferenčna zaščita motorjev, generatorjev in
transformatorjev
Measuring relays and protection equipment - Part 187-1: Functional requirements for
differential protection - Restrained and unrestrained differential protection of motors,
generators and transformers
Ta slovenski standard je istoveten z: EN IEC 60255-187-1:2021
ICS:
29.120.70 Releji Relays
SIST EN 60255-187-1:2021 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

SIST EN 60255-187-1:2021
SIST EN 60255-187-1:2021
EUROPEAN STANDARD EN IEC 60255-187-1

NORME EUROPÉENNE
EUROPÄISCHE NORM
September 2021
ICS 29.120.70
English Version
Measuring relays and protection equipment - Part 187-1:
Functional requirements for differential protection - Restrained
and unrestrained differential protection of motors, generators
and transformers
(IEC 60255-187-1:2021)
Relais de mesure et dispositifs de protection - Partie 187-1: Messrelais und Schutzeinrichtungen - Teil 187-1:
Exigences fonctionnelles pour la protection différentielle - Funktionsanforderungen für den stabilisierten und nicht
Protection différentielle avec et sans caractéristique de stabilisierten Differentialschutz von Motoren, Generatoren
retenue des moteurs, générateurs et transformateurs und Transformatoren
(IEC 60255-187-1:2021) (IEC 60255-187-1:2021)
This European Standard was approved by CENELEC on 2021-09-01. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60255-187-1:2021 E

SIST EN 60255-187-1:2021
European foreword
The text of document 95/465/FDIS, future edition 1 of IEC 60255-187-1, prepared by IEC/TC 95
“Measuring relays and protection equipment” was submitted to the IEC-CENELEC parallel vote and
approved by CENELEC as EN IEC 60255-187-1:2021.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2022-06-01
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2024-09-01
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 60255-187-1:2021 was approved by CENELEC as a
European Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards
indicated:
IEC 61850 (series) NOTE Harmonized as EN 61850 (series)
IEC 61850-7-4:2010 NOTE Harmonized as EN 61850-7-4:2010 (not modified)
IEC 61850-9-2 NOTE Harmonized as EN 61850-9-2
SIST EN 60255-187-1:2021
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod), the
relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60255-1 - Measuring relays and protection equipment EN 60255-1 -
- Part 1: Common requirements
IEC 61850-8-1 - Communication networks and systems for EN 61850-8-1 -
power utility automation - Part 8–1:
Specific communication service mapping
(SCSM) - Mappings to MMS (ISO 9506-1
and ISO 9506-2) and to ISO/IEC 8802-3
IEC 61869-2 - Instrument transformers - Part 2: Additional EN 61869-2 -
requirements for current transformers
IEC 61869-9 - Instrument transformers - Part 9: Digital EN IEC 61869-9 -
interface for instrument transformers

SIST EN 60255-187-1:2021
SIST EN 60255-187-1:2021
IEC 60255-187-1 ®
Edition 1.0 2021-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Measuring relays and protection equipment –

Part 187-1: Functional requirements for differential protection – Restrained and

unrestrained differential protection of motors, generators and transformers

Relais de mesure et dispositifs de protection –

Partie 187-1: Exigences fonctionnelles pour la protection différentielle –

Protection différentielle avec et sans caractéristique de retenue des moteurs,

générateurs et transformateurs

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.120.70 ISBN 978-2-8322-1003-9

SIST EN 60255-187-1:2021
– 2 – IEC 60255-187-1:2021 © IEC 2021
CONTENTS
FOREWORD . 11
1 Scope . 13
2 Normative references . 14
3 Terms and definitions . 14
4 Specification of the function . 18
4.1 General . 18
4.2 Input energizing quantities/energizing quantities . 19
4.2.1 General . 19
4.2.2 Connections . 19
4.3 Binary input signals. 19
4.4 Functional logic . 22
4.4.1 General . 22
4.4.2 Phase biased differential protection . 22
4.4.3 Biased restricted earth fault protection . 24
4.4.4 Compensation of energizing quantities . 25
4.4.5 Additional restraint or blocking methods . 26
4.5 Binary output signals . 27
4.5.1 General . 27
4.5.2 Start (pick-up) signals . 27
4.5.3 Operate (trip) signals . 27
4.5.4 Other output signals . 27
4.6 Additional influencing functions and conditions . 27
4.6.1 General . 27
4.6.2 Operation during CT saturation . 28
4.6.3 Switch onto fault . 28
4.6.4 Energizing quantity failure (CT supervision) . 28
4.6.5 Off-nominal frequency operation . 28
4.6.6 Geomagnetically induced currents (GIC) . 28
5 Performance specification . 29
5.1 General . 29
5.2 Effective and operating ranges. 29
5.3 Steady state accuracy tests in the effective range . 29
5.3.1 General . 29
5.3.2 Test related to the declared thermal withstand current . 30
5.3.3 Basic characteristic accuracy . 30
5.3.4 Ratio compensation accuracy . 30
5.3.5 Phase (vector) compensation validity . 31
5.3.6 Zero sequence compensation validity . 31
5.3.7 Harmonic restraint basic accuracy . 31
5.3.8 Basic accuracy of time delay settings . 31
5.3.9 Disengage time. 31
5.4 Dynamic performance in operating range . 32
5.4.1 General . 32
5.4.2 Typical operate time . 32
5.4.3 Relay stability for external faults . 32
5.4.4 Relay behaviour for internal fault preceded by an external fault . 33
5.5 Stability during magnetizing inrush conditions . 33

