Presented at PowerTech Conference,
Mumbai, India, October 1999
Enhancing of transmission capability of power corridors by means of series compensation
Matt Matele
ABB Power Systems AB
Västerås, Sweden
Abstract
Series compensation is a well established
technology, enabling an increase of power
transmission capacity as well as an increase of
steady-state as well as dynamic stability in long AC
power transmission corridors. In later years, the
incorporation of thyristor control has enabled
further flexibility of series compensation, adding
useful new applications to the technology.
The paper highlights the concepts of series
compensation and TCSC, and gives current
examples of the implication of the said technologies
in long distance AC power transmission systems in
the Nordic countries and in South America.
Introduction
For transmission of large amounts of electric
power, AC in the overwhelming majority of cases
is the established as well as the most cost effective
option at hand. In cases of long distance
transmission, as in interconnection of power
systems, care has to be taken for safeguarding of
synchronism as well as stable system voltages in
the interconnection, particularly for extreme load
conditions and in conjunction with system faults.
With series compensation, the viable distances of
AC power transmission become sufficiently large
to remove altogether the issue of distance as a
limiting factor for AC transmission in practice in
most cases. Series compensated AC power
corridors transmitting bulk power over distances of
well over 1.000 km are a reality today.
An increase of power transmission
Series compensation has been in commercial use
since the early 1960s. By ABB alone, more than
200 series capacitors have been installed
worldwide, at voltages ranging from the highest in
use for power transmission down to 10 kV
1
distribution systems. In total, it represents around
50.000 Mvar of reactive power.
Series
compensation
reduces
transmission
reactances at power frequency, which brings a
number of benefits for the user of the grid, all
contributing to an increase of the power
transmission capability of new as well as existing
transmission lines. These benefits include:
• An improvement in system stability.
• Improvement of voltage regulation and reactive
power balance.
• Improved load sharing between parallel lines.
• In many cases, a reduction in transmission
losses.
The impact of series compensation on power
transmission capability can be illustrated as in
Fig.1. Here, the quantity k is the degree of
compensation of the series capacitor, equal to the
relationship between the capacitive reactance of the
series capacitor (XC) and the inductive reactance of
the transmission line (XL). Ψ is the angular
difference between end voltages of the line.
For a fixed angular difference, the active power
transmission capability of the line increases as the
degree of compensation increases. Vice versa, for a
fixed amount of power transmission over the line,
the angular difference decreases as k increases,
which is a measure of increased dynamic stability
of the transmission system.
Whether a series capacitor is installed to bring
about an increase in power transmission capacity
or increased dynamic stability at a fixed power
transmission level, is purely a matter of application
in each particular case.
Interconnecting of power systems
ABB Power System´s experience includes
installations in power corridors where series
capacitors have helped to increase the power
transmission capability both of existing and of new
lines, thereby helping to reduce the need for
additional transmission facilities. In other cases, the
availability of interconnecting transmission links
has been raised by means of series compensation.
Whichever the situation, the installing of series
capacitors has meant considerable savings in costs,
as well as reductions of lead times and
environmental impact.
Power interconnections are becoming increasingly
widespread in various parts of the world, as
incentives for power exchange between countries as
well as parts of countries are growing. Examples
close at hand are UCPTE, the merger between
UCPTE and CENTREL, NORDEL, and the
cooperation between NORDEL and UCPTE.
In NORDEL, series compensation is playing a
crucial role for the successful interconnecting of
countries as well as regions within countries. This
is treated more in detail elsewhere in this paper.
Series capacitor scheme
Of course, a series capacitor is not just a capacitor
bank in series with the line. For proper functioning,
series compensation requires control, protection
and supervision facilities to enable it to perform as
an integrated part of any power system. Also, since
the series capacitor is working at the same voltage
level as the rest of the system, it needs to be fully
insulated to ground.
The main circuit diagram of a state of the art series
capacitor is shown in Fig.2. The main protective
device is a varistor, usually of ZnO type, limiting
the voltage across the capacitor to safe values in
conjunction with system faults giving rise to large
short circuit currents flowing through the line.
A spark gap is utilized in many cases as back-up
protection, to enable the by-pass of the series
capacitor in situations where the varistor is not
sufficient to absorb the excess current during a
fault sequence.
