IEC TS 63042-101:2019
(Main)UHV AC transmission systems - Part 101: Voltage regulation and insulation design
UHV AC transmission systems - Part 101: Voltage regulation and insulation design
IEC TS 63042-101:2019(E) specifies reactive power compensation design, voltage regulation and control, and insulation design for the coordination of UHV AC transmission systems.
General Information
Standards Content (Sample)
IEC TS 63042-101 ®
Edition 1.0 2019-01
TECHNICAL
SPECIFICATION
colour
inside
UHV AC transmission systems –
Part 101: Voltage regulation and insulation design
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.
IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.
About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.
IEC publications search - webstore.iec.ch/advsearchform Electropedia - www.electropedia.org
The advanced search enables to find IEC publications by a The world's leading online dictionary on electrotechnology,
variety of criteria (reference number, text, technical containing more than 22 000 terminological entries in English
committee,…). It also gives information on projects, replaced and French, with equivalent terms in 16 additional languages.
and withdrawn publications. Also known as the International Electrotechnical Vocabulary
(IEV) online.
IEC Just Published - webstore.iec.ch/justpublished
Stay up to date on all new IEC publications. Just Published IEC Glossary - std.iec.ch/glossary
details all new publications released. Available online and 67 000 electrotechnical terminology entries in English and
once a month by email. French extracted from the Terms and Definitions clause of
IEC publications issued since 2002. Some entries have been
IEC Customer Service Centre - webstore.iec.ch/csc collected from earlier publications of IEC TC 37, 77, 86 and
If you wish to give us your feedback on this publication or CISPR.
need further assistance, please contact the Customer Service
Centre: sales@iec.ch.
IEC TS 63042-101 ®
Edition 1.0 2019-01
TECHNICAL
SPECIFICATION
colour
inside
UHV AC transmission systems –
Part 101: Voltage regulation and insulation design
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.240.01 ISBN 978-2-8322-6456-0
– 2 – IEC TS 63042-101:2019 © IEC 2019
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Reactive power compensation for UHV AC transmission systems . 8
4.1 General principles . 8
4.2 Configuration of reactive power compensation – consider placing after
general functions . 8
4.3 Determining reactive power compensation . 9
4.3.1 Reactive compensation at UHV side . 9
4.3.2 Compensation at tertiary side of UHV transformers . 9
4.3.3 Reactive power compensation at UHV side . 10
4.3.4 Shunt capacitor configuration at tertiary side of UHV transformers. 11
4.3.5 Shunt reactor configuration at tertiary side of UHV transformers . 12
4.4 Controllable shunt reactor at UHV side . 13
4.4.1 General . 13
4.4.2 Capacity selection . 13
4.4.3 Tap-changer . 13
4.4.4 Response speed of CSR . 13
4.4.5 Control mode . 14
4.5 Other requirements for compensation at tertiary side of UHV transformers . 14
4.5.1 Configuration of shunt compensator banks . 14
4.5.2 Connection . 14
4.5.3 Dynamic reactive compensation . 14
5 Voltage regulation . 15
5.1 General . 15
5.2 Voltage regulation for UHV transformers . 15
5.2.1 Voltage regulation via transformer tap changes . 15
5.2.2 Selection of transformer taps . 15
5.2.3 Voltage selection for transformers . 15
5.2.4 Types of tap-changers . 15
5.2.5 UHV transformer tap range . 15
5.2.6 Selection of transformer tap position during operation . 15
6 Generator reactive power control . 16
6.1 General . 16
6.2 Coordination among reactive devices . 17
7 Insulation design and coordination procedure for transmission line and substation
design . 17
7.1 General . 17
7.2 Insulation design procedure . 18
7.3 UHV AC system overvoltage . 18
7.3.1 General . 18
7.3.2 Temporary overvoltage (TOV) . 19
7.3.3 Switching overvoltage (slow-front overvoltage) . 19
7.3.4 Lightning overvoltage (fast-front overvoltage) . 20
7.3.5 Very fast front overvoltage (VFFO) . 21
7.4 Reduction of insulation levels using overvoltage suppression measures . 21
7.4.1 General . 21
7.4.2 Overvoltage suppression using surge arrester with low protective level . 21
7.4.3 Resistor-fitted circuit-breakers with closing/opening resistor . 21
7.4.4 Damping effect of resistor-fitted disconnectors employed in GIS to
suppress VFFO. 22
7.4.5 Damping effect of AIS for suppressing VFFO . 22
7.4.6 Fast insertion of switchable or controllable shunt reactors . 22
7.4.7 Controlled switching . 22
