IEC 60255-121:2014

Measuring relays and protection equipment - Part 121: Functional requirements for distance protection

IEC 60255-121:2014 specifies minimum requirements for functional and performance evaluation of distance protection function typically used in, but not limited to, line applications for effectively earthed, three-phase power systems. This standard also defines how to document and publish performance tests. This standard covers distance protection function whose operating characteristic can be defined on an impedance plane and includes specification of the protection function, measurement characteristics, phase selection, directionality, starting and time delay characteristics. The test methodologies for verifying performance characteristics and accuracy are included in this standard. The standard defines the influencing factors that affect the accuracy under steady state conditions and performance characteristics during dynamic conditions. It also includes the instrument transformer requirements for the protection function. The general requirements for measuring relays and protection equipment are defined in IEC 60255-1.

Relais de mesure et dispositifs de protection - Partie 121: Exigences fonctionnelles pour protection de distance

L'IEC 60255-121:2014 spécifie les exigences minimales relatives à l'évaluation fonctionnelle et à l'évaluation des performances de la fonction de protection de distance typiquement utilisée, sans s'y limiter à des applications de lignes dans des systèmes électriques triphasés avec mise à la terre directe. La présente norme définit également comment documenter et éditer des essais de qualification. La présente norme couvre la fonction de protection de distance dont la caractéristique de fonctionnement peut être définie sur un plan d'impédance et inclut la spécification de la fonction de protection, les caractéristiques de mesure, la sélection de phase, la directionnalité, les caractéristiques de démarrage et de temporisation. Les méthodologies d'essai pour vérifier les caractéristiques de performance et de précision sont incluses dans la présente norme. La norme définit les facteurs d'influence qui affectent la précision dans des conditions de régime établi et les caractéristiques de performance pendant des conditions dynamiques. Pour la fonction de protection, elle inclut aussi les exigences relatives aux transformateurs de mesure. Les exigences générales relatives aux relais de mesure et aux équipements de protection sont définies dans la norme CEI 60255-1.

General Information

• Status: Published
• Publication Date: 06-Mar-2014
• ICS: 29.120.70 - Relays
• Technical Committee: TC 95 - Measuring relays and protection equipment
• Drafting Committee: MT 4 - TC 95/MT 4
• Current Stage: PPUB - Publication issued
• Start Date: 07-Mar-2014
• Completion Date: 15-Feb-2014

Overview

IEC 60255-121:2014 is an international standard published by the International Electrotechnical Commission (IEC) that outlines the functional requirements and performance evaluation criteria for distance protection functions in power systems. This standard primarily targets distance protection relays used in effectively earthed, three-phase power systems, though its application extends beyond just line protection. It defines essential specifications for characterizing distance protection functions via an impedance plane and details the associated measurement characteristics, phase selection, directionality, and timing properties.

Additionally, IEC 60255-121:2014 establishes comprehensive test methodologies to verify functional performance and accuracy. The document also covers influencing factors affecting relay precision under steady-state and dynamic system conditions and specifies the requirements for instrument transformers (CTs and VTs) used with the protection functions. This standard builds upon the general requirements for measuring relays and protection equipment defined in IEC 60255-1.

Key Topics

• Distance Protection Function Specification
  Defines how the distance protection relay identifies faulted phases, selects directionality, and executes protective logic. Key attributes such as starting conditions and time delay characteristics are described in detail.

• Operating Characteristics on the Impedance Plane
  Covers impedance-based relay characteristics like MHO, quadrilateral, polygonal, and circular zones as fundamental to distance protection operation.

• Performance Evaluation
  Provides guidelines for measuring the accuracy of relay characteristics during steady-state and dynamic conditions. Topics include transient overreach, operate times, and behavior during frequency deviations and harmonic interference.

• Input and Output Signal Requirements
  Specifies the types of electrical and binary input signals needed for relay energizing and logic processing, along with output signaling for pickup, operate, and other protective functions.

