EXPERIENCE WITH EQUIPMENT FOR SERIES AND SHUNT COMPENSATION

693
EXPERIENCE WITH EQUIPMENT
FOR SERIES AND SHUNT
COMPENSATION
WORKING GROUP
A3.33
JULY 2017

Members
G. LI, Convenor CN Z. XIANG, Secretary CN
X. WANG CN Z. LI CN
C. DAI CN A. JANSSEN NL
F. ROCHON CA J. A. FILHO BR
R. GOEHLER DE F. RICHTER DE
H. KAJINO JP F. GALLON FR
R. LE ROUX IE N. VAN STADEN ZA
P. DOPPLMAIR AT B. BHARGAVA US
H. SCHMITT DE M. KOSAKADA JP
M. GOTTI CH
Corresponding Members
H. ITO, Chairman SC A3 JP C. VAN DER MERWE ZA
J. FAN CN A. ALFREDSSON SE
R. J. MACLEOD US
WG A3.33
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EXPERIENCE WITH EQUIPMENT FOR SERIES
AND SHUNT COMPENSATION
ISBN : 978-2-85873-396-5

EXPERIENCE WITH EQUIPMENT FOR SERIES / SHUNT COMPENSATION
3
EXECUTIVE SUMMARY
As the series compensation techniques can increase transmission capability of transmission line power
transfer capability significantly, it has been widely applied in extra high voltage (EHV) bulk power
transmission systems. At the end of 2011, the first UHV series capacitor bank was put into service in
China, which increased the transmission capacity of a single circuit UHV transmission line up to 5 GW.
Due to the success of application the Chinese transmission companies are planning to install more
UHV series capacitors in the near future.
Recently thyristor controlled EHV series capacitor banks were developed and put into service. A fast
acting bypass device was also developed and applied to mitigate an excessive transient recovery
voltage (TRV) imposed on a circuit breaker as well as the MOV requirements. The applications of
shunt compensations such as shunt reactors, shunt capacitor banks, static var compensators (SVC)
are also increasing, which includes a shunt compensation connected to the windings of UHV and EHV
power transformers.
The experience and consequences with the equipment used for series and shunt compensations were
investigated focusing the following subjects:
 Field experience with equipment applied to a conventional as well as the states-of-art
series and shunt compensations along with the utilities policy for their applications;
 Impact of series compensation on temporary and switching overvoltages;
 Technical restraints in the application of series compensation;
 Impact of secondary arc extinction on the requirements for related equipment
including 4-legged shunt reactors and high speed earthing switches (HSES);
 Impact of new equipment on TRV requirments for a circuit breaker and
 Recommendations for standardisation, if necessary.
The investigations on equipment applied to series and shunt compensations are summarized below:
1. Series capacitor bank is mainly used to increase the power transmission capability of
transmission lines. Fixed Series Capacitors (FSC) are commonly used. A few Thyristor-
Controlled Series Capacitors (TCSC) are used in order to mitigate Sub-Synchronous
Resonance (SSR) and power frequency oscillations.
2. A fast bypass device is normally used to mitigate an excessive TRV and the Metal
Oxide Varistor (MOV) requirements. A forced trigger spark gap is commonly used for
most of FSCs. Other fast bypass devices such as a thyristor valve and a Fast
Protective Device (FPD) are also used.
3. Service experience of the Series Compensations is generally good. Only minor
problems were reported such as inadvertent gap operation. The problems can be
solved by a proper design, specifications and applications.
4. Some of MOV failures are caused by the existence of a moisture, or potential defects
in MOV elements. However the detailed reasons are still unclear.
5. Special attention has to be paid to a circuit breaker located at line side with Series
Compensations along with a control and protection circuitry, because the IEC standard
regards it as a special requirement.
6. Switching of shunt capacitors at tertiary side of UHV or EHV power transformers
shows some possibility to produce excessive overvoltage and overcurrent stress on
switchgear. The circuit breakers for these applications should be carefully studied and
selected.

4
CONTENT
EXECUTIVE SUMMARY............................................................................................................................... 3
1. INTRODUCTION............................................................................................................................... 6
2. EXPERIENCES OF SERIES COMPENSATION................................................................................ 9
2.1 DEVELOPMENT AND TECHNICAL ADVANCEMENT..................................................................................................... 9
2.2 EXPERIENCES FROM APPLICATIONS ............................................................................................................................11
2.2.1 Operation experiences of series capacitors in the eastern network of Canada......................................12
2.2.2 Operation experiences of series capacitors in China .....................................................................................16
2.2.3 Operation experience with series capacitors in Japan..................................................................................19
2.2.4 Operation experiences of series capacitors in South Africa .........................................................................23
2.2.5 Operation experiences of series capacitors in the TUCMAN project in Brazil..........................................24
3. TECHNOLOGY OF SHUNT COMPENSATION .........................................................................27
3.1 SHUNT COMPENSATION................................................................................................................................................27
3.2 FIXED SHUNT REACTORS ................................................................................................................................................28
3.2.1 Introduction ...............................................................................................................................................................28
3.2.2 Types of fixed shunt reactors ...............................................................................................................................29
3.2.3 Oil-immersed iron cored shunt reactors..............................................................................................................30
3.2.4 Dry-type air-cored shunt reactors.......................................................................................................................30
3.2.5 TRV and mitigation..................................................................................................................................................31
3.2.6 Shunt reactor grounding and secondary arc.....................................................................................................31
3.2.7 Shunt reactor applications at the tertiary side of power transformers.......................................................32
3.3 VARIABLE SHUNT REACTORS.........................................................................................................................................33
3.3.1 Introduction ...............................................................................................................................................................33
3.3.2 Shunt reactors with variable air gap..................................................................................................................33
3.3.3 Shunt reactors with a variable number of windings ........................................................................................34
3.3.4 Shunt reactors with the possibility to by-pass the inductive load .................................................................34
3.3.5 Shunt reactors with a variable degree of saturation......................................................................................35
3.3.6 Special cases............................................................................................................................................................36
3.4 OTHER REACTIVE COMPENSATION METHODS.........................................................................................................37
3.4.1 Synchronous condenser...........................................................................................................................................38
3.4.2 Static var compensator (SVC)...............................................................................................................................40
3.4.3 STATCOM .................................................................................................................................................................41
4. TECHNOLOGY OF AND REQUIREMENTS FOR COMPONENTS USED IN SERIES
COMPENSATION......................................................................................................................................43
4.1 MAIN CIRCUIT TOPOLOGY AND ITS COMPONENTS..............................................................................................43
4.2 BY-PASS TECHNOLOGY OF SERIES CAPACITORS...................................................................................................45
4.2.1 Spark gaps...............................................................................................................................................................45
4.2.2 Fast protective device ............................................................................................................................................50
4.2.3 MOVs.........................................................................................................................................................................54
4.2.4 By-pass switches for alternating-current series capacitors.............................................................................59
4.2.5 Thyristor protected series capacitors..................................................................................................................62
4.3 BY-PASS DISCONNECTORS FOR UHV SERIES CAPACITORS .................................................................................63
4.3.1 Technical requirement.............................................................................................................................................63
4.3.2 Prototype design.....................................................................................................................................................64
4.4 SECONDARY PROTECTION SCHEME RELATED TO SERIES COMPENSATION.....................................................65
4.4.1 Protection scheme of series capacitor.................................................................................................................67
4.4.2 Protection coordination between the series capacitors and lines.................................................................71

5
5. IMPACT OF REACTIVE POWER COMPENSATION ON THE REQUIREMENTS OF OTHER
EQUIPMENT................................................................................................................................................73
5.1 EFFECTS OF COMPENSATION EQUIPMENT ON THE TRANSIENTS IN POWER SYSTEMS...............................73
5.1.1 Line fault currents ....................................................................................................................................................73
5.1.2 Clearing of series-compensated line faults .......................................................................................................73
5.1.3 Other phenomena ...................................................................................................................................................76
5.2 INFLUENCE OF ELECTRO-MAGNETIC TRANSIENTS ON SECONDARY EQUIPMENT .........................................77
5.2.1 Electromagnetic Interference (EMI) with secondary system of series capacitor.........................................77
5.2.2 Very fast transients on SC platform due to spark gap discharge ...............................................................78
5.3 SWITCHING PHENOMENA AND REQUIREMENTS OF THE CAPACITOR BANK CIRCUIT AT THE TERTIARY
SIDE OF UHV TRANSFORMER ........................................................................................................................................80
5.3.1 Introduction ...............................................................................................................................................................80
5.3.2 Circuit configuration................................................................................................................................................80
5.3.3 Operation modes for the EMTP analysis............................................................................................................82
5.3.4 Analysis results under normal operating conditions.........................................................................................82
5.3.5 Analysis results of breaking capacitive current in a single phase fault (1LG) condition.........................86
5.3.6 The influence of circuit configuration and conditions to capacitive current switching duty .....................89
5.3.7 Capacitive switching duty for capacitor bank at the tertiary of UHV transformer..................................91
6. LIFE CYCLE MANAGEMENT .........................................................................................................93
6.1 RELIABILITY..........................................................................................................................................................................93
6.2 MAINTENANCE ..................................................................................................................................................................93
6.3 REPLACEMENT....................................................................................................................................................................93
6.4 UPGRADE OR RELOCATION...........................................................................................................................................94
7. TEST TECHNIQUES AND RESULTS FOR UHV SERIES COMPENSATION..............................97
7.1 TEST EXPERIENCES ON THE KEY COMPONENTS IN CHINA...................................................................................97
7.1.1 Tests on MOVs..........................................................................................................................................................97
7.1.2 Tests on spark gaps ............................................................................................................................................. 103
7.1.3 Fast Protective Device Type Testing................................................................................................................. 110
7.1.4 Tests on by-pass switch of series capacitors................................................................................................... 112
7.1.5 Tests on UHV by-pass disconnectors of series capacitors............................................................................ 122
7.1.6 Tests on capacitor bank switch on tertiary side of transformers................................................................ 125
7.2 SHORT-CIRCUIT CURRENT TEST OF UHV CIRCUIT-BREAKER FOR BREAKING TRANSMISSION LINE WITH
SERIES COMPENSATION.............................................................................................................................................. 129
7.2.1 Analysis of TRV on circuit-breaker during transmission line interruption with series compensation.... 129
7.2.2 Short-circuit current breaking test for circuit-breaker with higher TRV .................................................... 130
7.3 COMMISSIONING TEST EXPERIENCES OF THE UHV SERIES CAPACITOR FROM CHINA ............................. 131
7.3.1 Commissioning test contents and results........................................................................................................... 132
7.3.2 Simulation and analysis for the commissioning test....................................................................................... 139
7.3.3 National standardization of commissioning test ............................................................................................ 140
8. CONCLUSIONS........................................................................................................................... 141
APPENDIXA :LISTOFABBREVIATIONS..................................................................................... 142
APPENDIXB : REFERENCES.......................................................................................................... 143

