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7/23/2019 Review Paper on FACTS Devices
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A Review Paper on Various FACTS Devices For Enhancing Power System
Stability
Bakhtiar Khan, Sajid ul Haq, Abdul Wahab, Ghulam Hafeez, Hanan Ahmad, Muhammad Waqar, Sajid ul Haq,
Department of Electrical Engineering
Comsats Institite of Information Technology, Islamabad.
Abstract AC power control has been major problem since its invention. With increasing gap in demand and supply and
economic constraint, transmission lines are loaded to their neck. FACTS controller have been in use for power flow control
for some time but recently it has also being in use to enhance power system stability. This paper reviews on research and
development (R&D) on stability enhancement of power system using FACTS devices. In addition with review FACTS devices
installation and impact on power quality is summarized.
Keywords FACTS, UPFC, SVC, STATCOM, transient stability, TCSC, SSSC
1. Introduction
Flexible AC transmission systems (FACTS) technology is the application of power electronics in transmissionsystems. Flexible AC transmission systems (FACTS) controllers have been mainly used for solving various
power system steady state control problems and could be employed to enhance power system stability in
addition to their main function of power flow control. Power quality problems such as voltage regulation, power
flow control, low power factor, shortage of reactive power, poor voltage, voltage and current harmonics due to
sudden change in field excitation of synchronous alternator, sudden increased in load, sudden fault occur in thesystem and transfer capability enhancement are solved by FACTS controller.
The main purpose of this technology is to control and regulate the electric variables (voltage, phase angle and
transmission line impedance) and such effectively mitigate voltage sag in the power systems and to enhance the
power system stability. Facts Devices are also used for improving operation, control, planning & protection
from different performance point of view such as increasing the load ability, improve the voltage profile,
minimize the active power losses, increased the available power transfer capacity, enhance the transient and
steady-state stability, and flexible operations of power systems.
There are two generations for realization of power electronics-based FACTS controllers: the first generationemploys conventional thyristor-switched capacitors and reactors, and quadrature tap-changing transformers, the
second generation employs gate turn-off (GTO) thyristor-switched converters as voltage source converters
(VSCs).The first generation has resulted in the Static Var Compensator (SVC), the Thyristor- Controlled Series
Capacitor(TCSC), and the Thyristor-Controlled Phase Shifter (TCPS) . The second generation has produced the
Static Synchronous Compensator (STATCOM), the Static Synchronous Series Compensator (SSSC), the
Unified Power Flow Controller (UPFC), and the Interline Power Flow Controller (IPFC). The two groups of
FACTS controllers have distinctly different operating and performance characteristics. The thyristor-controlled
group employs capacitor and reactor banks with fast solid-state switches in traditional shunt or series circuit
arrangements. The thyristor switches control the on and off periods of the fixed capacitor and reactor banks and
thereby realize a variable reactive impedance. Except for losses, they cannot exchange real power with thesystem. The voltage source converter (VSC) type FACTS controller group employs self-commutated DC to AC
converters, using GTO thyristors, which can internally generate capacitive and inductive reactive power form
transmission line compensation, without the use of capacitor or reactor banks. The converter with energy storagedevice can also exchange real power with the system, in addition to the independently controllable reactive
power. The VSC can be used uniformly to control transmission line voltage, impedance, and angle by providingreactive shunt compensation, series compensation, and phase shifting, or to control directly the real and reactive
power flow in the line. The devices which are discussed in this paper are as follows.
1. Unified Power Flow Controller (UPFC)1
2. Stativ Var Compenator (SVC)2
3. Static Synchronous Compensator (STATCOM)3
4. Thyristor Controlled Series Capacitor (TCSC)4
5. Static Synchronous Series Compensator (SSSC)5
1 Studied and discussed by Bakhtiar Khan2 Studied and discussed by Sajid ul Haq3 Studied and discussed by Abdul Wahab and Ghulam Hafeez (including Introduction to the topic of review paper)4 Studied and discussed by Hanan Ahmad5 Studies and discussed by Muhammad Waqar
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2.
FACTS Devices
2.1 Unified Power Flow Converter (UPFC)
2.1.1 Introduction
Years ago, eletric power systems were relatively simple and self sufficeint in their design. The transmission of
power to distant locations was rare and the systems were designed with generous stability margins with respectto the power needs of that time. Moreover, it was a general view that the transients are of so short time and are
so fast that it couldn’t be controlled and the main focus was given to the steady state conditions. Since, the
control of power flow through transmissin lines require the control of following three transmission line
parameters.
1. Magintude of sending and receiving end voltage
2. Phase angle between the receving and sending end voltages
3. Transmission line impedace
Therefore, to control power flow, optimise system impedance and minimise voltage variation in steady state
conditions, mechanically-switched series and shunt compensators were employed in the design of transmissionline along with voltage regulating and phase shifting transformers tap-changers.
However, recent years have seen much more demand of electric power as well as the power system has become
more complex and deregulation in every industry including power industry has made the power system
vulnerable to much more other threats that were not observable years ago. The generation facilities and new
transmission lines are also not constructed with the same pace. Consequently, it put an urge to explore ways to
obtain better operating flexibility and utilization of existing power systems.