Finally, a circuit breaker is incorporated in the
scheme to enable the switching in and out of the
2
series capacitor as need may be. It is also
for extinguishing of the spark gap, or,
absence of a spark gap, for by-passing
varistor in conjunction with faults close
series capacitor (so-called internal faults).
needed
in the
of the
to the
Controllable series compensation
Though very useful indeed, conventional series
capacitors are still limited in their flexibility due to
their fixed ratings. By introducing control of the
degree of compensation, additional benefits are
gained.
In early types of controllable series capacitors,
mechanical circuit breakers are used to switch
segments of the capacitor in and out according to
need. This is adequate in most situations for power
flow control, but for applications requiring more of
dynamic response, its usefulness is reduced due to
the limitations associated with using circuit
breakers as switches.
The evolution of controllable series compensation
is shown in Fig.3. Here, the introduction of
thyristor
technology
has
enabled
strong
development of the concept of series compensation.
This is highlighted further elsewhere in the paper.
The Swedish case
The Swedish power transmission system is part of
the
synchronous
Nordic
power
system
interconnecting Sweden, Finland, Norway and the
eastern part of Denmark. The overall installed
capacity within Nordel amounts to some 90 GW.
Distances are vast within the cooperation, and large
amounts of power frequently have to be transported
over most considerable geographical lengths.
Placed in the middle, Sweden plays a multiple role
as contributor to the power balance, consumer of
power as well as yielder of territory for
transportation of power across the country between
other members of Nordel.
Sweden´s installed capacity is about 34 GW. The
country´s annual consumption of electric energy
amounts to some 140 Twh. The balance of
generation rests almost solely on hydro power
(45%) and nuclear (50%). Most of the hydro power
plants are located in the north of the country, while
the nuclear plants are found in the southern coastal
areas.
The main consumption areas are in the central and
south. A total of eight 400 kV transmission lines go
between the hydro plants in the north and the large
load areas in the centre and south (Fig.4). Each line
is up to 500 km long and all are series
compensated, with degrees of compensation
ranging up to 70%. The overall rating of the eight
series capacitors amounts to nearly 5000 Mvar.
Extensive series compensation
This extensive use of series compensation enables
up to 8000 MW of environmentally friendly hydro
power to be carried over the lines under stable
conditions. The alternative to series capacitors
would have been the construction of several
additional, very long 400 kV lines. This would have
been impossible in the real case due to political,
economical and environmental reasons.
The impact of the series capacitors is, in an
electrical sense, to make the transmission distances
between the generators in the north and the load
areas in the centre and south seem shorter. This is
of benefit to angular as well as voltage stability,
enabling power transmission to take place at levels
considerably exceeding the natural loading of the
lines. The power transmission capacity of the eight
series compensated 400 kV lines corresponds to a
loading equal to more than 2 x SIL (Surge
Impedance Loading).
An additional, very important benefit of the series
capacitors is an optimization of load sharing
between the parallel lines. Since the lines were
constructed at different times during three decades,
impedances vary from one line to the next. The
series capacitors, by the degree of compensation in
each individual case, make up for these differences
in line impedances and bring about the best
possible load sharing between the lines.
As a result, transmission losses have been
decreased quite significantly in comparison to the
uncompensated case. This in itself has helped to
pay for the series capacitors in a very reasonable
amount of time.
Operating experience
The series capacitors in the Swedish 400 kV grid
are all equipped with state of the art non-PCB, all
film, low loss capacitor units, ZnO varistor
overvoltage protection and fibre-optic platform to
ground signal transmission links (Fig.5). Their
service record is excellent. The overall failure rate
of capacitor units has been less than 0,1 % per
3
annum. Other faults have also been insignificant
and caused no interruption of service. A simple and
reliable design of protective and supervising
systems has contributed to this.
The series capacitors are located midline and
unattended. Inspection and maintenance are carried
out at regular intervals by visiting personnel.
Increase of cross border power transfer
As a consequence of deregulation of the electricity
supply industry of the Nordic countries, crossborder power trade is meeting with increasing
interest. A part of this process is to find the
quickest and most cost-effective ways of
reinforcing cross-border power transmission
corridors.
The usefulness of series compensation under these
circumstances has been proved again recently with
the installation of three new series capacitors in a
twin circuit 400 kV cross-border interconnection
north of the Bothnian Gulf between Sweden and
Finland (Fig.6), thereby enabling a substantial
increase of power exchange between the countries.
