International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Research Paper
COMPENSATION BY TCSC IN OPEN LOOP CONTROL
SYSTEM
1*
Sunita Tiwari, 2S.P. Shukla
1*
Address for Correspondence
Sr. Lecturer, Polytechnic,Durg
2
Professor, Bhilai Institute of Technology, Durg
ABSTRACT
The FACTS controllers clearly enhance power system performance, improve quality of supply and also provide an optimal
utilization of the existing resources. TCSC has been proposed to enhance the power transfer capability by changing the
reactive power distribution in the power system. This paper discusses the TCSC’s power enhancement capability. It has also
discussed the effect of TCSC on steady state and transient stability. A transmission line model equipped with TCSC that is
suitable for power transfer capability and transient stability analysis is proposed. This model is tested in a simple
transmission system for open loop control system on MATLAB 2007a software. Thyristor controlled series capacitors
(TCSC) in closed loop system have been widely studied by many researchers but in this model the effect of TCSC in open
loop control system is discussed. The simulation result shows that TCSC is capable of increasing power level and improving
transient stability.
KEYWORDS Transient stability, Power enhancement, FACT, TCSC
To examine the transient stability of the system with
I. INTRODUCTION
An increasingly competitive market where economic
and without TCSC, the same transmission line is
and environmental pressures limit their scope to
subjected to transient disturbances i.e. a circuit
expand transmission facilities. The optimization of
breaker, with specified switching time, is connected
transmission corridors for power transfer has become
in series with the transmission line and responses are
a great importance. In this scenario, the FACTS
observed.
technology is an attractive option for increasing
The main objective of this project is to demonstrate
system operation flexibility [1] , New developments in
how a TCSC influence the power of the load
high-current, high-power electronics are making it
connected to transmission line. The model developed
possible to control electronically the power flows on
in this project was verified by simulation studies for a
the high voltage side of the network during both
series compensated system
steady state and transient operation.
II.THYRISTOR
CONTROLLED
SERIES
One important FACTS component is the TCSC
COMPENSATOR
which allows rapid and continuous changes of the
It is obvious by series compensation technique that
transmission line impedance [ 2 ] . Active power flows
power transfer between two station can be affected
along the compensated transmission line can be
by adjusting the net series impedance of line. One
maintained at a specified value under a range of
such conventional and established method of
operating conditions. Fig. 1 is a schematic
increasing transmission line capability is to install a
representation of a TCSC module [2], which consists
series capacitor, which reduces the net series
of a series capacitor bank in parallel with a Thyristor
impedance, thus allowing additional power to be
Controlled Reactor (TCR). The controlling element is
transferred. Although this method is well known,
the thyristor controller, shown as a bidirectional
slow switching times is the limitation of its use.
thyristor valve.
Thyristor controllers, on the other hand, are able to
In this paper a short description of TCSC is given
rapidly and continuously control the line
along with the simulation of transmission line using
compensation over a continuous range with resulting
TCSC, a FACTS controller simulated in MATLABflexibility. Controller used for series compensation is
R2007a. Analysis of the simulated transmission line
the Thyristor Controlled Series Compensator
(compensated with TCSC) model shows that TCSC
(TCSC). TCSC controllers use thyristor-controlled
can enhance power level of transmission line and has
reactor (TCR) in parallel with capacitor segments of
the similar functions as a physical one.
series capacitor bank (Figure 1). The combination of
The simulation of transmission line at different load
TCR and capacitor allow the capacitive reactance to
conditions is done and the results show that the
be smoothly controlled over a wide range and
power transmitted through the line can be enhanced
switched upon command to a condition where the biwith the application of TCSC. Change in value of
directional thyristor pairs conduct continuously and
load affects the power level but, still, TCSC is
insert an inductive reactance into the line.
capable of increasing power level of the system in all
A TCSC is a series controlled capacitive reactance
conditions.
that can provide continuous control of power on the
Controlled series compensation can be applied
ac line over a wide range. The functioning of TCSC
effectively to damp power oscillations. For damping
can be comprehended by analyzing the behavior of a
power oscillations, it is necessary to optimize the
variable inductor connected in series with a fixed
applied compensation so as to counteract the
capacitor, as shown in Figure 1.
accelerating and decelerating swings of the disturbed
machine.
IJAET/Vol.III/ Issue I/January-March, 2012/175-179
International Journal of Advanced Engineering Technology
Fig.1. Thyristor Controlled Series Capacitor
(TCSC)
III.POWER SYSTEM STABILITY
Power system stability may be broadly defined as the
ability of a power system to remain in a state of
operating equillibrium under normal operating
conditions and to regain an acceptable state of
equilibrium after being subjected to a disturbance.[3]
Stability of power system has been a major concern
in system operation. The stability of a system
determines whether the system can settle down to the
original or close to the steady state after the transients
disappear. In general, power system stability is the
ability to respond to a disturbance from its normal
operation by returning to a condition where the
operation is again normal. [3]
A power system is said to be steady state stable for a
particular operating condition if, following any small
disturbance, it reaches a steady state operating
condition which is identical or close to the predisturbance operating condition.[3]
Transient stability is defined as the ability of the
power system to maintain synchronism when
subjected to a severe transient disturbance. A system
is transiently stable if it can survive the initial
disturbance but it is transiently unstable if it cannot
survive. For the transiently stable system, a large
disturbance suddenly occurs, the system angle spread
starts to increase but reaches a peak and then starts to
decline, making the system transiently stable. The
resulting system response involves large excursions
of generator rotor angles. Transient stability is
sometimes called first swing stability as the
instability often occurs during the first angle swing.[3]
IV.FUNDAMENTAL REACTANCE OF TCSC
The effective reactance of TCSC is given by
equations (1) and (2). [4] Equation (1) assumes that
the capacitor voltage is free from harmonics and
considers the only the TCR current harmonics.
 ( X 1 TCR ⋅ X C ) 
---------(1)

