1
Considerations for the Application of Thyristor
Controlled Series Capacitors to Radial Power
Distribution Circuits
M. N. Moschakis, E. A. Leonidaki, Student Member, IEEE, N. D. Hatziargyriou, Senior Member, IEEE
Abstract—This paper deals with the application of Thyristor
Controlled Series Capacitors (TCSCs) suitably rated for radial
distribution circuits. The various problems existing on long
distribution lines that can be alleviated by the connection of
variable series compensation, as a TCSC provides, are discussed.
An important feature is the capability of the TCSC to operate as
a short circuit current limiter. Thus, sensitive loads connected to
nearby substations will not experience any voltage sags caused by
faults on the distribution line where the TCSC is connected. The
benefits of the connection of a TCSC on a radial distribution
system are verified by means of the ElectroMagnetic Transients
for DC (EMTDC) simulation package.
Index Terms—Series compensated distribution lines, TCSC,
fault current limiter, thyristor switches, power quality, power
system simulation, sensitive loads, voltage sags.
T
I. INTRODUCTION
HYRISTOR controlled series capacitors (TCSCs) have
been used in transmission networks to control the
equivalent impedance of transmissions lines, and
therefore the power flow in the network [1], [2]. They have
renewed the interest in transmission line series compensation
because of their control system flexibility. Although
mechanical switching could, in principle, be applied to
achieve some flexibility, the fast electronic control and proven
reliability of the thyristors lead to maximum controllability of
the transmission system.
As regards distribution systems, many different solutions
have been proposed. Miske [3] and Souza et al [4] proposed
the use of fixed and GTO-controlled series capacitors in radial
distribution circuits, respectively. GTO or thyristor-controlled
series capacitors have the advantage of self-regulating,
continuous and instantaneous response. Some of the positive
effects of series compensation on a long radial power
distribution circuit when operating as a varying series
capacitance are:
M. N. Moschakis is with the Department of Electrical and Computer
Engineering, National Technical University of Athens, Athens, Greece,
(e-mail: mosxakis@power.ece.ntua.gr).
E. A. Leonidaki is with the Department of Electrical and Computer
Engineering, National Technical University of Athens, Athens, Greece,
(e-mail: eleonid@central.ntua.gr).
ƒ Increased power transmission capability by a decreased
total circuit reactance and improved voltage profile along
the circuit
ƒ Decreased circuit losses
ƒ Support during start of large asynchronous and
synchronous motors
ƒ Reduction of required reactive power input at the sending
end of a radial circuit
ƒ Reduced voltage fluctuations and voltage imbalance due to
load variations
In distribution systems, reactive power compensation is
typically provided by shunt-connected components, such as
SVC and STATCOM devices. Fixed series capacitors are
clearly the most economical solution, but their optimal
location is not easy to determine, when many loads are
connected to the distribution system. An appropriately rated
TCSC device can also be applied in radial power systems in
order to keep load voltage within given limits for several load
variations or to provide short circuit current limitation. More
specifically, the TCSC appears as variable impedance that
depends on thyristor firing angle. In this way the control
system sends firing order to the thyristors according to basic
scenarios in order to keep the load voltage between given
limits.
Furthermore, TCSC can operate in the inductive region to
reduce short-circuit currents. TCSC capability to operate as a
fault current limiter in transmission systems has already been
proposed. Karady [5] proposed a new configuration of TCSC
to obtain a combined short circuit current limiter, interrupter
and series compensator in steps. The series compensation is
controlled by the capacitors switching on and off, while the
current limitation and interruption is achieved by the insertion
of a resonant LC circuit in the transmission line. Godart et al
[6] conducted a study about the feasibility of TCSC for
distribution substations enhancement taking into consideration
load expansion. The capability of TCSC to operate as short
circuit current limiter when connected upstream the
distribution substation, was confirmed. Moreover, Yamazaki
et al [7] proposed and produced a prototype of a combined
TCSC system with an additional series current limiting
reactor.
N. D. Hatziargyriou is with the Department of Electrical and Computer
Engineering, National Technical University of Athens, Athens, Greece,
(e-mail: nh@power.ece.ntua.gr).
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In this paper, a TCSC model that operates both as a series
compensator and fault current limiter is studied. The TCSC
can be installed on a long distribution line, where the number
of faults is expected to be high to decrease the short-circuit
effect on neighboring loads. Thus, sensitive loads connected
to nearby substations do not experience voltage sags caused
by faults on the TCSC supported distribution line.
