EN IEC 62855:2021

SLOVENSKI STANDARD
01-september-2021
Jedrske elektrarne - Elektroenergetski sistemi - Analiza elektroenergetskih
sistemov (IEC 62855:2016)
Nuclear power plants - Electrical power systems - Electrical power systems analysis (IEC
62855:2016)
Kernkraftwerke - Elektrische Stromversorgung - Analyse der Stromversorgung (IEC
62855:2016)
Centrales nucléaires de puissance - Systèmes d’alimentation électrique - Analyse des
systèmes d’alimentation électrique (IEC 62855:2016)
Ta slovenski standard je istoveten z: EN IEC 62855:2021
ICS:
27.120.20 Jedrske elektrarne. Varnost Nuclear power plants. Safety
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 62855

NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2021
ICS 27.120.20
English Version
Nuclear power plants - Electrical power systems - Electrical
power systems analysis
(IEC 62855:2016)
Centrales nucléaires de puissance - Systèmes Kernkraftwerke - Elektrische Stromversorgung - Analyse
d'alimentation électrique - Analyse des systèmes der Stromversorgung
d'alimentation électrique (IEC 62855:2016)
(IEC 62855:2016)
This European Standard was approved by CENELEC on 2021-07-05. 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 62855:2021 E
European foreword
This document (EN IEC 62855:2021) consists of the text of IEC 62855:2016 prepared by IEC/TC 45
"Nuclear instrumentation".
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2022-07-05
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2024-07-05
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.
As stated in the nuclear safety directive 2009/71/EURATOM, Chapter 1, Article 2, item 2, Member
States are not prevented from taking more stringent safety measures in the subject-matter covered by
the Directive, in compliance with Community law.
In a similar manner, this European standard does not prevent Member States from taking more
stringent nuclear safety and/or security measures in the subject-matter covered by this standard.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN and CENELEC
websites.
Endorsement notice
The text of the International Standard IEC 62855:2016 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 60038 NOTE Harmonized as EN 60038
IEC 60364-5-52 NOTE Harmonized as HD 60364-5-52
IEC 60880 NOTE Harmonized as EN 60880
IEC 60964 NOTE Harmonized as EN IEC 60964
IEC 61225 NOTE Harmonized as EN IEC 61225
IEC 61508-1 NOTE Harmonized as EN 61508-1
IEC 61508-2 NOTE Harmonized as EN 61508-2
IEC 61508-3 NOTE Harmonized as EN 61508-3
IEC 61508-4 NOTE Harmonized as EN 61508-4
IEC 61513 NOTE Harmonized as EN 61513
IEC 62003 NOTE Harmonized as EN IEC 62003
IEC 62138 NOTE Harmonized as EN IEC 62138
IEC 62271-200 NOTE Harmonized as EN 62271-200
IEC 62305-1 NOTE Harmonized as EN 62305-1
IEC 62305-3 NOTE Harmonized as EN 62305-3
IEC 62305-4 NOTE Harmonized as EN 62305-4
IEC 62340 NOTE Harmonized as EN 62340
IEC 62566 NOTE Harmonized as EN 62566
IEC 62645 NOTE Harmonized as EN IEC 62645
IEC 63046 NOTE Harmonized as EN IEC 63046
ISO/IEC 27001 NOTE Harmonized as EN ISO/IEC 27001
ISO/IEC 27002 NOTE Harmonized as EN ISO/IEC 27002

IEC 62855 ®
Edition 1.0 2016-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Nuclear power plants – Electrical power systems – Electrical power systems

analysis
Centrales nucléaires de puissance – Systèmes d’alimentation électrique –

Analyse des systèmes d’alimentation électrique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.120.20 ISBN 978-2-8322-3589-8