X 1 TCSC = 
 j ⋅ ( X 1 TCR ⋅ X C ) 



2σ
 2 λ2 2 ⋅ cos 2 
 σ
 σ  σ  sin(σ )
⋅ λ ⋅ tanλ  − tan  −   −
X 2TCSC = − j ⋅ XC 1+ ⋅ 2  2

2 
 2
 2   2 
 π λ −1 λ −1 



---------(2)
Where


π
X1TCR = j ⋅ ω ⋅ L ⋅ 

(
)
−
σ
sin
σ


----------(3)
λ=
ω0
ωN
and ω 0
=
1
L⋅C
On the other hand equation (2) gives a more accurate
representation of reactance of TCSC by considering
IJAET/Vol.III/ Issue I/January-March, 2012/175-179
E-ISSN 0976-3945
the harmonics of both the capacitor voltage and the
TCR current. Intuitively, in the case of equation (2)
the extra charge injected into the capacitor during the
capacitive vernier mode increases the fundamental
component of voltage, increasing the effective TCSC
capacitive reactance as seen by the power system. As
a result equation (2) results in higher value of TCSC
reactance for a given value of conduction angle when
compared to equation (1), in addition be presenting a
more complete representation.
In the above equations, σ is the conduction angle, L is
the inductance of the TCR inductor, C is the
capacitance of the fixed capacitor, ωN is power
system frequency in radians per second and ω0 is the
resonant frequency of the TCSC circuit. Fig.2 [5]
shows the effective reactance of TCSC.
Fig.-2 Reactance Characteristics of TCSC
Simulation results match more closely to
characteristics drawn using equation (4.2). The
negative and positive portions of the characteristics
represent capacitive and inductive vernier modes of
operation.
V. MODES OF OPERATION IN STEADY STATE
By controlling the firing angle of the thyristors the
effective reactance of the TCR can be varied. This
variable TCR reactance in parallel with a fixed
capacitor allows the TCSC to operate in four
different modes; blocking mode; bypass mode;
capacitive boost mode; and inductive boost mode.[4 ]
[5] [6]
Blocking Mode:
When the thyristor valve is not triggered and the
thyristors are kept in non-conducting state, the TCSC
is operating in blocking mode. In this mode, the
TCSC performs like a fixed series capacitor.
Bypass Mode:
In bypass mode the thyristor valve is triggered
continuously and the valve stays conducting all the
time; so the TCSC behaves like a parallel connection
of the series capacitor with the inductor, Ls, in the
thyristor valve branch. In this mode, the resulting
voltage in the steady state across the TCSC is
inductive and the valve current is somewhat bigger
than the line current due to the current generation in
the capacitor bank. For practical TCSCs with XL/XC
ratio between 0.1 to 0.3 range, the capacitor voltage
at a given line current is much lower in bypass than
in blocking mode. Therefore, the bypass mode is
utilized as a means to reduce the capacitor stress
during faults.
Capacitive Boost Mode:
International Journal of Advanced Engineering Technology
In capacitive boost mode a trigger pulse is supplied to
the thyristor having forward voltage just before the
capacitor voltage crosses the zero line, so a capacitor
discharge current pulse will circulate through the
parallel inductive branch.
The discharge current pulse adds to the line current
through the capacitor and causes a capacitor voltage
that adds to the voltage caused by the line current.
The capacitor peak voltage thus will be increased in
proportion to the charge that passes through the
thyristor branch. The fundamental voltage also
increases almost proportionally to the charge. From
the system point of view, this mode inserts capacitors
to the line up to nearly three times the fixed
capacitor. This is the normal operating mode of
TCSC.
Inductive Boost Mode
In inductive boost mode, the circulating current in the
TCSC thyristor branch is bigger than the line current.
In this mode, large thyristor currents result and
further the capacitor voltage waveform is very much
distorted from its sinusoidal shape. The peak voltage
appears close to the turn on.
The poor waveform and the high valve stress make
the inductive boost mode less attractive for steady
state operation. This mode increases the inductance
of the line, so it is in contrast to the advantages
associated with the application of TCSC for