II. TCSC STEADY-STATE OPERATION
The basic circuit of TCSC is illustrated in Figure 1. It
consists of a capacitor in parallel with a thyristor-controlled
inductor. The control variable is the firing angle α of the
thyristors, with reference to the capacitor voltage zero
crossings. The thyristors are fired when the capacitor voltage
and current are opposite in polarity. This is equivalent to
thyristor firing angles between 90° and 180°.
Fig. 1. One-line diagram for TCSC circuit
The TCSC can operate in three different modes. In the
bypassed mode, the thyristor path is conducting continuously,
the capacitor is bypassed and the apparent impedance
becomes inductive. In the blocked mode, the thyristor path is
blocked continuously, which is equivalent to the fixed
capacitor reactance. Finally in the Vernier mode, the thyristor
path is partially conducting resulting in a flow current
circulating in the TCSC loop. Depending on the conduction
time of the thyristors, this current may have the same or
opposite direction with the internal capacitor current. In this
way the TCSC appears as an apparent reactance that may be
capacitive or inductive.
The steady-state curve of the apparent reactance of the
TCSC at fundamental frequency versus the firing angle is
equal to:
Fig. 2. Apparent impedance of TCSC.
behavior for at least one angle is observed. This asymptote
corresponds to the firing angle where resonance between
inductor and the capacitor reactance occurs. This firing angle
must be avoided, because it results in excessively high
voltages and currents in TCSC circuit. By changing the
reactor inductance, it is possible to increase the effective
bandwidth of control and have a lower circulating current
through the components [8][9].
Emphasis must be given on the rating of all parameters of
a real TCSC not only to achieve the desired capacitive range,
but also to select the appropriate components that are designed
to withstand system contingencies that cause high current
peaks without damage. The control system should take care to
bypass the TCSC, in case these ratings are exceeded.
Additional protection schemes are applied [2]. The thyristors
have a Break-over Diode protection scheme for overvoltage
protection. The capacitor is equipped with a gapless metaloxide varistor (MOV) surge arrester connected in parallel,
which operates whenever the capacitor voltage overcomes a
set limit. The behaviour of MOV across conventional
capacitor banks is extensively described in [10]. Finally, a
bypass breaker exists that operates whenever the device has to
be removed from the system.
ZTCSC = - XC + (XC+XLC)[(2σ+sin2σ)/ π ] 4X2LCcos2σ [(ktankσ-tanσ)/π] /XL
(1)
XLC =XC . XL / (XC-XL)
σ = π-α
wo 2 = 1/LC , w = 2πf , k=wo/w
The apparent impedance of a typical TCSC versus the firing
angle α is plotted in Fig. 2. In this figure an asymptotic
Fig. 3.1 TCSC voltage and current waveforms in capacitive region.
During normal operation, the TCSC generally operates in
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the capacitive region. The capacitor voltage and current as
well as the thyristor current for this operation are illustrated in
Fig. 3.1. During system faults, it is desirable to operate in the
inductive region to lower the fault current contribution (Fig.
3.2).
Fig. 3.2 TCSC voltage and current waveforms in inductive region.
Regarding the harmonic content of TCSC device, the
harmonic content of thyristor current is equal to:
IT ( n) =
2 A ⎡sin(n + 1)σ sin(n −1)σ ⎤ 2 Acosσ ⎡sin(k + n)σ sin(k − n)σ ⎤
+
−
+
π ⎢⎣ n + 1
n −1 ⎥⎦ π coskσ ⎢⎣ k + n
k − n ⎥⎦
(2)
where n=3,5,7,...
These harmonic currents circulate mainly inside the LC
circuit, because the system impedance for harmonic
frequencies is much greater than the capacitor impedance.
This can be derived considering that capacitor and system
impedance form a current divider for the thyristor current at
harmonic frequencies. In this way the capacitor and system
currents are given by the equations:
connected through a long line. This load fluctuates from 9 to
15 MVA at a regular basis leading to voltage fluctuations and
rapid voltage changes [13], which affect both this load and the
loads connected to nearby substations. At feeder 2, a sensitive
to voltage sags load is connected. The installation of this load
contains protection systems that trip immediately after even
shallow sags with only a short duration.
