An Overview of Facts Devices used for Reactive Power Compensation Techniques

An Overview of Facts Devices used for Reactive
Power Compensation Techniques
Aishvarya Narain
M.Tech Research Scholar
Department of Electrical Engineering
Madan Mohan Malviya University of Technology
Gorakhpur, UP, India
S. K. Srivastava
Associate Professor
Department of Electrical Engineering
Madan Mohan Malviya University of Technology
Gorakhpur, UP, India
Abstract— In the last two decades demand of power is
increasing rapidly but we have limited resources of power
generation resulting transmission line getting heavily loaded and
facing stability, voltage sag, reactive power issues. It is necessary
to implement the FACTS devices which are better solution for
power transmission problem. This paper describes about the
study of various reactive power compensation techniques needed
for any power system using FACTS devices such as SSSC, TCR,
TCSC, STATCOM and UPFC. A classification of FACTS
controller is also mentioned here. At the end comparison of
various FACTS devices are done.
Keywords— reactive power compensation, STATCOM, SVC,
fact controllers
I. INTRODUCTION
Today the power system is very complex and
interconnected; we need to improve power utilization to
maintain security and reliability. Some transmission line are
overloaded and some are loaded below the limit by which
voltage profile deteriorate and system stability decreases so
we need to control the power flow in the transmission line for
power transfer. The development of power electronic
technology has introduced Flexible AC transmission system
(FACTS) devices. [12]
The fact devices can overcome limitation of mechanically
controlled transmission system [1].power flow equation for
lossless transmission line is given by
𝑃𝑖𝑗 =
𝑉 𝑖 𝑉 𝑗
𝑋 𝑖𝑗
sin 𝛿𝑖𝑗 (1)
𝑉𝑖 and𝑉𝑗 Denotes the 𝑖 𝑡ℎ
and 𝑗 𝑡ℎ
bus voltage magnitudes, 𝛿
is bus voltage angle and 𝑋𝑖𝑗 Denotes line reactance. From
equation (1) we can say that power is a function of sending
and receiving end voltage, transmission line impedance and
phase angle between voltages. By controlling one of them we
can control active as well as reactive power. FACTS devices,
such as Static Synchronous Series Compensator (SSSC),
Static Var Compensator (SVC), Unified Power Flow
Controller (UPFC) and Static Synchronous Compensator
(STATCOM) can regulate bus voltages, phase angles and line
impedances of power transmission lines. These FACTS
controllers are voltage source converters use for controlling
power flow, enhancing the power transfer capability, reduce
the generation cost, and improve the stability and security of
the power system [2-3]
Electrical power system is always in transition between
equilibrium and steady state because unwanted disturbance
occur at any time. In this paper we concerned with this
dynamic situation of the system which can be stabilize by
Shunt and series capacitors and specially FACTS devices.
Reactive power compensation technique is important factor
for system stability and voltage sag during disturbance. Now
a day’s fact controller like STATCOM, SVC and TCR are
use for reactive power compensation.
II. CONCEPT OF REACTIVE POWER
“Power” refers to the energy related quantities flowing in
the transmission and distribution network. Instantaneously,
power is equal to the multiplication of voltage and current.
When voltage and current are not in phase.There are two
components:
 Real or active power is measured in watt
 Reactive (some times refer to as imaginary) power is
measured in Vars
The sum of active and reactive power is known as apparent
power. It is shown in fig1.
Fig.1: power triangle [7-8]
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
www.ijert.orgIJERTV4IS120071
(This work is licensed under a Creative Commons Attribution 4.0 International License.)
Vol. 4 Issue 12, December-2015
81

In AC circuits, energy is stored in inductor and
capacitor component, which results in the periodic reversal
of the direction of flow of energy between the supply and
therefore the load. Further explaining the reactive power
it can be said that in AC system, when the voltage and
current varies up and down at the same time then only
real power is transmitted and when there is time shift
between voltage and current then active and reactive both
power are transmitted. For average in time calculation
average active power causes the net flow of energy
whereas average reactive power is zero irrespective of the
network or state of the system. The amount of energy
flowing in one direction is equal to the amount of energy
flowing in another direction in case of reactive power so
we can say reactive power neither produce nor consumed.
But in real we measure reactive power losses and to
reduce it we have so many equipments for reactive power
compensation to reduce consumption of electricity and to
reduce cost. Energy store in capacitor in the form of
electric field so it is use to produce reactive power given
by equation (2). In AC circuit capacitive voltage is of
charging nature since capacitor opposes these changes
that cause the lag of voltage behind current in phase.