– 2 – IEC 62855:2016  IEC 2016
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 10
2 Normative references. 11
3 Terms and definitions . 11
4 Symbols and abbreviations . 12
5 Electrical power system analyses . 13
5.1 Overview of typical studies . 13
5.1.1 Transient stability analyses . 13
5.1.2 Load flow studies . 13
5.1.3 Transient and dynamic studies . 13
5.1.4 Short circuit studies . 14
5.1.5 Electrical protection coordination and selectivity . 14
5.1.6 Lightning protection studies . 14
5.2 Applicability of analyses to different plant states . 14
5.2.1 General . 14
5.2.2 Recommendations . 14
5.3 Selection, verification and validation of analytical tools . 15
5.3.1 General . 15
5.3.2 Recommendations . 16
5.4 Electrical power system model . 17
5.5 Grid and NPP connection . 17
5.6 Updating of system analyses . 17
5.7 Prerequisites for the performance of electrical studies . 17
5.8 Acceptance requirements . 18
6 Off-site power transient stability analyses . 18
6.1 General . 18
6.2 Recommendation . 18
6.3 Acceptance requirements . 18
7 AC on-site power system analyses . 18
7.1 General . 18
7.2 Load flow studies . 19
7.2.1 General . 19
7.2.2 Recommendations . 19
7.2.3 Acceptance requirements . 19
7.3 Transient studies . 19
7.3.1 General . 19
7.3.2 Faulted conditions . 19
7.3.3 Bus transfer studies . 20
7.3.4 Motor starting and reacceleration studies . 21
7.3.5 House load operation . 22
7.3.6 Voltage disturbances . 22
7.3.7 Voltage surge caused by switching and malfunctions . 23
7.3.8 Load sequencer studies . 23
7.3.9 Frequency studies . 24
7.4 Fault studies . 24

IEC 62855:2016  IEC 2016 – 3 –
7.4.1 Short-circuit studies . 24
7.4.2 Earth fault (degraded insulation) studies . 25
7.5 Electrical protection coordination studies . 25
7.5.1 Recommendations . 25
7.5.2 Acceptance requirements . 26
8 DC system and uninterruptible AC system analyses . 26
8.1 Load flow studies . 26
8.1.1 General . 26
8.1.2 Recommendations . 26
8.1.3 Acceptance requirements . 27
8.2 Transient studies . 27
8.2.1 Rectifier . 27
8.2.2 Inverter/UPS and bypass switch . 27
8.3 Fault studies . 28
8.3.1 Short circuit studies . 28
8.3.2 Earth fault (degraded insulation) studies . 28
8.4 Electrical protection coordination studies . 28
8.4.1 Recommendations . 28
8.4.2 Acceptance requirements . 29
9 Miscellaneous analyses . 29
9.1 Lightning protection studies . 29
9.1.1 General . 29
9.1.2 Recommendation . 29
9.1.3 Acceptance requirements . 30
9.2 Electromagnetic compatibility . 30
9.2.1 General . 30
9.2.2 Recommendation . 30
9.3 Harmonic studies . 30
9.3.1 General . 30
9.3.2 Recommendation . 30
9.4 Geomagnetic induced current (GIC) . 30
9.5 Ferroresonance studies . 30
Annex A (informative) Establishment of design bases for nuclear power plant
electrical power systems . 31
A.1 Overview. 31
A.2 Site electrical characteristics . 34
A.2.1 General . 34
A.2.2 Grid disturbances . 35
A.2.3 Short circuit power . 35
A.2.4 Lightning protection and insulation coordination . 36
A.2.5 Earthing characteristics . 36
A.3 Plant electrical characteristics . 37
A.3.1 General . 37
A.3.2 Main generator characteristics . 37
A.3.3 Standby AC power sources and alternate AC source . 37
A.3.4 DC power sources . 39
A.4 Conceptual electrical design criteria . 39
A.4.1 General . 39
A.4.2 Capacity of power sources . 39