The optimal position to connect a TCSC is at the sending
end of feeder 1. In this location, it enables reactive power
compensation and also reduces short-circuit currents
introduced by faults that occur on feeder 1. The number of
faults on feeder 1 is expected to be high, assumed
proportionally related with the length of a line.
As a basis for the specification of the design parameters of
the TCSC model, the parameters of a real TCSC of 15÷50 Ω
at 400kV have been used. The scaling factor applied to all the
constituent components is defined as the ratio of the model
impedance base to the system impedance base at the same
point of the circuit, as follows:
λ=
Z b mod
Z bsyst
(5)
The resistors and reactors of the real system are multiplied
by this scaling factor, while the capacitances are divided by it.
Consequently, each phase of the TCSC model consists of a
4760 µF capacitor and a 52 mH inductor with quality factor
equal to 110. The TCSC apparent impedance varies between 0.668 Ω (α=180°) and 0.1324 Ω (α=90°). The apparent
impedance of the applied TCSC versus the firing angle α is
shown in Fig. 2. Attention has been paid, so that only one
resonant point in the range of 90° to 180° exists, equal to
135.4°. In addition, the reactor size was properly selected, in
order to increase the effective control bandwidth in the
inductive range.
iC = i T . ZSYST/(ZSYST + ZC)
iac = i T . ZC/(ZSYST + ZC)
(3)
Assuming that the system impedance is infinite, the
harmonic thyristor currents flow only through the capacitor,
so the capacitor voltage at the n-harmonic is equal to:
VC(n) = iT(n) XC(n) = iT(n)/(n.w.C)
(4)
The harmonic content and magnitude of circulating currents
increases essentially as firing angle α gets nearer to the
resonant point. It is shown in [11] that the Total Harmonic
Distortion (THD) of transmission line current is below the
current distortion limits recommended in IEEE Std 519-1992
[12].
III. STUDY CASE
A. Power system and TCSC characteristics
The test system consists of two feeders fed by 150/20 kV
substations. At feeder 1 a nominal load of 10 MVA is
Fig. 4. Single-line diagram of the test network
B. TCSC as a varying series capacitance
When TCSC operates in the capacitive region, it can be
controlled so that the voltage at the terminals of load 1 and the
voltage across the feeder 2 (assuming that other consumers are
also connected to that feeder or will be connected in the
future) are kept constant. Simulations were performed using
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EMTDC to demonstrate the ability of TCSC to alleviate
voltage fluctuations caused by load fluctuations. The
simulation results are shown in Fig. 5. The load varies in steps
from 9 to 15 MVA, as shown in the upper diagram of Fig. 5.1.
The voltage at the terminals of load 1 with and without TCSC,
are shown in the middle diagram. The instantaneous response
of TCSC and the ability to control even rapid voltage changes
can be easily seen. The lower diagram of Fig. 5.1 presents the
variation of the control angle in order to keep the voltage at
the load terminals constant. Furthermore, as voltage across
the TCSC is proportional to the current, the higher current is
flown on the feeder, the more benefits are gained by the
connection of TCSC.
Fig. 5.3 Capacitor voltage and current, line current and control angle
following a load variation
C. TCSC as fault current limiter
Some of the positive effects of fault current limitation are:
Fig. 5.1 Voltage with and without TCSC after extreme symmetrical
load fluctuations
In Fig 5.2, THD of the load phase voltage and the line
current are presented. It can be seen that the harmonics
injected in the voltage and current are acceptable, according to
European Standard EN50160 [13].
Fig. 5.2 THD of phase voltage and line current.
In Fig 5.3 the capacitor voltage and current inside the
TCSC in the first 0.3 sec are shown. It should be noted that
after the first load variation at 0.1 sec, the TCSC circuit
gradually approaches steady state, which is reached when the
half of the inductor current is exactly symmetrical about the
zero crossings of capacitor voltage.
ƒ Mitigation of voltage sags, swells and outages
ƒ Longer life with higher reliability for nearby transformers
ƒ Limited inrush current (soft start), even for capacitive
loads
ƒ Mitigation of the effects of distributed generation at lower
voltages within Distribution Systems
When TCSC operates in the inductive region, it represents a
varying inductance, which can reach values much higher than
the nominal inductor value, as the control angle increases
towards the resonance value (Fig. 2). In cases of excessive
currents flowing in feeder 1, significant damping can be
effected by regulating accordingly the TCSC angle. Thus,
although operation near the resonance area should be avoided,
such operation of TCSC during a short circuit, lasting
essentially no longer than a few periods, e.g. 5 periods (100
ms for a 50 Hz system) is very beneficial. This time period
corresponds also to the required time for the normal operation
of a distribution protection system. This strategy is used in the
following simulations investigating the effect of fault current
limitation via TCSC. It is assumed that the TCSC model is
equipped with a MOV appropriate selected for overcurrent
and overvoltage protection of the device components.