• Testing Methodologies
  Establishes test procedures for functional verification including:

  • Rated frequency accuracy tests
  • Dynamic performance tests with fault simulation
  • Harmonic distortion impact analysis
  • Double infeed system evaluations
  • Instrument transformer interaction tests

• Instrument Transformer (CT, VT, CVT) Requirements
  Specifies the necessary performance criteria and limitations for current and voltage transformers that provide inputs to distance protection devices.

• Documentation and Reporting
  Details the format and content requirements for type test reports and technical documentation to ensure traceability and compliance.

Applications

IEC 60255-121:2014 is essential for manufacturers, testing laboratories, and power utilities engaged with electrical protection systems, particularly for:

• Distance Protection Relay Design and Manufacture
  Ensures that relay designs meet rigorous functional and performance standards needed for reliable fault detection on transmission and distribution lines.

• Power System Protection Coordination
  Helps engineers properly configure and evaluate distance protection functions to achieve selective clearing and minimize outage impact.

• Relay Testing and Certification
  Guides testing bodies through structured procedures to validate relay accuracy and dynamic behavior before deployment.

• System Reliability and Safety
  Supports utilities in improving power system stability and safety by verifying that distance relays respond appropriately under diverse fault and network conditions.

Related Standards

• IEC 60255-1 – General requirements for measuring relays and protection equipment. Provides baseline definitions, environmental, and mechanical criteria referenced within IEC 60255-121.

• IEC 60255 Series – Covers various aspects of protection relay standards such as measurement techniques, testing, and communication protocols.

• IEEE C37 Series – Offers complementary guidance and best practices in the protection of electric power systems, including distance protection principles.

Conclusion

IEC 60255-121:2014 is a pivotal standard that delivers thorough functional and performance criteria for distance protection devices in power systems. Its detailed specifications and test methodologies ensure robust fault detection, promote safety, and enhance power system reliability. Adherence to this standard supports manufacturers in product development, helps utilities optimize protection schemes, and provides a common framework for global compliance in power system protection.

Keywords: IEC 60255-121, distance protection, measuring relays, protection equipment, power system protection, relay testing, functional requirements, impedance plane, instrument transformers, performance evaluation, relay accuracy, distance relay standard.

Standards Content (Sample)

IEC 60255-121 ®
Edition 1.0 2014-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Measuring relays and protection equipment –
Part 121: Functional requirements for distance protection

Relais de mesure et dispositifs de protection –
Partie 121: Exigences fonctionnelles pour protection de distance

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IEC 60255-121 ®
Edition 1.0 2014-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Measuring relays and protection equipment –

Part 121: Functional requirements for distance protection

Relais de mesure et dispositifs de protection –

Partie 121: Exigences fonctionnelles pour protection de distance

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XG
ICS 29.120.70 ISBN 978-2-8322-1399-5

– 2 – IEC 60255-121:2014 © IEC 2014
CONTENTS
FOREWORD . 9
1 Scope . 11
2 Normative references . 11
3 Terms and definitions . 12
4 Specification of the function . 13
4.1 General . 13
4.2 Input energizing quantities/energizing quantities . 13
4.3 Binary input signals . 14
4.4 Functional logic . 15
4.4.1 Faulted phase identification . 15
4.4.2 Directional signals . 15
4.4.3 Distance protection function characteristics . 15
4.4.4 Distance protection zone timers . 16
4.5 Binary output signals . 16
4.5.1 General . 16
4.5.2 Start (pickup) signals . 16
4.5.3 Operate signals . 17
4.5.4 Other binary output signals . 17
4.6 Additional influencing functions/conditions . 17
4.6.1 General . 17
4.6.2 Inrush current . 17
4.6.3 Switch onto fault/trip on reclose . 17
4.6.4 Voltage transformer (VT) signal failure (loss of voltage) . 17
4.6.5 Power swings . 18
4.6.6 Behavior during frequencies outside of the operating range . 18
5 Performance specifications . 18
5.1 General . 18
5.2 Effective and operating ranges . 18
5.3 Basic characteristic accuracy under steady state conditions . 19
5.3.1 General . 19
5.3.2 Determination of accuracy related to time delay setting . 19
5.3.3 Disengaging time . 20
5.4 Dynamic performance . 20
5.4.1 General . 20
5.4.2 Transient overreach (TO) . 20
5.4.3 Operate time and transient overreach (SIR diagrams) . 21
5.4.4 Operate time and transient overreach (CVT-SIR diagrams). . 21
5.4.5 Typical operate time . 21
5.5 Performance with harmonics . 22
5.5.1 General . 22
5.5.2 Steady-state harmonics tests . 23
5.5.3 Transient LC oscillation tests . 23
5.6 Performance during frequency deviation . 23
5.6.1 General . 23
5.6.2 Steady state testing during frequency deviation . 23
5.6.3 Transient testing during frequency deviation . 23
5.7 Double infeed tests . 24