Instantaneous power in inductive circuit :
p =
Vmax Imax
2
cos θ(1 + cos ωt) +
Vmax Imax
2
sin θ sin 2ωt
……….(2)
P =Instantaneous power
Vmax =Maximum value of the voltage waveform
Imax =Maximum value of the current waveform
ω = angular frequency
=2πf
f=Frequency of the waveform
θ=angle by which the current lags the voltage in phase
t=time period
III. NEED OF REACTIVE POWER COMPENSATION
Reactive power generated by A.C source is stored in
capacitor or a reactor during a one fourth of a cycle and in
the next one fourth of the cycle its send back to the power
source. Therefore reactive power oscillates with twice
frequency of rated value (50 or 60 Hz) between the ac
source and capacitor or reactor. So we need reactive
power compensation to avoid this circulation between the
load and source.
The main reason in a system for reactive power
compensation is:
 voltage regulation
 to increase stability of the system
 to improve system power factor
 to increase utilization of machines and equipments
connected to the system
 to reduce losses of the system
 to prevent voltage sag and voltage collapse
Transmission line impedance and in generating system
most machines need lagging VAR results consumption of
reactive power which causes instability in the system and
transmission lines.
Fig.2: Reactive power compensation (supplied by capacitors)[14]
Fig.2 shows that reactive power compensation not only
reduces losses, regulate voltage, and increase system
stability but also helps in better transient response in case
of fault and disturbances. In recent times focus on the
compensation techniques increase rapidly and now
including better devices in this techniques compensation
is more effective. Reactive power should be supply near
the load or generators for economical purpose.
IV. FACTS DEVICES
Now a day as power demand increases the power
system is getting more complex to fulfill the requirements
within the acceptable quality and costs [4]. Generating
stations are located away from the load centres because of
economic and environmental issues. As the demand of
power increases uncertainty occur in the system operation
resulting reduction in stability and risk of blackouts. This
problem can be tackle by introducing high power
electronics controller in ac transmission networks. So
“flexible” operation in ac transmission network comes in
to role where changes can be done easily without
affecting the systems. FACTS (Flexible Alternating
Current Transmission System) are a family of power
electronics device which improves stability, power
transfer capability and controllability of ac system [5].
FACTS devices are combination of components power
system (like transformers, reactors, switches, and
capacitors) with power electronics components (like
various types of transistors and thyristors).we are capable
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to deal high power application (ten, hundreds and
thousands of MW) with the help of high current rating of
thyristors FACTS controller also improve the transient
and dynamic performance of the power system. Electronic
based switches use in FACTS device have less switching
time than the conventional mechanical switches. FACTS
devices using electronic based switches are more flexible
and fast reacting causes many advantages like
enhancement of transmission capacity, control of power
flow, improvement of transient stability, voltage stability
and control. With using of appropriate type and rating of
FACT devices transmission capacity enhance up to 40 –
50 %.
V. FACTS CONTROLLERS USED IN REACTIVE
POWER COMPENSATION TECQNICH
FACTS Controllers can be divided into four categories:[16]
• Series Controllers
• Shunt Controllers
• Combined series-series Controllers
• Combined series-shunt Controllers
Series controllers:
It may be capacitor or reactor or power electronic based
variable source. These controllers inject voltages in series
with the line. When voltage and current are in 90 degree
phase shift controller only supply or consume variable
reactive power. For any other phase real power also
considered. [15]
 Static Synchronous Series Compensator (SSSC)
 Interline Power Flow Controller (IPFC)
 Thyristor Controlled Series Capacitor (TCSC)
 Thyristor Switched Series Capacitor (TSSC)
 Thyristor Controlled Series Reactor (TCSR)
 Thyristor Switched Series Reactor (TSSR)
Among these fact devices TCSC is most commonly use
for reactive power compensation which is explains below.
Thyristor controlled series compensator (TCSC)
Kinney et al. proposed the concept of TCSC in 1994 in
[10]. TCSC is combination of TCR in parallel with FC
(fixed capacitor). SVC and TCR are shunt connected
controller where as TCSC is series connected controller.
That is why TCSC is always shown in single phase form
rather than three phase form. it has one or more sub
module. TCSC is used when increase damping is require
for large interconnecting system because it provide
variable capacitive reactance. To avoid sub synchronous
resonance it changes apparent impedance for sub
synchronous frequency. TCSC act as a fast active power
flow regulator as it changes the electrical length of
transmission line with approx no delay. The circuit
diagram is shown in Fig.3
Fig.3 TCSC circuit diagram
Shunt Controllers:
Like series controller shunt controller also may be
capacitor or reactor or power electronic based variable source
or a combination of these. Shunt controller inject current in
the system. When voltage and current are in 90 degree phase
shift controller only supply or consume variable reactive
power. for any other phase real power also considered.
 Static Synchronous Compensator (STATCOM)
 Static Synchronous Generator (SSG)
 Battery Energy Storage System (BESS)
 Superconducting Magnetic Energy Storage (SMES)
 Static Var Compensator (SVC):
 Thyristor Controlled Reactor (TCR)
 Thyristor Switched Reactor (TSR)
 Thyristor Switched Capacitor (TSC)
 Static Var Generator or Absorber (SVG)
 Thyristor Controlled Braking Resistor (TCBR)
STATCOM, SVC and TCR are more commonly use for
commercial and industrial purpose. These devices are one
by one explain below. [11]
Thyristor controlled reactor (TCR)
In 1982, Miller et al. Proposed Thyristor controlled reactor
[6]. It is shown in the figure it is a combination of two
antiparallel thyristor. Both thyristor conducts for the alternate
half cycle. It acts as a controllable susceptance. Inductance L
is important sub device of TCR. TCR is also fundamental
component of TCSC and SVC. It is also a shunt compensator.