The fault detection strategy used is based on the rate of
current increase. In this way, a fault is detected within a few
µs and the fault current limitation begins even before the first
5
peak is reached. The detection strategy is shown in Fig. 6.1.
The rate of the line current is compared with the instantaneous
line current. When this rate is above a preset limit (set to 200
pu/s for this case), the output of the rate limiter is presented in
Fig. 6.2. Thus, a signal is issued almost instantaneously and
after the first zero crossing of the thyristor current the
operation of the TCSC in the current limiting mode is
initiated.
The two middle diagrams show the line current with and
without a TCSC. The lower diagram shows harmonic
injection, when TCSC operates in the current limiting mode. It
can be seen that the THD is about 5%, well below the limits
set in EN50160, verifying the fact that TCSC does not inject
significant harmonics into the system.
Fig. 6.1 Fault detection strategy
Fig.7. TCSC operation sequence in current limiting mode
The TCSC can also operate independently at each phase,
hence, it can deal with asymmetrical faults. This means that
the faulted phase will operate in the current limiting mode
while the healthy phases will operate in the capacitive region.
To demonstrate the ability of TCSC to deal with asymmetrical
faults, the following simulations were performed.
1) Single- phase fault
Fig. 6.2 Fault detection logic in a three-phase fault
In Fig.7, the repetitive operation of TCSC in a three-phase
fault is shown. In the beginning, TCSC operates in the
capacitive region, a fault immediately after the TCSC (which
is the worst case) occurs at t=0.1 sec and it lasts until t=0.8
sec. The TCSC operates for 100 ms and the protection breaker
opens. The breaker recloses after 300 ms, the fault is still
present and TCSC operates again for 100 ms limiting the fault
current until the breaker opens for the second time. The
breaker recloses again after 300 ms, the fault has been cleared
and the TCSC returns in the capacitive region.
In the upper diagram in Fig. 7, the voltage at the terminals
of the sensitive load 2 with and without a TCSC is shown. It is
clearly shown that the voltage sag experienced by the
sensitive load 2 is significantly lower when TCSC is present.
The simulation results when a single-phase fault occurs
immediately after the TCSC are shown in Figures 8.1 and 8.2.
The fault occurs at t=0.025 sec and is cleared after 100 ms.
The load 1 at pre-fault time is 10 MVA. In Fig. 8.1 the load 2
voltage with and without TCSC is presented. In Fig. 8.2 it is
observed that the phase –c is also affected but continues to
operate in capacitive region. The voltage sag magnitude with
the TCSC reaches a value of 72.2% and lasts for 25.66 ms.
Furthermore, the THD of load 2 voltage is again within
acceptable limits.
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Fig. 8.1 Load 2 phase voltages with and without TCSC
Fig. 9.1 Load 2 phase voltages with and without TCSC
Fig. 8.2 Load 2 phase –a and –c line currents, THD of voltage Va
2) Phase-to-phase fault
Phase-to-phase faults (between phase -a and -b) are also
examined. The pre-fault conditions are the same as in the
previous case. Fault occurs at t=0.03 sec and is cleared after
100 ms. The voltage sag magnitude experienced by the load is
62.9% and its duration is 69 ms. The simulation results are
shown in Figures 9.1 and 9.2.
Fig. 9.2 Load 2 phase –a and –b line currents, THD of voltage Va
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IV. CONCLUSIONS
To date, TCSC systems have been proposed as an effective
means to increase power transfer capability of transmission
lines and rapid controllability for power swing damping and
stability in transmission systems. In addition, the ability of
TCSC to operate as a fault current limiter has been mentioned.
In this paper the application of an appropriately rated TCSC
system in distribution networks is proposed. A suitable TCSC
model is applied in a distribution test system and its
effectiveness in the mitigation of several power quality
problems is investigated. It is shown that TCSC operating as a
varying series capacitance can effectively alleviate load
voltage sags due to load fluctuations. In addition, it is shown
that TCSC operating in the inductive region contributes to
fault current limitation and maintenance of voltage at sensitive
loads in neighboring feeders close to its nominal value.