5.7.1 General . 24
5.7.2 Single line, double infeed system . 24
5.7.3 Double line, double infeed system . 24
5.8 Instrument transformer (CT, VT and CVT) requirements . 25
5.8.1 General . 25
5.8.2 CT requirements . 25
6 Functional tests . 29
6.1 General . 29
6.2 Rated frequency characteristic accuracy tests . 29
6.2.1 General . 29
6.2.2 Basic characteristic accuracy under steady state conditions . 30
6.2.3 Basic directional accuracy under steady state conditions . 43
6.2.4 Determination of accuracy related to time delay setting . 48
6.2.5 Determination and reporting of the disengaging time . 48
6.3 Dynamic performance . 50
6.3.1 General . 50
6.3.2 Dynamic performance: operate time and transient overreach
(SIR diagrams) . 51
6.3.3 Dynamic performance: operate time and transient overreach
(CVT-SIR diagrams) . 61
6.3.4 Dynamic performance: transient overreach tests. 65
6.3.5 Dynamic performance: typical operate time . 69
6.4 Performance with harmonics . 74
6.4.1 Steady state harmonics tests . 74
6.4.2 Transient oscillation tests (network simulation L-C) . 75
6.5 Performance during off-nominal frequency . 82
6.5.1 Steady state frequency deviation tests . 82
6.5.2 Transient frequency deviation tests . 85
6.6 Double infeed tests . 90
6.6.1 Double infeed tests for single line . 90
6.6.2 Double infeed tests for parallel lines (without mutual
inductance) . 96
6.6.3 Reporting of double infeed test results . 100
7 Documentation requirements . 101
7.1 Type test report . 101
7.2 Documentation . 101
Annex A (informative) Impedance characteristics . 102
A.1 Overview. 102
A.1.1 General . 102
A.1.2 Non-directional circular characteristic . 102
A.1.3 MHO characteristic . 102
A.1.4 Quadrilateral/polygonal . 104
A.2 Example characteristics . 106
A.2.1 General . 106
A.2.2 Non-directional circular characteristic (ohm) . 106
A.2.3 Reactive reach line characteristic . 106
A.2.4 MHO characteristic . 107
A.2.5 Resistive and reactive intersecting lines characteristic . 107
A.2.6 Offset MHO characteristic. . 108

