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.

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IEC TS 63042-101

Edition 1.0 2019-01


UHV AC transmission systems –
Part 101: Voltage regulation and insulation design

IEC TS 63042-101:2019-01(en)

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IEC TS 63042-101


Edition 1.0 2019-01





UHV AC transmission systems –

Part 101: Voltage regulation and insulation design




ICS 29.240.01 ISBN 978-2-8322-6456-0

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® Registered trademark of the International Electrotechnical Commission

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– 2 – IEC TS 63042-101:2019 © IEC 2019
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

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IEC TS 63042-101:2019 © IEC 2019 – 3 –
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

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– 4 – IEC TS 63042-101:2019 © IEC 2019


Part 101: Voltage regulation and insulation design

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IEC TS 63042-101, which is a Technical Specification, has been prepared by IEC technical
committee 122: UHV AC transmission systems.

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IEC TS 63042-101:2019 © IEC 2019 – 5 –
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.
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stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
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• withdrawn,
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• amended.
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– 6 – IEC TS 63042-101:2019 © IEC 2019
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.

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IEC TS 63042-101:2019 © IEC 2019 – 7 –

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
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
voltage deviation
difference between the actual voltage and nominal system voltage under continuous operating
network node
any point where two or more transmission lines meet
controllable shunt reactor
high voltage shunt reactor whose capacity can be adjusted
continuous controllable shunt reactor
high voltage shunt reactor whose capacity can be adjusted continuously
multi-stage controllable shunt reactor
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

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– 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
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
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
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.

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IEC TS 63042-101:2019 © IEC 2019 – 9 –
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.

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– 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
HR-total L B
is the capacity of shunt reactive power compensation required at both sides of the
UHV line because of this line;
 is the no-load charging reactive power of this UHV line;
is the compensation coefficient;
is normally obtained based on the overvoltage calculation and reactive power
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
 
is the compensation coefficient, which is normally close to 100 %;

 is the reactive power loss of the transmission line under no-load conditions,
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;
 is half of .
HR HR-total
is the total capacity of all reactors directly connected to the UHV lines at

the bus.
In general, for the receiving end should be higher than that for the sending end.
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.


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IEC TS 63042-101:2019 © IEC 2019 – 11 –
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
 is the total capacity of capacitive compensation.
 
Q = (Q −Q )+Q − Q
  (4)
Hhalf ∑ BH Hloss HC ∑ HR
 
 is the charging power of the UHV line connected to the primary side of the UHV
is the

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