Enhancement of voltage stability using static synchronous series compensator

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN
0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME
164
ENHANCEMENT OF VOLTAGE STABILITY USING STATIC
SYNCHRONOUS SERIES COMPENSATOR (SSSC) WITH PI
CONTROLLER-LLLG FAULT
B. Suresh Kumar
Assistant Professor, Department of Electrical Electronics Engineering, CBIT
ABSTRACT
This project presents the enhancement of voltage stability using Static Synchronous Series
Compensator (SSSC). The continuous demand in electric power system network has caused the
system to be heavily loaded leading to voltage instability. Under heavy loaded conditions there may
be insufficient reactive power causing the voltages to drop. This drop may lead to drops in voltage at
various buses. The result would be the occurrence of voltage collapse which leads to total blackout
of the whole system. Flexible AC transmission systems (FACTS) controllers have been mainly used
for solving various power system stability control problems. In this project, a static synchronous
series compensator (SSSC) is used to investigate the effect of this device in controlling active and
reactive powers as well as damping power system oscillations in transient mode. The transient mode
is created by LLLG fault. The PI controller is used to tune the circuit and to provide the zero signal
error. Simulations have been done in MATLAB/Simulink environment.
Keywords: Static Synchronous Series Compensator (SSSC), Proportional-Integral, Real and
Reactive Power Flow, Voltage Stability.
I. INTRODUCTION
In recent years, greater demands have been placed on transmission network and the increase
in demands will rise because of the increasing number of non utility generators and heightened
competition among utilities themselves. Increasing demands, lack of long term planning, and the
need to provide open access electricity market for generating companies and utility customers, all of
them have created tendencies toward less security and reduced quality of supply. The power systems
of today, by and large, are mechanically controlled.
There is widespread use of microelectronics, computers and high speed communications for
control and protection of present transmission systems; however, when operating signals are sent to
the power circuits, where the final power control action is taken, the switching devices are
mechanical and there is little high speed control. Another problem with mechanical devices is that
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN
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ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 4, Issue 5, July – August 2013, pp. 164-175
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165
control cannot be initiated frequently, because these mechanical devices tend to wear out very
quickly compared to static devices. New installations of power stations and other facilities are
primarily determined based on environmental and economic reasons. In addition, new transmission
lines are expensive and take considerable amount of time to construct. Given these conditions, in
order to meet ever-increasing load demands, electric utilities have to rely on power export/import
arrangements through the existing transmission system, deteriorating voltage profiles and system
stability in some cases. This situation has resulted in an increased possibility of transient, oscillatory
and voltage instability, which are now brought into concerns of many utilities especially in planning
and operation [1, 2]. Moreover, the trend of the deregulated power system has led to some
unexpected problems, such as voltage instability, etc.
In effect, from the point of view of both dynamic and steady state operation, the system is
really uncontrolled. Power system planers, operators, and engineers have learned to live with
this limitation by using a variety of ingenious techniques to make the system work effectively, but at
a price of providing greater operating margins and redundancies. These represent an asset that can
be effectively utilized with prudent use of FACTS technology on a selective, a needed basis. i.e.
Proportional Integral Controller, Real and Reactive Power Flow, Voltage Stability. The FACTS
devices (Flexible AC Transmission Systems) could be a means to carry out this function
without the drawbacks of the electromechanical devices such as slowness and wear. FACTS can
improve the stability of network, such as the transient and the small signal stability, and can reduce
the flow of heavily loaded lines and support voltages by controlling their parameters including series
impedance, shunt impedance, current, and voltage and phase angle. Controlling the power flows in
the network leads to reduce the flow of heavily loaded lines, increased system load ability, less
system loss and improved security of the system [3]. The static synchronous series compensator
(SSSC) FACTS controller is used to prove its performance in terms of stability improvement. A
Static Synchronous Series Compensator (SSSC) is a member of FACTS family which is connected
in series with a power system. It consists of a solid state voltage source converter (VSC) which
generates a controllable alternating current voltage at fundamental frequency. When the injected
voltage is kept in quadrature with the line current, it can emulate as inductive or capacitive reactance
so as to influence the power flow through the transmission line. While the primary purpose of a
SSSC is to control power flow in steady state, it can also improve transient stability of a power
system. Here PI controller is used to control the parameters of the power system [4].
II. BASIC OPERATIONAL PRINCIPLE OF SSSC
Fig.1 Functional Model of SSSC