6
1. INTRODUCTION
Series capacitors banks(SC) in power systems have many advantages when they will be used for a
long distance AC power transmission. The intention to applicate them is to increase the transmission
capacity of the AC transmission lines, to improve the system stability, to change the power flow in the
grid and to reduce an impact of geomagnetic storms. In the contrary, there are some disadvantages
such as a risk of sub-synchronous resonance (see 5.1.3.) and an increase of TRV peaks.
Most of the series compensations are installed as the fixed type design, which means in practically the
capacitive power is fixed. The fixed design is named as fixed series capacitor (FSC). When a thyristor
controlled reactor (TCR) is connected in parallel to the FSC, the FSC is named into a thyristor-
controlled series capacitors bank (TCSC). TCSCs are mainly used to dampen power flow oscillations
or to vary the power transfer in order to prevent resonance and subsynchronous resonance effects in
the transmission system, that can possibly happen when SCs with a larger degree of compensation
are used to ensure the safe and stable operation of the power system.
The biggest advantage of TCSC consists in the fact that the capacitive power can be adjusted to an
optimal value depending on the operating conditions. Despite of that big advantage the applicaton of
TCSCs is very limited compared to FSCs in power systems. The reason is that the equipment costs
are significantly higher. Due to only TCSCs will be installed in applicationes where resonance
problems exist or only a part of the series compensation is designed as TCSC, while the rest of the
series compensation is still a FSC.
The dielectric performance of the series capacitors is normally required to withstand a certain power
frequency voltage, which value is much lower than the rated voltage of the transmission lines due to
economical reason. When a fault occurs, a short-circuit current will flow through the SC and a power
frequency voltage imposes across the SC. An overvoltage protection device is generally applied to
protect the series capacitor bank by bypassing it to avoid an excessive stress.
For safe operation and cost reduction, the insulation of series capacitors is designed to withstand a
power frequency overvoltage which is much lower than the rated voltage of the transmission line. In
case of a high current flowing through the series capacitor (line fault or system fault) a power
frequency overvoltage will occur across the series capacitors. Therefore overvoltage protection
devices have to be connected in parallel to the series capacitors, which will generally bypass the SC,
regardless of whether the fault is internal (within the line where the series capacitor is installed) or
external (outside the line where the series capacitor is installed). Before metal oxide varistors (MOVs)
were developed and applicated, the protection device was only a self triggering spark gap (single or
multiple gaps), in parallel with by-pass switches. The reliability of the spark gaps were not sufficent
enough and the appliacation caused in a more frequent bypassing of the SCs, which leads to
disturbances in the power system.
In the 1980’s, the development of metal oxide varistor elements with an excellent non-linear V-I
characteristics took place. By using these elements in surge arresters the overvoltage protection
changed from the gapped type to the gap-less type. The varistor elements are currently used to form
large capacity surge arresters (MOVs) by an appropriate combination of the elements. The surge
arresters were connected in parallel to the series capacitors and will limit power frequency overvoltage
instead of short circuiting the series capacitor bank. The MOVs will absorb electric energy from the
power system. The capacity to absorb the energy is limited and lead to a sudden increase of the
temperature in the elements. To avoid a potential damage of the MOVs due to too many energy
absorption, spark gaps may be still needed in modern series capacitor banks. These used gaps are no
longer the traditional self-triggered paps. The overvoltage protection device based on MOV can avoid
frequent short circuits across the SC which is used to cause them by operations of a self-triggering
spark gap due to external faults, because the MOV will improve the operation reliability of the SC and
the power system stability by supressing the power frequency overvoltage effectively.

7
From the beginning of the 1990s new types of series capacitors, which were equipped with power
electronics (thyristor–controlled series capacitor [TCSC]) and a thyristor-protected series capacitor
(TPSC) have been used in series capacitor banks after the year 2000. By the new capacitor design
the spark gaps could be replaced by thyristor valves. The advantages are:
 A shorter time to by-pass the MOV, which leads to lower triggering voltage than compared
to the forced triggering spark gaps;
 A reduction of the MOVs capacity of energy absorbtion , and
 A reduction of the transient recovery voltage (TRV) across the line circuit-breakers.
In addition to the TPSC, where the thyristor valves are used to a fast by-passing device rather than the
spark gap, a further newly developed fast bypassing protection equipment has been developed in
order to avoid to applicate a spark gap. The development is well known as fast protective device
(FPD). The FPD is composed of a sealed plasma gap and a high speed bypass switch in parallel. The
working principle is: as soon as the capacitor group (i.e. the MOV) has to be bypassed, a control
system will simultaneously send a bypass command to both the plasma gap and the fast bypass
switch. The reacting time of the plasma gap is smaller than the making time of the bypass switch, due
to the plasma gap will firstly conduct the current and the speed bypass switch will immediately (some
ms later) follow and bypasses the plasma gap. The charcatersistic of a FPD is characterised by a fast
bypass time and a very small triggering voltage, which is also used to reduce the TRVs across the line
circuit-breakers.
A quick survey among the CIGRE Working Group (WG) A3.33 members shows 12 TCSC and 8 TPSC
installations. Currently series compensated AC transmission lines are installed in system with voltages
of 220 kV (in China), 275 kV (in Japan), 400 kV (in Sweden, South Africa and Turkey), 500 kV (in
Brazil, Canada, China, and the US) gradually to 765 kV (in Brazil, Canada, and South Africa) and
1000 kV (in China). With the increase of the system rated voltage, the capacity and also the rated
voltage of series capacitors themselves become higher. In general, the importance of series
compensation increases with the higher voltages of the transmission networks, and stricter
requirements are needed for the SCs.
Series compensation operated under heavy load conditions is often accompanied with shunt
compensation especially in case that it also operates under light load conditions. Four-legged shunt
reactors has been installed to transmission lines up to UHV levels including 1200 kV applications in
Russia. Shunt compensations with variable reactors are put in service up to 765 kV and under
consideration for 1100 kV transmission systems. Furthermore shunt compensation with both inductive
and capacitive loads, is applied by connecting both shunt reactor and shunt capacitor banks to tertiary
side of UHV and EHV transformers. WG A3.33 studied any particular subjects to be considered when
it is applied to UHV levels especially in case of connection to large capacity UHV and EHV
transformers. Alternative technologies related to shunt compensations are also described briefly.
WG A3.33 was established in May 2013 to investigate recent field experience with equipment used for
series and shunt compensations. Total 24 members from 12 countries participated in the WG A3.33.
The members are experts belonged to utilities, manufacturers, research institutions, and consultants.
The WG held a kick-off meeting in September 2013 and discussed the detailed tasks. An enquiry to
collect the members’ experience with equipment applied to series and shunt compensations was
drafted at the second WG meeting held in April 2014. The field experiences in different counties had
been presented one by one and a potential structure of Technical Brochure (TB) was also discussed
at the following meetings. The first draft of the TB was compiled in January, 2016, and discussed
actions to be required for improving the contents. The second TB draft was circulated among the
members in May, 2016, and reviewed by the end of July, 2016.
WG A3.33 collected updated field experiences with equipment applied to both series and shunt
compensations, especially the equipment used for SCs connected to UHV and EHV transmission
systems and for shunt compensation mainly connected to tertiary side of UHV and EHV power
transformers. WG A3.33 utilized the related information collected by WG A3.13 [1], A3.22 [2], [3],
A3.26 [4] and A3.28 [5] in order to avoid overlapped investigations.
WG A3.33 studied the electrical stresses imposed on the equipment during operations of series and
shunt compensations, and clarified the requirements for each equipment such as a bypass switch, a
spark gap, MOV, a circuit breaker, a capacitor, and some alternative devices for a spark gap. The WG
also studied the impact of series and shunt compensations on equipment requirement including
system overvoltages, short circuit currents and secondary arc current extinction after a fault clearing,
and summarizes consequential requirements for equipment, and provides some countermeasures to
decrease a potential risk for transmission systems and associated devices.

8
The Technical Brochure (TB) provides a brief introduction in the chapter one. The chapter two mainly
summarizes the field experiences with equipment applied to series compensation investigated in
several different countries. The chapter three deals with some developments related to shunt
compensations including both fixed and variable shunt reactors. The chapter four deals with different
technologies related to series capacitors and the requirements for associated components, including a
main circuit topology, control and protection system, spark gaps, MOVs, bypass switches used in
FPDs as well as TPSC technologies. The chapter five deals with electrical stresses and the
requirements for equipment related to shunt compensation connected to the tertiary side of UHV and
EHV transformers. The chapter six summarizes the utilities experience related to maintenance
practices, replacement, and upgrading of series capacitors. The chapter seven mainly summarizes
testing experience with individual equipment applied to UHV series capacitors, and with whole SC
system during the commissioning tests. Finally it provides the conclusion in the chapter eight. TB
mostly covers equipment / components used in series and shunt compensation. In the TB, equipment
is normally specified with the rating voltages, instead of the nominal system voltage.