– 4 – IEC 62855:2016  IEC 2016
A.4.3 Protection coordination . 39
A.4.4 Voltage transients and interruptions . 40
A.4.5 Capability for bus transfer . 40
A.4.6 Capability for motor start and reacceleration . 40
A.4.7 System earthing . 40
A.4.8 Capability of electrical equipment . 41
A.4.9 Electromagnetic interference . 41
A.4.10 Geomagnetic induced currents . 41
A.4.11 Ferroresonance . 41
A.5 Conceptual nuclear design criteria . 42
A.5.1 General . 42
A.5.2 Reliability and availability, single failure criterion . 42
A.5.3 Common cause failures (CCF) and common mode failures (CMF) . 43
A.6 Design bases analysis . 43
A.6.1 General . 43
A.6.2 Voltage . 44
A.6.3 Sizing of safety standby AC power sources . 45
A.6.4 Frequency. 46
A.6.5 Electrical consumers’ databases and power balances . 46
Annex B (informative) Guidelines for analytical studies . 48
B.1 Analytical studies methodology . 48
B.1.1 General . 48
B.1.2 Process . 48
B.2 Example of detailed level . 50
B.2.1 Purpose . 50
B.2.2 Analysis and basics . 50
B.2.3 Minimum required data . 51
B.2.4 Contribution of the study . 51
Annex C (informative) Verification of design bases and equipment specification . 52
Annex D (informative) Example of plant specific acceptance criteria . 55
Bibliography . 56

Figure A.1 – Input and process to determine the specific electrical design bases for a
nuclear power plant and verification analyses . 32
Figure A.2 – Relationship of the plant electrical power system, the off-site electrical
power system and the on-site electrical power system for a nuclear power plant . 33
Figure A.3 – Relationship of power supplies important to safety, safety power supplies,
and the preferred power supply for a nuclear power plant . 34
Figure A.4 – Typical voltage design bases . 44
Figure B.1 – Overview of analytical studies . 48
Figure B.2 – Phenomena . 49

Table C.1 – AC on-site power system analyses . 52
Table C.2 – DC system and uninterruptible AC system analyses . 53

IEC 62855:2016  IEC 2016 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
______________
NUCLEAR POWER PLANTS –
ELECTRICAL POWER SYSTEMS –
ELECTRICAL POWER SYSTEMS ANALYSIS

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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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 62855 has been prepared by subcommittee 45A: Instrumentation,
control and electrical systems of nuclear facilities, of IEC technical committee 45: Nuclear
instrumentation.
The text of this document is based on the following documents:
FDIS Report on voting
45A/1094/FDIS 45A/1100/RVD
Full information on the voting for the approval of this document 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.