In general, it is believed that TCSC systems can provide an
important control tool to distribution system engineers. Their
ability to reduce short-circuit currents can provide solutions to
many operational and power quality problems. However,
because of its high cost, the viability of such a device should
be further investigated.
V. REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
E. Larsen, C. Bowler, B. Damsky, S. Nilsson, “Benefits of ThyristorControlled Series Compensation”, CIGRE Paper 14/37/38-04, Paris
1992.
N. Christl, R. Hedin, et al. “Advanced Series Compensation (ASC) with
Thyristor Controlled Impedance”, CIGRE Session 1992, Paper
14/37/38-05, Paris, 1992.
S. Miske, “Considerations for the Application of Series Capacitors to
Radial Power Distribution Circuits”, IEEE Trans. Power Delivery, vol.
16, No.2, pp. 306-318, April 2001.
L. Souza, E. Watanabe, M. Aredes, “A GTO Controlled Series Capacitor
for Distribution Lines”, CIGRE Paper 14-201, Paris 1998.
G. Karady, “Concept of a Combined Short Circuit Limiter and Series
Compensator”, IEEE Trans. Power Delivery, vol. 6, No.3, pp. 10311037,, July 1991.
T. Godart, A. Imece, J. McIver, E. Chebli, “Feasibility of Thyristor
Controlled Series Capacitor for Distribution Substation Enhancements”,
IEEE Trans. Power Delivery, vol. 10, No.1, pp. 203-209, January 1995.
Y. Yamazaki, S. Sugimoto, S. Ogawa, H. Konishi and A. Kikuchi,
“Development of TCSC application to Fault Current Limiters”,
Electrical Engineering in Japan, vol. 140, No. 3, 2002, Translated from
Denki Gakkai Ronbunshi, vol. 121-B, No. 4, April 2001, pp. 514-519.
S. Helbing, G. Karady, “Investigation of an Advanced Form of Series
Compensation”, IEEE Trans. Power Delivery, vol. 9, No.2, pp. 939-947,
April 1994.
E. Larsen, K. Clark, S. Miske, J. Urbanek, “Characteristics and Rating
Considerations of Thyristor Controlled Series Compensation”, IEEE
Trans. Power Delivery, vol. 9, No.2, pp. 992-1000, April 1994.
G.J. Georgantzis, N.D.Hatziargyriou, E.A. Leonidaki, “Transient
Simulation of Series Compensated EHV Transmission Lines for Shortcircuit Studies”, IEEE MELECON ‘96, p. 1584-1587, Bari, Italy, May
1996.
E.A Leonidaki., N.D.Hatziargyriou, B.C.Papadias, G.J.Georgantzis,
"Investigation of Power System Harmonics and SSR Phenomena Related
to Thyristor Controlled Series Capacitors" , Proceedings of the 8th
International Conference on Harmonics and Quality of Power (ICHQP
’98), Athens, Greece, October 1998, pp. 848-852.
IEEE Std. 519-1992, "IEEE Recommended Practices and Requirements
for harmonic Control in Electrical Power systems", 1992.
European Standard EN 50160, Voltage characteristics of electricity
supplied by public electricity distribution networks, November 1994.
VI. BIOGRAPHIES
M. N. Moschakis received his Diploma in Electrical Engineering from the
National Technical University of Athens (NTUA), Greece, in 1998. Currently
he is a Ph.D student in NTUA. His scientific interests mainly concern Custom
Power Device Modeling and Evaluation, and Voltage Sag Stochastic
Assessment.
E. A. Leonidaki received her Electrical Engineering degree in 1992 from
NTUA. She is now with Public Power Corporation of Greece and she works
towards a PhD degree in Electrical Engineering Department of NTUA. Her
interests include FACTS modeling and control for power system dynamic
analysis and transient phenomena. She is student member of IEEE, member of
CIGRE and member of Technical Chamber of Greece.
N. D. Hatziargyriou received a Diploma in Electrical and Mechanical
Engineering from NTUA, and an Msc and PhD degree from UMIST,UK. He
is currently Professor at Power Division of the Electrical Engineering
Department of NTUA. His research interests include Modeling and Digital
Techniques for Power System Analysis and Control. He is Senior member of
IEEE and secretary of IEEE Greek section, member of CIGRE SC C6 and of
Technical Chamber of Greece.