SIST EN IEC 60255-181:2020

Measuring relays and protection equipment - Part 181: Functional requirements for frequency protection

IEC 60255-181:2019 specifies the minimum requirements for functional and performance evaluation of frequency protection. This document also defines how to document and publish performance test results.
This document covers the functions based on frequency measurement or rate of change of frequency measurements. This document also covers frequency protection where additional blocking elements are used.
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.

Messrelais und Schutzeinrichtungen - Teil 181: Funktionsanforderungen für den Frequenzschutz

Relais de mesure et dispositifs de protection - Partie 181: Exigences fonctionnelles relatives aux protections de fréquence

IEC 60255-181:2019 spécifie les exigences minimales relatives à l'évaluation fonctionnelle et à l'évaluation de la performance des protections de fréquence. Le présent document définit également comment documenter et publier les résultats des essais de performance.
Le présent document couvre les fonctions basées sur la mesure de fréquence ou la mesure de la dérivée de fréquence. Il couvre également la protection de fréquence lorsque des éléments de blocage complémentaires sont utilisés.
Le présent document définit les facteurs d'influence qui affectent la précision dans des conditions de régime établi et les caractéristiques de performance dans des conditions dynamiques. Les méthodologies d'essai pour la vérification des caractéristiques de performance et de précision figurent également dans le présent document.

Merilni releji in zaščitna oprema - 181. del: Funkcijske zahteve za frekvenčno zaščito

General Information

• Status: Published
• Publication Date: 15-Apr-2020
• ICS: 29.120.70 - Relays
• Technical Committee: ISS EIT.ERE - All-or-nothing electrical relays
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 30-Mar-2020
• Due Date: 04-Jun-2020
• Completion Date: 16-Apr-2020
• Ref Project: EN IEC 60255-181:2019 - Measuring relays and protection equipment - Part 181: Functional requirements for frequency protection

Overview

EN IEC 60255-181:2019 (CLC adoption of IEC 60255-181:2019) defines minimum functional and performance requirements for frequency protection used in power systems. The standard specifies how to evaluate, test and document protective functions that rely on frequency measurement or rate-of-change-of-frequency (ROCOF) measurements. It also addresses frequency protection schemes that include additional blocking or restraint elements, and identifies influence factors affecting accuracy in steady-state and dynamic conditions.

Key topics and requirements

• Function specification: definition of input energizing quantities, binary inputs/outputs, operating and reset characteristics for under‑ and over‑frequency and ROCOF functions.
• Performance specification: effective/operating ranges, accuracy related to the characteristic quantity, start times, operate time delay accuracy, disengaging time, reset hysteresis/reset ratio and performance with restraint/blocking.
• Dynamic behaviour tests: methods to measure performance during frequency ramps, sudden frequency changes, and ROCOF events.
• Disturbance and interference: requirements and test procedures to assess accuracy in the presence of harmonics, inter‑harmonics and sudden voltage phase or magnitude changes.
• Test methodology: detailed, reproducible procedures for determining start value accuracy, start time, operate time delay, disengaging time and hysteresis (includes example waveforms and test signal equations).
• Documentation: requirements for type test reports and user documentation - how to report and publish performance test results.
• Measurement algorithms: informative annexes covering frequency measurement methods (periodic, analysis algorithms, DFT, error minimization) and examples for ROCOF testing.

Practical applications

• Preparing type-test and factory-test procedures for frequency relays and protection equipment.
• Writing procurement and specification documents for utilities and OEMs that require validated frequency protection performance.
• Verifying compliance of under/over‑frequency and ROCOF relays with grid‑code or interconnection requirements.
• Guiding R&D and product design teams on measurable performance targets (accuracy, start/operate time, stability under harmonics).
• Supporting certification bodies and test laboratories in producing consistent, reproducible test reports.

Who uses this standard

• Protection engineers and relay designers
• Utility engineers and grid operators
• Relay manufacturers and OEMs
• Test laboratories and certification bodies
• Procurement and compliance teams involved in grid‑code conformance

Related standards

Normative and related references include IEC 60255-1, IEC 60255-24, IEC 61850 series, IEC 61869 series, and IEC 61000-2-4 - useful for broader relay functionality, communications and electromagnetic compatibility.