7.5 Coordination of design requirements . 22
7.5.1 General . 22
7.5.2 Transmission line. 22
7.5.3 Substation . 23
Annex A (informative) UHV multi-stage controllable shunt reactor . 24
Annex B (informative) General procedure for the selection of transformer tap positions . 26
Bibliography . 29
Figure 1 – Flowchart for reactive power compensation configuration . 9
Figure 2 – Flow chart for rational insulation specification for UHV . 18
Figure 3 – Overvoltage categorized by time domain . 18
Figure 4 – Overvoltage mechanism caused by back-flashover and direct lightning . 20
Figure A.1 – Illustrative example of a UHV project with an MCSR . 24
Figure B.1 – Schematic diagram of UHV transmission line . 26
Figure B.2 – Voltage profile of UHV line A-B while energized at substation . 27
Table A.1 – Impact of MSCR switching on voltage at station B . 25
Table B.1 – Lower limits of operating voltage for UHV substations . 28
– 4 – IEC TS 63042-101:2019 © IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
UHV AC TRANSMISSION SYSTEMS –
Part 101: Voltage regulation and insulation design
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
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
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
misinterpretation by any end user.
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
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.
The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a Technical
Specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• the subject is still under technical development or where, for any other reason, there is the
future but no immediate possibility of an agreement on an International Standard.
Technical Specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 63042-101, which is a Technical Specification, has been prepared by IEC technical
committee 122: UHV AC transmission systems.
The text of this Technical Specification is based on the following documents:
Enquiry draft Report on voting
122/60/DTS 122/70A/RVDTS
Full information on the voting for the approval of this Technical Specification 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 63042 series, published under the general title UHV AC
transmission systems, can be found on the IEC website.
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.
A bilingual version of this publication may be issued at a later date.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
considered to be useful for the correct
that it contains colours which are
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – IEC TS 63042-101:2019 © IEC 2019
INTRODUCTION
Large-scale power sources including renewable energy have recently been developed. To
meet the requirements for large power transmission capacity, some countries have
introduced, or are considering introducing, ultra high voltage (UHV) transmission systems,
overlaying these on the existing transmission systems at lower voltages such as 420 kV and
550 kV.
However, the introduction of UHV AC also presents many challenges to planners and
operators. One of the major challenges is the management and control of system voltage and
reactive power control. Reactive power control is normally used to address power frequency
voltage requirements and maintain the voltage under transient conditions. Suitable insulation
designs and coordination procedures are adopted in order to control transient overvoltages
and prevent damage to equipment.
The objective of UHV AC power system design is to achieve both economic efficiency and
high reliability, considering its impact on systems at lower voltages such as 420 kV and
550 kV. Long-distance transmission lines in particular generate a large amount of charging
reactive power (Mvar) that could cause the system voltage to rise significantly. For example,
when energizing a transmission line, the terminal voltage at the remote end could reach an
unacceptable level. Reactive power compensation is implemented to ensure that the UHV AC
system operates within an adequate voltage range under normal conditions and any
contingency conditions that the system is designed to withstand.
Moreover, effective insulation design that limits internal electric field stress is important for
minimizing and optimizing the size and structure of UHV AC transmission lines and substation
apparatus. This document provides technical specifications on insulation design and
coordination, reactive power compensation design and voltage regulation that are essential
for maintaining UHV AC transmission systems so that they operate safely and efficiently.