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IEC 60255-187-1:2021 © IEC 2021 – 3 –
5.6 Stability during overexcitation conditions . 33
5.7 Presence of harmonics on load . 33
5.8 Performance during saturation of current transformers . 33
5.9 Behaviour of differential protection with digital interface for the energizing
quantities . 34
6 Functional tests . 34
6.1 General . 34
6.2 Test related to the declared thermal withstand current . 35
6.3 Steady state accuracy tests in effective range . 35
6.3.1 General . 35
6.3.2 Basic characteristic accuracy . 37
6.3.3 Ratio (magnitude) compensation accuracy . 44
6.3.4 Phase (vector) compensation validity . 45
6.3.5 Zero sequence compensation validity . 47
6.3.6 Harmonic restraint basic accuracy test under steady state conditions at

nominal frequency . 50
6.3.7 Accuracy related to time delay setting . 52
6.3.8 Determination and reporting of the disengage time . 54
6.4 Dynamic performance tests . 55
6.4.1 General . 55
6.4.2 Operate time for double infeed network model (restrained operation) . 57
6.4.3 Operate time for double infeed network model (unrestrained operation) . 68
6.4.4 Operate time for radial single infeed network model (restrained
operation) . 73
6.4.5 Operate time for radial single infeed network model (unrestrained
operation) . 86
6.4.6 Reporting of typical operate time . 89
6.4.7 Stability for external faults . 95
6.5 Relay behaviour for internal fault preceded by an external fault . 112
6.5.1 General . 112
6.5.2 Application specific considerations: transformer differential . 112
6.5.3 Application specific considerations: biased restricted earth fault . 115
6.5.4 Application specific considerations: generator differential . 119
6.5.5 Reporting . 122
6.6 Stability during inrush conditions. 123
6.6.1 General . 123
6.6.2 Application specific considerations: transformer differential . 123
6.7 Stability during overexcitation conditions . 128
6.7.1 General . 128
6.7.2 Application specific considerations: transformer differential . 128
6.8 Performance with load harmonics . 133
6.8.1 General . 133
6.8.2 Application specific considerations: transformer differential . 133
6.8.3 Application specific considerations: generator or motor differential . 137
6.8.4 Application specific considerations: biased restricted earth fault . 140
6.8.5 Reporting . 142
7 Documentation requirements . 143
7.1 Type test report . 143
7.2 Other user documentation . 143

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Annex A (informative) Examples of phase (vector) compensation and zero sequence
compensation schemes . 144
A.1 General . 144
A.2 Y→d conversion . 145
A.2.1 Current conversion . 145
A.2.2 Three-phase fault at Y (star/wye) side . 146
A.2.3 Phase-phase fault at Y (star/wye) side . 147
A.2.4 Single-phase fault at Y (star/wye) side . 147
A.2.5 Three-phase fault at delta side . 148
A.2.6 Phase-phase fault at delta side . 149
A.2.7 Single-phase fault at delta side . 149
A.2.8 Ratio between start currents under different fault types . 152
A.3 d→Y conversion . 152
A.3.1 Current conversion . 152
A.3.2 Three-phase fault at Y (star/wye) side . 153
A.3.3 Phase-phase fault at Y (star/wye) side . 153
A.3.4 Single-phase fault at Y (star/wye) side . 153
A.3.5 Three-phase fault at delta side . 154
A.3.6 Phase-phase fault at delta side . 154
A.3.7 Single-phase fault at delta side . 155
A.3.8 Ratio between start currents under different fault types . 155
Annex B (normative) Calculation of mean, median and mode . 156
B.1 Mean . 156
B.2 Median . 156
B.3 Mode . 156
B.4 Example . 156
Annex C (normative) CT requirements . 157
C.1 General . 157
C.2 Transformer differential protection . 161
C.2.1 General . 161
C.2.2 Fault 1 . 161
C.2.3 Fault 2 . 162
C.2.4 Fault 3 . 162
C.3 Transformer restricted earth fault protection . 163
C.3.1 General . 163
C.3.2 Fault 1 . 163
C.3.3 Fault 2 . 164
C.3.4 Fault 3 . 164
C.4 Generator differential protection . 165
C.4.1 General . 165
C.4.2 Fault 2 . 165
C.4.3 Criteria and additional conditions . 166
C.5 Motor differential protection . 166
C.5.1 General . 166
C.5.2 Fault 1 . 166
C.5.3 Criteria and additional conditions . 166
C.5.4 Start of motor, security case . 167
C.5.5 Criteria and additional conditions . 167
C.6 Reporting . 167