Recent advancement in the field of power electronics has already made significant impact on AC transmission
system via the increasing use of thyistor-controlled static Var Compensators (SVCs). However, SVCs only
control the magnitude of the bus voltage on the selected terminals of transmission line while for complete
control on the power flow through transmission line, impedance and phase angle are also required to be under
control. Similarly, thyristor-controlled tap-changing phase shift transformers are also in use for control over phase shift and thyristor-controlled series compensators are employed for impedance control. But all these
different techniques combined, to give a complete control over the power flow through the transmission line, is
rather custom-designed, bulky in size and are of substancial cost with significant labour installation. Therefore,
they are not considered as an economic, flexible and volume-based solution for the objective of flexible AC
transmission systems.
2.1.2 UPFC: An overview
Unified Power Flow Controller (UPFC) is the most versatile and robust device among the FACTS devices. It is
able to control all the three parameters of power flow i.e. voltage of the bus, reactance of transmission line and
phase angle between two buses, either simultaneously or independently. These parameters are controlled by
controlling shunt compensation, in-phase voltage and quadrature voltage.
The UPFC is represented by the block diagram as shown in Figure 1. It is comprised up of two voltage source
converters (VSCs) coupled through a common DC link. The first VSC (Converter 1) is connected to the line
through a shunt coupling transformer whose primary winding is connected from line to ground and secondary
winding is conected with the converter 1 and the second VSC (Converter 2) is connected in series with the line
through a series coupling transformer whose primary winding is connected in series with the transmission line
and secondary winding is connected with converter 2. Both the converters are supplied DC voltage from a
capacitor bank connected as a common DC link between the two converters.
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Figure 1 Basic Circuit of UPFC
2.1.3 Operating Principle of UPFC
The basic operation of UPFC for power flow control is such that a voltage V pq is injected in series with the
transmission line by controlling the series converter (converter 2). This series injected voltage can be varied
from 0 to V pqmax. Moreover, this series converter can also vary the phase angle of the phasor V pq independently
from 0o to 360o. In this way, the series converter exhanges both the real and reactive power with the
transmission line. In the process, the series converter generates/absorbs the reactive power internally while the
real power is generated/absorbed by the dc energy storage device i.e. the capacitor.
The main purpose of the shunt connected Converter (Converter 1) to supply the real power demand of Converter
2. Converter 1 obtains this real power from the transmission line and maintains the voltage of the dc bus
constant. Thus, the losses of the two converters and that of the coupling transformers makes the net real power
drawn from the ac system. In addition to all this, the shunt converter also works like a STATCOM and it hasthe abillity to regulate the terminal voltage of the interconnected bus independently, by generating/absorbing
requisite amount of reactive power.
2.1.4 Literature Review
All the three parameters of transmission line can be controlled with UPFC. It was proved experimently that the
parameters for power flow control i-e voltage, phase angle and transmission line impedance can be controlledeither individually or in appropriate combinations at the series-connected output of UPFC while maintaining
reactive power support at its shunt-connected input [1]. To enhance the damping of power system, the
mechanism of the three control methods of a UPFC was also investigated [2]and it was shown that using simple
propotional feedback of rotor angle deviation of machine, transient swing can be reduced significantly. High
frequency power fluctuations induced by a UPFC were also investigated [3].
Various techniques and methodologies are used in literaure to study the steady-state and transient state response
of UPFC. A small-signal linearized dyanmic model, a steady-state model a state-space large-signal model of a
UPFC was developed assuming the power system to be symmetrical and operating under three-phase balanced
conditions [4]. Two other UPFC models were also developed that were incorporated in the Phillips – Heffron
model after linearizing [5], [6]. It was noticed that their are chances of negative interaction between the voltagecontrol of DC link capacitor of UPFC and PSSs in the power system, unless UPFC is equipped with a proper
damping controller [6]. Another effort in modeling UPFC was made by injecting current to improve dynamicresponse of power system [7]. In this model, a shunt current source and a series voltage source was used to
represent the equivalent circuit of UPFC. The property of symmetry of Ybus matrix was featured in the
presented model.
It was a general view that by adding a supplementary controller to the UPFC, power system damping can be
enhanced to a greater extent. Therefore, various control schemes were introduced for the purpose of damping
the oscillations. To damp the interarea mode of oscillations, an attempt was made to design a lead-lag controller
having conventional fixed-parameter for the UPFC installed in the tie-line of a two-area system [8]. With the
use of H ∞ control, much more robust control schemes were also presented [9], [10]. A PI controller having
multiple input and multipel output was also proposed [11]. It has been illustrated that if more than one UPFC
controller, such as a power flow controller, an AC voltage controller, and a DC voltage controller, weredesigned separately, the dynamic interactions among the various control channels may have a detrimental effect
on the system stability. A nonlinear control strategy for phase angle of the series branch of a UPFC and linearcontrol strategies for the other channels have been hybridized for stability enhancement of a multimachine
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system [12]. The adverse effect of DC voltage regulator on the damping characteristics of UPFC has been
addressed in [13]. In addition, different control channels of the UPFC have been evaluated using a
controllability index. Different intelligent damping controllers were also developed for a UPFC to damp both
local and interarea modes of oscillation for a multimachine system. The effectiveness of such controllers has
been demonstrated and reported with satisfactory success. [14] – [17]
2.1.5
Mathematical Modeling
The equivalent electrical model for UPFC in steady state is shown in figure 2 [18]. This model is developed for
a transmission line of 300 Km where the UPFC is placed just at the center of the transmission line. This model
was derived to study the characteristics of transmission line and UPFC in steady state and to derive a
mathematical model for the steady state case.
