Flexible AC Transmission System Controllers: A Review
Arsalan Masood1, Qadeer-ul-Hassan1, Anzar Mahmood*1
1
EE, COMSATS Institute of Information Technology Islamabad
Abstract. Development of power generation and transmission, in last ten years, has been inadequate due to
limited resources while power demand has increased significantly. Consequently existing transmission lines are
used near thermal stability limits under heavy loads and the system stability becomes a power transfer limiting
factor. Substantial expansion of generation as well as transmission system in order to accommodate the increased
demand is restricted by the environmental, political, social and regulatory constraints. In this environment,
Flexible Alternating Current Transmission System (FACTS) controllers open the door towards the advanced
control of power system at least for transmission lines. FACTS technology helps to explore some new
possibilities for flow control and improves the operational capability of existing and new transmission lines. This
paper presents a comprehensive review of major FACTS controllers and of their applications.
1 Introduction:
In recent years voltage stability has become a key matter of interest to operators, especially the power systems that
are heavily loaded and have shortage of reactive power. Voltage instability is a great threat to power system
protection, safety and reliability [1]. The power systems are getting more advanced and complex due to diverse
generation sources and transmission of power from these sources without modifying and adding additional
transmission capability, in some case, forces the system to operating under extremely overstressed situations.
Additionally, it has become difficult to meet the requirement of reactive power and to maintain the bus voltage
within adequate limits [2].
To improve overall efficiency, power system operators are forced to move away from the traditional/conventional
model of centralized generation, transmission and distribution to de-centralized and less regulated operations. This
global trend of deregulations hopes to make power system more efficient and competitive in open market
environment. This basically means that all aspects of power system engineering such as generation, transmission,
distribution and utilization of electric power must now become accustomed to new rules and regulations. In this
study, we will concentrate on the transmission part of the power system and issues related to it.
Due to limited expansion of transmission lines and increased generation issues like heavily loaded lines,
unscheduled power flow and power system stability are becoming more severe. To overcome these issues, new kind
of devices are introduced that can operate and control power flow in the power system quickly and efficiently and at
the same time mitigate the voltage stability issues. These devices are power electronics based and provide:
1. Phase angle control
2. Transmission line voltage control
3. Impedance Control
These challenges are encountered by power industry with the technology of FACTS and their use is preferred in
some studies [3]. Some of these new power electronics based devices can control all three parameters
1
,*Corresponding Author: Anzar Mahmood
Email: anzarmahmood@comsats.edu.pk, anzarmahmood@gmail.com
Web: http://www.njavaid.com/anzar.aspx
Ph: +92-3315079549
simultaneously as compared to the conventional devices that lack speed and controllability of multiple parameters at
the same time [4] [5].
2 POWER SYSTEM CONTROL:
2.1 Generation, Transmission, Distribution
Power system consisting of generation, transmission, distribution and consumption of electrical energy can be
detached into zones shown in Figure 1:
1
2
3
4
Generation
Transmission
Distribution
Distributed Generation
Fig. 1. Block Diagram of Generation, Transmission and Distribution.
These days power electronic based equipment is common in all zones [6], the emphasis of this paper is on
transmission zone, i.e. shifting the power from generation zone to consumption zone.
2.2 Power System Constraints
The power system constraints are many (listed below) and they put a limit over power transfer among areas or
region. The typical constraints are:
1 Thermal
2 Dynamic Voltage and voltage stability
3 Power System Oscillation Damping
4 Steady-State Power Transfer
5 Short Circuit Current and Other limitations
Some of the above constraints also influence the transmission system, hence a requirement for a solution to use with
the transmission lines with highest possible efficiency.
2.3 Power system controllability
To improve the performance of a power system there are three key variables that must be controlled. The three main
variables are:
1 Voltage
2 Angle
3 Impedance
AC network controllers used to improve the performance of a power system can be classified in two categories,
conventional network controller and FACTS controller. Overview of these controllers is shown in Fig. 2.
AC-Network
Controller
Conventional
Controllers
FACTS controllers
Thyristor
Valve
SVC
TCSC
DPFC
VSC
Hybrid
Controllers
STATCOM
STATCOM
(Without energy
storage)
(With energy
storage)
SSSC
Fault current
Limiter
Transformer
R,L,C
Switched Series
Compensation
Switched Shunt
Compensation
UPFC
Transformer LTC
IPFC
Synchronous
Condensor
PST
Fig. 2. Overview of Conventional network controller and FACTS Controllers.
Conventionally, equipment like switched shunt capacitor, series capacitor, phase shifting transformer etc. were used
to control these parameters. Most of conventional devices are just able to control one parameter at a time. With
FACTS controllers comes ability to control one or more parameter at a time. Some FACTS controllers such as
SSSC, UPFC and IPFC are capable of controlling all three parameters simultaneously.
To control voltage, conventionally switched shunt capacitor, Low tap changing transformers and synchronous
condenser were used. For impedance and angle control series capacitor and phase shifting transformers were used
respectively. Table 1 shows some conventional equipment used for enhancing power system control
Table. 1. Conventional Equipment for Enhancing Power System Control
Equipment
Switched-shunt capacitor
Series capacitor
Transformer LTC
Phase shifting transformer (PST)
Synchronous condenser
Impedance control
Voltage control