SIST EN 60255-187-1:2021
IEC 60255-187-1:2021 © IEC 2021 – 5 –
Annex D (informative) CT saturation and influence on the performance of differential
relays . 168
Annex E (informative) Guidance on dimensioning of CTs for transformer differential

protection . 173
E.1 General . 173
E.2 Example 1 . 174
E.2.1 General . 174
E.2.2 Verification of CT1 – Internal fault . 175
E.2.3 Verification of CT1 – External fault . 175
E.2.4 Verification of CT2 . 176
E.3 Example 2 . 177
E.3.1 General . 177
E.3.2 Dimensioning of CT1 . 178
E.3.3 Dimensioning of CT2 . 179
Annex F (informative) Examples of test procedures to determine CT sizing
requirements for differential protection . 181
F.1 General . 181
F.2 Test data . 183
F.2.1 General . 183
F.2.2 Network model for CT requirement tests for the transformer differential
protection . 183
F.2.3 Network model for CT requirement tests for the transformer restricted
earth fault protection . 187
F.3 CT data and CT models . 189
F.4 Test summary . 197
Annex G (normative) Ramping methods for testing basic characteristic accuracy . 199
G.1 General . 199
G.2 Pre-fault condition . 199
G.3 Pseudo-continuous ramp . 199
G.4 Ramp of shots. 201
Annex H (informative) Example of COMTRADE file for an evolving fault test case . 203
Annex I (normative) Definition of fault inception angle. 204
Bibliography . 205

Figure 1 – Explanatory diagram for start time, operate time and disengage time . 17
Figure 2 – Simplified biased differential functional block diagram . 18
Figure 3 – Primary current reference direction . 21
Figure 4 – Typical restrained element (biased) characteristic . 23
Figure 5 – Typical unrestrained element characteristic . 23
Figure 6 – Example of combined characteristic using restrained and unrestrained
elements . 24
Figure 7 – Basic error of the operating characteristic . 30
Figure 8 – Example of an operating characteristic in the I /I plane with a
DIFF REST
tolerance band . 37
Figure 9 – Test cases for differential characteristic basic accuracy . 39
Figure 10 – Example of a differential characteristic with test lines "a" to "h" . 40
Figure 11 – Machine differential protection . 40
Figure 12 – Test sequence for basic characteristic accuracy . 42

SIST EN 60255-187-1:2021
– 6 – IEC 60255-187-1:2021 © IEC 2021
Figure 13 – Machine restricted earth fault protection . 44
Figure 14 – Example for documenting the test results for differential relay
characteristic . 45
Figure 15 – Ratio (magnitude) compensation accuracy test . 46
Figure 16 – Secondary three-phase and double-phase injection for Winding 1

(example) . 47
Figure 17 – Secondary single-phase and three-phase injections for Winding 1
(example) . 49
Figure 18 – Zero sequence current injection on the Y side of the transformer . 50
Figure 19 – Zero sequence current injection on the delta side of the transformer . 50
Figure 20 – Example of a rated frequency harmonic restraint characteristic with
visualization of test lines . 53
Figure 21 – Sequence of events for testing the disengage time . 55
Figure 22 – Double infeed network model for operate time tests . 58
Figure 23 – Test sequence for double infeed network model – Restrained operation

(transformer) . 62
Figure 24 – Double infeed network model for operate time tests . 63
Figure 25 – Test sequence for double infeed network model – Restrained operation
(REF) . 66
Figure 26 – Double infeed network model for operate time tests . 67
Figure 27 – Test sequence for double infeed network model – Restrained operation
(generator). 70
Figure 28 – Test sequence for double infeed network model – Unrestrained operation

(transformer) . 73
Figure 29 – Single infeed network model for operate time tests . 74
Figure 30 – Test sequence radial single infeed network model – Restrained operation . 78
Figure 31 – Single infeed network model for operate time tests . 79
Figure 32 – Test sequence for radial single infeed network – Restrained operation
(generator). 82
Figure 33 – Single infeed network model for operate time tests . 83
Figure 34 – Test sequence for radial single infeed network – Restrained operation
(motor) . 86
Figure 35 – Test sequence for radial single infeed network – Unrestrained operation . 89
Figure 36 – Example of distribution of the operate time for one application . 93
Figure 37 – Operate time as a function of the off-nominal frequency values (effective

range is the specified range of ±10 % of nominal frequency) . 95
Figure 38 – Operate time as a function of the off-nominal frequency values (accuracy
range beyond the specified range of ±10 % of nominal frequency . 96
Figure 39 – Double infeed network model for stability tests . 97
Figure 40 – Sequence of fault injection for stability due to external faults (transformer) . 100
Figure 41 – Double infeed network model for stability tests . 101
Figure 42 – Sequence of fault injection for stability due to external faults (REF) . 104
Figure 43 – Double infeed network model for stability tests . 105
Figure 44 – Sequence of fault injection for stability due to external faults (generator) . 108
Figure 45 – Double infeed network model for stability tests . 109
Figure 46 – Sequence of fault injection for stability due to external faults (motor) . 112