Figure 2 Electrical Equivalent of UPFC
According to [19], UPFC can be represented as voltage sources representing the fundamental component of the
two converters while impedances as the leakage impedances of the two coupling transformers. Based on theoperating principle of UPFC and using some knowledge of network theory, the real and reactive power flow
between bus-i and bus-j can be written as [20]
2 2( ) 2 cos( ) [ cos( ) sin( )] ( cos sin )ij i T ij i j ij T j j T ij T j ij T j i j ij ij ij ij P V V g V V g V V g b V V g b ( 1 )
2 ( / 2) [ sin( ) cos( )] ( sin cos )ij i T ij i T ij T j ij T i i j ij ij ij ijQ V I V b B V V g b V V g b ( 2 )
Where1
ij ij
ij ij
g jbr jx
andq
I is the reactive current flowing into the shunt transformer to improve the shunt
connected bus of the UPFC.
Similarly, the real and reactive power flows from bus-j to bus-i in the transmission line with UPFC installed on
it is written as
2 [ cos( ) sin( )] ( cos sin ) ji j ij j T ij T j ij T j i j ij ij ij ij P V g V V g b V V g b ( 3 )
2 ( / 2) [ sin( ) cos( )] ( sin cos ) ji j ij j T ij T j ij T j i j ij ij ij ijQ V b B V V g b V V g b ( 4 )
The real and reactive power injections at bus-i with system loading ( ) can be written as
0 (1 )b
i Gi Di ij
j N
P P P P
( 5 )
0 (1 )b
i Gi Di ij
j N
Q Q Q Q
( 6 )
Where 0
Di P and 0
DiQ are the initial real and reactive power demands. Gi P and GiQ are the real and reactive power
generations on the bus-i respectively. b N is the number of system buses and is the sensitivity of system
loading. In equation (5), uniform loading with the same power factor at all the load buses has been considered
and the increase in the loading is assumed to be taken care by the slack bus whereas any sharing of generationamongst the generators can be easily incorporated in this model.
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2.1.6 Conclusion
In this study, the most versatile member of FACTS family, UPFC, was briefly discussed. The study includes the
working principle of UPFC, literature review and a mathematical model for steady state. From the literature
review, it is deduced that UPFC is very efficient and robust in enhancing power system transient and steady
state stability. It was noticed in the introductory section that complete control over the flow of power through a
transmission line is possible only if the the voltage magnitude, phase angle and transmission line impedance areunder control simultaneously and later on, it was observed that UPFC is able to control all these parameters
independently or in possible combinations. Therefore, UPFC is a very favourite option for power system
engineers to control power flow over designated transmission line and to enhance power system stability. To
summarize, transient stability is improved and faster steady state is achieved when UPFC is used at an
approprate location in the transmission lines.
2.2 Static Var Compensator (SVC)
2.2.1 Introduction
Static var Compensator (SVC) is a flexible AC transmission system (FACTS) controller. SVC is the latest
technology in power electronics switching devices.[21] It mostly used in the transmission of electric power
system to control the voltage and power flow of the system. it also regulate the voltage level of transmission
line. Both capacitive and inductive shunt reactive power sources is controlled by SVC and improve the overall
system efficient. SVC is made from the following components showen in figure 3 [22].
Figure 3 Basic model of SVC
1. Coupling transformer2. Thyristor valves
3. Reactors
4. Capacitors
The voltages is regulated by SVC at its terminals by controlling the amount of reactive power injected into orabsorbed from the power system [23].The SVC generates reactive power (SVC capacitive) When system
voltage is low. It absorbs reactive power (SVC inductive) When system voltage is high. Static VAR
Compensator is a shunt connected FACTS controller, and enhance stability dynamic and transient disturbances
in power systems. When a three phase fault occur in power system.[24]the damping of power system
oscillations is analyzed with the analyzation of the effects of SVC on transient stability performance of a power
system. SVC is installed in midpoint power system and also may be installed at the end of line.
2.2.2 Literature review
For the purpose of review paper from literature we carried out two most important and database which is named by two different categories called the IEEE/IEE electronic library and Science Direct electronic databases. This
survey is stared from1990 tan complete in 2004.the overall survey period is divided into three stages. 1990 –
1994, 1995 – 1999, and 2000 – 2004 [25]. The number of publications discussing FACTS applications to different
power system studies has been recorded. The results of the survey are shown in Figure 4. It is clear that the
applications of FACTS to different power system studies have been drastically increased in last five years. This
observation is more pronounced with the second generation devices as the interest is almost tripled. This shows
more interest for the VSC-based FACTS applications. The results also show a decreasing interest in TCPS whilethe interest in SVC and TCSC slightly increase. Generally, both generations of FACTS have been applied to
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different areas in power system studies including optimal power flow, economic power dispatch, voltage
stability, power system security, and power quality.
Figure 4 Classification of power system stability
2.2.3 Types of SVCs
There are two basic types of SVCs, each having a different combination of the Components.
1) SVC of the TCR-FC type
2) SVC of the TCR-TSC type.
2.2.3.1 SVC of the TCR-FC type
The SVC of the TCR-FC type consists of a TCR, which absorbs reactive power from the ac power system to
which the SVC is connected, and several FCs, which supply reactive power to the system connected to theSVC[26]. The simplified single-wire circuit diagram of an SVC of the TCR-FC type is illustrated in Figure 5.
Figure 5 Simplified single-wire circuit diagram of an SVC of the TCR-FC type.
2.2.3.2 SVC of the TCR-TSC type
The SVC of the TCR-TSC type consists of a TCR, which absorbs reactive power from the ac power system
connected to the SVC, and several TSCs, which supply reactive power to the ac power system connected to the
SVC. The simplified single-wire circuit diagram of an SVC of the TCR-TSC type is illustrated in Figure 6.