– 4 – IEC 60255-121:2014 © IEC 2014
Annex B (informative) Informative guide for the behaviour of timers in distance
protection zones for evolving faults . 110
Annex C (normative) Setting example . 112
Annex D (normative) Calculation of mean, median and mode. 115
D.1 Mean . 115
D.2 Median . 115
D.3 Mode . 115
D.4 Example. 115
Annex E (informative) CT saturation and influence on the performance of distance
relays . 116
Annex F (informative) Informative guide for testing distance relays based on CT
requirements specification . 119
F.1 General . 119
F.2 Test data . 120
F.3 CT data and CT model . 121
Annex G (informative) Informative guide for dimensioning of CTs for distance
protection . 125
G.1 General . 125
G.2 Example 1 . 126
G.3 Example 2 . 128
Annex H (normative) Calculation of relay settings based on generic point P expressed
in terms of voltage and current. 131
H.1 Settings for quadrilateral/polygonal characteristic . 131
H.2 Settings for MHO characteristic . 133
Annex I (normative) Ramping methods for testing the basic characteristic accuracy . 134
I.1 Relationship between simulated fault impedance and analog quantities . 134
I.2 Pre-fault condition . 134
I.3 Phase to earth faults . 134
I.4 Phase to phase faults. . 136
I.5 Ramps in the impedance plane . 139
I.5.1 Pseudo-continuous ramp . 139
I.5.2 Ramp of shots . 140
Annex J (normative) Definition of fault inception angle . 143
Annex K (normative) Capacitive voltage instrument transformer model . 145
K.1 General . 145
K.2 Capacitor voltage transformer (CVT) . 145

Figure 1 – Simplified distance protection function block diagram . 14
Figure 2 – Basic accuracy specification of an operating characteristic . 19
Figure 3 – Basic angular accuracy specifications of directional lines . 20
Figure 4 – SIR diagram – Short line average operate time . 22
Figure 5 – Fault positions to be considered for specifying the CT requirements . 26
Figure 6 – Test procedure for basic characteristic accuracy . 31
Figure 7 – Calculated test points A, B and C based on the effective range of U and I . 32
Figure 8 – Modified points B’ and C’ based on the limited setting range . 32
Figure 9 – Position of test points A, B, C, D and E in the effective range of U and I . 33
Figure 10 – Position of test points A, B’, C’, D and E in the effective range of U and I . 33

Figure 11 – Quadrilateral characteristic showing ten test points . 34
Figure 12 – Quadrilateral characteristic showing test ramps. 35
Figure 13 – Quadrilateral characteristic showing accuracy limits . 36
Figure 14 – Quadrilateral/polygonal characteristic showing accuracy limits . 37
Figure 15 – MHO characteristic showing nine test points . 37
Figure 16 – MHO characteristic showing test ramps . 38
Figure 17 – Accuracy limits for MHO characteristic . 39
Figure 18 – Basic directional element accuracy tests . 44
Figure 19 – Directional element accuracy tests in the second quadrant. 45
Figure 20 – Directional element accuracy tests in the second quadrant. 46
Figure 21 – Directional element accuracy tests in the fourth quadrant . 46
Figure 22 – Directional test accuracy lines in the fourth quadrant . 47
Figure 23 – Position of the three-phase fault for testing the disengaging time . 49
Figure 24 – Sequence of events for testing the disengaging time . 50
Figure 25 – Power system network with zero load transfer . 51
Figure 26 – Dynamic performance: operate time and dynamic overreach (SIR diagram) . 55
Figure 27 – SIR diagram for short line: minimum operate time . 56
Figure 28 – SIR diagram for short line: average operate time . 57
Figure 29 – SIR diagram for short line: maximum operate time . 57
Figure 30 – Dynamic performance tests (SIR diagrams). 59
Figure 31 – SIR diagram for long line: minimum operate time . 61
Figure 32 – SIR diagram for long line: average operate time . 62
Figure 33 – SIR diagram for long line: maximum operate time . 62
Figure 34 – Dynamic performance: operate time and dynamic overreach (CVT-SIR
diagram) . 64
Figure 35 – CVT-SIR diagram for short line: minimum operate time . 66
Figure 36 – CVT-SIR diagram for short line: average operate time . 66
Figure 37 – CVT-SIR diagram for a short line: maximum operate time . 67
Figure 38 – Fault statistics for typical operate time . 70
Figure 39 – Frequency distribution of operate time . 73
Figure 40 – Ramping test for harmonics . 75
Figure 41 – Steady-state harmonics test . 77
Figure 42 – Simulated power system network . 78
Figure 43 – Flowchart of transient oscillation tests . 79
Figure 44 – Simulated voltages (U , U , U ) and currents (I , I , I ) . 81
L1 L2 L3 L1 L2 L3
Figure 45 – Transient oscillation tests – Operate time . 82
Figure 46 – Test points for quadrilateral characteristics . 83
Figure 47 – Test points for MHO characteristic . 83
Figure 48 – Test ramp direction for quadrilateral characteristic . 83
Figure 49 – Test ramp direction for MHO characteristic . 84
Figure 50 – Steady-state frequency deviation tests . 86
Figure 51 – Short line model for frequency deviation test . 87
Figure 52 – Flowchart of transient frequency deviation tests . 89