SIST EN 60255-187-1:2021
IEC 60255-187-1:2021 © IEC 2021 – 7 –
Figure 47 – Double infeed network model for internal fault preceded by an external fault . 112
Figure 48 – Double infeed network model for internal fault preceded by an external
fault test . 116
Figure 49 – Double infeed network model for internal fault preceded by an external
fault test . 119
Figure 50 – Power transformer inrush current waveform . 124
Figure 51 – Comparison of waveforms . 125
Figure 52 – Connection for the relay when current is injected from Y winding . 126
Figure 53 – Connection for the relay when current is injected from delta winding . 127
Figure 54 – Power transformer overexcitation current waveform injected from Y
winding . 129
Figure 55 – Overexcitation current waveform injected from delta winding . 129
Figure 56 – Comparison of the waveforms injected from Y winding . 130
Figure 57 – Comparison of the waveforms injected from delta winding . 131
Figure 58 – Three-phase overexcitation current waveform injected from Y winding . 132
Figure 59 – Three-phase overexcitation current waveform injected from delta winding . 133
Figure 60 – Test with superimposed harmonics on load – Transformer protection . 133
Figure 61 – Three-phase load current waveform on the Y side of the transformer with
superimposed harmonics . 137
Figure 62 – Three-phase load current waveforms on the delta side of the YNd1
transformer with superimposed harmonics . 137
Figure 63 – Test with superimposed harmonics on load . 138
Figure 64 – Test with superimposed harmonics on load – Restricted earth fault protection . 140
Figure A.1 – Example of a transformer . 144
Figure A.2 – Current vectors . 145
Figure A.3 – Three-phase injection at Y (star/wye) side . 147
Figure A.4 – Phase-phase injection at Y (star/wye) side . 147
Figure A.5 – Single-phase injection at Y (star/wye) side . 148
Figure A.6 – Three-phase injection at delta side . 149
Figure A.7 – Phase-phase injection at delta side . 149
Figure A.8 – Internal single-phase fault at delta side with neutral grounding transformer
in the system . 150
Figure A.9 – Single-phase injection at delta side . 150
Figure A.10 – External single-phase fault at delta side with neutral grounding
transformer inside protected zone . 151
Figure C.1 – Fault positions to be considered for specifying the CT requirements . 160
Figure C.2 – Fault positions to be considered for transformer differential protection. 161
Figure C.3 – Fault positions to be considered for the restricted earth fault protection . 163
Figure C.4 – External fault position to be considered for the generator differential

protection . 165
Figure C.5 – Internal fault position to be considered for the motor differential protection . 166
Figure D.1 – Fault positions to be considered for specifying the CT requirements . 170
Figure D.2 – Additional fault position to be considered in case of summation of currents . 170
Figure E.1 – Transformer differential relay example 1 . 174
Figure E.2 – Transformer differential relay example 2 . 177
Figure F.1 – Network models and fault positions for transformer differential protection . 184

SIST EN 60255-187-1:2021
– 8 – IEC 60255-187-1:2021 © IEC 2021
Figure F.2 – Network models and fault positions for transformer restricted earth fault
protection . 187
Figure F.3 – Excitation characteristic for the high-remanence basic CT . 191
Figure F.4 – Magnetization curve for the high-remanence type basic CT . 193
Figure F.5 – Secondary current at the limit of saturation caused by the AC component

with no remanent flux in the CT . 194
Figure F.6 – Secondary current in case of maximum DC offset . 194
Figure F.7 – Excitation characteristics for non-remanence and high-remanence type
basic CTs . 196
Figure F.8 – Magnetization curve for non-remanence type basic CTs. 197
Figure G.1 – Secondary injected currents for the simulation of a through load of 30 % . 200
Figure G.2 – Pseudo-continuous ramp in the restraining current – Differential current
plane in the time domain . 201
Figure G.3 – Ramp of shots showing differential step change and the time step . 202
Figure G.4 – Ramp of shots with binary search algorithm . 202
Figure I.1 – Graphical definition of fault inception angle . 204

Table 1 – Example of effective and operating ranges of differential protection . 29
Table 2 – Frequencies for steady state accuracy tests when the frequency effective

range is equal to ±5 % of nominal frequency . 36
Table 3 – Frequencies for steady state accuracy tests when the frequency effective
range is larger than ±5 % of nominal frequency . 36
Table 4 – Example frequencies for steady state accuracy tests when the frequency
effective range is narrower than ±5 % of nominal frequency . 36
Table 5 – Test points for differential characteristic basic accuracy . 38
Table 6 – Test lines on the differential characteristic (Figure 10) . 39
Table 7 – Basic characteristic accuracy . 43
Table 8 – Example of start ratios resulting from phase (vector) compensation . 47
Table 9 – Example of start ratios resulting from zero sequence compensation . 50
Table 10 – Test points for rated frequency harmonic restraint . 51
Table 11 – Reporting example of test results for harmonic restraint basic accuracy test . 52
Table 12 – Results of time delay tests . 53
Table 13 – Reported time delay . 53
Table 14 – Results of disengage time for all the tests . 55
Table 15 – Frequencies for dynamic performance tests when the frequency operating

range is equal to ±10 % of nominal frequency . 55
Table 16 – Frequencies for dynamic performance tests when the frequency operating
range is wider than ±10 % of nominal frequency . 55
Table 17 – Example frequencies for dynamic performance tests when the frequency
operating range is narrower than ±10 % of nominal frequency . 56
Table 18 – Double infeed network model . 58
Table 19 – Source impedances for double infeed network model – Restrained
operation (e.g. 50 Hz ± 10 % operating range) . 59
Table 20 – Double infeed network model .
...