increasing the line loadability by decreasing the line
impedance. Meanwhile, this mode is useful during
short circuits to decrease the fault current. This mode
is normally used as a current-limiting system, helping
to reduce the voltage sag during the faults.
V. TCSC MODELING USING SIMULINK
E-ISSN 0976-3945
uncompensated line (ii) line equipped with TCSC (at
three different firing angle).
In second condition, the load is changed, making it
more inductive and the results are identified in both
the condition i.e. when line is compensated (at one
particular firing angle) and when line is not
compensated.
In third condition, load is again changed, making it
more resistive, the results are identified in both the
condition, when line is compensated (at one
particular firing angle) and when line is not
compensated.
For analyzing the effect of TCSC on transient
stability of transmission system, transient disturbance
is applied on line in both the conditions, when it is
uncompensated, and when it is compensated with
TCSC (at three different firing angle) is observed and
results are compared.
Condition-I
(When load is P = 10 KW and QL = 1KVar )
CASE-1 Single phase transmission system
Fig. 4 Active, Reactive Power
(without compensation)
Table -1 Active/Reactive power output
CASE-2 Simulation of tr. Line with TCSC:
(i)When firing angle is 1500
Table-2 Active/Reactive power output
Figure 3. Model of SMIB system using
TCSC
The complete system has been represented in terms
of SIMULINK blocks in a single integral model.
SIMULINK is a software tool associated with
MATLAB, used for modeling, simulating and
analyzing dynamical systems. Single Machine
Infinite Bus (SMIB) system with all the required
components is modeled and is described. Simulink
model of SMIB system with TCSC has been shown
in Figure 3.
VII. SIMULATION RESULTS
For analyzing the effect of TCSC on transmission
system, three conditions of line is taken. In first
condition, at particular load, power transfer capability
of line is noted and the results are compared for (i)
IJAET/Vol.III/ Issue I/January-March, 2012/175-179
Fig. 5 Active, Reactive Power at α=150o
International Journal of Advanced Engineering Technology
(ii). When firing angle, α = 1620
Table-3 Active/Reactive power output
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in series with the transmission line and responses are
observed.
Case-1.When line is uncompensated:
(iii)When firing angle, α = 1730
Table -4 Active/Reactive power output
Condition-II
(When load is P = 10 KW and QL =10 KVar)
Table-5 Active/Reactive power output
Condition –III
(When load is P = 10 KW and QL=100 Var)
Table- 6 Active/Reactive power output
It is clear from above simulations, for all the cases of
transmission line and all the conditions of load when
line is series compensated by TCSC, the transmission
capacity of line gets increased. It is also concluded
that transmission capacity can be controlled by
operating the model at different firing angle.
Moreover TCSC can be operated in capacitive mode
as well as inductive mode whenever it is required.
VIII. TRANSIENT STABILITY IMPROVEMENT
BY TCSC
After the application of transient disturbances, if
power oscillations persist for longer period and the
amplitude of oscillation is also high, then the system
is called unstable. To improve stability of the system
it is required that oscillations should damp fast.
Controlled series compensation can be applied
effectively to damp power oscillations. For damping
power oscillations it is necessary to optimize the
applied compensation so as to counteract the
accelerating and decelerating swings of the disturbed
machine.
To examine the transient stability of the system with
and without TCSC, the same transmission line is
subjected to transient disturbances i.e. a circuit
breaker, with specified switching time, is connected
IJAET/Vol.III/ Issue I/January-March, 2012/175-179
Fig.-6 Power oscillation diagram
The amplitudes of oscillations are
1st positive Peak = above 740 MW
1st negative Peak = below 640 MW
2nd positive Peak = above 719 MW
2nd negative Peak = below 670 MW