UHV AC TRANSMISSION SYSTEMS –
Part 101: Voltage regulation and insulation design
1 Scope
This part of IEC 63042 specifies reactive power compensation design, voltage regulation and
control, and insulation design for the coordination of UHV AC transmission systems.
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 60071-1, Insulation co-ordination – Part 1: Definitions, principles and rules
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
voltage deviation
difference between the actual voltage and nominal system voltage under continuous operating
conditions
3.2
network node
any point where two or more transmission lines meet
3.3
controllable shunt reactor
CSR
high voltage shunt reactor whose capacity can be adjusted
3.4
continuous controllable shunt reactor
CCSR
high voltage shunt reactor whose capacity can be adjusted continuously
3.5
multi-stage controllable shunt reactor
MCSR
type of controllable shunt reactor, based on the principle of high impedance transformers
whose reactive power output usually varies in discrete stages and is achieved by controlling
transistors, circuit-breakers and other devices
– 8 – IEC TS 63042-101:2019 © IEC 2019
4 Reactive power compensation for UHV AC transmission systems
4.1 General principles
An appropriate amount of reactive power supply should be planned and installed in UHV AC
systems to meet the system voltage regulation requirements and reduce the amount of
unintended reactive power transfers between different network nodes/voltage levels.
A sufficient amount of reactive power supply with flexible capacity, including an adequate
amount of reactive power reserve, should be maintained.
The capacity, type and location of reactive power compensators should be selected to
improve power transmission capabilities and enhance system stability limits.
Planning and design of reactive power compensators for UHV AC systems should meet the
overvoltage limit requirements for UHV AC systems.
A compensation ratio of between 90 % to 110 % is considered reasonable in planning reactive
compensation to minimize the reactive power exchange between UHV and lower voltage level
systems. The compensation should be judiciously implemented between line and bus reactive
compensation so that it is able to control voltage during various switching operations and to
prevent oscillations due to high levels of compensation.
4.2 Configuration of reactive power compensation – consider placing after general
functions
In general, reactive power compensation should be distributed at the primary, secondary and
tertiary side of the UHV transformer based on the overall requirements for voltage regulation
and to minimize the overall cost. The principle of locating reactive power compensation at the
primary and secondary sides of the UHV transformer is the same except for the cost of
reactive power compensation and its effectiveness in regulating voltage at the primary side of
the UHV transformer. In this way, they are treated in the same manner.
The major processes in configuring reactive power compensation for UHV AC systems are as
follows:
a) Identify the range of likely active power flow across the UHV line, then calculate and
analyse the characteristics of reactive power and voltage profiles along the UHV line,
taking into account charging reactive power produced by UHV lines and reactive power
loss under different power flow conditions. Simulations need to be repeated for each
scenario to determine the compensation that keeps the voltage within acceptable limits.
One of the methods for this is to determine the compensation required at each bus by
using a static Var compensator (STATCOM) with a large range. The calculated output of
the STATCOM that maintains bus voltage at 1,0 p.u. is the required compensation at that
bus.
b) Select UHV transformer tap positions to avoid overvoltage under a range of operating
conditions taking into account UHV substation location, number of transmission lines
connected, and system operation mode.
c) Select capacity and location of UHV line shunt reactors with the following considerations:
1) limiting temporary overvoltage and reducing secondary arc current;
2) balancing charging power of lines and flexibly controlling bus voltage.
d) Identify total and unit capacity of compensators installed on the tertiary side of the
transformer. Total capacity should be selected to reduce the reactive power exchange
between different voltage levels and maintain bus voltage within the admissible range; the
selection of single bank capacity should take into account the maintaining of voltage
fluctuations induced by the switching of a single capacitor bank or reactor within a
reasonable range. Set the dynamic reactive power limits provided by generators within the
desired capability range.
e) Check whether the dynamic reactive power reserve provided by generators is adequate
within their reactive power capability range. If it is adequate, then the process stops;
otherwise return to d).
Figure 1 shows the process of configuring reactive power compensation.