166
Fig.1 shows a functional model of the SSSC where the dc capacitor has been replaced by an
energy storage device such as a high energy battery installation to allow active as well as reactive
power exchanges with the ac system. The SSSC's output voltage magnitude and phase angle can be
varied in a controlled manner to influence power flows in a transmission line. The phase
displacement of the inserted voltage , with respect to the transmission line current , determines
the exchange of real and reactive power with the ac system [5].
In Fig.2, it is assumed that the SSSC losses are zero and, therefore, the series injected
voltage is in perfect quadrature with the line current, leading or lagging. The operating conditions
that limit the SSSC operation from the power system point of view are also depicted inFig.2. The
SSSC can increase as well as decrease the power flow in the transmission line by simply reversing
the operation from capacitive to inductive mode. In the inductive mode, the series injected voltage is
in phase with the voltage drop developed across the line reactance, thus, the series compensation has
the same effect as increasing the line reactance. If the series inserted voltage magnitude is larger than
voltage drop across the uncompensated line, i.e., , the power flow will reverse. This fact can
limit the SSSC operation to values of , as in practice, it would be unlikely to use the SSSC
for power reversal. Also, if the rating of the SSSC controller is high, it is possible to increase or
decrease the receiving end voltage above or below the typical operating voltage range of 0.95p.u.–
1.05p.u, but with possible negative consequences for other system devices.
The SSSC output current corresponds to the transmission line current, which is affected by
power system impedance, loading and voltage profile, as well as by the actions of the SSSC. Thus,
the relationship between the SSSC and the line current is complex. The fundamental component of
the SSSC output voltage magnitude is, on the other hand, directly related to the dc voltage that is
either constant or kept within certain limits, depending on the chosen design and control of the
SSSC. The SSSC output voltage phase angle is correlated to the line current phase angle by plus or
minus few degrees for example, to account for changes in the dc voltage. It has to be noted that the
injected SSSC voltage is different from the SSSC output voltage ,due to the voltage drop or
rise across the series transformer reactance, i.e.,
(1)
Where, the negative sign corresponds to capacitive operation, while the positive sign
corresponds to inductive operation of the SSSC and stands for the series transformer reactance.
This voltage difference between the injected and output SSSC voltage can be small in the case of
small transmission line currents, but it can be significant in high loading conditions [6].
Fig.2 Series Compensation by a SSSC

167
The active and reactive power exchanged between the SSSC and the transmission line can be
calculated as follows
(2)
(3)
Where, represents the angle between the injected SSSC voltage and transmission line
current. Inspection of the equations (2) and (3), considering that the angle between the SSSC output
voltage and line current is approximately 90o
, shows that the SSSC real power should be small
compared to the reactive power. This is expected, since the real power going into the SSSC is used
only to cover for the losses and charging of the dc capacitor, i.e.
(4)
The losses in the SSSC circuit are due to the transformer windings and especially due to the
switching of the GTO valves.
III. CONTROL SYSTEM OF SSSC
SSSC is similar to the variable reactance because the injected voltage and current to the
circuit by this device are changing depend upon to the system conditions and the loads
entering/getting out. For responding to the dynamic and transient changes created in system, SSSC
utilizes the series converter. One side of the converter is connected to the AC system and the other
side is connected to a capacitor and battery which in the system we assume DC source as battery. If a
dynamic change in system will be occurred, SSSC circuit works such that according to the control
circuit, the energy of battery will be converted to the ac form by converter and then injecting this
voltage to the circuit the changes will be damped appropriately. For controlling the powers, first,
sampling from the voltage and current is reactive powers of bus 2 are calculated using their voltage
and current in dq0 references and compared with the determined reference and the produced error
signal is given to the PI controllers. Adjusting parameters of the PI controllers, we are trying to
achieve the zero signal error, such that powers can follow the reference powers precisely. Then, the
output of the controllers are transformed to the abc reference and given to the PWM.
Fig.3 Scheme of the SSSC
Fig.3 shows a single line diagram of a simple transmission line with an inductive
reactance, , connecting a sending end voltage source, Vs, and a receiving end voltage source,
respectively [7].