With the series capacitors in operation, the power
transmission
capacity
of
the
existing
interconnection has been increased by one third,
from 900 MW uncompensated to 1.200 MW,
without any need for building of an additional line.
The impact of the series capacitors in this case is
an increase of voltage stability of the
interconnection at steady-state as well as transient
grid conditions.
Isovaara series capacitor
The series capacitor at Isovaara forms part of a
circuit stretching some 250 km between the hydro
power resources at Letsi on the Swedish side and
Petäjäskoski on the Finnish side of the border. It is
rated at 515 Mvar at 400 kV and was
commissioned in late 1997. Its degree of
compensation is 70%, and the rated current is 1800
A (Fig.7).
Main circuit protection is performed by ZnO
varistors rated around 100 MJ (3-phase value). In
case of internal faults, the series capacitor is
temporarily by-passed by means of a forced
triggered spark gap.
Conditions on platform are monitored and
controlled from the ground at all times by means of
optical current transformers (OCT), sending all
necessary information between platforms and
ground through optical fibre communication, not
requiring any auxiliary power on platform.
Since the Isovaara series capacitor is located out on
the line and normally unmanned, it is essential that
the proper function of the series capacitor can be
supervised as well as controlled if necessary
remotely. This is performed by means of a Station
Control and Monitoring system (SCM), allowing
remote control and supervision of the series
capacitor over the public telephone network
(Fig.8). With this, the nearest Network Control
Centre communicates with the series capacitor by
means of a Gateway Station and a File Transfer
Protocol. It is thus possible to retrieve event data
files as well as transient fault recorder files for
supervision and analysis at the control centre, and
if needed, also to return subsequent corrective
commands.
Brazil: North-South Interconnection
A current example of AC interconnection of
separate power systems within one country is found
in Brazil. There are two main power systems in the
country at present which are not interconnected, the
North System and the South System. These are
mainly hydroelectric, comprising more than 95% of
the nation´s total volume of power generation and
consumption. Feasibility studies have been
performed regarding an interconnection of the two
systems, and a decision has been made to go ahead
and build the transmission corridor. Both AC and
DC alternatives have been assessed, and decided in
favour of the AC option. It consists of a single 500
kV compact circuit (to be doubled at a later stage),
more than 1.000 km long and series compensated in
several places along the line. The start of operation
is targeted to the beginning of 1999. The power
transmission capability of the corridor will be 1300
MW.
The “North-South Interconnection” will have the
purpose of exploiting the hydrologic diversity
between the systems, and power flow will occur in
both directions, depending on current hydrologic
conditions. As a consequence, the risk of energy
deficiency in conjunction with the rapidly growing
energy demand experienced by the country at
present will be reduced.
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The AC option is highly attractive as it facilitates
the making of inexpensive hydro energy available
to a rapidly growing federal economy as well as to
future development over a vast area having great
economical potential. Several hydroelectric plants
are expected to be built along the same route in the
coming two decades, to be connected to 500 kV
AC.
The integration of the national power system will
also have other related benefits, as for instance a
reduction of the required spinning reserve.
ABB Power Systems was entrusted by Eletronorte
of Brazil to supply, install and commission a total
of six 500 kV series capacitors for the project, five
of which fixed and one thyristor-controlled (Fig.9).
All in all, about 1100 Mvar of series capacitors
have been supplied and are presently under
commissioning.
TCSC for POD
The ABB Power Systems thyristor-controlled series
capacitor
(TCSC)
in
the
North-South
Interconnection, the first of its kind to be installed
in Latin America, is located at the Imperatriz
substation at the northern end of the
interconnection. It has the task of damping lowfrequency inter-area power oscillations between the
power systems on either side of the interconnection.
These oscillations (0,2 Hz) would otherwise
constitute a hazard to power system stability.
The function of the TCSC as a power oscillation
damper can be explained from the expression for
power transmission as a function of angular
difference and transfer reactance:
P = V1V2 sin ψ / X
When active power oscillations build up over a
transmission corridor, the angular difference
between the end voltages varies periodically with
time, as well. If a mechanism is devised to
introduce a “counter-oscillation” in some other
member of the formula, i.e. the line reactance, the
two oscillations can be made to cancel each other
out.