SLOVENSKI STANDARD
01-november-2021
Merilni releji in zaščitna oprema - 187-1. del: Funkcijske zahteve za diferenčno
zaščito - Omejena in neomejena diferenčna zaščita motorjev, generatorjev in
transformatorjev
Measuring relays and protection equipment - Part 187-1: Functional requirements for
differential protection - Restrained and unrestrained differential protection of motors,
generators and transformers
Messrelais und Schutzeinrichtungen - Teil 187-1: Funktionsanforderungen für den
stabilisierten und nicht stabilisierten Differentialschutz von Motoren, Generatoren und
Transformatoren
Relais de mesure et dispositifs de protection - Partie 187-1: Exigences fonctionnelles
pour la protection différentielle - Protection différentielle avec et sans caractéristique de
retenue des moteurs, générateurs et transformateurs
Ta slovenski standard je istoveten z: EN IEC 60255-187-1:2021
ICS:
29.120.70 Releji Relays
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 60255-187-1

NORME EUROPÉENNE
EUROPÄISCHE NORM
September 2021
ICS 29.120.70
English Version
Measuring relays and protection equipment - Part 187-1:
Functional requirements for differential protection - Restrained
and unrestrained differential protection of motors, generators
and transformers
(IEC 60255-187-1:2021)
Relais de mesure et dispositifs de protection - Partie 187-1: Messrelais und Schutzeinrichtungen - Teil 187-1:
Exigences fonctionnelles pour la protection différentielle - Funktionsanforderungen für den stabilisierten und nicht
Protection différentielle avec et sans caractéristique de stabilisierten Differentialschutz von Motoren, Generatoren
retenue des moteurs, générateurs et transformateurs und Transformatoren
(IEC 60255-187-1:2021) (IEC 60255-187-1:2021)
This European Standard was approved by CENELEC on 2021-09-01. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60255-187-1:2021 E

European foreword
The text of document 95/465/FDIS, future edition 1 of IEC 60255-187-1, prepared by IEC/TC 95
“Measuring relays and protection equipment” was submitted to the IEC-CENELEC parallel vote and
approved by CENELEC as EN IEC 60255-187-1:2021.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2022-06-01
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2024-09-01
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 60255-187-1:2021 was approved by CENELEC as a
European Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards
indicated:
IEC 61850 (series) NOTE Harmonized as EN 61850 (series)
IEC 61850-7-4:2010 NOTE Harmonized as EN 61850-7-4:2010 (not modified)
IEC 61850-9-2 NOTE Harmonized as EN 61850-9-2
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod), the
relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60255-1 - Measuring relays and protection equipment EN 60255-1 -
- Part 1: Common requirements
IEC 61850-8-1 - Communication networks and systems for EN 61850-8-1 -
power utility automation - Part 8–1:
Specific communication service mapping
(SCSM) - Mappings to MMS (ISO 9506-1
and ISO 9506-2) and to ISO/IEC 8802-3
IEC 61869-2 - Instrument transformers - Part 2: Additional EN 61869-2 -
requirements for current transformers
IEC 61869-9 - Instrument transformers - Part 9: Digital EN IEC 61869-9 -
interface for instrument transformers

IEC 60255-187-1 ®
Edition 1.0 2021-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Measuring relays and protection equipment –

Part 187-1: Functional requirements for differential protection – Restrained and

unrestrained differential protection of motors, generators and transformers

Relais de mesure et dispositifs de protection –

Partie 187-1: Exigences fonctionnelles pour la protection différentielle –

Protection différentielle avec et sans caractéristique de retenue des moteurs,

générateurs et transformateurs

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.120.70 ISBN 978-2-8322-1003-9

– 2 – IEC 60255-187-1:2021 © IEC 2021
CONTENTS
FOREWORD . 11
1 Scope . 13
2 Normative references . 14
3 Terms and definitions . 14
4 Specification of the function . 18
4.1 General . 18
4.2 Input energizing quantities/energizing quantities . 19
4.2.1 General . 19
4.2.2 Connections . 19
4.3 Binary input signals. 19
4.4 Functional logic . 22
4.4.1 General . 22
4.4.2 Phase biased differential protection . 22
4.4.3 Biased restricted earth fault protection . 24
4.4.4 Compensation of energizing quantities . 25
4.4.5 Additional restraint or blocking methods . 26
4.5 Binary output signals . 27
4.5.1 General . 27
4.5.2 Start (pick-up) signals . 27
4.5.3 Operate (trip) signals . 27
4.5.4 Other output signals . 27
4.6 Additional influencing functions and conditions . 27
4.6.1 General . 27
4.6.2 Operation during CT saturation . 28
4.6.3 Switch onto fault . 28
4.6.4 Energizing quantity failure (CT supervision) . 28
4.6.5 Off-nominal frequency operation . 28
4.6.6 Geomagnetically induced currents (GIC) . 28
5 Performance specification . 29
5.1 General . 29
5.2 Effective and operating ranges. 29
5.3 Steady state accuracy tests in the effective range . 29
5.3.1 General . 29
5.3.2 Test related to the declared thermal withstand current . 30
5.3.3 Basic characteristic accuracy . 30
5.3.4 Ratio compensation accuracy . 30
5.3.5 Phase (vector) compensation validity . 31
5.3.6 Zero sequence compensation validity . 31
5.3.7 Harmonic restraint basic accuracy . 31
5.3.8 Basic accuracy of time delay settings . 31
5.3.9 Disengage time. 31
5.4 Dynamic performance in operating range . 32
5.4.1 General . 32
5.4.2 Typical operate time . 32
5.4.3 Relay stability for external faults . 32
5.4.4 Relay behaviour for internal fault preceded by an external fault . 33
5.5 Stability during magnetizing inrush conditions . 33

