2004 International Conference on
Power System Technology - POWERCON 2004
Singapore, 21-24 November 2004��
Studies for the Integration of a TCSC in a
Transmission System
Lutz Kirschner, Gerhard H. Thumm
other countries with long distance power transmissions TCSCs
are in commercial operation or under construction.
Abstract-- Series Capacitors are widely used in long distance
transmission systems to improve stability of power transmission.
Thyristor- controlled series capacitors provide additional benefits
for the transmission system compared to fixed series capacitors.
During a project to realize a series capacitor several studies are
required, starting with planning studies, studies that investigate
possible interaction with the surrounding system, and finally the
capacitor component design. Different parties are involved in
these studies, as information is required from the transmission
system on one side, but also from the series capacitor and its
components. This paper describes the studies and and shows
typical results.
II. SYSTEM STUDIES
A series compensation project starts with a number of system studies that show the benefit of the series capacitor for the
transmission system and describe the overall characteristic. In
a second stage the interaction with the surrounding system has
to be investigated. During a project the results of studies can
be compared with tests involving the actual control system and
a digital system representation [5], and eventually with on-site
measurements to demonstrate the effect of the TCSC for the
transmission system.
Index Terms—Power transmission, series compensation,
thyristor-controlled series compensation (TCSC), stability,
FACTS
T
A. Loadflow Studies
Loadflow situations in a transmission system vary over a
wide range due to various parameters. Seasonal effects like
rain and temperature on one side have a big influence on loads
as well as on generation capacity. On the other side contractual
conditions can end up in restrictions and basic conditions for
power flow. Load requirements vary with daily and weekly
cycles. Load forecast is a big challenge, with a time span from
minutes until planning studies reaching over several years,
resulting in investment in systems, when bottlenecks or economic advantages have been identified. Under all the situations which are to be investigated, voltages and currents in the
system shall not exceed limits as given from components and
systems like transmission lines, transformers, generators, etc.
Series compensation is a system which can help in critical
situations, so that restrictions in power transfer can be avoided.
I. INTRODUCTION
HE thyristor-controlled series capacitor (TCSC) is one
system out of the family of FACTS systems, which has
proven its benefits for transmission systems in several
installations [1]. In addition to a fixed series capacitor (FSC)
the TCSC is able to vary the effective capacitor impedance
within a very short time, thus increasing stability not only in
steady state operation and during first swing. The TCSC is also
able to actively damp power oscillations in the transmission
system by variation of the impedance between the generators
involved.
In longitudinal series compensated transmission systems a
specific interaction between the series capacitor and thermal
generators came up, the sub-synchronous resonance (SSR).
The TCSC is able to mitigate SSR just by operation with constant impedance under phase angle control, active counteraction to damp SSR is possible as well. These additional benefits justify the higher costs for a TCSC in comparison to an
FSC installation.
The first TCSC is in commercial operation since more than
ten years in USA [2]. TCSC installations in Brazil have
demonstrated their capability to stabilize a transmission system
with a length of more than 1000 km, which could not be operated safe and stable without series compensation [3]. Another
TCSC installation is in operation since one year in China,
within a project to transfer electric power over a distance of
more than 1000 km on parallel AC- and DC-lines [4]. Also in
B. Stability Study and Power Oscillation Study
Series capacitors in general are able to reduce the risk of
power oscillations by reducing the impedance between
generators or networks involved in the oscillation. This effect
will be investigated in a stability study. Additionally a TCSC
can damp power oscillations by varying its impedance. This
effect will be demonstrated in a subsequent power oscillation
damping study, where the operating range of the TCSC and its
dynamic behavior is incorporated.
From these studies a typical diagram for a TCSC can be
drawn, showing the TCSC operating range as base for TCSC
design, see Fig. 1.
Lutz Kirschner, MIEEE, and Gerhard H. Thumm are with SIEMENS AG,
Erlangen, Germany, PTD H166, 91050 Erlangen, PO Box 3220, Paul Gossen
Str.
100,
Germany
(e-mail:
lutz.kirschner@siemens.com
and
gerhard.thumm@siemens.com).