Angle control




With the development of FACTS controllers one or more parameters can be controlled simultaneously. Table 2
explains what parameter/s each device can control.
Table. 2. FACTS Controllers for Enhancing Power System Control
Equipment
Impedance control
Static synchronous Compensator (STATCOM)
Static Var Compensator (SVC)

Thyristor Controlled Series Compensator (TCSC)

Static Synchronous series Compensator (SSSC)

Unified power flow controller (UPFC)

Interline Power flow controller (IPFC)
Voltage control


Angle control






3 Classification and description OF FACTS
Facts devices as shown in Fig. 2 are classified in the literature as first, second and third generation based on their
functionality as well as technical feature. Another classification of FACTS devices is depending on their connection
to the network. Based on second classification FACTS devices can be differentiated in four categories i.e. series
controllers, shunt controllers, series to series controllers and series to shunt controllers. These two classifications are
independent, as many devices of a group of first classification may belong to the other group of second
classification. In this study we are reviewing devices on their first classification.
3.1 FIRST GENERATION:
First generation devices uses thyristor valve with devices like SCR. some of these devices can exchange
active/reactive power but are not able to generate reactive power and some can generate or absorb reactive power
but can’t exchange reactive power.
3.1.1
Static VAR Compensator (SVC):
This device provides reactive power quickly to HV transmission lines thus enhancing the line performance. The
word “static” indicates that it has no moving part such as circuit breakers. This SVC device was designed for
impedance matching so that power system come closer to unity power factor. If the reactive load of power system is
leading, the SVC will consume VARs mainly using thyristor controlled reactors, however if the load is lagging, the
capacitor banks are switched in automatically offering greater control of system voltage.
3.1.2
Thyristor-Controlled Phase Shifter (TCPS)
In this control method the phase shift angle is observed as a non-linear function of rotor angle and speed. But, when
we talk about electrical power system with more than one alternators, the angle computed of one alternator as
compared with system angle will not be very significant [8].
3.1.3
Thyristor Controlled Series Capacitor (TCSC)
Thyristor-Controlled Reactor (TCR) is used in shunt with capacitor bank in TCSC. The arrangement of linking TCR
with capacitor bank in shunt will permit the control of capacitive reactance over a wide range. Similarly linking
TCSC with transmission line in series will gives the opportunity of controlling the line impedance. TCSC is a first
generation FACTS device which is an economical and effective way of solving the transient stability problem as
well as problems of dynamic, steady state voltage stability in transmission networks [9] [10].
3.2 SECOND GENERATION
Second generation devices can exchange active and reactive power as well as capable of absorbing or generating
these automatically.
3.2.1
Static Compensator (STATCOM)
A STATCOM or static Compensator is a shunt connected device used on AC transmission systems and is a good
alternative of conventional static VAR compensator. It belongs to the second generation of FACTS family and is
based on power electronics voltage source converters (VSC). As it is connected in parallel it is also called shunt
connected controller. The output current of STATCOM can be regulated autonomously without any regard for the
system voltage, independent of the detail that it is inductive or capacitive. Usually it is used to support voltage
regulation and in power networks of reduced power factor. It can provides dynamic stability and active AC power