IEC 60255-187-1:2021 © IEC 2021 – 3 –
5.6 Stability during overexcitation conditions . 33
5.7 Presence of harmonics on load . 33
5.8 Performance during saturation of current transformers . 33
5.9 Behaviour of differential protection with digital interface for the energizing
quantities . 34
6 Functional tests . 34
6.1 General . 34
6.2 Test related to the declared thermal withstand current . 35
6.3 Steady state accuracy tests in effective range . 35
6.3.1 General . 35
6.3.2 Basic characteristic accuracy . 37
6.3.3 Ratio (magnitude) compensation accuracy . 44
6.3.4 Phase (vector) compensation validity . 45
6.3.5 Zero sequence compensation validity . 47
6.3.6 Harmonic restraint basic accuracy test under steady state conditions at

nominal frequency . 50
6.3.7 Accuracy related to time delay setting . 52
6.3.8 Determination and reporting of the disengage time . 54
6.4 Dynamic performance tests . 55
6.4.1 General . 55
6.4.2 Operate time for double infeed network model (restrained operation) . 57
6.4.3 Operate time for double infeed network model (unrestrained operation) . 68
6.4.4 Operate time for radial single infeed network model (restrained
operation) . 73
6.4.5 Operate time for radial single infeed network model (unrestrained
operation) . 86
6.4.6 Reporting of typical operate time . 89
6.4.7 Stability for external faults . 95
6.5 Relay behaviour for internal fault preceded by an external fault . 112
6.5.1 General . 112
6.5.2 Application specific considerations: transformer differential . 112
6.5.3 Application specific considerations: biased restricted earth fault . 115
6.5.4 Application specific considerations: generator differential . 119
6.5.5 Reporting . 122
6.6 Stability during inrush conditions. 123
6.6.1 General . 123
6.6.2 Application specific considerations: transformer differential . 123
6.7 Stability during overexcitation conditions . 128
6.7.1 General . 128
6.7.2 Application specific considerations: transformer differential . 128
6.8 Performance with load harmonics . 133
6.8.1 General . 133
6.8.2 Application specific considerations: transformer differential . 133
6.8.3 Application specific considerations: generator or motor differential . 137
6.8.4 Application specific considerations: biased restricted earth fault . 140
6.8.5 Reporting . 142
7 Documentation requirements . 143
7.1 Type test report . 143
7.2 Other user documentation . 143

– 4 – IEC 60255-187-1:2021 © IEC 2021
Annex A (informative) Examples of phase (vector) compensation and zero sequence
compensation schemes . 144
A.1 General . 144
A.2 Y→d conversion . 145
A.2.1 Current conversion . 145
A.2.2 Three-phase fault at Y (star/wye) side . 146
A.2.3 Phase-phase fault at Y (star/wye) side . 147
A.2.4 Single-phase fault at Y (star/wye) side . 147
A.2.5 Three-phase fault at delta side . 148
A.2.6 Phase-phase fault at delta side . 149
A.2.7 Single-phase fault at delta side . 149
A.2.8 Ratio between start currents under different fault types . 152
A.3 d→Y conversion . 152
A.3.1 Current conversion . 152
A.3.2 Three-phase fault at Y (star/wye) side . 153
A.3.3 Phase-phase fault at Y (star/wye) side . 153
A.3.4 Single-phase fault at Y (star/wye) side . 153
A.3.5 Three-phase fault at delta side . 154
A.3.6 Phase-phase fault at delta side . 154
A.3.7 Single-phase fault at delta side . 155
A.3.8 Ratio between start currents under different fault types . 155
Annex B (normative) Calculation of mean, median and mode . 156
B.1 Mean . 156
B.2 Median . 156
B.3 Mode . 156
B.4 Example . 156
Annex C (normative) CT requirements . 157
C.1 General . 157
C.2 Transformer differential protection . 161
C.2.1 General . 161
C.2.2 Fault 1 . 161
C.2.3 Fault 2 . 162
C.2.4 Fault 3 . 162
C.3 Transformer restricted earth fault protection . 163
C.3.1 General . 163
C.3.2 Fault 1 . 163
C.3.3 Fault 2 . 164
C.3.4 Fault 3 . 164
C.4 Generator differential protection . 165
C.4.1 General . 165
C.4.2 Fault 2 . 165
C.4.3 Criteria and additional conditions . 166
C.5 Motor differential protection . 166
C.5.1 General . 166
C.5.2 Fault 1 . 166
C.5.3 Criteria and additional conditions . 166
C.5.4 Start of motor, security case . 167
C.5.5 Criteria and additional conditions . 167
C.6 Reporting . 167