Keywords: EN IEC 60255-181, IEC 60255-181:2019, frequency protection, frequency relays, ROCOF, under/over frequency, performance tests, measuring relays, relay test methodology, harmonics, grid protection.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2020
Merilni releji in zaščitna oprema - 181. del: Funkcijske zahteve za frekvenčno
zaščito
Measuring relays and protection equipment - Part 181: Functional requirements for
frequency protection
Ta slovenski standard je istoveten z: EN IEC 60255-181:2019
ICS:
29.120.70 Releji Relays
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 60255-181

NORME EUROPÉENNE
EUROPÄISCHE NORM
April 2019
ICS 29.120.70
English Version
Measuring relays and protection equipment - Part 181:
Functional requirements for frequency protection
(IEC 60255-181:2019)
Relais de mesure et dispositifs de protection - Partie 181: Messrelais und Schutzeinrichtungen - Teil 181:
Exigences fonctionnelles relatives aux protections de Funktionsanforderungen für den Frequenzschutz
fréquence (IEC 60255-181:2019)
(IEC 60255-181:2019)
This European Standard was approved by CENELEC on 2019-04-03. 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, 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
© 2019 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60255-181:2019 E

European foreword
The text of document 95/402/FDIS, future edition 1 of IEC 60255-181, 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 60255-181:2019.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2020-01-03
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2022-04-03
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.

Endorsement notice
The text of the International Standard IEC 60255-181:2019 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 60255-24 NOTE Harmonized as EN 60255-24
IEC 61000-2-4:2002 NOTE Harmonized as EN 61000-2-4:2002 (not modified)
IEC 61850-7-4 NOTE Harmonized as EN 61850-7-4
IEC 61850-9-2 NOTE Harmonized as EN 61850-9-2
IEC 61869-9:2016 NOTE Harmonized as EN 61869-9:— (not modified)

Under preparation. Stage at time of publication: FprEN 61869-9:2016.
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 60050-103 -  International Electrotechnical Vocabulary - - -
Part 103: Mathematics - Functions
IEC 60050-447 -  International Electrotechnical Vocabulary - - -
Part 447: Measuring relays
IEC 60050-601 -  International Electrotechnical Vocabulary. - -
Chapter 601: Generation, transmission and
distribution of electricity - General
IEC 60255-1 -  Measuring relays and protection equipment EN 60255-1 -
- Part 1: Common requirements
IEC 61850 series Communication networks and systems for EN 61850 series
power utility automation
IEC 61869 series Instrument transformers EN 61869 series

IEC 60255-181 ®
Edition 1.0 2019-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Measuring relays and protection equipment –

Part 181: Functional requirements for frequency protection

Relais de mesure et dispositifs de protection –

Partie 181: Exigences fonctionnelles relatives aux protections de fréquence

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.120.70 ISBN 978-2-8322-6496-6

– 2 – IEC 60255-181:2019 © IEC 2019
CONTENTS
FOREWORD . 6
1 Scope . 8
2 Normative references . 9
3 Terms and definitions . 9
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 . 14
4.4.1 Operating characteristics . 14
4.4.2 Reset characteristics . 17
4.5 Additional influencing functions/conditions . 18
4.5.1 General . 18
4.5.2 Specific characteristics for under/over frequency function . 18
4.5.3 Specific characteristics for rate of change of frequency (ROCOF)
function . 19
4.6 Binary output signals . 19
4.6.1 General . 19
4.6.2 Start (pick-up) signal . 19
4.6.3 Operate (trip) signal . 19
4.6.4 Other binary output signals . 19
5 Performance specification . 20
5.1 General . 20
5.2 Effective and operating ranges . 20
5.3 Accuracy related to the characteristic quantity . 21
5.4 Start time for under/over frequency function . 21
5.5 Start time for rate of change of frequency (ROCOF) function . 21
5.6 Accuracy related to the operate time delay setting . 22
5.7 Disengaging time . 22
5.8 Reset hysteresis and reset ratio . 22
5.9 Accuracy related to restraint/blocking elements . 23
5.10 Performance with harmonics . 23
5.11 Stability in case of sudden voltage change (phase shift and magnitude shift) . 23
5.12 Voltage input requirements . 23
6 Functional test methodology . 24
6.1 General . 24
6.2 Determination of steady state errors related to the characteristic quantity . 26
6.2.1 Accuracy of the start value . 26
6.2.2 Reset hysteresis or reset ratio determination . 32
6.3 Determination of the start time . 41
6.3.1 General . 41
6.3.2 Under/over frequency . 41
6.3.3 Rate of change of frequency . 47
6.4 Determination of the accuracy of the operate time delay . 50
6.4.1 General . 50
6.4.2 Description of test method . 50
6.4.3 Reporting of the operate time delay accuracy . 52