when connected to source, but most commonly it is used to provide voltage stability in power system [11]. Figure 3
shows circuit diagram of static compensator (STATCOM) without energy storage.
VAC
3-Phase
Shunt
Transformer
Voltage Source
Convertor
VSC
Vdc
DC Capacitor
Fig. 3. Shunt Connected Controller
STATCOM system with energy storage system is shown in the fig 4. As shown in the fig interface provides
coupling of Dc side of the STATCOM and energy storage which can be of any kind like photovoltaic systems or
capacitor banks. STATCOM with energy storage system also provides transient and dynamic stability.
VS
VR
VAC
Shunt
Transformer
VSC
VDC
DC
Capacitor
Interface
Energy Storage System (ESS)
Fig. 4. STATCOM with storage
3.2.2
Static Synchronous Series Compensator (SSSC)
SSSC works similarly like static compensator. The VSC in SSSC is serially connected through transformer to a
transmission network as shown in fig 5. In order to regulate active power flow, SSSC is capable of injecting voltage
in quadrature with sending or receiving line end voltage. For reactive power, it does not absorb reactive power from
the AC system because having a DC capacitor itself forms the reactive power requirement. This makes it capable of
regulating both active and reactive power flow [12] [13] [14]. Furthermore, if we want to just balance or maintain
the reactive power, quite small energy source which provides a continuous voltage could be used. If our aim is of
controlling the phase angle of voltage injected, it is possible only if energy source is big enough.
Vs
I
Series
Transformer
VSC
VDC
DC Capacitor
Fig. 5. Circuit diagram of SSSC
VR
3.2.3
Unified Power Flow Controller (UPFC)
UPFC is one of the very complex and advanced FACTS controllers. It is one of the most adaptable and versatile
FACTS device ever used to enhance the operation of power system [15]. The concept of UPFC was proposed by
Gyugi in 1991.It can control all the parameters such as voltage, phase angle and impedance, individually and
simultaneously. It is a blend of STATCOM and SSSC. Primarily, it is used to control power flow in transmission
line. Secondarily voltage control, transient stability improvement, and oscillation damping can also be done
individually or simultaneously by it in an adaptive fashion [16] [17].
UPFC is based on one dc link which operates two switching inverters as shown in figure 6. Inverter 1 provides or
absorbs the real power accordingly to dc link which will be coupled to transmission line through parallel connected
transformer; after it is converted back to ac. Inverter 2 performs the key function of UPFC, it injects AC voltage
with controllable phase angle and magnitude, which is connected in parallel with transmission line [18] [19]. There
are two terminals due to common dc link. AC terminal, in which inverter 2 generates reactive power and DC
terminal in which real power is exchanged and is converted in to dc power.
Power
Source
Load
T2
T1
Series
Compensator
Shunt
Compensator
VSC1
VSC2
DC
Capacitor
VDC
Control Scheme of
STATCOM and SSSC
Fig. 6. Basic UPFC scheme
3.2.4
Interline Power Flow Controller (IPFC)
The Interline Power Flow Controller, which was initially introduced by Gyugyi in 1998 and used as a solution to the
difficulty of compensating multi transmission lines at a substation. In other words, the IPFC provides many VSCs
attached at the similar DC terminal and all offer series compensation for its individual transmission line. In this
scheme, the power optimization of the whole transmission network can be achieved in the way of suitable power
wheeling via shared DC link from over-loaded power lines to under-loaded power lines. A basic IPFC contains two
VSCs as demonstrated in Fig 7. Each inverter injects series voltage to compensate transmission line and shared DC
link is denoted using a bi-directional link for real power transmission among these two voltage sources [20].