IEC 60255-187-1:2021 © IEC 2021 – 5 –
Annex D (informative) CT saturation and influence on the performance of differential
relays . 168
Annex E (informative) Guidance on dimensioning of CTs for transformer differential

protection . 173
E.1 General . 173
E.2 Example 1 . 174
E.2.1 General . 174
E.2.2 Verification of CT1 – Internal fault . 175
E.2.3 Verification of CT1 – External fault . 175
E.2.4 Verification of CT2 . 176
E.3 Example 2 . 177
E.3.1 General . 177
E.3.2 Dimensioning of CT1 . 178
E.3.3 Dimensioning of CT2 . 179
Annex F (informative) Examples of test procedures to determine CT sizing
requirements for differential protection . 181
F.1 General . 181
F.2 Test data . 183
F.2.1 General . 183
F.2.2 Network model for CT requirement tests for the transformer differential
protection . 183
F.2.3 Network model for CT requirement tests for the transformer restricted
earth fault protection . 187
F.3 CT data and CT models . 189
F.4 Test summary . 197
Annex G (normative) Ramping methods for testing basic characteristic accuracy . 199
G.1 General . 199
G.2 Pre-fault condition . 199
G.3 Pseudo-continuous ramp . 199
G.4 Ramp of shots. 201
Annex H (informative) Example of COMTRADE file for an evolving fault test case . 203
Annex I (normative) Definition of fault inception angle. 204
Bibliography . 205

Figure 1 – Explanatory diagram for start time, operate time and disengage time . 17
Figure 2 – Simplified biased differential functional block diagram . 18
Figure 3 – Primary current reference direction . 21
Figure 4 – Typical restrained element (biased) characteristic . 23
Figure 5 – Typical unrestrained element characteristic . 23
Figure 6 – Example of combined characteristic using restrained and unrestrained
elements . 24
Figure 7 – Basic error of the operating characteristic . 30
Figure 8 – Example of an operating characteristic in the I /I plane with a
DIFF REST
tolerance band . 37
Figure 9 – Test cases for differential characteristic basic accuracy . 39
Figure 10 – Example of a differential characteristic with test lines "a" to "h" . 40
Figure 11 – Machine differential protection . 40
Figure 12 – Test sequence for basic characteristic accuracy . 42

– 6 – IEC 60255-187-1:2021 © IEC 2021
Figure 13 – Machine restricted earth fault protection . 44
Figure 14 – Example for documenting the test results for differential relay
characteristic . 45
Figure 15 – Ratio (magnitude) compensation accuracy test . 46
Figure 16 – Secondary three-phase and double-phase injection for Winding 1

(example) . 47
Figure 17 – Secondary single-phase and three-phase injections for Winding 1
(example) . 49
Figure 18 – Zero sequence current injection on the Y side of the transformer . 50
Figure 19 – Zero sequence current injection on the delta side of the transformer . 50
Figure 20 – Example of a rated frequency harmonic restraint characteristic with
visualization of test lines . 53
Figure 21 – Sequence of events for testing the disengage time . 55
Figure 22 – Double infeed network model for operate time tests . 58
Figure 23 – Test sequence for double infeed network model – Restrained operation

(transformer) . 62
Figure 24 – Double infeed network model for operate time tests . 63
Figure 25 – Test sequence for double infeed network model – Restrained operation
(REF) . 66
Figure 26 – Double infeed network model for operate time tests . 67
Figure 27 – Test sequence for double infeed network model – Restrained operation
(generator). 70
Figure 28 – Test sequence for double infeed network model – Unrestrained operation

(transformer) . 73
Figure 29 – Single infeed network model for operate time tests . 74
Figure 30 – Test sequence radial single infeed network model – Restrained operation . 78
Figure 31 – Single infeed network model for operate time tests . 79
Figure 32 – Test sequence for radial single infeed network – Restrained operation
(generator). 82
Figure 33 – Single infeed network model for operate time tests . 83
Figure 34 – Test sequence for radial single infeed network – Restrained operation
(motor) . 86
Figure 35 – Test sequence for radial single infeed network – Unrestrained operation . 89
Figure 36 – Example of distribution of the operate time for one application . 93
Figure 37 – Operate time as a function of the off-nominal frequency values (effective

range is the specified range of ±10 % of nominal frequency) . 95
Figure 38 – Operate time as a function of the off-nominal frequency values (accuracy
range beyond the specified range of ±10 % of nominal frequency . 96
Figure 39 – Double infeed network model for stability tests . 97
Figure 40 – Sequence of fault injection for stability due to external faults (transformer) . 100
Figure 41 – Double infeed network model for stability tests . 101
Figure 42 – Sequence of fault injection for stability due to external faults (REF) . 104
Figure 43 – Double infeed network model for stability tests . 105
Figure 44 – Sequence of fault injection for stability due to external faults (generator) . 108
Figure 45 – Double infeed network model for stability tests . 109
Figure 46 – Sequence of fault injection for stability due to external faults (motor) . 112