– 6 – IEC 60255-121:2014 © IEC 2014
Figure 53 – SIR diagrams for frequency deviation tests – average operate time . 90
Figure 54 – Network model for single line tests . 91
Figure 55 – Line to earth fault . 92
Figure 56 – Line to line fault . 92
Figure 57 – Line to line to earth fault . 92
Figure 58 – Three-phase fault . 93
Figure 59 – Network model for parallel lines tests . 98
Figure 60 – Network model for current reversal test . 99
Figure A.1 – Non-directional circular characteristic with directional supervision . 102
Figure A.2 – MHO characteristic . 103
Figure A.3 – Quadrilateral/polygonal characteristics . 104
Figure A.4 – Non-directional circular characteristic (ohm) . 106
Figure A.5 – Reactive reach line characteristic . 107
Figure A.6 – MHO characteristics . 107
Figure A.7 – Resistive and reactive intersecting lines characteristics . 108
Figure A.8 – Offset MHO . 108
Figure B.1 – The same fault type evolving from time delayed zone 3 (position 1) into
time delayed zone 2 (position 2) after 200 ms . 110
Figure B.2 – Phase to earth fault in time delayed zone 3 (position 1) evolving into
three-phase fault in the same zone (position 2) after 200 ms . 111
Figure C.1 – Setting example for a radial feeder . 112
Figure C.2 – Phase to earth fault (LN) . 113
Figure C.3 – Phase to phase fault (LL) . 114
Figure E.1 – Fault positions to be considered for specifying the CT requirements . 117
Figure F.1 – Fault positions to be considered . 119
Figure F.2 – Double source network . 120
Figure F.3 – Magnetization curve for the basic CT . 122
Figure F.4 – Secondary current at the limit of saturation caused by AC component with
no remanent flux in the CT . 123
Figure F.5 – Secondary current in case of maximum DC offset . 123
Figure G.1 – Distance relay example 1 . 126
Figure G.2 – Distance relay example 2 . 128
Figure H.1 – Quadrilateral/polygonal characteristic showing test point P on the reactive
reach line . 131
Figure H.2 – Quadrilateral distance protection function characteristic showing test
point P on the resistive reach line. . 132
Figure H.3 – MHO characteristic showing test point P . 133
Figure I.1 – Three-line diagram showing relay connections and L1N fault . 135
Figure I.2 – Voltage and current phasors for L1N fault . 135
Figure I.3 – Voltages and currents for L1N fault, constant fault current . 136
Figure I.4 – Voltages and currents for L1N fault, constant fault voltage . 136
Figure I.5 – Three-line diagram showing relay connections and L1L2 fault . 137
Figure I.6 – Voltage and current phasors for L1L2 fault . 138
Figure I.7 – Voltages and currents for L1L2 fault, constant fault current . 138
Figure I.8 – Voltages and currents for L1L2 fault, constant fault voltage . 139

Figure I.9 – Pseudo-continuous ramp distance relay characteristic on an impedance
plane . 140
Figure I.10 – Pseudo-continuous ramp showing impedance step change and the time step 140
Figure I.11 – Ramp of shots distance relay characteristic on an impedance plane . 141
Figure I.12 – Ramp of shots showing impedance step change and the time step . 142
Figure I.13 – Ramp of shots with binary search algorithm . 142
Figure J.1 – Graphical definition of fault inception angle . 143
Figure K.1 – CVT equivalent electrical circuit . 145
Figure K.2 – Transient response of the 50 Hz version of the CVT model . 147