0-7803-8610-8/04/$20.00 © 2004 IEEE
1
XTCSC
Subject of the power oscillation damping study is to find an
optimum strategy to damp oscillations with the TCSC, as they
can occur after defined fault scenarios. This includes the control strategy, the selection of an input signal, and the size of
the TCSC.
As an example Figure 2 shows the transferred power
through the transmission line in the North-South-Interconnection in Brazil. After tripping of a generator a power oscillation
arises. Without any action to damp the oscillation the system
would become instable. With the TCSCs with the power
oscillation damping (POD) control active the oscillation is
damped within a few cycles.
Experience shows that in most cases the active power
flowing through the series compensated line is the most suitable signal to detect the power oscillation. It can be calculated
from line current and voltage at the series capacitor location.
Both are local signals. The transmission of signals for example
from a generator normally is not safe enough to rely on during
contingency situations.
cont 30 min 10 s
3.0
cap
.
pu
2.0
1.2
1.0
1.0
ind.
0.0
- 0.2
1.5
2.0
linecurrent
10 s
Fig. 1. TCSC operating range as function of line current
Generally a series capacitor is designed for overloads. In
the relevant IEC and IEEE standards [6, 7] such overload
cycles are proposed, however each system may require its specific overload cycles depending on the system characteristic
during contingency situations. Accordingly overloads in the
range of minutes to hours are specified to cover stresses from
overcurrents. A short time overload for 10 s for example is
feasible to cover stresses from power swings, as they may
occur immediately after fault clearing or during power oscillations.
Typically a stability study investigates the most severe
system faults based on the performance of line protection, also
including scenarios with component failures, which are very
rare to occur. In general it is required that the transmission
system remains stable in such situations. The most severe disturbance of active power unbalance can be expected from
three-phase faults with a duration of 100 ms typically. Sometimes a longer fault duration is simulated taking into account
for example the effect of a stuck breaker, non-successful autoreclosure or protection failures.
III. INTERACTION STUDIES
Interaction between a TCSC and the surrounding system
occurs in several ways. The series capacitor forms together
with the inductive line impedance a resonant circuit, which can
interact with other resonant circuits or oscillating systems. At
line opening the line breaker sees a voltage between its terminals that can be influenced by the series capacitor. The TCSC
generally generates harmonics. These harmonics may not
exceed given limits. The TCSC design and the selection of the
steady state operating point have influence on the harmonics in
the system. A study is very difficult, as the impedance area of
the system on both sides of the TCSC must be taken into
account including contingency situations. The harmonic impedance area also must be valid over a certain range of harmonic frequencies. The calculations have to be verified by
harmonic measurements during the commissioning period.
Transmitted Power [MW]
Transmitted Power [MW]
A. SSR study
The presence of series capacitors in a power system may
cause sub-synchronous resonance (SSR). The dangerous impact of this phenomenon has first been noticed at the damage
of two generator shafts at the Mojave Power Station (California, USA) in 1970 and 1971. Since then, considerable effort
has been made to analyze the phenomenon and seek ways to
prevent damages in the future.
In a power system, conditions for sub-synchronous resonance may arise from the presence of a series capacitor in an
otherwise inductive electrical circuit. A system fault or sudden
load change may then excite transient power oscillations at
sub-synchronous frequencies. The frequencies depend on the
degree of line compensation and typically range between 10
and 45 Hz. If the frequency corresponds to an undamped natural torsional frequency of a generator shaft, the electrical
power system interacts with the shaft and causes sub-synchronous resonance.
Fig. 2. Example of a power oscillation, damped by TCSCs
2
HP
LP1
GEN
LP2
analysis therefore is to determine whether a series resonance
condition exists at a sub-synchronous frequency.