IEC 60255-181:2019 © IEC 2019 – 3 –
6.5 Determination of disengaging time . 53
6.5.1 General . 53
6.5.2 Under/over frequency . 53
6.5.3 Rate of change of frequency . 56
6.6 Performance with harmonics . 58
6.6.1 General . 58
6.6.2 Accuracy of the under/over frequency start value in the presence of
harmonics . 58
6.6.3 Accuracy of the ROCOF start value in the presence of harmonics . 63
6.7 Stability in the case of sudden voltage change (phase shift and magnitude
change) . 65
6.7.1 General . 65
6.7.2 Performance in case of voltage phase shift and magnitude change . 65
6.7.3 Performance in case of voltage magnitude drop and restoration . 68
7 Documentation requirements . 70
7.1 Type test report . 70
7.2 Other user documentation . 71
Annex A (normative) Test signal equation with constant frequency variation (df/dt) . 72
Annex B (normative) Calculation of mean, median and mode . 73
B.1 Mean . 73
B.2 Median . 73
B.3 Mode . 73
B.4 Example. 73
Annex C (informative) Example of frequency measurement and calculation . 74
C.1 Definitions. 74
C.2 Signal observation model . 74
C.3 General requirements on frequency measurement . 76
C.3.1 General requirements on frequency measurement . 76
C.3.2 Periodic algorithm . 76
C.3.3 Analysis algorithm . 78
C.3.4 Error minimization algorithm . 79
C.3.5 Discrete Fourier transformation (DFT) . 82
Annex D (informative) Performance with inter-harmonics . 84
D.1 General . 84
D.2 Proposed test: accuracy of the under/over frequency start value . 84
D.2.1 Description of the generated frequency ramp . 84
D.2.2 Protection function settings . 85
D.2.3 Test points and calculation of frequency accuracy in the presence of
inter-harmonics . 86
D.2.4 Reporting of frequency accuracy in the presence of inter-harmonics . 86
Annex E (informative) Management of sudden frequency change without discontinuity
in voltage waveform . 87
Bibliography . 90

Figure 1 – Operate time and operate time delay setting . 11
Figure 2 – Simplified protection function block diagram . 13
Figure 3 – Underfrequency independent time characteristic . 15
Figure 4 – Overfrequency independent time characteristic . 16

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Figure 5 – ROCOF independent time characteristic (for negative or positive ROCOF) . 16
Figure 6 – Explanatory diagram for start, operate, disengage and reset . 18
Figure 7 – Example of test method for overfrequency . 26
Figure 8 – Example of test method for positive ROCOF function . 29
Figure 9 – Frequency ramps for assessing the reset hysteresis for overfrequency
functions . 33
Figure 10 – Frequency ramps for assessing the reset hysteresis for underfrequency

functions . 33
Figure 11 – Test method for measurement of reset value for ROCOF functions:
example for positive ROCOF function . 37
Figure 12 – Start time measurement of overfrequency with sudden frequency change . 42
Figure 13 – Start time measurement of overfrequency with constant slope frequency
ramp . 43
Figure 14 – Example of start time reporting for under/over frequency protection
function . 47
Figure 15 – Start time measurement of positive ROCOF function . 48
Figure 16 – Histogram for the start time test results for ROCOF. 50
Figure 17 – Operate time delay measurement of overfrequency and positive ROCOF . 51
Figure 18 – Disengaging time measurement of overfrequency with sudden frequency
change . 54
Figure 19 – Disengaging time measurement of overfrequency with constant slope
frequency ramp . 54
Figure 20 – Disengaging time measurement of ROCOF . 56
Figure 21 – Histogram for the disengaging time test results for ROCOF . 58
Figure 22 – Example of an increasing pseudo-continuous ramp for overfrequency

functions . 59
Figure 23 – Voltage signal with superimposed harmonics . 61
Figure 24 – Representation of the input energizing quantity (voltage, RMS) injection
sequence . 67
Figure 25 – Representation of the input energizing quantity (voltage, RMS) injection
sequence with the power system frequency values . 69
Figure C.1 – Zero-crossing algorithm . 77
Figure C.2 – Level-crossing algorithm . 77
Figure D.1 – Example of an increasing pseudo-continuous ramp for overfrequency
function . 84
Figure E.1 – Example of voltage waveform without discontinuity at to = 0,02 s. 88
Figure E.2 – Example of voltage waveform with discontinuity at to = 0,02 s . 89