9
2. EXPERIENCES OF SERIES COMPENSATION
Long distance bulk power transfer often requires an increase of transmission line power transfer
capacity and an adequate sharing of a power flow among parallel circuits. Series capacitor banks that
can partially compensate the series inductance of transmission lines are one of solutions to cope with
an increasing demand. In addition, they can improve the power system transient stability and the
voltage control along transmission lines. Since the 1950s, series compensation has been applied to
EHV transmission systems and recently to UHV transmission systems. Field experiences with
equipment applied to series compensations were investigated. This chapter describes the progress in
equipment developments and operation experiences of series capacitors.
2.1 DEVELOPMENT AND TECHNICAL ADVANCEMENT
Before the 1980s, series capacitor banks were mainly protected by a spark gap. Due to technological
progress, for example, reliable capacitor manufacturing, power electronic evolutions, advanced control
and protection techniques, the applications of series compensation has been increased rapidly,
especially in those countries which require long distance and large capacity AC transmission lines. The
demand for reliable series compensation initiated new developments, such as a protection system in
application of large capacity MOV and a thyristor-controlled reactor. A fixed series capacitor bank with
the MOV protection is widely used in AC power grids. Thyristor controlled reactor applications led to
the development of thyristor controlled series capacitors (TCSC) and thyristor-protected series
capacitors (TPSC). TCSCs are usually used in combination with fixed series capacitors (FSCs) to
continuously adjust the line impedance to maintain a constant power transfer and to supress power
flow oscillations, or to vary power transfer in order to prevent resonance and sub-synchronous
resonance effects in EHV transmission system.
Series compensation is generally applied to heavily loaded transmission lines, where the reactive
power consumption is relatively large. In contrary, lightly loaded or unloaded transmission lines
produce reactive power that needs to be compensated by a shunt reactor. Shunt compensation,
inductive and capacitive, is described in the Chapter 3. Figure 2.1 shows a typical transmission line
with series and shunt compensations (inductive).
A power capacitor is one of
main equipment used in series
and shunt compensations.
Nowadays, it is recognized as
a well mature product
manufactured with
environmental friendly
materials.
Before the 1980s, a spark gap
was self-discharged or self-
triggered device, which was
discharged when any transient
voltage across the gap
exceeded the specified value.
The main gap consist usually
of two gap devices connected
in series with an opitimized
split of voltage. In order to
minimize the influence of gap
operating conditions (for
example, air density which
varies with the air pressure
and the ambient temperature,
will affect the characteristic of
discharge voltage) a sealed Figure 2.1: Simple three-phase scheme of a series / shunt
compensated double circuit overhead line

10
small triggering-discharge gap with a linear resistor in series is often connected in parallel to one of
the gap stages. In case of line fault occurrence, the series capacitor bank was by-passed, irrespective
of an internal or external fault. Consequently, the gap triggering will affect the operational availability
of transmission system.
In the 1980s the manufacturing techniques to produce a large capacity MOV/MOSA had been
advanced by optimization of the material components as well as the baking process to form the
homogeneous ZnO boundaries in the element, which can contribute to improve the reliability of MOV
and its energy absorption capacity. The large capacity MOVs have been applied widely to SCs to
protect them against power frequency overvoltages.
The SCs applied with MOVs were normally equipped with a spark gap in order to avoid excessive
stress imposed on the MOVs. Because the level of power frequency overvoltage is suppressed by the
MOVs, the self-discharge voltage of the spark gap has to be adjusted to a level slightly higher than
the MOVs protection level. The spark gap is triggered immediately only on an internal fault, or when
the energy dissipation of the MOVs is close to the specified value. The MOV applications can prevent
SC’s from being bypassed in case of external faults. Eventually, external faults will not affect the
power transfer of the healthy lines. Currently, SCs are commonly equipped with this power frequency
overvoltage protection scheme.
A spark gap applied to SCs must be externally triggered type when MOV is used to protect the power
frequency overvoltage, since the overvoltage is limited by the MOVs, the overvoltages never exceeds
the MOV voltage restriction level. An electronic triggering circuit with optical wires is typically used for
a spark triggering gaps on the SC platform. The electronic triggering circuit needs a reliable power
supply as follows:
1) Laser power which is supplied through optical fiber from the ground potential;
2) Electric power which is supplied from line current transformers at the platform potential;
3) Both laser power and electric power
In the early 1990’s, flexible alternative current transmission systems (FACTS) such as a Static
synchronous compensator (STATCOM) was introduced in AC power systems. Based on the power
electronic devices, a new thyristor controlled reactors were also used in SCs, realizing thyristor
controlled series capacitors (TCSCs). The thyristor controlled reactors are put in parallel to (a part of)
the capacitors. The equivalent capacitance of a TCSC can be controlled by its control system according
to the defined control strategies, thus continuously controlling a power flow on transmission line. This
technology can be used both to adjust the power flow on parallel lines and to mitigate resonance
effects or to suppress sub-synchronous power oscillations in power systems. TCSCs have been used in
the USA, Brazil, Sweden, China, and other countries. Since a thyristor valve used for TCSC make it
more expensive than a conventional fixed SC (FSC), the applications of TCSC are still limited as
compared with FSC. According to the WG A3.33 survey, there are totally 12 TCSCs in operation in
different countries including one full TCSC application in China’s 220 kV transmission systems [6].
In order to bypass the series capacitor faster than a spark gap, in order to mitigate TRV imposed on
of a circuit breaker after a fault clearing, new SC protection scheme using a thyristor valve was
developed, and applied in the US around 2000 and later. In the SCs called thyristor protected series
capacitors (TPSCs), a spark gap is replaced by a thyristor valve. Since a thyristor valve acts faster and
more accurate than a spark gap, the energy absorbing requirement for MOV is much smaller than that
of SC with a conventional spark gap. Consequently, the amount of MOV elements can be considerably
reduced, even though the costs of a thyristor valve are much higher than a spark gap. Up to now, 8
TPSC installations in 3 substations are applied in the US.
Around 2008, another fast discharge device, called the fast protective device (FPD), was developed,
and applied in SCs. This technology with a fast discharge device uses a fast making switch in parallel
with a sealed plasma triggering gap. The devices can operate at very low SC voltage.
Since a fast making switch together with a plasma triggering device will expose the equipment to the
transients with higher energy, a careful consideration to design an EM compatible equipment such as
a power supply source is required.

11
There are mainly 3 principles for a triggering spark gap:
1) The principle uses a trigatron. The trigatron is consist of a sealed spark gap with spark
plugs imbedded in the spherical electrodes. Some of the trigatrons have both sides of the
electrodes equipping with a spark plug, while some of them have only one electrode at
low voltage side equipping with a plug. Due to triggering scheme with trigatrons in which
both low voltage and high voltage electrodes are equipped with trigger plugs, the forced
discharge voltage of the gap will have advantages with no polarity effect and lower
voltage operations.
2) The principle uses high voltage source. It apply high voltage across the gaps connected in
series. When one of the gaps with higher voltage sharing is discharged, the entire gap will
be discharged immediately.
3) The principle uses a plasma injector on one of electrodes of the gap, which can trigger to
discharge the entire electrode of the gap by a plasma shooting.
2.2 EXPERIENCES FROM APPLICATIONS
Some examples of the application of series compensation have been given in Technical Brochure 336
(2007) of CIGRE WG A3.13 [1]. In the appendix of that Technical Brochure 336 the applied
technology and the service experience is described for the following countries: Sweden, Turkey,
Canada, Brazil, USA, Chile and Mali / Senegal. The technologies applied are series capacitor banks
protected:
- by triggered gaps and MOVs,
- by gaps without MOVs,
- by dual-gap / dual-breaker schemes,
- by MOVs without triggered gap or by thyristors that by-pass the capacitors.
The described series capacitor banks are used at voltage levels from 220 kV up to and including 735
kV. The degree of compensation varies several tens up to 100 percent. Some series capacitor banks
were equipped with thyristor controlled reactors in parallel with the capacitor bank in order to get a
flexible degree of compensation. The series capacitor banks were located in overhead lines, most of
the time installed in a substation, but there are also utilities, which install the series compensation in
the middle of an overhead line.
The service experience reported is rather good. Only minor problems have been mentioned during
decades of service, which were as example inadvertent gap operation, a few problems with MOV,
problems with control circuits, problems with early technologies of capacitors, with distance
protection, with control systems and with hydraulic drives. By improvemnets in design, specification
and application,
fundamental issues have
been solved for which as
example are described:
sub-synchronous
resonance, increased
TRV stresses for the line
circuit-breakers and
reverse voltage or
current which must be
handled by the distance
protection.
Since that time series
compensation was also
applied by a number of
other countries and by
other utilities (e.g. South
Africa, China, USA).
Moreover, series
compensation has been
applied in China at UHV-
level, referring to Figure 2.2.
Figure 2.2 Three-phase series capacitor bank in China, under
construction, applied to a single circuit 1000 kV overhead line