EXC
40
φ2
D2R
J2
φ3
D3R
J3
φi
DiR
Ji
35
φN
DNR
JN
30
impedance [ohms]
φ1
D1R
J1
k1,2
D1,2H
k2,3
D2,3H
k3,4
D3,4H
ki-1,i
Di-1,iH
ki,i+1
Di,i+1H
kN-1,N
DN-1,NH
25
20
15
10
Fig. 3. Multi-mass-spring oscillator of a turbine-generator unit
Ji
moment of inertia
ki,j
torsion spring constant
DiR speed-proportional frictional damping
H
Di,j hysteresis damping, proportional to the turning speed
differential
torque angle
i
5
0
0
10
1
10
fre q ue n c y [ H z ]
10
2
80
60
40
phase[degrees]
The shaft structure has a number of inherent resonance frequencies, so-called natural frequencies or eigenvalues. Each
eigenvalue is associated with an eigenvector that indicates the
condition, i.e. speed and angle displacement, at each location
along the shaft, necessary in order to excite the natural frequency. The local displacement is typically plotted in a mode
shape diagram. For example, the shaft system has a natural
frequency at a certain frequency, at which the low pressure
section LP2 is in complete phase opposition to the other sections. Once excited with this frequency it would swing against
the rest of the shaft sections. This example demonstrates that,
if the frequency ftorque of the sub-synchronous electrical torque
is near to one of the natural shaft frequencies fmj, and if its
component in phase with the rotor speed deviation exceeds the
inherent damping torque of the rotating system, sub-synchronous resonance occurs. The torque oscillations then rapidly
increase and may cause severe damage to the shaft.
Studying SSR interaction in a series compensated transmission system can be split in two steps. The first step is a
frequency scanning study, in which the frequency-dependent
impedance of the electrical system is calculated for various
situations with varying loads and also for a lot of contingency
cases. The impedance of interest is when looking from the
investigated generator into the system, where all generators are
represented by their subtransient impedance in the direct axis,
xd”.
Fig. 4 shows an example of a frequency dependent impedance calculation. In this example a series resonance condition
occurs at 19 Hz.
Note that this series resonance is a resonance of only the
electrical power network. If the machine shaft also had a natural mode at 41 Hz (60 Hz minus 19 Hz) the resonance could
lead to SSR and cause damage to the machine shaft. This is
mostly determined by the damping characteristic of the oscillating mode. However since the overall damping depends on
both the damping of the electrical network and the damping of
the shaft system, details of the machine behavior at the resonance frequency can only be determined in a more detailed
SSR analysis study. The purpose of the frequency scans in this
20
0
-20
-40
-60
-80
10
0
1
10
frequency [Hz]
10
2
Fig. 4. Example of a Frequency Scan
The network impedance generally varies with system loading and with operating conditions other than normal operation,
e.g. contingency situations. Therefore the frequency scans
have to be carried out for each inspected generator at various
system conditions, which are then presented in such a diagram.
The second step of an SSR study is the so called time
domain analysis. A program suited for computing electromagnetic transients may be used to simulate the transient time
response at a variety of system events. Time-domain simulation uses full three-phase electrical representation of network
and generators, and permits detailed modeling of the multimass shaft systems. Detailed representation of nonlinear
effects is also possible. SSR can be identified by observing the
time response of the torques at a particular shaft system. If
they persist or grow in time, then the system almost surely has
an SSR problem. Due to the amount and detail of data, time
domain simulation is mostly used to verify the existence of an
SSR problem after identification with one of the other
methods.
B. TRV study
The TRV (transient voltage recovery) study deals with
voltage stresses of breakers immediately after opening. Of
special interest is this voltage at fault clearing in long distance
transmission systems together with series compensated transmission lines. When a fault occurs on a series compensated
transmission line, the protection of the series capacitor is
allowed to operate, as soon as components tend to become
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• Arching: most faults on a high voltage transmission system
are not solid metal connections between phases but result
in arcs. An arc dissipates electric energy and, therefore,
adds resistive damping to high-frequency oscillations.
Correct representation of the arc bears an effect on the
TRV levels that can be experienced by the circuit breakers.
By the same argument, the arc produced within the breaker
chambers also contribute to the damping.
• Power system: during the fault, the entire power system in
the vicinity is affected and in a state of transient oscillation
so that, when the faulted line is isolated, the oscillations
continue until a steady post-fault state is gained. Since
TRV are voltages across the line breakers, the system oscillations also affect the level of TRV. As a consequence it
is not only important to analyze the affected transmission
line with regards to TRV, but also to consider the entire
power system in the vicinity in all its oscillatory detail.
overstressed. As in most cases the bypass device short-circuits
the series capacitor with a time delay of about 1 ms, this happens clearly before the line breaker opens. Nevertheless an
influence can be seen, as the pre-fault steady state current in a
series compensated transmission system will be higher than
without series capacitors.