Figure 1 – Flowchart for reactive power compensation configuration
4.3 Determining reactive power compensation
4.3.1 Reactive compensation at UHV side
Reactive power compensation at the UHV side (primary side) refers to equipment that is
directly connected to the UHV AC line or bus, including fixed capacity and controllable shunt
reactors. UHV shunt reactive power compensation is mainly used to compensate the charging
power of a UHV transmission line, limit temporary overvoltage and limit voltage to below the
maximum operation voltage in transmission line energization. In addition, a shunt reactor with
a neutral point reactor can be used to limit secondary arc current.
A shunt reactor connected to UHV transmission lines is used for reactive power compensation
and overvoltage limiting. For substations with some short lines, the shunt reactor is normally
connected to the bus, which is mainly used to compensate the charging power of the UHV
transmission line.
4.3.2 Compensation at tertiary side of UHV transformers
Reactive power compensation connected at the tertiary side of UHV transformers mainly
includes shunt capacitors, shunt reactors and static Var compensators (STATCOM), which are
mainly used to meet the reactive power compensation requirements of the UHV AC system, to
reduce the transformer‘s reactive power loss, and to regulate the system voltage.
– 10 – IEC TS 63042-101:2019 © IEC 2019
4.3.3 Reactive power compensation at UHV side
For a shunt reactor connected to the terminal of transmission lines, its capacity can be
calculated by Formula (1) below.
Q k×Q
(1)
HR-total L B
where
Q
is the capacity of shunt reactive power compensation required at both sides of the
HR-total
UHV line because of this line;
Q
is the no-load charging reactive power of this UHV line;
B
k
is the compensation coefficient;
L
k
is normally obtained based on the overvoltage calculation and reactive power
L
balance, which is normally less than 0,85, to avoid oscillations during switching. If
the line is short and line reactors are not required then the reactive power
requirement can be considered in the bus reactive compensation. The
requirement of a shunt reactor on the line has to be determined by the Ferranti
effect during energization and temporary overvoltage studies. The nearest Mvar to
the calculated value can be considered. In general, it is the compensation at each
terminal of the transmission line.
For the shunt reactor connected to the bus, the capacity can be calculated by Formula (2).
Q =k (Q −Q )− Q
(2)
∑ ∑
bus B B X HR
where
k
is the compensation coefficient, which is normally close to 100 %;
B
Q
is the reactive power loss of the transmission line under no-load conditions,
x
which is nearly zero;
is the sum of charging power and reactive power loss of half the line length
(Q −Q )
∑ B X
of all transmission lines connected to the bus;
Q Q
is half of .
HR HR-total
Q
is the total capacity of all reactors directly connected to the UHV lines at
∑
HR
the bus.
k
In general, for the receiving end should be higher than that for the sending end.
B
Furthermore, reactors at the generator bus and receiving end bus for light load conditions
should be available. The determination of line and bus reactors as described above should be
tested through simulation.
=
To gainfully utilize the line reactor when the line is not in service, a disconnector can be
provided between the line termination and the point of connection of the line reactor. When
the line is out of service, the disconnectors can be opened and the line reactor can be used
like a bus reactor without the line. Provision of circuit-breakers can be considered when
reactive compensation is not required for temporary overvoltage.
4.3.4 Shunt capacitor configuration at tertiary side of UHV transformers
The configuration of tertiary shunt capacitors should compensate for the reactive power loss
of transformers and half of the net reactive power loss of transmission lines connected at the
primary and secondary side of the UHV transformer.
The capacity of tertiary shunt capacitors can be calculated according to Formula (3) to
Formula (8).
Q =Q −Q −Q
cap Tloss Hhalf Mhalf
(3)
where
Q
is the total capacity of capacitive compensation.
cap
Q = (Q −Q )+Q − Q
(4)
Hhalf ∑ BH Hloss HC ∑ HR
where
Q
is the charging power of the UHV line connected to the primary side of the UHV
BH
transformer;
Q
is the reactive power loss of the UHV line connected to the primary side of the
Hloss
UHV transformer;
Q
is the series compensation capacity of the UHV line connected to the primary side
HC
of the UHV transformer;
Q is the capacity sum of all reactors connected to the UHV lines at the primary side
∑ HR
of the UHV transformer.