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Figure 6 Simplified single-wire circuit diagram of an SVC of the TCR-TSC type.
2.2.4 Methodology
2.2.4.1
Development of Transient Stability Analysis
When sudden change in disturbance is occur in power system, the transient stability analysis investigate the
stability of the power system. The transient stability analysis having two combine solution to solve this problem
one is algebraic equations which is performed by numerical solution and other one is differential equations
solution [27]. Although significant improvements have been made in the application of numerical andcomputational methods to the transient stability calculation, the computational demands are rising rapidly at the
same time. Therefore there is a continual search for faster and accurate solutions to the transient stability
problem.
2.2.4.2 With and without SVC Transient Stability Evaluation
The following steps are involves for algorithm of the transient stability studies with FACTS (SVC) devices
1. Read the data for lines, transformers and shunt capacitors.
2. Make matrix of admittance ,YBUS
3. Show and write generator data (Ra,XdXq, H, D etc).
4. From the load flow results, write steady state bus data. ([V], [δ], [Pload], [Qload
], [Pgen], [Qgen
]).
5. Calculates and write the number of steps for different conditions of fault such as fault existing time, line
outage time before auto-reclosing, simulation time etc
6. Modify,YBUS admittance matrix by adding the load and generator admittances.
For generator admittance matrix of bus ‗i‘
Yii = Yli + Ygi
Ygi =
1
Rgi + jXdi
For load admittance matrix bus ‗i‘
Yii = Yli + YLi
Where
YLi =PLi + jQ
Li
|V2|
7. Calculate fault impedance and also show bus impedance with modify from.
8. Initial conditions should be calculated and constants needed in solving the DAEs of generators, AVR etc. is
also known.
9. Calculate the network equation iteratively in each time step of the problem.
10. For Xd − Xq models calculates Vd − Vq using the obtained voltages and rotor angles.
11. Calculates electric power outputs of the generator.
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12. The time step is advanced by the current time step.
13. Keeping generator mechanical power output as constant and Solves the generator swing equations using by
trapezoidal rule of integration
14. Solves the AVR equations
15. Solves the SVC. The bus current injection vector is modified with SVC injection currents. Then network
equation is again solved using[28] [YBUS] [V] = [ Iinj ].
2.2.5 Different purpose of SVC:
The unsymmetrical load is balance by SVC and to support the railway voltage in the case of a feeder station trip
when two sections have to be fed from one station which is primary working. The second purpose of the SVCs
is to maintaining unity power factor during normal operation [29]. Thirdly, the SVCs alleviate harmonic
pollution by filtering the harmonics from the traction load.
2.2.6 V-I characteristics of SVC
Figure 7 VI characteristics of SVS
2.2.7 Design of SVC by Adaptive Control
The SVC, which comprises a fixed capacitor and a thyristor-controlled inductor, which improve generator
damping without deterioration in the voltage profile under severe disturbance conditions. We obtained betterdynamic performance, a supplementary proportional-integral (PI) controller, of which the parameters are find by
eigenvalue assignment, is incorporated in the static VAR compensator. By further improving the damping
characteristics over a wide range of operating conditions [30], an adaptive controller using model reference
adaptive control (MRAC) is also developed.
2.2.8 Conclusion
Instabilities in power system are created due to long length of, interconnected grid, transmission lines, changing
system loads and line faults in the system. These instabilities causes in reduced line flows or even line trip. SVC
FACTS controller stabilize transmission systems with increased transfer capability and minimize risk of line
trips. Financial benefit from SVC FACTS controller comes from the additional sales due to increasedtransmission capability, excesses wheeling charges due to increased transmission capability and due to delay in
investment of high voltage transmission lines or even provide new power generation facilities [31]. Also, in a
deregulated market, the improved stability in a power system substantially reduces the risk for forced outages,
thus reducing risks of lost revenue and penalties from power contracts.
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2.3
Thyristor Controlled Series Capacitor (TCSC)
There has been a considerable increase in electrical power generation harnessing wind energy. To ensure bulktransfer of power smoothly, the large electrical stations generating power would be connected to the power
system using series compensation. The subsynchronous resonance issues arise due to induction generator effect(
IG) and the possible torsional interactions ( IR). A thyristor controlled series capacitor (TCSC) which in the
first place is used to enhance the capability of the power transferred through the transmission line also mitigates
the sub synchronous resonance issues greatly. The TCSC helps to damp out the sub synchronous
oscillations[32].
Due to limited resources and some environmental issues some of the transmission lines are heavily loaded and
as a consequence the transfer of power is limited through the line to ensure the system stability. Using a
thyristor controlled series capacitor (TCSC) in a power system enhances the power carrying capacity and the
control of a transmission line by changing the impedance of a transmission line. Moreover the application of a
TCSC reduces the oscillations of active power, enhances voltage transient and increases dynamic stability and
overcome the issue of SCR issue ( Sub synchronous issue). Before the application of a TCSC in a power system
it is necessary to carry out a thorough power flow analysis of a power system and carrying out a thorough
analysis of TCSC [33].
The compensation employing fixed series compensation is an economically possible way of improving power
carrying capacity of a transmission line. The issues associated with a fixed capacitance can be avoided by
employing a TCSC which consists of a series capacitor connected with a parallel thyristor controlled reactor
(TCR). The firing angle of a thyristor is changed to change the voltage across the series capacitor and thus in
this way the capacitance is varied . Using a line current as a reference, a TCR offers a different reactance of a
TCSC for a specific firing angle as compared with a voltage referencing of a capacitor [34].