Table 1 – Frequency protection designation . 8
Table 2 – Example of effective and operating ranges for over/under frequency
protection . 20
Table 3 – Example of effective and operating ranges for ROCOF protection . 20
Table 4 – Test points for under/over frequency function . 28
Table 5 – Reporting of the frequency accuracy . 28
Table 6 – Reporting of the frequency accuracy (alternative solution) . 29
Table 7 – Test points for ROCOF function . 31
Table 8 – Reporting of ROCOF accuracy . 32

IEC 60255-181:2019 © IEC 2019 – 5 –
Table 9 – Test points of reset hysteresis for under/over frequency function . 35
Table 10 – Reporting of the reset hysteresis for over/under frequency functions . 36
Table 11 – Test points of reset value for ROCOF function . 40
Table 12 – Reporting of the reset value for ROCOF function . 40
Table 13 – Test points of start time for overfrequency function . 44
Table 14 – Test points of start time for underfrequency function . 45
Table 15 – Reporting of start time for under/over frequency functions . 46
Table 16 – Test points of start time for ROCOF function . 49
Table 17 – Reporting of typical start time for ROCOF function . 50
Table 18 – Test points to measure operate time delay . 52
Table 19 – Test points for accuracy of the operate time delay . 52
Table 20 – Reporting of operate time delay accuracy for under/over frequency
functions . 53
Table 21 – Test points of disengaging time for overfrequency function . 55
Table 22 – Test points of disengaging time for underfrequency function. 55
Table 23 – Reporting of disengaging time for over/under frequency functions . 56
Table 24 – Test points of disengaging time for ROCOF function . 57
Table 25 – Typical disengaging time for ROCOF protection . 58
Table 26 – Superimposed harmonics . 60
Table 27 – Test points for under/over frequency function in the presence of harmonics . 63
Table 28 – Test points for ROCOF function in the presence of harmonics . 64
Table 29 – Under/over frequency settings for stability tests with voltage
drop/restoration . 70
Table D.1 – Superimposed inter-harmonics . 85
Table D.2 – Test points for under/overfrequency function in the presence of inter-
harmonics . 86

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 181: Functional requirements for frequency protection

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60255-181 has been prepared by IEC technical committee 95:
Measuring relays and protection equipment.
The text of this International Standard is based on the following documents:
FDIS Report on voting
95/402/FDIS 95/409/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
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.

IEC 60255-181:2019 © IEC 2019 – 7 –
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document 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.
– 8 – IEC 60255-181:2019 © IEC 2019
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 181: Functional requirements for frequency protection

1 Scope
This part of IEC 60255 specifies the minimum requirements for functional and performance
evaluation of frequency protection. This document also defines how to document and publish
performance test results.
This document covers the functions based on frequency measurement or rate of change of
frequency measurements. This document also covers frequency protection where additional
blocking elements are used.
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.
The frequency functions covered by this document are shown in Table 1:
Table 1 – Frequency protection designation
IEEE/ANSI C37.2 IEC 61850-7-4 logical
function numbers nodes
Underfrequency protection 81U PTUF
Overfrequency protection 81O PTOF
Rate of change of frequency protection (ROCOF) 81R PFRC