The capacity of the series capacitor is obtained as follows:
Q = I ×X (5)
HC H HC
where
I
is the rated current connected to the UHV line at the primary side of the UHV
H
transformer;
X is the reactance of series compensation connected to the UHV line at the primary
HC
side of the UHV transformer.
– 12 – IEC TS 63042-101:2019 © IEC 2019
Q = (Q −Q )+Q − Q
Mhalf ∑ BM Mloss MC ∑ MR
(6)
where
Q
is the charging power of the lines connected to the secondary side of the UHV
BM
transformer;
Q
is the reactive power loss of the lines connected to the secondary side of the UHV
Mloss
transformer;
Q
is the series compensation capacity of the lines connected to the secondary side
MC
of the UHV transformer;
is the capacity of the reactor connected to the line at the secondary side of the
Q
MR
UHV transformer.
The capacity of the series capacitor is obtained as follows:
Q =I ×X (7)
MC M MC
where
I
is the rated current of the line connected to the secondary side of the UHV
M
transformer;
X
is the series compensation reactance of the line connected to the secondary side
MC
of the UHV transformer.
Transformer loss is expressed as follows:
Q =(S U ) ×X (8)
Tloss N N
where
Q
is the reactive power loss of the transformers;
Tloss
S
is the apparent power of the UHV transformer;
N
U
is the voltage effective value of the UHV transformer;
N
X is the reactance of the UHV transformer.
4.3.5 Shunt reactor configuration at tertiary side of UHV transformers
In general, the charging power of transmission lines should almost be compensated by UHV
and tertiary connected shunt reactors.
The capacity of tertiary shunt reactors is calculated by Formula (9).
Q = Q − Q
∑ ∑
rea BH HR (9)
where
Q
is the capacity of the tertiary shunt reactors;
rea
Q
is the charging power of the UHV line connected to the primary side of the UHV
BH
transformer;
Q is the capacity of the reactor connected to the UHV lines at the primary side of the
HR
UHV transformer.
4.4 Controllable shunt reactor at UHV side
4.4.1 General
A controllable shunt reactor (CSR) can be used to meet the requirement of limiting temporary
overvoltage and balancing the charging power of the UHV transmission line under a range of
UHV line loading conditions, such as light and heavy loads.
A CSR is composed of two parts, fixed and controllable. At present, there are two types of
CSRs: multi-stage and continuous controllable shunt reactors. Multi-stage controllable shunt
reactors (MCSR) vary the reactive power output in discrete steps, whereas continuous
controllable shunt reactors (CCSR) vary the output smoothly.
4.4.2 Capacity selection
The CSR can have a value equal to the compensation required to control parameters as
mentioned in 4.3.3 and to cater to heavy load conditions the same can be made variable.
4.4.3 Tap-changer
For the MCSR, voltage fluctuations caused by shifting tap-changers should be within the
admissible voltage deviation range. The tap-changer range should be selected according to
the voltage regulation requirements and overall cost.
4.4.4 Response speed of CSR
The response speed of a CSR should meet the requirements for overvoltage control,
secondary arc current limiting and rapid voltage regulation.
For a CSR connected to transmission lines, the response speed should meet the
requirements for temporary overvoltage control and secondary arc current limiting. For a CSR
connected to the bus, the same response speed may be required if voltage regulation is
considered necessary for temporary overvoltage control.
Where the CSR is connected to a transmission line and a single phase auto-reclose scheme
is adopted, the response speed should be able to meet the requirements for secondary arc
extinguishing.
In addition, the response speed of a CSR should also be able to meet the requirements for
suppressing sharp voltage fluctuations caused by system faults.
– 14 – IEC TS 63042-101:2019 © IEC 2019
4.4.5 Control mode
4.4.5.1 General
CSRs can be configured to operate in automatic and manual control modes. Automatic control
mode includes temporal, voltage-based and reactive power loss based control modes. When a
single phase fault occurs on the connected transmission line or in cases of load rejection,
temporary control should be employed. Otherwise, the CSRs can be configured to operate in
automatic and manual control modes.