A TCSC is a variable capacitor that is connected in series with a line. The insertion response test of a TCSC
reveals that when it is switched in a vernier mode into a transmission line then then switching transients are
damped out quickly. When TCSC is simply connected as a capacitor then voltage transient has a fairly large
component of oscillation that damps out slowly. Vernier firing helps to achieve steady state with in few cycles
after insertion and the oscillations are also less severe. Thus the control of a TCSC is quite effective and deals
with many types of faults [35].
TCSC mitigates the SSR ( sub synchronous reactance ) issue by changing the series capacitance continuously.
In other words TCSC has a variable capacitance. The thyristor controlled series capacitor at sub synchronousfrequency would reveal characteristics impedance of an inductor provided that middle of firing angles or
conduction angles lie at zero crossing of the voltage across the capacitor. The relationship developed between
the voltage called as sub synchronous voltage across the TCSC and the sub synchronous current flowing
through it reveals that if the firing angles of the valves of thyistors is symmetrical with respect to the zero
crossing of the capacitor voltage then TCSC doesnot lead to SSR issue. If the magnitude of the subsynchronous
frequency is relatively small as compared to the system frequency then the magnitude of the voltage across thecapacitor i.e subsynchronous voltage is approximately equal to zero. The application of TCSC is safe to use. But
if the magnitude of the subsynchronous frequency is comparable to the system frequency then the role of
subsynchronous voltage appearing across the TCSCR cannot be neglected any further. Hence the use of TCSC
requires a careful consideration [36].
FACTS devices are employed in a power system to use our transmission infrastructure effectively. A TCSC
offers a flexible control of the power flowing through the lines. It enhances the power transfer capability and
improves the stability of our system. There are two approaches for synchronization of TCSC. One way is to
synchronize a TCS I,e firing angles or conduction angles of the valves of a thyristor with respect to the zeros of
the capacitor voltage of a TCSCR. And the second method employs the synchronization with zero of the line
current [37].
In case of voltage synchronization the conduction angle varies from 90 to 180 degree and in case of current
synchronization the firing angles increases from 90 to 270. For step change in conduction angles the transient
response is faster with current synchronization as compared with voltage synchronization of TCSC. Irrespective
of which method of synchronization of TCSC was used, experiments indicates that instability occurs under each
scheme of synchronization.
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TCSC is connected in series with a line. It consists of a series capacitor shunted by a reactor. The use of TCSC
is more effective as compared with fixed capacitance. The use of thyristors provides more flexibility whena
TCSC is connected in series with a line and TCSC helps to counterbalance the inductance of a transmission line.
Voltage stability is increased and the voltage dips in a power system are reduced to a great extent. The
application of a TCSC in a power system is of significant importance in operation and control of power system.
Besides that TCSC reduces the fault current and improving the dynamic and transient stability of a power
system to a great system. It enhances the power flow through the transmission lines. The values of inductanceand the capacitance of a TCSC should be carefully chosen in order to avoid negative effects on the power
system. Before actually applying a TSCS in a system it is necessary to study the steady state behavior of aTCSC. The mathematical model can be established by employing the firing angles or conduction angles of a
thyristor to enhance the power flow through the transmission lines. The desired value of the parameter in order
to determine the operating performance of a TCSC “w” should lie in between 2.2 and 2.7 [38].
A thyristor controlled series capacitor has a number of benefits to offer. The TCSC alters the reactance of a
transmission line. Inshort it changes the apparent impedence of line. In this way it effects and changes the power
flow through a network. If the parameters of TCSC are set up correctly then the performance of the system is
better with TCSC than it was before the installation of TCSC in power system. The effects of TCSC on the
reliability of power system can be investigated by making a model. The model can be simplified using statespace approach. In order to determine system reliability when a TCSC is connected in power system it is
necessary to operate the TCSC in various operational conditions. Some of the possible conditions includedifferent locations of installation of a TCSC and different thermal limits of the transmission line. The difference
between the TCSC in working conditions and in failure mode indicates that ideal model of TCSC can be used to
determine the reliability of a power system provided that TCSC is reliable enough also [39].
The problems associated with long distribution can be resolved effectively using a variable series capacitance i.e
a TCSC. A TCSC can be operated as current limiter in case of short circuit. The installation of TCSC would notallow the voltage sag on the lines where the sensitive loads are connected to a nearby substation in case of
occurrence faults . EMTDC ( electromagnetic transients for DC) package can be used to stimulate the benefits
of a connection of a TCSC. So the capacity to carry power by a transmission line is comparably increased,
power swings can be damped out and TCSC could be used a fault current limiter. The voltage dips due to
fluctuating loads can be lowered considerably by changing the series capacitance. When TCSC is operated in
an inductive region it limits the fault current and the voltage can be kept constant at the loads. The high power
quality issues could be oversome to some extent by using TCSC in power system [40].
A thyristor controlled series capacitor can be used for closed loop control of power flow in a constant power and
constant angle modes. The transient stability and small signal characteristics of a power system are influenced
by the controller of power flow. The constant angle mode has negative impacts on the first swing stability and
damping of the system. On the other hand the constant power mode has positive impacts on the damping of
system and first swing stability [41].
With the integration of renewable energy and market deregulation the electromechanical stability of power
system is challenged. A thyristor controlled series capacitor can be used to control the stability of a system
during large disturbances during the time of transients. The TCSC can be used to dynamically control the
impedance of a line. In short the TCSC can provide the dynamic series compensation to overcome the transients
in a power system. This remedial action in turn increases the security and stability of a power system [42].