168
The real ad reactive power (P and Q) how at the receiving end voltage source given by the
expressions
(5)
Q = (6)
Where and are the magnitudes and and are the phase angles of the voltage sources and
respectively. For simplicity, the voltage magnitudes are chosen such those = = V and the
difference between the phase angles is
(7)
A SSSC limited by its voltage and current ratings, is capable of emulating a compensating
reactance (both inductive and capacitive) in series with the transmission line inductive
reactance .Therefore, the expressions for power flow given as
(8)
(9)
Where is the effective reactance of the transmission line between its two ends, including the
emulated variable reactance inserted by the injected voltage source of the Static Synchronous Series
Compensator (SSSC). The compensating reactance is defined to be negative when the SSSC
is operated in an inductive mode and positive when the SSSC is operated in a capacitive mode.
PI controller will eliminate forced oscillations and steady state error resulting in operation of
on-off controller and P controller respectively. However, introducing integral mode has a negative
effect on speed of the response and overall stability of the system.
IV. TWO MACHINE POWER SYSTEM MODEL
The dynamic performance of SSSC is presented by real time voltage and current waveforms.
Using MATLAB software the system shown in Fig.2, has been obtained [3]. In the simulation one
SSSC has been utilized to control the power flow in the 500 kV transmission systems.
Fig.4 Two machine Power System Model
This system which has been made in ring mode consisting of 4 buses (B1 to B4) connected to
each other through three phase transmission lines L1, L2-1, L2-2 and L3 with the length of 280, 150,
150 and 50 km respectively. System has been supplied by two power plants with the phase-to-phase
voltage equal to 13.8 kV. Active and reactive powers injected by power plants 1 and 2 to the power
system are presented in per unit by using base parameters =100 MVA and =500 kV, which active
and reactive powers of power plants 1 and 2 are (24-j3.8) and (15.6-j0.5) in per unit, respectively.

169
Fig.5 Two Machine System with SSSC
The SSSC is connected at bus 2. And the effect of SSSC in this transmission line was
observed. The bus 2 is at the middle of the transmission line so that it was selected as candidate bus
to connect the SSSC.
The SSSC is connected to control the active and reactive powers. And also could fairly
improve the transient oscillations of the system.
The Static Synchronous Series Compensator (SSSC), one of the key FACTS devices, consists
of a voltage-sourced converter and a transformer connected in series with a transmission line. The
SSSC injects a voltage of variable magnitude in quadrature with the line current, thereby emulating
an inductive or capacitive reactance. This emulated variable reactance in series with the line can then
influence the transmitted electric power. In this the SSSC is used to damp power oscillation on a
power grid following a three-phase fault.
The power grid consists of two power generation substations and one major load centre at bus
B3. The first power generation substation (M1) has a rating of 2100 MVA; representing 6 machines
of 350 MVA and the other one (M2) has a rating of 1400 MVA, representing 4 machines of 350
MVA. The load centre of approximately 2200 MW is modeled using a dynamic load model where
the active & reactive power absorbed by the load is a function of the system voltage. The generation
substation M1 is connected to this load by two transmission lines L1 and L2. L1 is 280-km long and
L2 is split in two segments of 150 km in order to simulate a three-phase fault (using a fault breaker)
at the midpoint of the line. The generation substation M2 is also connected to the load by a 50-km
line (L3). When the SSSC is bypass, the power flow towards this major load is as follows: 664 MW
flow on L1 (measured at bus B2), 563 MW flow on L2 (measured at B4) and 990 MW flow on L3
(measured at B3).
The SSSC, located at bus B2, is in series with line L1. It has a rating of 100MVA and is capable of
injecting up to 10% of the nominal system voltage. This SSSC is a phasor model of a typical three-
level PWM SSSC. On the AC side, its total equivalent impedance is 0.16p.u on 100 MVA. This
impedance represents the transformer leakage reactance and the phase reactor of the IGBT bridge of
an actual PWM SSSC.
V. SIMULATION RESULTS WITHOUT SSSC
First, power system with two machine and four buses has been simulated in MATLAB
environment, and then powers and voltages in all buses have been obtained. The results have been
given in Table.1. Using obtained results bus 2 has been selected as candidate bus to which the SSSC
be installed. Therefore,the simulation results has been focused on bus 2.