3rd positive Peak = above 690 MW
3rd negative Peak = above 680 MW
Case-2.When line is compensated with TCSC:
(ii)When firing angle is 1500:
Fig.-7 Power oscillation diagram at 1500
The amplitudes of oscillations are-:
1st positive Peak = above 1580 MW
1st negative Peak = below 1490 MW
2nd positive Peak = above 1550 MW
2nd negative Peak = below 1530 MW
(ii)When firing angle is 1620:
Fig.-8 Power oscillation diagram at 1620
The amplitudes of oscillations:
1st positive Peak = above 1600 MW
1st negative Peak = below 1500 MW
2nd positive Peak = above 1570 MW
2nd negative Peak = below 1540 MW
(iii)When firing angle is 1730
Fig.-9 Power oscillation diagram at 1730
International Journal of Advanced Engineering Technology
The amplitudes of oscillations are1st positive Peak = above 1550 MW
1st negative Peak = below 1450 MW
2nd positive Peak = above 1510 MW
2nd negative Peak = below 1540 MW
Comparison and discussion for stability:
Table-7 Oscillation Time
uncompensated line. Moreover the amplitude of
oscillations is lower in case of compensated line.
Now it is well proven that TCSC improves stability
of system.
REFERENCE
Seconds
0.3
0.25
0.2
0.15
0.28
0.26
0.24
0.1
0.2
Seconds
0.05
0
Seconds
Uncompensated
146 (with159
TCSC)
(with 170
TCSC)
(with TCSC)
1
2
3
4
0.28
0.26
0.24
0.2
E-ISSN 0976-3945
Fig.10 Stability diagram for transmission line
By comparing the four cases of power oscillations, it
is observed that oscillations damp faster when the
firing angle of thyristor is 170o, taking only 0.2
second and the amplitude of oscillations is
comparatively low. But, when the line is
uncompensated, oscillations damp after 0.28 seconds
and the amplitude of oscillations is high as compared
to compensated line. It is proven that TCSC improves
transient stability of the system.
X. CONCLUSIONS:
This paper
analyzes the effect of TCSC on the
power flow through the buses with resistive and
inductive loads. The simulation results show one of
the salient features of TCSC, i.e., enhancement of
power by operating TCSC in capacitive region power
as it is the important issues of power transmission
system.
Table 8 Comparison of power (MW)
It is observed from the table 8 that without
compensation, the transmission line transfers 685
MW. When this transmission line is series
compensated with fixed capacitor, line transfers 1290
MW. With TCSC operated in capacitive region , the
power transfer capability of line increases and it
becomes 1550 MW which is 2.3 times more (126%)
than the power when line is not compensated
(i.e..685MW). By comparing the four cases of power
oscillations, it is observed that oscillations damp
faster in compensated line as compared to
IJAET/Vol.III/ Issue I/January-March, 2012/175-179
1. N. H. Hingorani, "Flexible AC transmission systems,"IEEE
Spectrum p. 4045, Apr. 1993.
2. N. Cbristl, R. Hcdin, K. Sadck, P. Llitzelberger, p. E.
Krduse, S. M. McKcnna, A. H. Maiitaya, and D. Togerson,
"Advanced series compensiuion (ASC) with thyristor controllcd
impedance," Paper14/37/38-05.
3. “MATLAB Based Simulation of TCSC FACTS Controller”
Preeti Singh, Mrs.Lini Mathew, Prof. S. Chatterji ,N.I.T.T.T.R.
Chandigarh. RIMT-IET, Mandi Gobindgarh. March 29, 2008.
4. “The Impact of FACTS Devices on Digital Multi-functional
Protective Relays” Mojtaba Khederzadeh, 2002.
5. “Identification of Thyristor Controlled Series Capacitor
(TCSC)”Erivelton G. Nepomuceno1, Ricardo H. C. Takahashi1,
Luis A. Aguirre1, Oriane M. Neto2.
6. “Power Quality Enhancement by TCSC Application to
Mitigate the Impact of Transformer Inrush Current”
Mojtaba Khederzadeh, Senior Member, IEEE 2008 IEEE.
7 “Selection of TCSC parameter:Inductor and Capacitor”IEEE
2011, S. Meikandasivam, Rajesh Kumar Nema, and Shailendra
Kumar Jain.
Paper presented
(i) Sunita Tiwari and S.P.Shukla “Implementation of TCSC on
a Transmission Line model to analyze the variation in Power
Transfer capability”, BITCON, National conference,Nov.2008.
(ii) Sunita Tiwari and S.P. Shukla “Thyristor-Controlled Series
Capacitor and its application on Transmission System
to
improve Transient stability” AICON, National conference,
CSIT, Durg, Feb.2009.