For lightly loaded transmission line it is use for limiting
voltage rise. Circuit diagram is shown in Fig.4
Fig.4: circuit diagram of TCR
ISSN: 2278-0181
83

Static VAR Compensator (SVC)
Static VAR compensator used for high voltage power
system. There are many advantages using SVC like it
improve system stability, reduce losses, maintain the line
voltage variation within limits and better utilization of
equipments.
It consists of shunt capacitors and shunt reactors.
Shunt reactor and TCRs is to prevent voltage rise under
low load and no load condition. Static capacitor and TSCs
(Thyristor switch capacitor) are to prevent voltage sag for
peak load. There are two combination uses in practise
first one is TCR parallel with fixed capacitor (FC) and
another one is TSC in parallel with TCR. If compare with
TCR it is better than TCR as TCR only generate reactive
power but SVC not only generate but also absorb reactive
power. Fig 5 shows the structure of SVC.
Fig.5:Static Var Compensator
Static compensator (STATCOM)
The concept of STATCOM is proposed in [9].
STATCOM consist a voltage source controller and
shunt connected transformer. it is a voltage source
converter that dc power into ac power of variable
phase angle and magnitude. It supply desire reactive
power by varying the phase angle and magnitude.
For industrial application unity power factor can be
obtain by using it. The basic structure of STATCOM
is shown in Fig.6
Fig.6: Basic Structure Of The STATCOM [13]
Combined series-series Controllers
It is a combination of series controller. For multiline
transmission system this controller control in coordinated
manner. It can be unified controller because it separately
compensates the power line. It is also called “Interline Power
Flow Controller” because it balances both real and reactive
power. The term unified means dc terminal of controller are
connected together for transfer of real power.
Combined series-shunt Controllers
It is a combination of series and shunt controller
which control in a coordinated manner or it may be
unified power flow controller. It injects current into
system with shunt part of it and voltage with series part of
the controller. When it works as unified real power
exchange occurs via power link.
Example of combine series and shunt controller
 Unified Power Flow Controller (UPFC)
 Thyristor Controlled Phase Shifting Transformer
(TCPST)
 Interphase Power Controller (IPC)
VI. COMPARISON OF VARIOUS FACTS DEVICES
Table1. Shows the comparison among SVC, STATCOM,
UPFC, TCSC and SSSC on the basis of their load flow,
voltage control ,transient stability and dynamic stability in the
form of low, medium, and high.
TABLE I. COMPARISON OF VARIOUS FACTS DEVICES
S NO
FACTS
Device
Load flow
Voltage
control
Transient
stability
Dynamic
stability
1 SVC LOW HIGH LOW MEDIUM
2 STATCOM LOW HIGH MEDIUM MEDIUM
3 UPFC HIGH HIGH MEDIUM MEDIUM
4 TCSC MEDIUM LOW HIGH MEDIUM
5 SSSC LOW HIGH MEDIUM MEDIUM
On comparison it is found that UPFC used for higher
load flow and voltage control where as STATCOM is used
for voltage control in small distribution system and the UPFC
shows better results for power system stability improvement
compared to the other FACTS devices such as SVC, TCSC,
and SSSC.
VI. CONCLUSION
In this paper we have studied about the need of reactive
power compensation and the various FACTS devices used for
compensation. The working principle of STATCOM, TCR,
SVC, and TCSC has been described. Finally the comparison
of these fact devices is done. From the comparison it is found
that UPFC is better for voltage control and lad flow but for
low level application STATCOM is also shows better results.
From this it can be say that FACT controller will play a very
vital role for reactive power compensation in electrical power
system.
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REFERENCES
[1] Pranesh Rao and M. L. Crow, “STATCOM Control for Power System
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[3] B.M. Zhang and Q.F. Ding, “The Development of FACTS and
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system
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[7] T. Gönen, Electric Power Distribution System. New York:
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[10] G.Sundar , S.RamaReddy,“Digital Simulation of D-Statcom for voltage
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[11] Zhiping Yang, Chen Shen, Mariesa L. Crow, Lingli Zhang, An
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[12] L. Dong, M.L. Crow, Z. Yang, C.Shen, L.Zhang, A Reconfigurable
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[13] C. L. Wadhwa, Electrical Power Systems, New Age International
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[14] Hadi Saadat, Power System Analysis,WCB McGraw Hill, 1999.
[15] Narain Hingorani & L. Gyugi, Understanding FACTS, Concepts and
Technology of Flexible AC Transmission Systems, IEEE Press, 2000.
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