The voltage across the breaker build-up after current interruption is termed recovery voltage. Due to the circuit’s inductive and capacitive characteristics on both sides of the breaker,
the recovery voltage generally has high frequency components
superimposed to its fundamental sinusoidal shape, which typically decay within a few cycles. Hence, the attribute
“transient” was added to qualify the term recovery voltage.
High frequency components are particularly responsible for
exceeding the dielectric capabilities in the contact gap.
The objective of TRV analysis therefore is to determine the
fastest initial build-up of the voltage after current interruption.
While many textbook examples of TRV analysis rely on a
simplified RLC equivalent circuit of the system in order to
determine the highest frequency components in the initial
voltage rise, a high-voltage series-compensated transmission
system shows a much higher degree of complexity than could
possibly be represented by means of an RLC circuit. The
following is a list of influences that affect TRV levels in a
high-voltage series-compensated transmission system:
• Transmission lines: a high-voltage transmission system
typically involves long transmission lines that extend over
hundreds of kilometers. Due to finite wave traveling times,
distinct voltage shapes such as TRV may travel forth and
back a line, producing a harmonic oscillation on its own
(e.g. trapped line charge).
• Series compensation: as already described, a series capacitor is equipped with protective devices such as metal-oxide
varistor (MOV), a triggered spark-gap, and a bypassswitch. Since TRV is a process of fault clearing, these
protective devices have already responded to the fault
situation. Their response has a tremendous impact on the
TRV levels: e.g. a bypassed series capacitor does not interact with the harmonics in the circuit.
• Fault sequence: the instant at which the fault occurred, at
which voltage and currents, and the location of the fault influence some of the parameters, that are contributing to the
currents across the line breakers, and therefore have an
effect on TRV. By the same argument, it is important to
consider the fault type, i.e. single-phase-to-ground, threephase, phase-to-phase with or without ground. Obviously,
there are multiple possibilities of fault development, e.g. a
single-phase fault that develops into a three-phase fault.
Another influence is how long a fault lasts before the circuit breakers start opening their contacts.
• Fault clearing: note that it matters if the faulted line is
interrupted in only one phase or in all three phases. The
opening of the line breakers at one substation may not start
at exactly the same time as at the other terminal. There may
be a few milliseconds of a difference due to different protection equipment, communication signals or different
types of breakers.
The number of cases, which has to be investigated to determine worst-case TRV levels for the circuit breakers is
immense, particularly since the task involves detailed representations of stochastic phenomena such as the arcs at the fault
location or in the breaker chambers.
Evaluation and interpretation of results can be in a statistical diagram, showing the distribution of TRV levels. Besides
this the fact, that the allowable level or TRV is exceeded or
not, is of course of interest. For these cases it is necessary to
evaluate, under which conditions this TRV level occurs and
how likely such a fault is.
A comparison with TRV stresses in situations without series
comparison is difficult, as the load flow situation is different
due to the capacitor. However, as a general rule it can be
stated that TRV is higher in cases with a series capacitor. This
is especially the case at system faults far away from the
breaker under investigation. In these cases the line together
with the series capacitor forms an oscillating circuit. Also
when the series capacitor is equipped with a fast bypass device
acting clearly faster than the line breaker under investigation,
an increase in TRV can be found. These fast bypass devices
operate within some milliseconds after a severe fault, whereas
the time to open a line breaker is clearly longer.
C. Line protection study
The line protection system has to take into account that the
impedance as seen from the substation into the line has an
additional component from the series capacitor, which is capacitive in contrary to the inductive line impedance. Especially
at a TCSC this impedance may vary, and no information regarding the capacitor impedance will be sent to the line protection system. Line protection and series capacitor protection
shall be able to operate independent from each other. This
means for the line protection, that it must be designed for a
series compensated line.
Generally the communication between the two protection
systems is limited. In most cases the series capacitor protection
system will receive a signal, when the line opens, and accordingly the bypass switch of the series capacitor will be closed.