12
According to the survey of this CIGRE WG, there are more application experiences coming from
Canada, China, Japan as well as South Africa, and these experiences are as shown in the following.
2.2.1 Operation experiences of series capacitors in the eastern network of Canada
Worldwide, the eastern power network of Canada is one of the utilities with the largest use of series
compensation (SC) on its transmission network. SC contributes significantly to voltage control, stability
improvement and reducing the impact of geomagnetic storms in transmission networks. Another great
benefit of having series compensation on their network is that it allows maximization of the transfer
capability of their transmission lines. It has become more and more complicated to build additional
lines due to social and environmental impact. Series compensation permits the avoiding of or the
delay for the need of new line projects which has a positive economic impact for utilities.
2.2.1.1 Overview of SC on the eastern network of Canada
Main characteristics of series compensation installed on the eastern network are summarized in Table
2.1. The first SC platform was installed on a 120 kV line in 1986 in order to increase the transfer
capability between Joutel and Figuery substations. This installation consists on a fixed capacitor bank
protected only by an air gap. The discharge current of the capacitor bank occurring during a fault is
attenuated by a damping circuit. This installation is still in operation today.
Table 2.1 Series capacitor installations in the eastern power grid of Canada
Substation Voltage
network
N
line
Type
of SC
Nominal
current In
Zc % of
SC
P
1
Protective
Level
2
Year of
operation
Ultimate
Stage
(kV) (A rms) () (MVAR) (p.u.) (A rms)
Montagnais 735 3 Fixed 2300 30 40 476 2.6 1993 NO
Arnaud Nord 735 3 Fixed 2200 25 34 363 2.5 1993 NO
Arnaud Sud 735 3 Fixed 2200 25 44 363 2.5 1993 2500
Saguenay 735 1 Fixed 1900 22 26 238 2.5 1993 2500
Périgny 735 1 Fixed 1900 22 16 238 2.5 1992 2500
Bergeronnes-1 735 3 Fixed 2560 21
36
412.8 2.55 1991 2560
Bergeronnes-2 735 3 Fixed 2929 25.71 662 2.3 2006 2560
Abitibi 735 3 Fixed 2300 25 36 397 2.5 1995 2900
Albanel 735 1 Fixed 2200 16 19 232 2.5 1995 2800
Albanel 735 2 Fixed 2600 16 23 324 2.5 1995 3000
Chamouchouane 735 3 Fixed 2057 21.87 34 277.7 2.5 1994 2800
Chibougamau 735 3 Fixed 2200 22.4 30 325,3 2.4 1994 2800
LaVérendrye North 735 3 Fixed 2852 31.81 37 776,2 2.4 1995 3100
Némiscau 735 3 Fixed 2200 16 19 232 2.5 1995 3000
Jacques-Cartier 735 2 Fixed 2800 28 35 658,6 2.3 2011 3200
Kamouraska 330 4 Fixed 1250 41 60 192,2 2.1 1987 NO
Des Hêtres 230 1 Fixed 1000 36 60 108 2.3 2007 NO
Joutel 120 1 Fixed 410 50 59 25 2.5 1986 NO
1 : Reactive Power is per line (for a three phase line). Total MVAR installed in the networks shall be calculated with the
number of lines (N line) compensated.
2 : 1 p.u. = In X Zc
The second series compensation project was installed on a 330 kV network at Kamouraska substation
on the fourth line between Rivière-du-Loup and Lévis substations. Those lines serve the Gapesian
area which includes the Madawaska and Eel River interconnections within the 184 km transmission
line. This series compensation project was strategic to reinforce this corridor by improving the voltage
stability in this area and also increasing the transmission line capability. Series compensation is
located at 45 km from the nearest substation and consists of a fixed series capacitor bank protected
by MOV and conventional controlled air gap. The fourth line is compensated at 60%. It was the first
project (at the eastern network) using MOV for capacitor bank protection. This installation has been in
operation since 1987. No future expansion was planned at the origin. However, overload capability will
be increased from 1.35 p.u. for 30 minutes to 1.5 p.u. for 30 minutes in the coming years.
Service continuity and system reliability became a major issue at the eastern network during the 80’s
following major network outages and also due to the increasing exporting to neighboring networks. In
response to its concerns, the eastern network in 1989 launched a vast reinforcement network
program. At the same time, basic design criteria have been adopted to fulfill the requirements of
NPCC (North-Eastern Power Coordinating Council) with additional eastern network criteria to ensure
more robustness. The culmination of this program is summarized by the massive addition of series
compensation on 32 transmission lines (735 kV) from 1993 to 1996, for a total of approximately

13
11000 Mvars. The technology used for all these installations is based on fixed series capacitor banks
(with external or internal fuses) protected by MOVs and conventional controlled spark gaps. The
majority of platforms are installed on line terminals (substations) except for 2 installations which are
located approximately in the middle of the line. Presently, the degree of compensation for these lines
varies from 16% to 44%.
An additional project of 108 Mvars of SC located on a single 145 km line between Rapide-Blanc and
Des Hêtres substation was put in service in 2007. This project was required due to an integration of a
new hydro power plant in this 230 kV grid area which required increasing the power transmission
capability. The distinctiveness of this installation is that it’s the first one that uses a fast protective
device, FPD, instead of the more conventional controlled spark gap as protective device.
The latest series compensation project in the eastern 735 kV network was put in service in 2011. It
consists in the addition of 658 Mvars series capacitor banks on two transmission lines between
Chamouchouane and Jacques-Cartier substations for network reinforcement and stability
enhancement. Same technology is used: fixed capacitors banks protected by MOVs and conventional
controlled spark gaps. Localization of series compensation projects in 735 kV transmission network is
shown in Figure 2.3.
2.2.1.2 Return of experience
More than 20 years of series compensation operation on the eastern network of Canada has
demonstrated a real benefit for proper operation of their long transmission lines, in particular in terms
of voltage control, reliability and stability. Another benefit of SC is that it has allowed delaying the
addition of new lines - which is at this time very complicated for utilities due to negative perception
from the population. The following paragraphs give an overview of performances and problems
occurring in service and the solutions that have been implemented to counter them.
2.2.1.2.1 Capacitor units
Special service conditions such as very low temperatures occurring in SC locations are a real challenge
for equipment. For capacitor units, a relatively difficult endurance test at minimum temperature of -50
C (cold duty test) was performed to verify the dielectric strength under maximum stress. This test
has had a significant impact on capacitor design since manufacturers had to incorporate safety
margins in order to pass the test successfully. Another issue is that a lot of installations were planned
in the same time frame on the 735 kV network and then, manufacturers proposed the same unit to
cover the needs of several installations, resulting in an oversizing of capacitor units in some locations.
On the other hand, it should be admitted that network stresses in series capacitors are much less
severe than for shunt capacitor banks since the series capacitors often operates below 1 p.u.
Figure 2.3 Series compensation in 735 kV network

14
continuously. However, they are strongly needed during network faults or during overload periods
which is not a situation that happens frequently.
Furthermore, if electrical performance of capacitor banks is well satisfied, there are still some weak
points to be addressed regarding capacitor units. Maintenance activities have reported the
replacement of a few capacitor units due to oil leakage at bushing interface; degradation of external
fuse covers and rust on capacitor casings.
Overload capability of capacitors for SC installations on the 735 kV network have been established at
1.5 p.u. for 30 minutes and for installation on 315 kV network, it was 1.35 p.u. for 30 minutes. After
some years of operation, this requirement seems insufficient to cover all contingencies for a few SC
installations. In fact, network evolution by addition of new generation (hydro power and windfarm)
and an increasing energy demand have imposed a revision of these requirements.
Two alternatives were considered: an upgrade of SC installations affected by new requirements or an
investigation on capacitor units to verify if the existing units installed on SC platforms have some
margin which can allow a higher overload such as 1.6 p.u. or more for 30 minutes. This second
alternative had a great economic benefit since it delays an upgrade project. However, using
equipment close to their limits should not compromise reliability of the grid. A rigorous approach was
developed to confirm the effective overload available for different designs of series capacitors: cold
duty tests at -50 C on complete capacitor units have been performed to confirm if additional overload
capabilities were really available. Overload may also have an impact on MOVs and their energy margin
was also verified. Moreover, for these installations where overload capability had to be increased,
capacitance of all capacitor units was measured in order to detect if any capacitor elements had failed.
In such case, the capacitor was replaced by a spare part in order to have a capacitor bank in a very
good condition before increasing the overload capability of a specific capacitor bank.
The additional overload capability was made available to operators in a few specific installations where
capacitor overload capabilities were confirmed by test, MOV’s margin were verified based on design
studies and where capacitor units were verified on site by capacitance measurements. Furthermore,
maintenance activities have been reinforced for capacitor banks which are submitted to an increased
overload.
2.2.1.2.2 MOVs
Concerning MOVs, only one problematic situation has occurred during the first year of operation in
one specific substation. There were a few MOV failures in service but not related to internal faults and
energy absorption. In fact, the MOV units had seen only low current. Some hypotheses were analyzed
to explain this phenomenon, such as influence of electrical fields on MOVs due to the proximity of
busbars on the platform, and also, influence of icing conditions or snow deposits on voltage
distribution along MOV columns since these events occurred during such environmental icing
conditions. The dismantling of MOV units has revealed the cause of previous failures: cracks were
observed on some MO disks due probably to mechanical impact during the long transport by train. At
that time, all MOV units were removed from the platform for inspection, dismantled, rebuilt as
necessary and tested for proper matching and re-installed in service. This solution was effective since
no more MOV failures occurred in this substation following this intervention.
2.2.1.2.3 Controlled spark gap
At the beginning of SC operation, the eastern network observed some by-passes not associated with
fault conditions. After investigations it appeared that the immunity of conventional spark gaps to
environmental conditions was not sufficient to insure their proper behavior. Moreover, inspection
performed on SC platforms had shown the penetration of snow inside the housing of spark gaps. This
situation was the starting point of a research study conducted with the eastern network research
laboratory and with the collaboration of manufacturers to find a solution to avoid untimely operation
of spark gaps.
The first part of the study made by the eastern network and the laboratory consisted in analyzing
dielectric withstand and electric field under several atmospheric conditions on different electrode types
used in SC. This study was important since it gave a result, what the maximum electrical field
between electrodes should be at protective voltage level to reduce the risk of natural ignition of the
spark gaps. This value of maximum electrical field is now a part of the technical specification.