This functional document is applicable to frequency functions embedded in a protection relay
but also to other physical devices which include frequency protection in their functionality (for
example, trip units in a low-voltage circuit breaker or inverters associated with photovoltaic or
storage systems).
This document does not cover synchronizing or synchronism-check functions.
This document does not specify the functional description of additional features often
associated with frequency functions such as undervoltage blocking, df/dt or ∆f/∆t supervision,
current supervision or power supervision (f/P function). Only their influence on the frequency
protection function is covered in this document.
Frequency and rate of change of frequency measurement outputs provided by protection
devices are not in the scope of this document.
Additionally, this document does not explicitly cover the frequency relays based on current as
the input energizing quantity but the principles covered by this document can be extended to
provide guidance for these applications.
The general requirements for measuring relays and protection equipment are defined in
IEC 60255-1.
IEC 60255-181:2019 © IEC 2019 – 9 –
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 60255-1, Measuring relays and protection equipment – Part 1: Common requirements
IEC 60050-103, International Electrotechnical Vocabulary – Part 103: Mathematics –
Functions
IEC 60050-447, International Electrotechnical Vocabulary – Part 447: Measuring relays
IEC 60050-601, International Electrotechnical Vocabulary – Chapter 601: Generation,
transmission and distribution of electricity – General
IEC 61850 (all parts), Communication networks and systems for power utility automation
IEC 61869 (all parts), Instrument transformers
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-103,
IEC 60050-447, IEC 60050-601, 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
input energizing quantity
energizing quantity which either by itself constitutes the characteristic quantity or helps to
constitute it
Note 1 to entry: For the frequency protection function, the input characteristic quantity could be voltage.
[SOURCE: IEC 60050-447:2010, 447-03-02, modified – The note to entry has been replaced
by a new note.]
3.2
characteristic quantity
electric quantity, or one of its parameters, the name of which characterizes a measuring relay
or protection equipment and the values of which are the subject of accuracy requirements
Note 1 to entry: For underfrequency protection and overfrequency protection, the characteristic quantity is
frequency; for rate of change of frequency protection (ROCOF), the characteristic quantity is rate of change of
frequency.
[SOURCE: IEC 60050-447:2010, 447-07-01, modified – The examples have been replaced by
a new note to entry.]
– 10 – IEC 60255-181:2019 © IEC 2019
3.3
characteristic curve
curve which represents the relationship between the theoretical specified operate time and
the characteristic quantity
3.4
setting value of the characteristic quantity
G
S
value of the characteristic quantity used as the reference for the definition of the
characteristic curve
3.5
start value
value of the characteristic quantity at which a measuring relay or protection equipment starts
(picks up)
Note 1 to entry: Start value is also called "pick-up value".
3.6
reset value
value of the characteristic quantity at which a measuring relay or protection equipment resets
3.7
start time
time interval between the instant a specified change is made in the value(s) of the input
energizing quantity(ies) which will cause the measuring relay or protection equipment in initial
condition or reset condition to start and the instant it starts
Note 1 to entry: Start time is also called "pick-up time".
3.8
operate time
time interval between the instant a specified change is made in the value(s) of the input
energizing quantity(ies) which will cause the measuring relay or protection equipment in initial
condition or reset condition to operate and the instant it operates
Note 1 to entry: The operate time of the protection function is the sum of the start time and the operate time delay
setting.
3.9
operate time delay setting
intentional time delay defined by a user setting which is activated by the start signal to assert
the operate signal
Note 1 to entry: The operate time of the frequency protection function is the sum of the start time (pick-up time)
and the operate time delay setting. The difference between operate time and operate time delay setting is specified
in Figure 1. Figure 1 is based on a sudden frequency change only to simplify the definition of start time initialization.

IEC 60255-181:2019 © IEC 2019 – 11 –

a
Take underfrequency protection as an example.
Figure 1 – Operate time and operate time delay setting
3.10
disengaging time
time interval between the instant a specified change is made in the value(s) of the input
energizing quantity(ies) which will cause the measuring relay or protection equipment in
operate condition to disengage and the instant it disengages
3.11
reset time
duration between the instant a specified change is made in the value(s) of the input
energizing quantity(ies) which will cause the measuring relay or protection equipment to reset
and the instant it resets
[SOURCE: IEC 60050-447:2010, 447-05-06, modified. See figure 6.]

3.12
reset hysteresis
absolute value of the difference between the reset value and the start value of the protection
function
3.13
reset ratio
ratio between the reset value and the start value of the protection function
Note 1 to entry: Reset ratio is used for rate of change of frequency, as defined in 4.4.2 and 5.8.