– 6 – IEC 62855:2016  IEC 2016
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.
IEC 62855:2016  IEC 2016 – 7 –
INTRODUCTION
a) Technical background, main issues and organisation of the Standard
The principal function of the electrical power system is to support the safe operation of a
nuclear power plant (NPP) in all modes of operation. A subset of the electrical power system
is essential for supporting nuclear safety functions at various voltage levels. This subset is
critical for all plant states and events requiring plant cool-down in a controlled manner. A
reliable power system is critical for maintaining control to power, control and monitor plant
safety functions. This is required to support the barriers that prevent radiological releases
during design basis accidents and design extension conditions.
International Standards and National safety codes provide guidance on acceptable
requirements for safe and reliable operation of electrical distribution systems. Compliance
with these safety codes and standards generally provides reasonable assurance for the
correct electrical functionality and capability of these systems in the nuclear power plant
(NPP).
The design basis of the electrical power systems in a NPP should be established by
consideration of the following elements:
• nuclear design criteria, defence in depth approach, safety classification, design basis
conditions (DBC) and design extension conditions (DEC);
• requirements for transmission system operating limits, grid safety, grid code, plant
performance and operating limits;
• architecture and specification of the electrical power systems;
• sizing of main components and systems such as unit auxiliary and standby transformers,
switchgear, cables, motors and standby alternating current (AC) and direct current (DC)
power sources;
• load allocations and load power balance;
• load flow calculations;
• coordination of characteristics (voltage, current and short circuit current);
• support system requirements during postulated DBCs;
• design verification including verification analyses.
An example of design bases considerations for electrical power systems is provided in
informative Annex A.
Guidelines and an example of analytical methods are detailed in informative Annex B. The
relationship between analyses and verification of design bases and equipment specification is
given in informative Annex C. An example of plant specific acceptance criteria (see 5.8) is
given in informative Annex D.
It is intended that the Standard will be used by operators of NPPs (utilities), systems
evaluators and by licensors.
b) Situation of the current Standard in the structure of the IEC SC 45A standard series
IEC 62855 is a third level IEC SC 45A document covering the topic of electrical power
systems analysis.
This standard supports the guidance provided in the IAEA Safety Guide SSG-34 related to the
design of electrical power systems for nuclear power plants.
This standard is related to
– 8 – IEC 62855:2016  IEC 2016
• IAEA Nuclear Energy Series NG-T-3.8 dealing with electric grid reliability and interface
with nuclear power plants, and
• IEC 61513 establishing general requirement for I&C systems important to safety used in
nuclear power plants.
For more details on the structure of the IEC SC 45A standard series, see item d) of this
introduction.
c) Recommendations and limitations regarding the application of this standard
To ensure that the Standard will continue to be relevant in future years, the emphasis has
been placed on issues of principle, rather than specific technologies.
d) Description of the structure of the IEC SC 45A standard series and relationships
with other IEC documents and other bodies documents (IAEA, ISO)
The top-level documents of the IEC SC 45A standard series are IEC 61513 and IEC 63046 .
IEC 61513 provides general requirements for I&C systems and equipment that are used to
perform functions important to safety in NPPs. IEC 63046 provides general requirements for
electrical power systems of NPP; it covers power supply systems including the supply
systems of the I&C systems. IEC 61513 and IEC 63046 are to be considered in conjunction
and at the same level. IEC 61513 and IEC 63046 structure the IEC SC 45A standard series
and shape a complete framework establishing general requirements for instrumentation,
control and electrical systems for nuclear power plants.
IEC 61513 and IEC 63046 refer directly to other IEC SC 45A standards for general topics
related to categorization of functions and classification of systems, qualification, separation,
defence against common cause failure, control room design, electromagnetic compatibility,
cybersecurity, software and hardware aspects for programmable digital systems, coordination
of safety and security requirements and management of ageing. The standards referenced
directly at this second level should be considered together with IEC 61513 and IEC 63046 as
a consistent document set.
At a third level, IEC SC 45A standards not directly referenced by IEC 61513 or by IEC 63046
are standards related to specific equipment, technical methods, or specific activities. Usually
these documents, which make reference to second-level documents for general topics, can be
used on their own.
A fourth level, extending the IEC SC 45 standard series, corresponds to the Technical
Reports, which are not normative.
The IEC SC 45A standards series consistently implements and details the safety and security
principles and basic aspects provided in the relevant IAEA safety standards and in the
relevant documents of the IAEA nuclear security series (NSS). In particular, this includes the
IAEA requirements SSR-2/1, establishing safety requirements related to the design of nuclear
power plants (NPP), the IAEA safety guide SSG-30 dealing with the safety classification of
structures, systems and components in NPP, the IAEA safety guide SSG-39 dealing with the
design of instrumentation and control systems for NPP, the IAEA safety guide SSG-34 dealing
with the design of electrical power systems for NPP and the implementing guide NSS 17 for
computer security at nuclear facilities. The safety and security terminology and definitions
used by SC 45A standards are consistent with those used by the IAEA.
____________
Under preparation. Stage at the time of publication: IEC ANW 63046:2016.