Table 1 – Example of effective and operating ranges of distance protection . 18
Table 2 – Recommended levels of remanence in the optional cases when remanence
is considered . 27
Table 3 – Basic characteristic accuracy for various points (quadrilateral/polygonal) . 42
Table 4 – Overall basic characteristic accuracy (quadrilateral/polygonal) . 42
Table 5 – Basic characteristics accuracy for various points (MHO) . 42
Table 6 – Overall basic characteristic accuracy (MHO) . 42
Table 7 – Basic directional accuracy for various fault types . 47
Table 8 – Basic directional accuracy e . 47
α
X
Table 9 – Results of disengaging time for all the tests . 50
Table 10 – Short line SIR and source impedance for selected rated current and
frequency . 53
Table 11 – Short line SIR and source impedances for other rated current and
frequency . 54
Table 12 – Long line SIR and source impedances for selected rated current and
frequency . 59
Table 13 – Long line SIR and source impedances for other rated current and frequency . 60
Table 14 – Short line CVT-SIR source impedance. 63
Table 15 – Transient overreach table for short line . 68
Table 16 – Transient overreach table for long line. 68
Table 17 – Transient overreach table for short line with CVTs . 69
Table 18 – Typical operate time . 71
Table 19 – Typical operate time . 71
Table 20 – Typical operate time . 72
Table 21 – Typical operate time (mode, median, mean) . 73
Table 22 – Steady state harmonics test . 75
Table 23 – Capacitance values . 78
Table 24 – Quadrilateral/polygonal basic characteristic accuracy at f and f . 85
min max
Table 25 – MHO basic characteristic accuracy at f and f . 85
min max
Table 26 – Tests without pre-fault load . 94
Table 27 – Tests with pre-fault load . 95
Table 28 – Current reversal test . 98
Table 29 – Evolving faults (only one line affected) . 99
Table 30 – Evolving faults (both lines affected) . 100
Table 31 – Double infeed test results . 101

– 8 – IEC 60255-121:2014 © IEC 2014
Table F.1 – Magnetization curve data . 122
Table G.1 – Fault currents . 127
Table G.2 – Fault currents . 128
Table J.1 – Fault type and reference voltage . 144
Table K.1 – Parameter values for the 50 Hz version of the CVT model . 146
Table K.2 – Parameter values for the 60 Hz version of the CVT model . 146

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 121: Functional requirements for distance protection

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International Standard IEC 60255-121 has been prepared by IEC technical committee 95:
Measuring relays and protection equipment.
This standard cancels and replaces IEC 60255-16.
The text of this standard is based on the following documents:
FDIS Report on voting
95/319/FDIS 95/321/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 10 – IEC 60255-121:2014 © IEC 2014
A list of all parts in the IEC 60255 series, published under the general title Measuring relays
and protection equipment, 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.
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 121: Functional requirements for distance protection

1 Scope
This part of IEC 60255 specifies minimum requirements for functional and performance
evaluation of distance protection function typically used in, but not limited to, line applications
for effectively earthed, three-phase power systems. This standard also defines how to
document and publish performance tests.
This standard covers distance protection function whose operating characteristic can be
defined on an impedance plane and includes specification of the protection function,
measurement characteristics, phase selection, directionality, starting and time delay
characteristics.
The test methodologies for verifying performance characteristics and accuracy are included in
this standard. The standard defines the influencing factors that affect the accuracy under
steady state conditions and performance characteristics during dynamic conditions. It also
includes the instrument transformer requirements for the protection function.
The distance protection functions covered by this standard are as follows:
IEC 61850-7-4
IEEE/ANSI C37.2
Function numbers Logical nodes
Phase distance protection 21 PDIS
Earth (ground) distance protection 21G PDIS
This standard does not specify the functional description of additional features often
associated with digital distance relays such as power swing blocking (PSB), out of step
tripping (OST), voltage transformer (VT) supervision, switch onto fault (SOTF), trip on reclose
(TOR), the logic fo
...