The region known as signal stability region van be greatly enhanced by employing FACTS devices in our power
system. Even on the lower portion of the PV curve the system tends to remain stable. Using a TCSC to keep the
voltage stable i.e constant at the loads the ratio of load time constant to time constant of a thyristor controlled
series capacitor does not has a significant impact on voltage stability region of the power system [43].
The thyristor controlled series capacitor can be used to develop an inductive reactance with in subsynchronous
frequency. The TCSC is able to enhance the transmission power capacity and at the same time reduce the fault
levels [44].
Using angle stability enhancement like TCSC can increase the stability of the voltage in case of occurrence of
faults [45].
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2.4 Static synchronous Compensator:
STATCOM is a shunt type FACTS device which is used in power system primarily for the purpose of voltage
and reactive power control. In [46], a current source converter (CSC) based static synchronous compensator
(STATCOM) which is a shunt flexible AC transmission system (FACTS) device; impact is described for small
and large transient stability in an interconnected power system.
That paper investigates the impact of a noveland robust pole-shifting controller for CSC-STATCOM to improve the transient stability of the multi machine
power system. The proposed algorithm utilizes CSC based STATCOM to supply reactive power to the test
system to maintain the transient stability in the event of severe contingency. Below the CSC based STATCOM
representation is shown.
Figure 8 STATCOM
In emerging electric power systems, enlarged communication often lead to the situations where the structure nolonger remains in secure operating region. In [47], the author presents an application of fuzzy control to
determine the control signal of static compensator (STATCOM) for improvement of power system stabilityA
fuzzy logic based supplementary controller for Static Compensator (STATCOM) is developed which is used for
damping the rotor angle oscillations and to improve the transient stability of the power system. Generator speedand the electrical power are chosen as input signals for the fuzzy logic controller (FLC). A Standard 3-phase, six
bus system is taken as test system to evaluate the FACTS device (STATCOM) performance for proposed
controllers PI and fuzzy with Power System Stabilizer in multi machine System.
Damping of low frequency electromechanical oscillations is very important for a safe system operation. To
solve this problem, [48] presents a novel methodology for tuning STATCOM based damping controller in order
to enhance the damping of system low frequency oscillations. The design of STATCOM parameters are
considered an optimization problem according to the time domain-based objective function solved by a Honey
Bee Mating Optimization (HBMO) algorithm that has a strong ability to find the most optimistic results.
In [49], paper presents seeker optimization algorithm (SOA) to design the parameters of PSS and STATCOMcoordinately to improve more stability of power system. The SOA is used to achieve the best optimal results to
minimize the objective function of the optimization problem. Simulations are carried out on a two-area Kundur
and 39-bus New England power systems, and simulation results confirm the efficiency of the proposed method
to stabilize power system oscillations.
In [50], paper the issues related with distance protection of transmission lines with STATCOM connected atmid-point are addressed used to improve the power transfer capability of transmission lines. Use of FACTS
devices in transmission lines affects protection system very drastically. To mitigates the problems associated
With distance protection, a novel algorithm based on synchronized measurement for adaptive Relay setting is
proposed.
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Owing to the constantly increased electricity demands and transactions, power systems are becoming more
vulnerable to voltage instability generally incurred by over-utilized transmission facilities or any contingency. In
order for transmission networks to accommodate more power transfers with less expansion cost, proper
installation of Flexible AC Transaction Systems (FACTS) is a promising way. The power system loading
margin enhancement problem to determine an optimal Static Synchronous Compensator (STATCOM)
installation scheme can be formulated as a mixed discrete – continuous nonlinear optimization problem (MDCP).
In [51], to improve the efficiency of solving the MDCP, an ordinal optimization (OO) STATCOM installationstrategy is proposed to seek a good enough solution rather than the Optimal solution.
With increase in electric power demand, transmission lines were forced to operate close to its full load and due
to the drastic change in weather conditions, thermal limit is increasing and the system is operating with less
security margin. To meet the increased power demand, a doubly fed induction generator (DFIG) based wind
generation system is a better alternative. For improving power flow capability and increasing security
STATCOM can be adopted. As per modern grid rules, DFIG needs to operate without losing synchronism
called low voltage ride through (LVRT) during severe grid faults. A STATCOM is coordinated to the system forobtaining much better stability and enhanced operation during grid fault. In [52], author mitigates voltage and
limits surge currents to enhance the operation of DFIG during symmetrical and asymmetrical faults. The system
performance with different types of faults like single line to ground, double line to ground and triple line to
ground was applied and compared without and with a STATCOM occurring at the point of common coupling
with fault resistance of a very small value at 0.001 Ω.
In [53], author presents singular value decomposition (SVD)-based approach to assess and measure the
controllability of the poorly damped electromechanical modes by STATCOM different control channels. Powersystem stability enhancement via STATCOM-based stabilizers is thoroughly investigated in this paper. The
coordination among the proposed damping stabilizers and the STATCOM internal ac and dc voltage controllers
has been taken into consideration. The design problem of STATCOM-based stabilizers is formulated as an
optimization problem. For coordination purposes, a time domain-based multi objective junction to improve the
system stability as well as ac and dc voltage regulation is proposed.