15
The second part of the study performed in collaboration with manufacturers and University of
Chicoutimi consisted in designing and testing snow barriers which could limit snow penetration in
housings without compromising the performance on voltage recovery after an operation of the spark
gap. In fact, to limit snow penetration, the openings of the enclosure should be covered but these
openings are essential to let the ionized gases leave the space between the electrodes. A compromise
between performance on expected voltage recovery time and limitation of snow penetration was
necessary. The solution developed consisted of air vents installed on all openings of housings and
adapted for each housing design. The efficiency of this solution was tested in laboratory and also in
outdoor conditions with a special set-up for wind, ice or snow. Furthermore, the requirement
concerning the delay between a spark gap operation and a full voltage recovery was reviewed in order
to allow the series capacitor to be re-inserted after a successful line auto reclosing. A test was
conducted on each spark gap enclosure design with snow barriers installed to demonstrate that the
electrical performance with respect to recovery voltage was fulfilled.
Records of spark gap operations on the 735 kV grid since their installation are shown in Figure 2.4.
This graph includes all operations due to: untimely sparkover from unknown causes, problems with
protection systems, equipment failure, normal operation, human error and gap testing. The majority of
gap operations are due to normal operation.
2.2.1.2.4 Fast protective device (FPD)
The first generation of fast protective device was installed in 2003 on one phase of SC of 315 kV
network at Kamouraska substation. It was a pilot project conducted in collaboration between the
manufacturer and the eastern network. Initially, the original air-gap was kept in operation in parallel
with the FPD. This prototype was in operation for a few years only. Unfortunately, it never operated
since no fault occurred in this phase during that period.
A few years later a new generation of FPD was installed on a series compensation project on 230 kV
network at Des Hêtres substation. It was not a network requirement since there was no need for TRV
reduction in this project but rather as a technology proposed by the manufacturer in replacement of
conventional air-gap. The immunity against environmental conditions is a great benefit for network
operation since with this device untimely gap operation can be avoided. An extensive type test
program to verify the robustness and proper functions of the system was performed before
Figure 2.4 Number of gap operations per year for all 735 kV installations
0
5
10
15
20
25
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
Number of gap operations per year from 1991 to 2013

16
commissioning, with particular attention to low temperature as a service condition. Some adjustments
were necessary to fulfill their requirements. The FPD is in operation since 2007 and service experience
is very positive. This device is equipped with good supervision features which make maintenance
easier. Only one internal fault has occurred since its installation and the FPD has operated properly.
2.2.2 Operation experiences of series capacitors in China
2.2.2.1 Application history and main purpose of series compensation
China began to apply series compensation techniques in its power lines in the early 1970’s. There
were a total of two applications, one was on a 220 kV power line located in Zhejiang Province, while
the other was on a 330 kV line in Shaanxi Province. Since the failure rate of power capacitors was, at
that time, too high due to old manufacturing technology, and poisonous impregnating PCB liquid used
in power capacitors, the two SC applications were removed only after a few years of operation.
Since the 1980’s, the economy began to develop rapidly in the eastern and southern coastal areas of
China, which required a great amount of electric energy. However, these areas were short of energy
resources. Electric power had to be generated transporting coal by rail from far away provinces. There
were abundant hydro resources in the south-western area, and coal resources and other large energy
bases in the western area of China. Therefore, the rapid economic development of the southeast
coastal area resulted in a demand for the electric power provided from the western region. The
central and north areas also needed development and electric power provided from the western areas.
Although the point-to-point power transfer used by the DC transmission scheme is suitable for long
distance and even for ultra long distance and for bulk capacity power transmission, it cannot provide
electric power sufficiently for the development needs of the central area of China. In fact, long
distance AC power transmission has to be applied in addition to DC power transmission. Therefore,
both AC and DC power transmission has been rapidly developed in China.
China is highly populated and the land resources are very scarce, especially in the eastern and
southern part of China. A power transmission corridor is always a big problem for power companies
that usually has to be solved by reducing the number of transmission lines. The first 500 kV Sanbao
SC project was born from such a background. This AC power transmission project began from
Yangcheng Power plant in south-eastern Shanxi province, and ends at the 500 kV Huaiyin substation
in Jiangsu province - as shown in Figure 2.5. The total length of the transmission corridor is about 700
km, and it is required to transmit 2100 MW of electric power. There are two other stations in between
Yangcheng and Huaiyin in this corridor. One is Dongming switching station and the other is Sanbao
substation. There are three 500 kV transmission lines between Yangcheng power plant and Dongming
switching station, and only two transmission lines between Dongming switching station and Sanbao
substation. The length of each line between Yangcheng and Dongming is about 256 km, and that
between Dongming and Sanbao is about 269 km. From Sanbao substation, there are two 500 kV lines
going to Huaiyin substation, and one line going to Renzhuang substation.
Figure 2.5 Single line diagram for Sanbao series compensation project
In order to increase the stable power transfer capacity of the transmission lines, fixed SC was installed
in Sanbao substation on each of the two lines that links Sanbao substation and Dongming switching

17
station, referring to Figure 2.5. According to investigation, the main purpose of all fixed SC’s installed
in China’s EHV power lines is to increase the stable power transmission capacity.
Besides of fixed SC’s, TCSC’s were also installed in 500 kV Pingguo substation, 500 kV Fengtun
substation as well as 220 kV Chengxian substation to eliminate sub-synchronous resonance in the
power grid.
2.2.2.2 Application status of series compensation
Series capacitor applications in the 500 kV and 220 kV power grids of China are listed briefly in Table
2.2. All the series capacitors function or functioned as expected, although a few of them have been
upgraded or stopped operation due to the enhancement of the 500 kV power grid.
Table 2.2 Series capacitor applications in China’s 500 kV and 220 kV power grids
Station Name of Line
Number
of sets
Comp
Degree
Type
Capa-
city
(Mvar)
Current
(A)
Line
Voltage
(kV)
Gap
type
MOV
Energy
(MJ/Phase)
Operation
Date
ChengXian Cheng-Bi 1 50% TCSC 95.3 1210 220 Gap 10 12/2004
SanBao
Dong-San I&II 2 40% FSC 500 2360 500 Gap 50 11/2000
Dong-san III 1 41.4% FSC 528 2360 500 Gap 48 07/2006
FengTun
Yi-Feng I&II 2 30% FSC 544 2330 500 Gap 40 10/2007
Yi-Feng I&II 2 15% TCSC 326 2330 500 Gap 33 10/2007
HunYuan
Tuo-Yuan I&II 2 46.6% FSC 466 2400 500 Gap 45 04/2008
Tuo-Yuan III 1 45.5% FSC 466 2400 500 Gap 45 04/2008
Tuo-Yuan IV 1 44.5% FSC 466 2400 500 Gap 45 04/2008
Yuan-An 2 41.3% FSC 539 2400 500 Gap 55 11/2010
Yuan-Ba I 1 35.3% FSC 359 2400 500 Gap 65 11/2010
Yuan-Ba II 1 34.9% FSC 359 2400 500 Gap 65 11/2010
ChengDe Shang-Cheng 2 45% FSC 493 2700 500 Gap 52 03/2009
Xindu
Jin-Xin 2 35% FSC 380 2700 500 Gap 82 08/2009
Xin-Shi 3 35% FSC 297 2700 500 Gap 84 08/2009
GuYuan
Han-Gu 2 40% FSC 417 3000 500 FPD 94 10/2010
Gu-Tai 2 45% FSC 663 3000 500 Gap 84 10/2010
TongYu Zhan-Li 1 40% FSC 358 2700 500 Gap 66 12/2015
YanShan Wen-Da 2 30% FSC 435 2700 500 Gap 26 03/2009
GuiLin Gui-Xian 4 25% FSC 415 3000 500 Gap 25 01/2010
Jianshui Hong-Mo 2 50% FSC 590 3000 500 Gap 95 04/2009
WanQuan
Feng-Wan 2 35% FSC 259 2400 500 Gap 06/2003
Wan-Shun I&II 2 45% FSC 444 2400 500 Gap 06/2003
Wan-Shun III 1 45% FSC 444 2400 500 Gap 08/2006
WuTai Shen-Bao 2 35% FSC 575 2700 500 Gap 33 11/2008
DaFang Da-Fang 2 35% FSC 375.6 2100 500 Gap 30 06/2001
HeChi Qing-He 2 50% FSC 762 2400 500 Gap 67 11/2003
PingGuo Tian-Ping 2
35%
5%
FSC
TCSC
350
55
2000 500 Gap
30
6
06/2003
Puti Er-Pu 3 40% FSC 315 2200 500 Gap 11/2006
FengJie Wan-Long 2 35% FSC 610 2400 500 Gap 35 08/2006
BoShang
De-Bo 2 50% FSC 620 2700 500 Gap 05/2010
Bo-Mo 2 50% FSC 425 2700 500 Gap 05/2010
BaiSe
Ma-Bai 1 50% FSC 542 2400 500 Gap 12/2005
Luo-Bai I 1 50% FSC 670 2700 500 Gap 12/2005
Luo-Bai II 1 50% FSC 670 2700 500 Gap 11/2007
HeZhou Liu-He 2 40% FSC 390 2400 500 Gap 75 06/2009
TongBao 2 25% FSC 135 3000 500 Gap 32 08/2012
Yulin Yu-Mao 2 42% FSC 286 2400 500 Gap 85 02/2010
2.2.2.3 Faults experienced on series capacitors
Nowadays all series capacitors are serving very well in power grids, though faults occurred during
system commissioning or during early days of operation for several SC’s. Those faults are briefly
introduced in the following:
A) Self-triggering of spark gaps

18
Sanbao series capacitors were put into operation in December, 2000. At 6:25:23 of December 1st,
2002, temporary fault occurred on phase B of Dong-San Line I. The spark gap of phase B self-
triggered during this fault. There was no abnormality found with the main gap itself, but several dead
birds within the main gap shell. The conclusion is that birds initiated the discharge of the gap during
line fault [7].
Three self-triggering events occurred in the gaps of Dafang series capacitors between the years 2003
through 2005. Checking and analysis show that it was the altitude that caused the gap’s self-
triggering. Elevation affects the air density, and then affects the self-discharge voltage of the main
gaps. The altitude of the Dafang series capacitor yard is 910 m, that corresponds to the self-discharge
voltage correction factor of 1.12. According to experimental research result, the distance of the gaps
of Dafang series capacitors was not corrected according to the altitude effect, and the self-discharge
voltage was lower. There was no more self-discharge event occurred after elevation effect corrections
to the gap distance of the spark gaps.
B) Reliable triggering problems:
The spark gap of Dafang series capacitors experienced an unexpected triggering during permanent
artificial single phase to ground fault test [8], [9]. The reason was found that the test method used by
test engineers on a varistor numbered FR4 of the triggering circuit was incorrect, that affected the
normal life of the varistor. This varistor was damaged during the artificial grounding test, and thus
affected the function of the triggering circuit. After the artificial fault test, this varistor was replaced.
Any of the elements used in series capacitors, even a small element, may affect the normal
functioning of the SC if tested with incorrect method, and should be replaced before system
commissioning, eliminating the risk of normal operation of the SC’s.
C) Burn out of the series capacitor bank
Referring to [10], the burn-out event occurred for phase A of two series capacitors installed in a 500
kV substation. A single line diagram is presented in Figure 2.6 which may help to explain the event.