On the other side, at a severe failure of the series capacitor e.g.
4
when the bypass switch does not close, the line has to be
tripped.
on different conditions. Provided the steady state stress for the
thyristor-valve allows the additional stress from bypass operation, the thyristor-valve can be used as fast bypass device. Like
in the thyristor-protected series capacitor (TPSC) the thyristorvalve can replace the triggered spark gap. When the protection
strategy of the TPSC is acceptable, the energy rating of the
varistor can be considerably reduced [8].
To find out maximum transient stresses a number of transient stress simulations is necessary to find out the worst case
system fault. The sequence during each simulation run is based
on the strategy of line protection during and after system fault.
The number of system faults for which the components shall
be designed determines mainly the varistor rating. Auto-reclosure for example is usual in many cases, but this may be
different for single-phase faults and multi-phase faults. At
single-phase faults it is possible to open the line breakers only
in the faulty phase and to reclose after a dead time of typically
1 s. As the most probable system fault is due to lightning
strokes, it is most likely to clear the fault. This results in minimum disturbance for the remaining system, as the active power
transfer continues during the dead time at a lower level. In
other systems the faulty line opens in all three phases at singlephase faults. At multi-phase faults the line will be opened in
three phases, and again it depends on the protection strategy,
whether auto-reclosure is foreseen for multi-phase faults or
not.
The series capacitor protection system is able to react in a
similar way, i.e. it is possible that at single phase-faults only
one phase will be bypassed. Before re-closing the line breaker
the series capacitor returns into operation with the risk of a
second stress, when the fault is still persistent (non-successful
auto-reclosure). In other cases all three phases are bypassed
and they only return to operation after the success of reclosure
has been proven. All possible combinations between these two
extreme cases are possible. In most cases the varistor shall be
designed to withstand stresses from to subsequent external or
internal faults in one, two, or three-phases. This covers worst
case stresses from a fault with non-successful auto-reclosure or
from two subsequent faults. Accordingly the bypass damping
circuit has to be designed, so that it is able to withstand
stresses from two subsequent capacitor discharges from the
maximum voltage.
Another result from this study are the parameters for capacitor protection to distinguish between external and internal
faults, as only at internal faults the protection system is allowed to bypass the capacitor. Only when the varistor current
or the accumulated energy exceeds the maximum value that
has been found during the study at any external fault, the protection system interprets a system fault as internal and initiates
a bypass action.
Insulation coordination is a part of the component design
study, as it is focused on the insulation coordination of all
components of the series compensation. The insulation levels
between phases and ground are given from the surrounding
system or substation. Based on the maximum voltage across
the components, which has been found during the transient
stress calculation, the insulation of all components, the
creepage distances of housings, bushings and post insulators,
and the distances between the components can be determined.
IV. COMPONENT DESIGN STUDY
The component design study, of course, has to be done by
the manufacturer of the series compensation system, as he is
responsible for the component design and series capacitor performance. On the other side the study is based on system
requirements.
Figure 5 shows the basic single line diagram of a TCSC.
The design of the main components includes capacitor, varistor, the thyristor-valve and the reactor, and the bypass devices
with the associated bypass damping circuit. Generally the
essential data for disconnects, earthing switches, CTs etc. are
already defined from customer’s side.
bypass disconnect
platform disconnect
line
current
CT
varistor
capacitor
thyristor controlled
reactor branch
fault to
platform
CT
damping circuit
triggered spark gap
bypass switch
Fig. 5. Principal single line diagram of a TCSC
The first step of the component design covers steady state
stresses. It takes into account continuous operation and operation during overload situations for a limited time, see Fig. 1. In
a TCSC installation steady state operation is defined with a
boost factor of 1.1 or 1.2 at nominal current. This defines
nominal capacitor voltage. The TCSC also can operate continuously at higher boost factors but with lower line current, so
that the capacitor voltage does not exceed the rated value, see
the dark area in Fig. 1. The continuous operating range together with overloads defines the voltage rating for the varistor. The thyristor-valve and the reactor in series to the thyristor-valve must be designed to cover the associated stresses
continuously. This also determines the rating of the cooling
system for a water cooled thyristor-valve.