170
Table.1 Voltage, Current, Active and Reactive Powers at all buses without SSSC
Time (sec)
Fig.6 Active Power of bus 2 wihout SSSC
Time (sec)
Fig.7 Reactive Power of bus 2 without SSSC
Time (sec)
Fig.8 Voltage of bus 2 without SSSC
Activepower
(p.u)
Reactivepower(p.u)Voltage(p.u)

171
Time (sec)
Fig.9 Current of bus 2 without SSSC
VI. SIMULATION RESULTS WITH SSSC
Table.2 Voltage,Curent,Active and Reactive Powers at all buses with SSSC,when SSSC Connected
at bus 2
Time (sec)
Fig.10 Active Power of bus 2 with SSSC
Current(p.u)
Activepower(p.u)

172
Time (sec)
Fig.11 Reactive Power of bus 2 with SSSC
Time (sec)
Fig.12 Voltage of bus 2 with SSSC
Time (sec)
Fig.13 Current of bus 2 with SSSC
Table.3 Comparision between the Valuses of the System with and without SSSC,when SSSC
Connected to bus 2
Reactivepower(p.u)
Current(p.u)
Voltage(p.u)

173
SSSC is controlling the active and reactive powers, beside these could fairly improve the
transient oscillations of system. Obtained results of bus 2 had proven that the stability of power
system parameters has been increased.
When SSSC is connected at bus 4. The results are as follows.
Time (sec)
Fig.14 Active power of bus 4 with SSSC
Time (sec)
Fig.15 Reactive power of bus 4 with SSSC
Time (sec)
Fig.16 Voltage of bus 4 with SSSC
Reactivepower(p.u)
Voltage(p.u)

174
Time (sec)
Fig.17 Current of bus 4 with SSSC
Table.5.4 Comparison between the Values of the System with and without SSSC, when SSSC
Connected at bus 4
CONCLUSION
Accoding to the obtained results of bus 2 had proven that the stability of power system
parameters has been increased and also observed that the transient stability of the system also
increased. It has been found that the SSSC is capable of controlling the flow of power at a desired
point on the transmission line. It is also observed that the SSSC injects a fast changing voltage in
series with the line irrespective of the magnitude and phase of the line current. After installation of
SSSC, besides controlling the power flow in bus 2, the transient oscillations of system also
improved.
When SSSC is connected at bus 4, it is observed that the power system parameters have been
increased at bus 4 also.
According to the obtained simulation results it is observed that SSSC is capable of controlling
the flow of power at a desired point on the transmission line. It is also observed that by installing the
SSSC, active power, reactive powers will be damper faster compare to the mode without SSSC.
REFERENCES
[1] B. H. Lee and K. Y. Lee, "A Study on Voltage Collapse Mechanism in Electric Power
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[3] N. G. Hingorani, L. Gyugyi, “Understanding FACTS: Concepts and Technology of Flexible
AC Transmission Systems”, New York: IEEE Press, 2000.
Current(p.u)

175
[4] M. Faridi, H. Maeiiat, M. Karimi, P. Farhadi and H. Moslesh (2011) “Power System Stability
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[5] Kalyan K. Sen, (1998) “SSSC- Static Synchronous Series Compensator (SSSC): Theory,
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