In [54], the optimal location of a static synchronous compensator (STATCOM) and its coordinated design with
power system stabilizers (PSSs) for power system stability improvement are presented in this paper. First, thelocation of STATCOM to improve transient stability is formulated as an optimization problem and particle
swarm optimization (PSO) is employed to search for its optimal location. Then, coordinated design problem ofSTATCOM-based controller with multiple PSS is formulated as an optimization problem and optimal controller
parameters are obtained using PSO. A two-area test system is used to show the effectiveness of the proposed
approach for determining the optimal location and controller parameters for power system stability improvement
.In [55], author proposes and validates models to accurately represent static synchronous shunt compensators
(STATCOM) in voltage and angle stability studies of powers systems. The proposed STATCOM stability
models are justified based on the basic operational characteristics of this flexible AC transmission system
(FACTS) controller for both phase and PWM control strategies. These models are first validated by means of
EMTP simulations on a test system, and then are implemented into two different programs used to study voltage
and angle stability issues in the system.
In [56], author deals with the stability problem at the inverter end of a HVDC link with STATCOM (Static
Compensator), when connected to a weak AC system which has the stability enhancement for power instabilityand commutation failures. The HVDC stability problem is tackled with a STATCOM which not only provides a
rapid recovery from power, harmonic stability and commutation failures but also offers a lower cost filter design
for the HVDC system.
The electric power infrastructure that has served huge loads for so long is rapidly running up against many
limitations. Out of many challenges it is to operate the power system in secure manner so that the operation con-
straints are fulfilled under both normal and contingent conditions. Smart grid technology offers valuable
techniques that can be deployed within the very near future or which are already deployed nowadays. Flexible
AC Transmission Systems (FACTS) devices have been introduced to solve various power system problems. In
[57], author presents a technique for determining the proper rating/size of FACTS devices, namely the StaticSynchronous Compensator (STATCOM), while considering contingency cases. The paper also verifies that the
weakest bus determined by eigenvalue and Eigen- vectors method is the best location for STATCOM. The
rating of STATCOM is specified according to the require reactive power needed to improve voltage stability
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under normal and contingency cases. Two case system studies are investigated: a simple 5-bus system and the
IEEE 14-bus system.
The [58], presents the design of a non-linear controller to prevent an electric power system losing synchronism
after a large sudden fault and to achieve good post fault voltage level. By Direct Feedback Linearization (DFL)
technique robust non-linear excitation controller is designed which will achieve stability enhancement and
voltage regulation of power system. By utilizing this technique, there is a possibility of selecting various controlloops for a particular application problem. This method plays an important role in control system and power
engineering problem where all relevant variables cannot be directly measured.
Wind power injection into an electric grid affects the power quality due to the fluctuation nature of the wind and
the comparatively new types of its generators. The power arising out of the wind turbine when connected to a
grid system generates power quality problems. The concerned power quality measurements are: active power,
reactive power, voltage sag, voltage swell, flicker, harmonics, and electrical behavior of switching
operation. For the control of these problems a FACTS device STATIC COMPENSATOR (STATCOM) isconnected at a point of common coupling [59]. The STATCOM will reduce the harmonics in the grid current
by injecting superior reactive power in to the grid. This is because of reactive power drops off STA TCOM is
linear with voltage. Here a bang-bang control scheme has been implemented with the STATCOM to achieve
fast dynamic response for the reduction of harmonics in grid current. Bang-bang controller is simple and
reliable. A STATCOM can improve power system performance in such areas as the following: thedynamic voltage control in transmission and distribution systems, the power-oscillation damping in powertransmission systems, the transient stability, the voltage flicker control and the control of not only reactive
power but also (if needed) active power in the connected line.
Main factor causing voltage instability is the inability of the power system to maintain a proper balance of
reactive power and voltage control. The driving force for the voltage instability is the load. Using the shunt
compensating devices, the reactive power balance of the power system can be maintained. In [60], author
compares the performance of shunt capacitor, SVC and STATCOM in the improvement of static voltage
stability. Issues related to shunt compensation, namely rating of the compensating device and its location are
also considered.
Figure 9 Equivalent Electrical Model
In [61], investigations are carried out to explore the impact of a midpoint static synchronous compensator
(STATCOM) on the coordination between the generator distance phase backup protection (function 21) and the
generator capability curves. The results of these investigations have shown that the midpoint STATCOM has an
adverse effect on such coordination Such an impact varies according to the fault type, the fault location, and the
generator loading. This paper proposes the use of the support vector machines’ classification technique for
generator phase backup protection to enhance the coordination between such protection and generator capability
curves in the presence of a midpoint STATCOM.
In [62], presents a new technique for improving the fault ride through (FRT) capability of self-excited induction
Generator (SEIG)-based wind parks by implementing fault current limiters (FCLs) using the electromagnetic
transient program simulation program (PSCAD/EMTDC). A non-inductive high-temperature superconducting
coil of an FCL is developed comprising its major components, operation control algorithm, sequence of events
and fault detection techniques. The test system is adopted with an integrated 80 MW SEIG-based wind park that
comprises a static synchronous compensator (STATCOM) at each wind turbine. A novel damping voltagecontrol algorithm for the STATCOM is presented for improving the FRT and damping the power system
oscillation. FCLs are installed in series with the high voltage (HV) side of the substation transformers of the
wind park. The operation of the FCLs is tested in the proposed system to demonstrate its superior performance
for reducing the high fault currents and improving the FRT capability.
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2.5 Static Synchronous Series Compensator:
2.5.1 Introduction
Initially power system is controlled by mechanical system like switchgears [63]. Mechanical system is slow as
switchgear takes very large time as compared power electronic based circuits. In order to control power system
efficiently, flexible AC transmission line (FACTS) are devised so that flow of power can be controlled. If
transmission system is loaded at their near full capability, generating reserve can be decreased. By decreasing
reserves, capital investment made for these reserves can be decreased.