Figure 2.6 Single line diagram for a substation with SCs and the related lines
One day at 15:59, phase A of the 500 kV line I-III tripped due to single phase to ground fault on
phase A of the line, causing single phase reclosing to activate. Since the fault was still detected, the
line was tripped. According to the message from the fault distance detector, the first single phase to
ground fault occurred close to middle of the line. But the fault corresponding to the line reclosing was
a phase to phase fault close to substation I. 2.1 s after the fault of line I-III, another single phase to
ground fault was detected at phase A of the 500 kV line II-I, line protection activated and phase A
was tripped, single phase reclosing activated but failed, and the line was tripped out. Fault distance
detection message showed that the fault was located close to substation I. After the faults, the
personnel of the substation on duty found that the series capacitors of phase A of both on line I-III
and line II-I were burning.
Analysis of the series capacitor fault of line I-III showed that the main reason for this fault was that
the time delay of the unbalance current protection of the capacitor banks was too long for the first
single phase to ground fault. This fault caused several series capacitor units to be damaged, to leak
and burn with free hot metal vapour spreading to the platform of Phase B, which caused a short
circuit between the SC platforms of phases A and B when line reclosing was activated.
For the SC fault of line II-I, the sequence of events (SOE) information showed damage to the series
capacitor units occurred during reclosing of line I-III which activated the unbalance current protection.
However, the damaged capacitor units were leaking and burning. The activation of the unbalance
current protection had no effect on the leaking and burning of the capacitor banks. The permanent

19
fault formed between the platform of phase A and ground by the burning capacitor dielectric leaking
towards ground. Inspection to the SC platform showed that the Line CT cracked and the unbalance CT
exploded.
D) MOV Failures
Since 2008, several MOV pressure-relief faults occurred in the 500 kV power grids in China [11]. Most
of the surge arrester damages were caused by internal moisture. However, those MOV damages as
mentioned above did not vent. And fault recordings showed that the energy dissipated was much
lower than that of the MOV allowed. In fact, the true reason which caused the MOV failure is, in fact,
not found, yet [11].
2.2.2.4 FPD Operation Experience in the Electric Power Grid in North China
There are 4 sets of series capacitors installed in the 500 kV Guyuan switching station (referring to
Table 2.1), located about 180 km northwest of Beijing. 2 sets of 417 Mvar series capacitors are on the
Han-Gu double circuit lines, compensating 40% of the line reactance, while the other 2 sets of 663
Mvar series capacitors are on the Gu-Tai double circuit lines which compensate 45% of the line
reactance. Rated current of the 4 sets of series capacitors is 3000 A.
In the 4 sets of series capacitors, 2 sets on the Han-Gu double circuit lines are equipped with FPD’s
instead of ordinary spark gaps, while the other 2 sets (on the Gu-Tai double circuit lines) are equipped
with ordinary spark gaps. There were two faults related to power supply transformers of TCU of the
FPD’s since October 2010 when the series capacitors were put into operation. The first fault occurred
on the SC of Han-Gu line II on November 17, 2010, and the second occurred on the SC of Han-Gu
Line I on November 19, 2010. Both faults caused three phase by-passing of the SC’s. Investigations
showed that it was the power supply transformer fault that caused the SC by-passing. After the power
supply transformer was replaced, no further faults occurred.
As a new type of fast overvoltage protective device, it was the first time for such a FPD to be used on
the SC platforms in China. Therefore, it is normal that malfunction was encountered during the initial
operation period. Nowadays, they work well on the lines.
2.2.3 Operation experience with series capacitors in Japan
The electric power network in Kansai region supplies electricity to the western part of Japan covering
several metropolitan cities such as Osaka, Kobe and Kyoto (see Figure 2.8). The major generating
power plants concentrate on the northern parts of the Kansai region, opposite to the major demand of
the metropolitan cities located in the southern part of Kansai region. The transmission networks are
composed of a double circuit link surrounding the metropolitan cities connected to relatively long radial
transmission lines that can transmit the electricity from remote large capacity generating stations to
the demand areas. The nuclear power plants had been producing about 50% of the regional demands
before the Tohoku region Pacific Coast earthquake in 2011. They also have large capacity hydraulic
power plants including Kurobe No.4 power station.
The national-first 275 kV Daikurobe transmission line with series capacitor banks (Figure 2.7) in Japan,
planned to increase the power transmission capability, was put into service in October 1973 [12].
Figure 2.8 shows the distance of the power transmission lines between Johana switching station to
Kitaosaka substation which is about 240 km long with double circuits. The compensation degree of the
Daikurobe transmission lines at the first stage was 22%, which increased a power transmission
capability from 500 MW to 636 MW in 1973. The compensation degree was upgraded to 50% and
increased its transmission capability to 1140 MW in 1984.

20
Figure 2.7 275kV series capacitors banks
Regarding the series capacitor components,
conventional spark-over gaps were initially used for the
protection devices of the series capacitor bank at the
first step (corresponding to the group 2 bank) and the
designs were modified to use MO varistors without a
gap in 1984 (corresponding to the group 1 bank). Since the renewal of protection equipment of
capacitor banks, group 2 was replaced to use MO varistor protection and group 1 was replaced to use
gap protection. The dissipated energy of each MO varistor was about 45 MJ parallel connected across
the capacitor banks. Since Johana switching station is located in a snowy district, where height of
snowfalls usually attains 2 m depth. The ambient temperature was specified as a maximum of 40 C
and minimum of -20 C. The seismic requirements for the design of insulators were provided by three
sinusoidal half-cycles of 0.3 gravity accelerations based on Japanese standards.
The ratings of the capacitor bank are shown in Table 2.3. Table 2.4 indicates the protection scheme
for capacitors against over voltages and a single line diagram of series capacitors. In the group 2
system, the spark-over of protective gaps gives the primary protection scheme for capacitors against
overvoltage. Firstly, the protective gap is discharged by voltage build up immediately. 58 ms after the
fault, the vacuum switch is closed and non-linear resistors limit the transient over voltage across the
capacitors. After CB clearing, the arc in the protective gap is extinguished by the blast of compressed
air. The capacitor will be re-inserted when the spark gap extinguished, 1.5 to 3 cycles after the fault is
cleared. Re-insertion transient overvoltages after re-energizing the series capacitors can be
suppressed by non-linear resistor inserted by vacuum switch. In the group 1 system, the protection
scheme of modified protection devices simply uses MO varistors without a gap. MO varistors limit the
transient over voltage across the capacitors as well as re-insertion transient voltage. By-pass switch
by-passes the capacitors within 3 cycles after the fault.
Table 2.3 Ratings of 275kV series capacitor installation
Items Initial Present
Capacitor bank Compensation degree 22% 50%
Reactance per phase 20.4  46.0 
Rated normal current 900 A 1200 A
Rated voltage of capacitors per phase 18.4 kV 55.2 kV
Rated Capacity 49.6 Mvar 200 Mvar
Spark-over gap Spark-over voltage per one gap 34.2kV -
Rated Short-time withstand current 10000 A -
By-pass switch Rated voltage 24 kV 72 kV
Rated current 1200 A 1200 A
Making time - 33 ms
The field tests, carried out in 1973 to confirm the effectiveness of the protection system for series
capacitor banks included power stability tests, capacitor insertion tests, sub-harmonic oscillation tests,
and artificial fault tests. The transient phenomena caused by the capacitor re-insertion on the
transmission line were also measured. A series of commissioning tests showed successful
demonstrations before the commission.
Figure 2.8 275 kV Daikurobe
transmission lines

21
(1) Power stability tests were carried out with single (one circuit is open) and double circuit
transmission lines in the case of both series compensated and uncompensated conditions (with
and without the series capacitor by-passed). The power was gradually increased until the power
loaded on the lines started an unstable power swing. Figure 2.9 shows typical unstable
phenomena observed when the load capacity was 470 MW on the single circuit line with the
series capacitor. As a countermeasure, Power System Stabilizer (PSS) was installed at each
power generation station at Kurobe River. This was to improve the power stability by controlling
the signal of generator’s automatic voltage regulator (AVR), which can increase the generator
damping force by adjusting the parameters such as angular velocity variation (ω), power
frequency variation (f) and power output variation (P).
Table 2.4 Initial protection scheme of the series capacitor
(2) Switching overvoltages were measured during 24 opening and closing operations of the by-pass
switch across the series capacitors. The overvoltages between the capacitors were less than 2.0
p.u., the installation design level of the equipment (the protection voltage is set as 34.2 kV).
(3) Sub-harmonic oscillations were induced on the transmission line with series capacitors, provided
no load transformer was energized on this line. The oscillations originated due to the biased
voltage of series capacitors causing the exciting inrush current of the transformer. The
Spark-over gap protection (group 2) MO varistors without a gap (group 1)
SinglediagramProtectscheme
Figure 2.9 Unstable power swing during 470 MW loading on single circuit
A: Unit capacitor groups
B: Current limiting reactor
C: Protective gap
D: By-pass switch stationary and
moving contact
E: Voltage dividing capacitors
F: Pilot gap
G: Non-linear resistors
H: Vacuum switch
I: Power gap air control Vacuum
switch operator and excessive
arcing time detector
J: Capacitors unbalance
protection
K: Insulation-flashover protection
L: Platform power transformer
SrC: Unit capacitor groups
BPS: By-pass switch
MOV: MO varistors
DR: Damping reactor