The second step of the component design study deals with
transient stresses. They determine the design of all main components, for example the maximum capacitor voltage occurring at a severe system fault, the maximum voltage across the
varistor, the associated current, and the energy the varistor
accumulates during a system fault scenario. The protection
strategy with the thyristor-valve as fast bypass device depends
5
V. CONCLUSION
The TCSC is a powerful and flexible system that provides
benefits especially for long distance power transmission
systems. The project of such an installation requires a number
of studies that have to be carried out from the system owner’s
side and from the manufacturer. This requires information
from both parties, as they both together have the knowledge to
allow a good presentation of transmission system and series
capacitor during the studies, and guarantees study results that
are as realistic as possible.
VI. REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Kirschner, Thumm, “Design and Experience with Thyristor-Controlled
and Thyristor-Protected Series Capacitors”, presented at the IEEE
PowerCon 2002, Kunming, China
Christl, Luetzelberger, and Sadek, “System Studies and Basic Design
for an Advanced Series Compensation Scheme”, presented at the International Conference on Advances in Power System Control, Operation
and Management APSCOM, Hong Kong, 1991
Gama, Leoni, Gribel, Fraga, Eiras, Ping, Ricardo, Cavalcanti,, and
Tenorio, “Brazilian North-South Interconnection - Designing of Thyristor Controlled Series Compensation (TCSC) to Damp Inter-Area
Oscillation Mode”, presented at Sepope 1998, Brazil
Kirschner, “Design Aspects of the Chinese 500 kV Thyristor-Controlled
Series Compensation Scheme TCSC Tian Guang”, presented at the 2nd
International Conference on Electric Utility Deregulation, Restructuring
and Power Technologies, IEEE DRPT, HongKong 2004
Retzmann, Claus, Kuhn, Kumar, Lei, Baran, Forsyth, Maguire,
“Advanced Fully Digital TCSC Real-Time Simulation”, presented at
CEPSI 2001
Series Capacitors for Power Systems, Part 1: General, International
Standard, IEC 143-1
IEEE Standard for Series Capacitors in Power Systems, IEEE 824-1995
Kirschner, Bohn, Sadek, “Thyristor-Protected Series Capacitors. Part I:
Design Aspects”, presented at the Cigré XERLAC Conference 2003,
Argentina
VII. BIOGRAPHIES
Lutz Kirschner, Senior Project Engineer, MIEEE, received his Diploma in
Electrical Engineering 1992 from the University of Aachen, Germany. He
joined SIEMENS company in the HVDC Department as a system design
engineer. He was involved with technical and commercial design of HVDC
converter stations. He got his project experiences from the North-American
Texas-Welsh Converter, the Chinese Tian-Guang Converter and the Asian
Thailand-Malaysian Converter projects. Since 1995 he is also responsible for
Fixed and Thyristor Controlled Series Capacitor design and was busy in the
brazilian Furnas/Eletronorte series capacitor project as well as the northamerican Lexington/Valley and TCSC Tian Guang project carrying out the
basic design system studies. He is working on the design of Thyristor
Protected Series Capacitor systems (TPSC) and FACTS devices. In the FSC
Sao Joao do Piaui project he carried out the Final Basic Design Studies comprising the transient fault calculation and main component ratings. His special
fields are time domain digital simulations and system studies. Since 1998 he
is a member of IEEE.
Dr. Gerhard H. Thumm, Senior Project Engineer, received his Diploma in
Electrical Engineering in 1977 from the Technical University of Stuttgart,
Germany, and the Dr.-Ing. degree in 1991. From 1977 to 1982 he worked at
the High Voltage Laboratory of the University in Stuttgart. He joined
SIEMENS AG, the system planning department in 1982, and changed to a
group for Reactive Power Compensation in 1985. He was involved with technical design of Static Var Compensators (SVC), and was responsible for the
design of several SVCs in Great Britain, Australia and USA. Since 1995 he is
also responsible for the design Fixed and Thyristor Controlled Series Capacitors, and was engaged in the Brazilian Furnas/Eletronorte series capacitor
project Interligacao I, several fixed series capacitors in South Africa, China,
and India, and in the system studies for the Tian Guang TCSC project.
6