4.4.5.2 Voltage control mode
Under normal operating conditions a CSR regulates its reactive power output (in stages for
MCSR and smoothly for CCSR) based on the deviation in the actual operating voltage from a
reference voltage. If, for example under system faults, the voltage exceeds the upper or lower
voltage limits, it can rapidly be increased to the maximum or decreased to the minimum of its
capacity.
4.4.5.3 Reactive power loss control mode
In this mode, the total reactive power loss of transmission lines and UHV transformers can be
calculated automatically and CSR reactive power output can be regulated to help ensure that
the change in total reactive power loss between two successive calculations does not exceed
a pre-determined threshold.
4.4.5.4 Temporary control
For a CSR connected to a transmission line, this mode will allow it to increase its output to the
maximum to limit secondary arc current when a single phase fault occurs on the connected
transmission line or to control the temporary overvoltage in cases of load rejection.
4.4.5.5 Manual control
A CSR can switch to the manual control mode under certain conditions such as maintenance,
tests.
4.5 Other requirements for compensation at tertiary side of UHV transformers
4.5.1 Configuration of shunt compensator banks
Configuration of individual tertiary shunt capacitor or reactor banks should ensure that voltage
step change caused by the switching in/out of individual banks does not exceed that caused
by the on-load tap-changers (OLTC) of the UHV transformers.
4.5.2 Connection
Tertiary shunt capacitors and reactors should have auto switching functions with circuit-
breakers installed for each capacitor/reactor bank.
4.5.3 Dynamic reactive compensation
Dynamic reactive compensation, such as STATCOM, can be installed at places with UHV
transmission lines or inter-area tie lines with frequent power flow changes, and UHV
substations with inadequate reactive compensation or voltage regulation capabilities. The
response speed should satisfy the requirements for temporary voltage control and rapid
reactive power regulation.
5 Voltage regulation
5.1 General
The UHV AC system should operate within the admissible voltage deviation range.
The voltage regulation range and regulation method for transformer taps in the UHV AC
system should be properly selected according to the grid structure and operating conditions.
5.2 Voltage regulation for UHV transformers
5.2.1 Voltage regulation via transformer tap changes
Changing the tap position of a transformer is one of the voltage regulation methods and it is
commonly used to regulate the reactive power distribution and voltage level. If tap-changers
are not used, reactive power compensation should be sufficient to control voltage.
5.2.2 Selection of transformer taps
Selection of transformer taps should meet the requirements for voltage control at power plant
and substation buses, taking into consideration rated voltage, regulation mode, voltage range
and tap values.
5.2.3 Voltage selection for transformers
The rated voltage at the UHV side of a step-up and step-down transformer should be
determined via calculation and analysis.
5.2.4 Types of tap-changers
There are two types of UHV transformer tap-changers: on-load and de-energized tap-changer
(DETC). Selection of tap-changer type should be based on the system operation conditions
and system analysis. An on-load tap-changer should be used in conditions of large voltage
variations.
5.2.5 UHV transformer tap range
The regulation range of transformer taps is determined via system analysis. The upper limit
for UHV transformer taps should be selected to avoid over-excitation, and the lower limit
should be selected to avoid overcurrent.
The range of individual taps should be determined to help ensure that voltage step change
caused by each tap does not exceed the permissible range, which is generally considered to
be 2,5 %. The range of an individual tap is normally up to 0,65 % (on-load) and 1,25 % to 2,5 %
(no-load).
5.2.6 Selection of transformer tap position during operation
In actual operation, the selection of transformer taps should consider the impact of
transmission line energization and de-energization, and the power flow of transmission lines
and transformers under heavy loads or light loads.
The configuration procedure is as follows:
a) Initially identify the voltage operating range for both primary and secondary buses, taking
into consideration equipment insulation and tolerance capabilities, as well as the voltage
regulation capacity of the grid.