22
overvoltage caused by sub-harmonic oscillation may become a problem like the ferro-resonance
phenomenon. Figure 2.10 shows the mechanism of sub-harmonic oscillation. The sub-harmonic
oscillation occurs as follows:
Step 1: After closing the switch, source voltage is applied to the transformer.
Step 2: The transformer reaches magnetic saturation, and excitation current flow to series
capacitor.
Step 3: The transformer reaches magnetic saturation, excitation current flows to series
capacitor and the voltage occurs over the capacitor.
Step 4: After half cycle, the transformer reaches magnetic saturation again. As a result, voltage
oscillation continues.
Transient network analyzer (TNA) studies show the generation of sub-harmonic oscillations for
Daikurobe trunk line. Field test conditions are shown in Figure 2.11, and one or two transformers of
Shin Aimoto substation were energized from Kita Osaka substation. A typical oscillograph is shown in
Figure 2.12.
Figure 2.12 Typical oscillograph of sub-harmonic oscillation tests
One seventh to one ninth sub-harmonic oscillation were observed during the test, referring to Table
2.5. The maximum phase voltage across the series capacitors was 18.7 kV. The protective gap and
NOTE: Transformers of Aimoto substation were energized from Kita Osaka substation by closing “circuit breaker 01”
Figure 2.11 Single line diagram of sub-harmonic oscillation tests
(a) Equivalent circuit (b) Waveforms
Figure 2.10 Mechanism of sub-harmonic oscillation

23
sub-harmonic protection did not operate during the commissioning test. And TNA simulation results
show a good agreement with the field test.
Table 2.5 Sub-harmonic oscillation study
TNA study Field test results
Exciting inrush current of transformer 450 A 454 A
Maximum voltage across series capacitors 21.3 kV / per phase 18.7 kV / per phase
Harmonics based on 60 Hz 1/9 1/9 to 1/10
Wave shape Sinusoidal Sinusoidal
Note: One transformer was energized.
There are two control sequences to avoid the sub-harmonics by operating the by-pass switches, the
series capacitors are not used in the following conditions.
1) The load capacity was less than 20%.
2) The sub-harmonics with 1/3 power frequency was detected.
A single phase and 3-phase artificial ground fault test were conducted to verify the effectiveness of
the capacitor protection and to study the related transient phenomena when the series capacitor was
re-inserted on the lines. The spark-over discharge of the protective gap was initiated in the range of
33.1 to 35.0 kV. The maximum overvoltage across the series capacitor was 16.5 kV and the longest
fault clearing time was 28 ms after the fault occurrence. The proper protection for series capacitor
banks were successfully demonstrated in the field.
The first 275 kV series capacitor banks in Japan were successfully commissioned in 1973 and it has
shown good service experience for more than 40 years without major trouble. The protection and
transmission equipment of the capacitor banks were replaced once. IEEE has acknowledged this
series capacitor bank as the IEEE milestone in 2010.
2.2.4 Operation experiences of series capacitors in South Africa
The electricity supply in South Africa is dominated by a large generation pool in the Mpumalanga
province. Although the other provinces have some local generation, in order to satisfy the demand in
all the other provinces a high voltage transmission network is required. The Western Cape, Eastern
Cape, Northern Cape, Free State and North West provinces are supplied from a central backbone of
the transmission grid known as the Cape Corridor. The Cape Corridor is a portion of the transmission
network that stretches from Mpumalanga down to the Western Cape, approximately 1400 km. The
large distance and loading requires very high voltages and hence the corridor has 765 kV and 400 kV
transmission lines. However the current 765 kV network only extends half way along the corridor. Two
key factors are driving the strengthening of the Cape Corridor. The first is the requirement for
network security and the second is the high forecast demand in the greater Cape area.
The preferred option was to implement the 765 kV series compensation and establish a Hydra-Gamma
765 kV line. The solution to provide the overall
best improvement in transfer limits, was the
lowest cost option, had the least environmental
impact, resulted in a significant reduction in
system losses and provided the best project
timelines to achieve the required increased
capacity.
A total of six series capacitors have been
implemented in the 765 kV national grid in South
Africa, to strengthen the power transmission
network in the Western Cape region. The series
compensated transmission corridor is shown in
Figure 2.13 [13]. The installations allows the
utility more flexibility and reduces its reliance on
existing local power generation. The series
capacitors are located at four sites along the Cape
Corridor, with ratings of the series capacitors Figure 2.13 765 kV series compensated
transmission corridor

24
ranging from close to 450 Mvar up to more than 1300 Mvar:
 Alpha 1 & 2, 2 x 446 Mvar
 Beta 1 & 2, 2 x 1340 Mvar
 Perseus, 893 Mvar
 Mercury, 1119 Mvar
The main circuit design is based on
single segment schemes in four of the
series capacitors (Alpha 1&2, Perseus
and Mercury) as shown in Figure 2.14
[14].
In the remaining two (Beta 1&2), due to
their sizes (each 1340 Mvar), sub-
division into dual segments schemes has
been applied as shown in Figure 2.15.
The main technical data of the series capacitors are listed in Table 2.6.
Figure 2.15 Dual segment series capacitor scheme
Table 2.6 Main technical parameters of the series capacitors
Parameter Alpha ( per SC ) Beta ( per SC ) Mercury Perseus
System voltage ( kV ) 765 765 765 765
Rated reactive power ( Mvar ) 446 1340 1119 893
Rated capacitor current ( A ) 3150 3150 3150 3150
Rated capacitor reactance ( Ω ) 15.0 2 x 22.5 37.6 30.0
Overload current for 30 minutes ( A ) 4253 4253 4253 4253
Rated cap bank voltage ( kV ) 48.0 2 x 71.8 119.6 95.7
Installed MOV, including 10%
redundancy ( MJ / phase )
20.9 2 x 25.4 62.1 98.1
2.2.5 Operation experiences of series capacitors in the TUCMAN project in Brazil
The Tucurui-Manaus (TucMan) project has a big importance for the electrical power system from
Brazil because with this project it is possible to connect the region “Amazonia” in the SIN (National
Interconnected System). Before the implementation, many studies evaluated the technical options
more viable and with less impact considering the criteria to avoid the construction of the transmission
lines in the areas with legal protection. As the Amazonia region has a global importance, the
implementation of any project in this region is a challenge and request adequate technologies to
minimize the interferences with the system. Figure 2.16 shows with more detail the region of Brazil
where the project was implemented.
Figure 2.14 Single segment series capacitor scheme

25
Figure 2.16 Region of implementation from Tucurui-Manaus project
Regarding the series compensation, the project TucMan comprises 18 FSCs connected in the 500 kV
double transmission line, with a total length of 1459 km (per line) and 5 substations. The line
segment Xingu-Tucurui is 70% compensated with FSCs installed in the Xingu substation. Other
segments are compensated also in 70% but with the FSCs split in 2 substations as shown in Figure
2.17. The compensation degree per FSC in this case is 35%.
Figure 2.17 Substations and FSC locations of the TUCMAN project
The main data of the FSCs are described Table 2.7:
The series compensation solution adopted is a fixed series capacitors installation (FSC) with a
capacitor bank and a MOV (metal oxide varistor) assembly in parallel. The MOV protects the capacitor
against over-voltages during and after faults in the transmission system. A triggered spark gap acts as
a fast by-pass device to protect series capacitor and varistor from excessive overload. The current is
then commutated into a by-pass switch in parallel to the spark-gap. The by-pass damping circuit limits
current stresses during capacitor discharge. It is made up of a reactor and a resistor in parallel. To
protect the resistor from continuous voltage stresses, a small auxiliary gap is connected in series to
the resistor. The by-pass damping circuit is arranged in series to the capacitor to reduce the voltage
drop across the by-pass disconnect, when it is closed. This allows opening and closing of the by-pass
disconnect with the lowest voltage drop possible. Figure 2.18 shows the simplified single line diagram
with the main components and a photo of one of the FSC installations.
Further information regarding TCSC experience, studies and the 500 kV FSC installation of Fengjie in
China are presented in references [15], [16] and [17].

26
Table 2.7 Main technical parameter of the FSCs
Subs
tation
Transmission
line
Length
(km)
FSC
Compensation
degree (%)
Nominal
current (A)
3 phase power
(Mvar)
MOV energy
without spares
(MJ)
Xing
u
Xingu-Tucurui
(L1)
264 FSC1 70 3146 1462.2 110
Xingu-Tucurui
(L2)
264 FSC2 70 3146 1462.2 110
Xingu- Jurupari
(L1)
263 FSC3 35 3024 624.12 50
Xingu- Jurupari
(L2)
263 FSC4 35 3024 624.12 50
Juru
pari
Xingu- Jurupari
(L1)
263 FSC5 35 3024 623.02 54
Xingu- Jurupari
(L2)
263 FSC6 35 3024 623.02 54
Jurupari-
Oriximina (L1)
374 FSC7 35 2700 699.40 14
Jurupari-
Oriximina (L2)
374 FSC8 35 2700 699.40 14
Orixi
mina
Jurupari-
Oriximina (L1)
374 FSC9 35 2700 699.40 13
Jurupari-
Oriximina (L2)
374 FSC10 35 2700 699.40 13
Oriximina-
Silves (L1)
334 FSC11 35 1900 340.6 26.6
Oriximina-
Silves (L2)
334 FSC12 35 1900 340.6 26.6
Silve
s
Oriximina-
Silves (L1)
334 FSC13 35 1900 340.6 30.7
Oriximina-
Silves (L2)
334 FSC14 35 1900 340.6 30.7
Silves- Lexuga
(L1)
224 FSC15 35 1840 214.2 19.5
Silves- Lexuga
(L2)
224 FSC16 35 1840 214.2 19.5
Lexu
ga
Silves- Lexuga
(L1)
224 FSC17 35 1840 214.2 19.7
Silves- Lexuga
(L2)
224 FSC18 35 1840 214.2 19.7
a) Simplified single line diagram b) Site installation
Figure 2.18 The FSC in TucMan project, Brazil