IEC 62855:2016  IEC 2016 – 9 –
IEC 61513 and IEC 63046 have adopted a presentation format similar to the basic safety
publication IEC 61508 (all parts) with an overall life-cycle framework and a system life-cycle
framework. Regarding nuclear safety, IEC 61513 and IEC 63046 provide the interpretation of
the general requirements of IEC 61508-1, IEC 61508-2 and IEC 61508-4, for the nuclear
application sector. In this framework, IEC 60880, IEC 62138 and IEC 62566 correspond to
IEC 61508-3 for the nuclear application sector. IEC 61513 and IEC 63046 refer to ISO as well
as to IAEA GS-R-3 and IAEA GS-G-3.1 and IAEA GS-G-3.5 for topics related to quality
assurance (QA). At level 2, regarding nuclear security, IEC 62645 is the entry document for
the IEC SC 45A security standards. It builds upon the valid high level principles and main
concepts of the generic security standards, in particular ISO/IEC 27001 and ISO/IEC 27002; it
adapts them and completes them to fit the nuclear context and coordinates with IEC 62443
(all parts). At level 2, regarding control rooms, IEC 60964 is the entry document for the IEC
SC 45A control rooms standards, and IEC 62342 is the entry document for the IEC SC 45A
ageing management standards.
NOTE 1 It is assumed that for the design of I&C systems in NPPs that implement conventional safety functions
(e.g. to address worker safety, asset protection, chemical hazards, process energy hazards) international or
national standards would be applied.
NOTE 2 IEC SC 45A domain was extended in 2013 to cover electrical systems. In 2014 and 2015 discussions
were held in IEC SC 45A to decide how and where general requirement for the design of electrical systems were to
be considered. IEC SC 45A experts recommended that an independent standard be developed at the same level as
IEC 61513 to establish general requirements for electrical systems. Project IEC 63046 is now launched to cover
this objective. When IEC 63046 will be published this NOTE 2 of the introduction of IEC SC 45A standards will no
longer be valid.
– 10 – IEC 62855:2016  IEC 2016
NUCLEAR POWER PLANTS –
ELECTRICAL POWER SYSTEMS –
ELECTRICAL POWER SYSTEMS ANALYSIS

1 Scope
IEC 62855 provides the electrotechnical engineering guidelines for analysis of AC and DC
electrical power systems in nuclear power plants (NPPs) in order to demonstrate that the
power sources and the distribution systems have the capability for safe operation and shut
down of the NPP, bringing it to a controlled state after an anticipated operational occurrence
or accident conditions and finally reaching a safe state.
The analytical studies discussed in this document provide assurance that the design bases
are satisfied to meet their functional requirements under the conditions produced by the
applicable design basis events. The studies provide assurance that the electrical power
system is capable of supporting safety functions during all required plant conditions.
NOTE The safety functions are described in IAEA Specific Safety Requirements SSR-2/1 related to the design of
the nuclear power plants.
Analytical studies validate the robustness and adequacy of design margins and demonstrate
the capability of electrical power systems to support plant operation for normal, abnormal,
degraded and accident conditions.
The analyses are used to verify that the electrical power system can withstand minor
disturbances and that the consequences of major disturbances or failures do not degrade the
capability of the electrical power systems to support safe shutdown of the plant and maintain
the plant in shutdown condition.
The analyses are performed with one or more of
• simulation tools (software and hardware) that have been verified and validated,
• hand calculations, and
• tests.
This document provides guidance on the types of analyses required to demonstrate that the
plant's auxiliary power system can perform the required safety functions. This document does
not provide specific details on how the analysis should be conducted.
This document does not cover digital controllers (such as controllers for rectifiers, inverters,
sequencers and electrical protection devices) used in electrical power systems. IEC 61513
gives recommendations that apply to the electronic controls and protective elements of the
electrical power systems.
This document does not include environmental conditions (i.e. temperature, humidity, etc.) or
external events (seismic, flooding, fire, high energy electromagnetic pulse, etc.) that may
impact equipment sizing or protection requirements. The external events lightning and
geomagnetic storms are included.
This document does not cover additional or unique requirements for stand-alone power
system, such as power supplies for security measures in NPPs. Pertinent clauses of this
document may be used as a guideline for such systems.