IEC 60255-187-1:2021 © IEC 2021 – 7 –
Figure 47 – Double infeed network model for internal fault preceded by an external fault . 112
Figure 48 – Double infeed network model for internal fault preceded by an external
fault test . 116
Figure 49 – Double infeed network model for internal fault preceded by an external
fault test . 119
Figure 50 – Power transformer inrush current waveform . 124
Figure 51 – Comparison of waveforms . 125
Figure 52 – Connection for the relay when current is injected from Y winding . 126
Figure 53 – Connection for the relay when current is injected from delta winding . 127
Figure 54 – Power transformer overexcitation current waveform injected from Y
winding . 129
Figure 55 – Overexcitation current waveform injected from delta winding . 129
Figure 56 – Comparison of the waveforms injected from Y winding . 130
Figure 57 – Comparison of the waveforms injected from delta winding . 131
Figure 58 – Three-phase overexcitation current waveform injected from Y winding . 132
Figure 59 – Three-phase overexcitation current waveform injected from delta winding . 133
Figure 60 – Test with superimposed harmonics on load – Transformer protection . 133
Figure 61 – Three-phase load current waveform on the Y side of the transformer with
superimposed harmonics . 137
Figure 62 – Three-phase load current waveforms on the delta side of the YNd1
transformer with superimposed harmonics . 137
Figure 63 – Test with superimposed harmonics on load . 138
Figure 64 – Test with superimposed harmonics on load – Restricted earth fault protection . 140
Figure A.1 – Example of a transformer . 144
Figure A.2 – Current vectors . 145
Figure A.3 – Three-phase injection at Y (star/wye) side . 147
Figure A.4 – Phase-phase injection at Y (star/wye) side . 147
Figure A.5 – Single-phase injection at Y (star/wye) side . 148
Figure A.6 – Three-phase injection at delta side . 149
Figure A.7 – Phase-phase injection at delta side . 149
Figure A.8 – Internal single-phase fault at delta side with neutral grounding transformer
in the system . 150
Figure A.9 – Single-phase injection at delta side . 150
Figure A.10 – External single-phase fault at delta side with neutral grounding
transformer inside protected zone . 151
Figure C.1 – Fault positions to be considered for specifying the CT requirements . 160
Figure C.2 – Fault positions to be considered for transformer differential protection. 161
Figure C.3 – Fault positions to be considered for the restricted earth fault protection . 163
Figure C.4 – External fault position to be considered for the generator differential

protection . 165
Figure C.5 – Internal fault position to be considered for the motor differential protection . 166
Figure D.1 – Fault positions to be considered for specifying the CT requirements . 170
Figure D.2 – Additional fault position to be considered in case of summation of currents . 170
Figure E.1 – Transformer differential relay example 1 . 174
Figure E.2 – Transformer differential relay example 2 . 177
Figure F.1 – Network models and fault positions for transformer differential protection . 184

– 8 – IEC 60255-187-1:2021 © IEC 2021
Figure F.2 – Network models and fault positions for transformer restricted earth fault
protection . 187
Figure F.3 – Excitation characteristic for the high-remanence basic CT . 191
Figure F.4 – Magnetization curve for the high-remanence type basic CT . 193
Figure F.5 – Secondary current at the limit of saturation caused by the AC component

with no remanent flux in the CT . 194
Figure F.6 – Secondary current in case of maximum DC offset . 194
Figure F.7 – Excitation characteristics for non-remanence and high-remanence type
basic CTs . 196
Figure F.8 – Magnetization curve for non-remanence type basic CTs. 197
Figure G.1 – Secondary injected currents for the simulation of a through load of 30 % . 200
Figure G.2 – Pseudo-continuous ramp in the restraining current – Differential current
plane in the time domain . 201
Figure G.3 – Ramp of shots showing differential step change and the time step . 202
Figure G.4 – Ramp of shots with binary search algorithm . 202
Figure I.1 – Graphical definition of fault inception angle . 204

Table 1 – Example of effective and operating ranges of differential protection . 29
Table 2 – Frequencies for steady state accuracy tests when the frequency effective

range is equal to ±5 % of nominal frequency . 36
Table 3 – Frequencies for steady state accuracy tests when the frequency effective
range is larger than ±5 % of nominal frequency . 36
Table 4 – Example frequencies for steady state accuracy tests when the frequency
effective range is narrower than ±5 % of nominal frequency . 36
Table 5 – Test points for differential characteristic basic accuracy . 38
Table 6 – Test lines on the differential characteristic (Figure 10) . 39
Table 7 – Basic characteristic accuracy . 43
Table 8 – Example of start ratios resulting from phase (vector) compensation . 47
Table 9 – Example of start ratios resulting from zero sequence compensation . 50
Table 10 – Test points for rated frequency harmonic restraint . 51
Table 11 – Reporting example of test results for harmonic restraint basic accuracy test . 52
Table 12 – Results of time delay tests . 53
Table 13 – Reported time delay . 53
Table 14 – Results of disengage time for all the tests . 55
Table 15 – Frequencies for dynamic performance tests when the frequency operating

range is equal to ±10 % of nominal frequency . 55
Table 16 – Frequencies for dynamic performance tests when the frequency operating
range is wider than ±10 % of nominal frequency . 55
Table 17 – Example frequencies for dynamic performance tests when the frequency
operating range is narrower t
...