27
3. TECHNOLOGY OF SHUNT COMPENSATION
Shunt compensation of reactive power can be of a capacitive or an inductive nature. Shunt reactors
provide inductive compensation. This chapter deals mainly with shunt reactors directly connected to
the EHV or UHV level and shunt reactors connected on the tertiary winding of a power transformer.
They will be treated in section 3.2.7. Shunt reactors may also be connected to a bus in a substation or
directly to an overhead line or a cable. In the following sections the technology of shunt
compensation will be described. Shunt reactors may have a fixed value, as discussed in section 3.2, or
variable inductance, as discussed in section 3.3. Shunt compensation methods by means of rotating
equipment (synchronous condensers) or through power electronic devices (SVC and STATCOM) are
described in section 3.4.
Nowadays, switching overvoltage and severe TRV stresses are mitigated by the application of MOSA’s
and controlled switching. By controlling the arcing time of the shunt reactor circuit breaker in such a
way that current interruption is avoided at small contact gaps, re-ignitions can be prevented. Re-
ignitions lead to very fast and possibly very high switching overvoltage that may harm both circuit
breaker and shunt reactor. In the early days, pre-insertion (opening) resistors have been used.
Nowadays it is strongly recommended by experts to apply MOSA for shunt reactor switching.
Controlled switching and RC-snubber circuits (see 3.2.7) can also improve the transient voltage
stresses.
3.1 SHUNT COMPENSATION
Shunt compensation is widely used in power grids for many purposes. High voltage shunt reactors are
usually directly connected to AC transmission lines, mostly at one or both line ends, or to the buses in
substations in order to reduce power frequency overvoltage during line switching. Traditional HV
shunt reactors are of a constant reactance. During AC power system operation, traditional HV shunt
reactors absorb reactive power from the system, and become a reactive “burden” to the power
system. In recent years, the need for adjustable shunt reactors has increased and their use in AC
power systems is growing. There are several methods to vary the reactance as will be described in
section 3.3. The high voltage shunt reactors are either continuously or stepwise controllable. Due to
developments in power electronics, most developments are with thyristor or IGBT based variable
shunt reactors.
Converter stations for HVDC power transmissions absorb great amount of reactive power during
operation. In order to keep the bus voltage constant, and improve the bus voltage waveform, high
voltage filters and / or capacitor banks are designed and installed in AC filter switch yards, and
connected through circuit-breakers to converter station’s HV busbars. Those filters and / or capacitor
banks work not only as reactive power generators, but also as high ordered harmonic filters.
Shunt capacitor banks and reactors can be designed and installed at the transformer tertiary side to
generate or absorb reactive power according to the power system operation requirement. Since the
voltage at tertiary side of a transformer is the lowest when compared to the other two sides, the
compensation is usually called low voltage compensation. Switching phenomena associated with shunt
capacitor banks connected to the tertiary side of large transformers will be discussed in section 5.3.
In order to make compensation quickly and continuously adjustable, thyristor-switched capacitors
(TSC) plus thyristor controlled reactors (TCR) are normally used in compensation schemes, and are
often called static var compensators (SVCs). SVCs usually generate reactive power instead of consume
reactive power.
In recent years, with the technological advancement in power electronics, fully controllable power
electronic switching devices such as MOSFET’s and IGBT’s became more mature, and another kind of
static var generation, which is called STATCOM, appeared and has been applied in power distribution
systems. STATCOMs can be operated either reactively or capacitively, and interchange their operation
modes quickly. Since STATCOMs do not use power capacitors or inductors as their main reactive
power compensation devices. They use fully controlled power electronic devices to perform fast
switching which results in a nearly sinusoidal waveform profile (depending on the number of levels),
and they use electrolytic capacitors of huge capacity to maintain internally a constant DC voltage.
Both SVCs and STATCOMs are out of the scope of working group A3.33, and will only be mentioned in
short in section 3.4.

28
At the end of section 3.2 brief attention is given to four-legged shunt reactors; i.e. shunt reactors with
a neutral reactor between the star-point and earth. It is used for secondary arc extinction at EHV and
UHV lines, where single pole auto-reclosing (SPAR) is applied. More information is given in [2][3][5].
3.2 FIXED SHUNT REACTORS
3.2.1 Introduction
Fixed shunt reactors are used to support the voltage profile on lightly loaded overhead lines or to
compensate cables. Fixed shunt reactors can be connected directly to the line, to the tertiary side of a
transformer or to the bus, as shown in Figure 3.1. Statistical data for the rated voltages of shunt
reactor applications can be found in Figure 3.2.
Shunt reactor connected to the tertiary of the transformer
Shunt reactor directly connected to the bus
Shunt reactor directly connected to the line
Figure 3.1 Connection of shunt reactors
Figure 3.2 Shunt reactor distribution among voltage levels [18]
The growing difficulties to receive the right of way for conventional overhead lines are forcing utilities
to plan their new HV AC lines with several cable sections along the whole distance. Each of these
cable sections needs continuous compensation by a shunt reactor. A typical capacitive power demand
for a 10 km long 420 kV cable is the range of 140 Mvar.
Fixed shunt reactors for overhead lines are normally subjected to a switched duty. They will be
energized during light load conditions (night times or during any other circumstance in the grid which
causes a high operating voltage). Especially with larger distances (long lines or large networks) the
Ferranti-effect will cause a voltage rise at the receiving end (Figure 3.3).

29
Figure 3.3 HV-Line voltage profile [19]
The duty of shunt reactors is more and more changing from the classic duty, energized during the
night, to several switching operations a day. This is caused by the increasing amount of renewable
energy generation, mainly wind and solar, and the liberalization and deregulation of the energy
sector.
3.2.2 Types of fixed shunt reactors
Fixed shunt reactors can be split into oil-immersed iron-cored reactors and air-core reactors.
Traditional applications up to 110 kV are realized as air-core and oil-immersed iron-cored shunt
reactors. Above 110 kV only oil-immersed iron-cored shunt reactors were used. Nowadays, due to
environmental and cost reasons, air-core shunt reactors are also used above 110 kV. Installations up
to 345 kV have been recognized [20]. Currently applications up to 500 kV are technically feasible.
Air-core shunt reactors above 245 kV have to be realised as two coils or three coils connected in
series due to limitations in voltage surface stress. Figure 3.4 shows a 345 kV 20 Mvar application.
Figure 3.4 20 Mvar 345 kV air-core shunt reactors

30
Figure 3.5: 150 Mvar 420 kV shunt reactor
Depending on the specific project requirements oil-immersed or air-cored shunt reactors have
different advantages or disadvantages as shown in Table 3.1. Which should be the preferred solution
is therefore subject to the combination of the different requirements.
Table 3.1 Comparison between the two types of shunt reactors
Oil-immersed iron-cored Air-core
Environmentally friendly - +
Basic Initial cost - +
Required space + -
Voltages above 500 kV + -
Cost of fire protection system - +
Protection system - +
Losses + -
Spare costs - +
Maintenance costs - +
Cost of civil work & oil containment system - +
3.2.3 Oil-immersed iron cored shunt reactors
Amongst others the major innovations and trends in oil immersed reactors are:
 Alternative insulation liquids [21]
 Improved gapped core construction to reduce noise
 Compensation of geomagnetically induced currents [22]
Shunt reactors are applied up to UHV applications (up to 1200 kV). Currently (at the moment of wring
thate technical brochure) oil-immersed iron-cored shunt reactors are addressed under CIGRE A2
Working Group A2.48.
3.2.4 Dry-type air-cored shunt reactors
Until some years ago air-core dry-type shunt reactors were only applied up to a maximum voltage of
110 kV. Therefore the most important trend is the application of air cored shunt reactors to voltage
levels up to 500 kV. Other innovation trends are:

31
 More compact solutions for shunt reactors above 110 kV
 Even better acoustic performance
Different than oil-immersed iron cored shunt reactors the limiting factor for air-core reactors is not
power, but mainly inductance. Reactors with lower power rating, meaning high inductance, are more
difficult to realize than high power ratings. The reason is that for air core reactors it is more difficult to
achieve high inductances due to the absence of the iron-core.
3.2.5 TRV and mitigation
Normal circuit-breakers are designed to interrupt large inductive currents during short-circuit faults on
the grid. Because of the low energy in the arc when switching small inductive currents the breakers
more easily extinguish the arc and break the current before current zero, which can lead to higher
chopping currents. The CIGRE paper [20] describes the details of TRV and the usage of RC damping
circuits to limit the TRV on the shunt reactor breaker, as shown in Table 3.2. Furthermore it describes
the fact that it is not necessary a good choice to use breakers suitable for higher voltage levels to
withstand the TRV. The shunt reactor breaker should be chosen based on its chopping current
behaviour. Another way to limit the TRV and to avoid re-striking is described in paper [23] which
presents specially developed breakers with longer arcing times for inductive load switching.
Table 3.2 Measures to reduce TRV and RRRV
Use breakers with low chopping currents
Connect (R)C-damping circuit parallel to the shunt
Use breakers with longer arcing times
Use point on wave (controlled) switching
Generally the oscillation frequency of the TRV voltage and therefore also the rate of rise of recovery
voltage is higher for air-core dry-type reactors than for oil-immersed iron-cored reactors. This results
from the lower stray capacitance of air-core reactors due to the absence of iron-core and tank.
On the other hand the transient voltage distribution inside the coil of an air-cored shunt reactor is
normally more linear due to higher series capacitance and lower ground capacitance. The non-linearity
factor (as shown by equation 3.1) is a function of series and ground capacitance [24].
Equation 3.1
Where: g Non-linearity factor of winding
Ce Capacitance of winding to ground
Cs Series capacitance of winding
3.2.6 Shunt reactor grounding and secondary arc
In HV transmission lines often single-phase auto-reclosing (SPAR) is used to limit the outage times.
In case of a line-fault only the faulty phase is opened by the circuit-breaker. Since the other phases
are still carrying current and voltage, there is a voltage / current induced into the faulty phase which
can create the so called secondary-arc which develops in the arc-channel of the primary-arc. One
solution to limit this phenomenon is to use 4 legged shunt reactors. These shunt reactors are
equipped with an additional impedance in the neutral. The additional impedance increases the overall
zero sequence impedance of the line (seen from the fault location) and thus limits the magnitude of
the secondary arc current.
A disadvantage of this solution is, that the TRV of the shunt reactor breaker will increase when having
additional impedance on the neutral side [25], as shown in Figure 3.6.


coth

g
C
C
s
e

EXPERIENCE WITH EQUIPMENT FOR SERIES AND SHUNT COMPENSATION

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