IEC 62855:2016  IEC 2016 – 11 –
Redundancy in the power system design can increase the availability of electrical power to
critical plant equipment. Performing a probabilistic risk assessment (PRA) is a method of
assessing system availability and optimizing design for high reliability. This document does
not cover improving the reliability of NPP electrical power systems using statistical or diverse
and redundant schemes.
Requirements for safeguards of personnel involved with installation, maintenance and
operation of electrical systems and general personal safety are outside the scope of this
document. General guidance for lightning protection of equipment is provided in relevant
clauses of this document.
This document is intended to be used:
• for verification of the design of new nuclear power plants,
• for demonstrating the adequacy and impact of major modifications of electrical power
systems in operating nuclear power plants, and
• where there is a requirement to assess and establish operating limits and constraints for
existing plants.
Pertinent parts of this document can be used as guidance for decommissioning stages.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
alternate AC source
power source reserved for the use for the power supply to the plant during total loss of all
non-battery power in the safety power systems (station blackout) and other design extension
conditions
Note 1 to entry: Figure A.2 gives a graphical representation.
3.2
house load operation
operation of a nuclear power plant to supply power only to its own electrical loads
3.3
design extension conditions
postulated accident conditions that are not considered for design basis accidents, but that are
considered in the design process of the facility in accordance with best estimate methodology,
and for which releases of radioactive material are kept within acceptable limits.
Design extension conditions include conditions in events without significant fuel degradation
and conditions with core melting
[SOURCE: IAEA SSR-2/1:2012, definitions revised as DS462]

– 12 – IEC 62855:2016  IEC 2016
3.4
loss of off-site power
simultaneous loss of electrical power to all unit safety buses, requiring the standby AC power
sources to start and supply power to the safety buses
Note 1 to entry: DC systems and uninterruptible AC systems safety buses are not included.
3.5
power balance
steady state active and reactive power required by the electrical power system
Note 1 to entry: This document provides the basis for sizing electrical equipment (switchboards, cables, power
sources, transformers, batteries, rectifiers/inverters, etc.).
3.6
preferred power supply
power supply from the transmission system to the safety classified electrical power system,
comprising transmission system, switchyard, main generator, distribution system up to the
safety classified electrical power system
Note 1 to entry: Some portions of the preferred power supply are not part of the safety classification.
3.7
standby AC power source
power source, capable of supplying the necessary power in anticipated operational
occurrences and accident conditions, in the event of the loss of off-site power and main
generator
3.8
station blackout
plant condition with complete loss of all AC power from off-site sources, from the main
generator and from standby AC power sources important to safety to the essential and non-
essential switchgear buses
Note 1 to entry: DC power supplies and uninterruptible AC power supplies may be available as long as batteries
can supply the loads.
Note 2 to entry: Alternate AC supplies are available.
3.9
total harmonic distortion
ratio of the rms value of the harmonic content of an alternating quantity to the rms value of the
fundamental component of the quantity
[SOURCE: 60050-551:1998, 17-06]
3.10
transmission system operator
party responsible for providing and operating networks for long-distance transmission of
electricity as well as regional distribution and responsible to ensure the system security with a
high level of reliability and quality
4 Symbols and abbreviations
AC alternating current
AOO anticipated operational occurrences
CCF common cause failure
CMF common mode failure
DBA design basis accident
IEC 62855:2016  IEC 2016 – 13 –
DBC design basis conditions
DC direct current
DEC design extension conditions
EMC electromagnetic compatibility
LOOP loss of off-site power
NPP nuclear power plant
PPS preferred power supply
PRA probabilistic risk assessment
QA quality assurance
SSC structures, systems and components
THD total harmonic distortion.
TSO transmission system operator
UPS uninterruptible power supply
V&V verification and validation
5 Electrical power system analyses
5.1 Overview of typical studies
5.1.1 Transient stability analyses
Grid disturbances, such as a short circuit on a transmission line, sudden loss of generation, or
the loss of a large load, may cause instability. If such disturbances are not cleared rapidly,
instability may ultimately lead to loss of parts or
...