2.5.2 Overview
Two generation of FACTS base controller have been existed. Fixed capacitors and inductors or thyristor
controlled capacitors and inductors form the first batch of FACTS controller. Static Var Compensator (SVC),
Thyristor Controlled Series Capacitor (TCSC) and Thyristor Controlled Phase Shifter(TCPS). The thyristor
switch converter also known as voltage source converter form the second generation. Static Synchronous Series
Compensator in addition with Static Synchronous Compensator (STATCOM), Unified Power Controller(UFPC) and Interline Power Flow Converter (IPFC) form the second generation of FACTS controller. Both first
and second generation devices are different in their operation, but are used for power flow control [64].
Usage of power electronic based converter for reactive power shunt compensation is being studied for long time.
In 1989, Synchronous Voltage Source for series reactive power compensation is proposed [65].
Power flow depends directly upon sending and receiving end voltages, the angle difference between them and
inversely proportional to impedance between them. Inductance and capacitance is added in order to increase or
decrease the reactance as per given circumstances [66]. Conventionally reactance is controlled by capacitors and
inductors. Now power electronic based solid state devices are proposed. One of them is Static Synchronous
Series Compensator (SSSC), produces nearly sinusoidal voltage in series of transmission line. Voltages injected
by SSSC are nearly in quadrature with line current. A part of voltage in phase with series current is used
compensate for losses by compensator. Voltage in quadrature of line current depicts inductive and capacitive
nature of compensator in series with line. When voltage leads line current, compensator behaves capacitive
nature and when voltage lags line current, compensator behaves inductive nature of reactance.
2.5.3 SSSC Installation
Different controllers are in commercial use for power flow control especially STATCOM but SSSC is only intesting stage. Convertible Static Compensator (CSC) is commissioned at Marcy 345 kv substation of New York
Power Authority. Although it is configured in STATCOM, it has been tested in SSSC configuration. In SSSC
configuration it was designed to produce constant voltage in series with line current [67].
Two 160 Mvar United Power Flow Converter (UPFC) is also installed by American Electric Power in
Kentucky. One of them is also operated as SSSC, while other one as STATCOM [68].
2.5.4 Power System Stability Enhancement
Kumkratug and Haque propose control strategy to increase stability region in power system using SSSC.Comparison is made between this nonlinear Control and Linear Control by others. Results are drawn to show
increase in power system stability [69].
Mihalic studied the mathematical model of SSSC and compared it with controllable series compensation (CSC).
Results from time based detailed simulation confirmed the superiority of SSSC based on theoretical
assumption[70].
SSSC is applied on IEEE bus systems for power flow analysis in steady state. In power flow analysis one of the
following parameters can be used for steady state control: (1) real power; (2) reactive power; (3) bus voltage;
and (4) impedance [71].
Frequency oscillation in inter-area mode is one of basic dynamic problem. SSSC is extended its use to damp the
frequency oscillation in interconnected power system [72].
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2.5.5 Voltage Enhancement
Static voltage stability margin is studied is assessed using STATCOM, TCSC and SSSC respectively and their
results are compared [73].
Heavy loaded system leads to voltage instability. In loaded system power demand increases which consequently
drop the voltage on bus. This drop may be cascaded to other busses. Cascaded voltage drop causes black out ofwhole system. Kumar and Easwarlal studied effect of SSSC in controlling reactive power hence voltage drop in
system. In addition to reactive power, damping on system is also studied [74].
2.5.6 FACTS and Market
With each passing day demand in electrical energy is increasing. Industrialization and commercialization of power system has broken the monopoly of single utility provider. Laws have been passed to deregulate the
system and make market open. Utilities are in ever struggle to make the best from existing system without
investing extra penny to generation. FACTS provided itself for better managed transmission and distribution
system. Kazemi and Andami have discussed extensively impacts of FACTS devices on competitive power
market and their benefits [75].
2.5.7
Stability of RES
Renewable energy sources comprise of solar parks, wind farms and tidal based electric generation. Renewable
energy sources produce green energy but has great impact on stability of power system.Offshore wind farm fed
synchronous generator using SSSC is studied. Logic was proposed for damping synchronous generator. After
performing different disturbances it was evaluated that SSSC provide damping in offshore wind farm fed
synchronous generator [76].
2.5.8 Power Quality Improvement using FACtS
Power quality problems are voltage sags, voltage swell, flicker and harmonics. FACTS devices can also be used
to improve power quality by removing harmonics. Harmonics are one of the core problems of power quality.
Current Harmonics are produced non-linear load [77]. Current harmonics when come in contact with system
impedance form voltage harmonics. Both current and voltage harmonics cause problem in power system. Normally SVR is used for power quality improvement but has limitation because of low voltage. In this studySTATCOM is used at point of common coupling (PCC). By injecting var in line STATCOM reduces harmonics
in system. Renewable energy sources especially wind power generator affects power quality due to natural
constraint depend upon air speed. Power quality of wind turbine using STATCOM is measured and results
confirmed betterment of power quality.
Facts devices have been in use on high voltage side but customer is on lower side. DVR injects voltage in series
with a distribution feeder in order to increase the level of power quality. Results are analyzed using
DSTATCOMS for protection of voltage sags [78].
2.5.9 Conclusion
In this review power system stability enhancement is extensively studied. Salient features of SSSC and impacton power system is addressed. Utility experience is also summarized and power quality enhancement is also
discussed.
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