– 12 – IEC 60255-181:2019 © IEC 2019
3.14
operating range
range for which the measuring relay under specified conditions is able to perform its intended
function(s) according to the specified requirements
Note 1 to entry: When accuracy requirements have to be met, see effective range (IEC 60050-447:2010,
447-07-08).
Note 2 to entry: A minimum level of input energizing quantity, such as voltage, is required in order to calculate the
power frequency correctly.
[SOURCE: IEC 60050-447:2010, 447-03-16, modified – Note 2 to entry has been added.]
3.15
effective range
part of the operating range of an input energizing quantity or characteristic quantity within
which the accuracy requirements are met
Note 1 to entry: This note applies to the French language only.
[SOURCE: IEC 60050-447:2010, 447-07-08]
3.16
rate of change of frequency protection
ROCOF
protection function intended to operate when frequency changes by a given amount per unit of
time
Note 1 to entry: This note applies to the French language only.
3.17
period
T
smallest positive difference between two values of the independent variable at which the
values of a periodic quantity are identically repeated
Note 1 to entry: If ft() denotes a periodic quantity, then ft( +=T ) ft() .
Note 2 to entry: The term "period duration" is sometimes used in the case of a function of time.
Note 3 to entry: The symbol T is mainly used for the period when the independent variable is time.
[SOURCE: IEC 60050-103:2009, 103-06-01]
3.18
frequency
f
reciprocal of the period
Note 1 to entry: The symbol f is mainly used when the period is a time. The symbol ν (nu) is mainly used in optics.
Note 2 to entry: For a sinusoidal waveform, frequency is the first derivative of the phase angle
(see IEC/IEEE 60255-118-1: 2018). A detailed description can be found in Annex D.
[SOURCE: IEC 60050-103:2017, 103-06-02, modified – Note 2 to entry has been replaced by
a new note.]
3.19
power frequency
conventionally, the values of frequency used in the electricity supply systems
[SOURCE: IEC 60050-601:1985, 601-01-05]

IEC 60255-181:2019 © IEC 2019 – 13 –
4 Specification of the function
4.1 General
An example of the protection function with its inputs, outputs, measuring element, time delay
characteristics and functional logic is shown in Figure 2. The manufacturer shall provide the
functional block diagram of the specific implementation.

Figure 2 – Simplified protection function block diagram
4.2 Input energizing quantities / energizing quantities
The input energizing quantities are the measuring signals, for example, voltages. In the case
of analogue entries, their ratings and relevant standards are specified in IEC 60255-1. Input
energizing quantities can be acquired via direct connection with primary conductors (for
example, low-voltage busbars) or from instrument transformers – for example, voltage
transformer, VTs, according to IEC 61869 (all parts) – or as a data packet over a
communication port using an appropriate communication protocol (such as that of
IEC 61850-9-2 and, more specifically, IEC 61869-9).
The protection function documentation shall state the type of input energizing quantities used
by the protection function. Examples are:
• single or multi phase-to-earth or phase-to-neutral voltage measurement;
• single or multi phase-to-phase voltage measurement;
• phase (line) currents.
The manufacturer shall specify which energizing quantities are used for the operation of the
frequency protection. As illustrated in Figure 2, the energizing quantities can differ to that of
the input energizing quantities, for example:
• use of phase-to-earth or phase-to-phase voltage;
• use of two different voltage sources;
• use of derived signals from phase quantities, for example, positive sequence voltage or
calculated phase-to-phase voltages, etc.;
• use of current, such as phase current or positive sequence current, etc.
The manufacturer shall specify which characteristic quantity is used for the operation of
frequency protections. Examples are:
• power frequency measurement;
• rate of change of frequency (df/dt) measurement (ROCOF).

– 14 – IEC 60255-181:2019 © IEC 2019
Power frequency and df/dt quantities can be measured over a different time period (for
example, one or several power frequency cycle(s)). This time period can be fixed or
adjustable according to a user setting. When this time period is adjustable, the setting value
has an impact on the time characteristic (for example, start time).
The manufacturer shall advise if the frequency or ROCOF measurement is impacted by a user
setting (for example, frequency averaged over selectable number of cycles). In such case, the
manufacturer shall declare the impact of these settings on the function performances and
recommend settings for the relevant functional tests and for applications.
All of these characteristic quantities are based on a basic frequency measurement. Different
algorithms exist and a selection of them are described in Annex C.
When the protection device is not equipped with voltage inputs, or when voltage signals are
not present or under a blocking threshold, frequency measurement could be performed on line
current inputs. Such a feature is generally used to manage the frequency tracking and not
recommended to manage frequency protections. The manufacturer shall declare if phase
currents are used as input energizing quantities within their frequency protections.
Performances of frequency protection based on phase current are not covered by type tests
defined in Clause 6, but the test principle may be extended to provide guidance for these
applications.
4.3 Binary input signals
If any binary input signals (externally or internally driven) are used, the
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