The TCSC technology offers precisely this
possibility. By introducing a time-varying element
to the degree of compensation, the resulting transfer
reactance XL - XC(t) can be made time-varying in a
periodic way, as well. With proper control of the
TCSC, very effective power oscillation damping
can be achieved (Fig.10).
Imperatriz TCSC
The TCSC at Imperatriz has been installed to
increase the damping of power oscillations between
the two systems.
The characteristics of the Imperatriz TCSC are
displayed in Fig.11.The boost level, defined as the
ratio between the virtual reactance of the series
capacitor and the physical capacitor reactance
(XTCSC / XC), is a key factor. It is a measure of the
amount by which the reactance of the series
capacitor can be virtually augmented in order to
counteract system power oscillations.
Boost is achieved by controlled current injection
into the mainstream from the inductor in parallel
with the capacitor (Fig.12). The boost level can be
varied continuously between 1 and 3. Expressed in
terms of degree of compensation, k can be
controlled over a range between 5% and 15%. At
rated line current, the nominal boost level has been
set to 1,20.
The thyristor valve (Fig.13) is mounted at platform
level. It is water cooled and utilizes indirect light
triggered thyristors.
10 seconds overload current
3000 A
Conclusion
Series compensation is a useful tool for the
improvement of power transmission capacity as
well as steady-state and dynamic stability of long
AC power transmission systems, for example in
conjunction with power interconnection corridors
between countries as well as between regions
within countries.
In many cases, series compensation offers a cost
effective as well as time saving alternative to
building of new or additional transmission lines.
Extensive experience all over the world has
demonstrated the the merits of the technology for
many years.
The recent introduction of thyristor control has
further widened the scope and usefulness of series
compensation and added new valuable benefits
such as damping of active power oscillations
between weakly interconnected power generating
areas.
Literature
[1] R. Grünbaum: “Series capacitors improve
power transmission”. Modern Power Systems,
October 1988.
The valve is rated at 1500 A continuous current
and 3000 A for 10 seconds. Furthermore, since the
valve has to perform as back-up protection of the
TCSC in extreme situations where the main ZnO
overvoltage protection is reaching its thermal limit,
it needs to be able to withstand fault currents of up
to 40 kA (peak) for about 60 ms, equal to the time
it takes the by-pass breaker to close and take over
the fault current.
[2] José L. Alonso et al: “Stability enhancement of
a 500 kV power transmission system by means of
series compensation”. IEEE SPT PE 04-02-0123,
Stockholm, 1995.
The main data of the Imperatriz TCSC can be
summarized as follows:
[4] Carlos Gama et al: “Brazilian North-South
Interconnection - designing of thyristor controlled
series compensation (TCSC) to damp inter-area
oscillation mode”. VI SEPOPE, Salvador-BA,
May, 1998.
Maximum system voltage
550 kV
Nominal reactive power
107 Mvar
Physical capacitor reactance
13,27 Ω
Nominal boost
1,20
Nominal degree of compensation
5%
Boost level range
1-3
Rated current
1500 A
Rated continuous voltage
23,9 kV
30 minutes overload current
2250 A
5
[3] O.J. Hjemås and M. Noroozian: “Application of
thyristor controlled series capacitor for damping of
electromechanical
oscillations”.
EPE-97,
Trondheim, Sept. 1997.
Fig1: Impact of series compensation.
C
Z
D
C Capacitor bank
Fig.5: Series capacitor, 400 kV, 550 Mvar.
Z Metal oxide varistor
D Discharge damping reactor
G Forced triggered spark gap
G
B By-pass breaker
B
Fig.2: Main circuit diagram.
Norwegian
Sea
Porjus
Isovaara
Letsi
Sweden
Norway
Finland
Fig.6: Swedish-Finnish series compensated
kV cross-border interconnection.
Fig.3: Evolution of controllable series
compensation.
Fig.7: Isovaara 400 kV series capacitor.
Fig.4: Swedish series compensated 400 kV
grid.
6
400
Fig.8: Station control and monitoring system.
Fig.11: TCSC impedance-current
characteristic, Imperatriz.
Fig.12: Thyristor-controlled inductor for TCSC
boost.
Fig.9: North-South Interconnection, Brazil.
Fig.10: TCSC power oscillation damper
control scheme.
Fig.13: Thyristor valve of TCSC.
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