– 16 – IEC TS 63042-101:2019 © IEC 2019
b) Calculate the voltage variation induced by transmission line energization and de-
energization under different tap-changer schemes, then select the scheme in which
voltage variation does not exceed the permissible range.
NOTE While charging with no load, voltage along the line will increase due to the charging capacitive current
of the line flowing through the line inductance. For a lossless line, voltage at the open circuit terminal can be
derived by Formula (10).
E
S
U=
(10)
cos(βl)+Y Z sin(βl)
end c
where
β is the propagation constant, indicated by β=ω L C ;
o o
L
is the inductive reactance of the line per unit length;
o
C
is the charging capacitor of the line per unit length;
o
E
is the voltage at the bus where the transmission line is energized;
s
l indicates total line length;
U is the voltage at the terminal;
Y
is the susceptance of the UHV shunt reactor at the open circuit terminal;
end
L
o
Z
is the wave impedance, expressed by .
Z =
c
c
C
o
c) Evaluate whether different transformer tapping schemes can meet the regulation
requirements for variable operating conditions according to the regulation capabilities of
existing voltage regulation and reactive power compensation devices.
d) As per the analysis and calculation in b) and c), identify transformer tap configuration.
6 Generator reactive power control
6.1 General
The generators in a UHV AC system should have rapid response in reactive power output and
strong control capabilities. They are an important source of regulation for the UHV AC system
voltage and reactive power distribution. A generator connected to the UHV AC system should
be capable of generating and absorbing reactive power.
Generators in a UHV AC system are generally located far from the load centre. The charging
reactive power of the line is large and excessive overvoltage may occur; thus, generators are
required to have a certain leading phase capability to control the bus voltage at a reasonable
level. In general, a generator connected to the UHV AC system should have 0,95 leading and
0,85 lagging capabilities under rated power. This means that generators with a rated capacity
of 1 000 MW should be capable of absorbing about 300 Mvar of reactive power and
generating about 600 Mvar.
6.2 Coordination among reactive devices
The UHV AC system should have a reactive power reserve with rapid response. Rapid
response reactive capacity should be made reserve capacity in reactive power supply in
running generators, shunt capacitors and dynamic reactive power compensation devices, so
that there will be a rapid increase in reactive power output to maintain stable operation of the
power system in the case of excessively low voltage caused by insufficient reactive power in
the grid.
There are many reactive devices which would respond to changes in grid parameters. It is not
desirable for all the resources to respond together. The sources which would participate for
an event should be finalized depending on the response speed, variable range, etc., and
depending on the grid event, and the voltage change system should respond. At any point in
time sufficient reserve in both fixed and variable controllers should be available to cater to
grid events like line/generator/compensating device outages.
7 Insulation design and coordination procedure for transmission line and
substation design
7.1 General
Economical and highly reliable transmission lines and substation equipment with
environmental considerations are essential in a UHV AC system. UHV AC systems should be
formulated to maintain an adequate voltage level. Overvoltage is mainly generated by
lightning and the switching of circuit-breakers/disconnectors so it is necessary to suppress
this overvoltage to within the required insulation levels.
If system design were carried out to counter each phenomenon, such as lightning overvoltage
and switching overvoltage, individually for substations and transmission lines, the overall
network system would become too redundant. To avoid this, insulation coordination is
necessary. IEC 60071-1 standardizes rated insulation levels. This document focuses on UHV
and specifies UHV design procedures in selecting appropriate insulation parameter values in
reference to IEC 60071-1.
For most instances of the highest voltage for equipment, several rated insulation levels are
standardized to allow for the application of different performance criteria or overvoltage
patterns. The selection should be made by considering a system configuration which
characterizes the degree of exposure to lightning and switching overvoltages, and the type of
overvoltage limiting devices.
To reduce the size of transmission and substation equipment, surge arresters with low
protection levels, as well as circuit-breakers with closing and/or opening pre-insertion
resistors, are commonly applied to suppress overvoltage.
Insulation design for a UHV AC system should achieve high reliability considering
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.
Loading comments...