Therefore, power flow control ability of IPFC is the identical to UPFC. The only alteration in IPFC is that the active
power required by inverter 1 is compensated by additional series inverter 2 utilizing additional line in place of shunt
inverter in UPFC [21].
TA
RA
Line A
LA
VRA
VS
VSC
1
CDc
Vdc VSC
2
RB
Line B
LB
VRB
TB
Fig. 7. Interline power flow Controller with Two inverters.
4 FACTS ADVANTAGES, DISADVANTAGES AND APPLICATIONS
To complete this review of FACTS devices, an overview of flexible AC transmission devices applications to
problems of power system is discussed in this part.
4.1 To Ideal Power Flow
In previous few years, researchers established new procedures or algorithms/process of resolving the ideal power
flow problem. FACTS controllers are one of the major inventions of researchers during these years. Different
FACTS controllers are used to improve the control and increase the power transfer capacity. In load flow studies,
the thyristor-controlled FACTS controllers such as TCSC and SVC are modelled as impedance controlled devices in
a transmission system [22] [23] [24] [25]. However, controllers based on VSC such as SSSC and IPFC, shunt
controllers like STATCOM and combination of these two; shunt and VSC based controller, like UPFC are more
complex and are modelled as source controllable devices [14] [22] [26] [27] [28]. The Interline Power Flow
Controller (IPFC) is quite similar but cost effective than UPFC. It is a kind of the VSC based FACTS Controllers
which uses multi-line Transmission System for efficient power flow management.
4.2 To De Centralized Electricity Market
At the present time, electricity demand is increasing rapidly. With no new major projects of enhancing or reinforcing
power transmission system or networks, it is necessary to construct new power transmission systems. However, a
number of factors such as cost, environment and difficulties in obtaining right of way have continuously delayed
such construction. As a result, existing transmission lines are operated on overload conditions. As the electricity
market is decentralizing, creating an atmosphere of competiveness in open market. FACTS controllers can be a
substitute to ease the power flow in overloaded transmission lines, causing improved load ability, low system loss,
improved network stability and less production cost by controlling the power flow. The advancement in supply
industry of electricity is presenting new areas of power system operation associated with de centralized market.
Commercial pressures of getting better results from existing transmission networks indicates a significant role for
management using FACTS devices and energy storage.
4.3 Advantages Of Facts
The practical assistances of FACTS and versatility in resolving problems like transient and dynamic stability, load
flow current and voltage control are explained in Graph and table given below.
3.5
3
2.5
SVC
2
STATCOm
1.5
TCSC
UPFC
1
0.5
0
Load Flow
Voltage Control
Transient Stability
Dynamic Stability
Fig. 8. Practical Advantages of the main FACT devices
Table. 3. Practical Advantages of The Main Facts Devices
Load flow current
Voltage control
SVC
*
***
STATCOM
*
***
TCSC
**
*
UPFC
***
***
*As asterisk increases it represents better results.
Transient stability
*
**
***
**
Dynamic stability
**
**
**
**
The conventional answers of these problems are less expensive in comparison of FACTS devices, but they are not as
versatile as FACTS.
5 CONCULSION
In this paper different FACTS devices or controllers are reviewed, compared and discussed. In previous few years,
experimental installation of FACTS controllers on transmission lines are successfully done to improve voltage
stability and power flow [29] [30] [31] [32]. But, the significant up front cost of FACTS controllers remains high as
the main hindrance to their common use. It is hoped that in future FACTS devices adaption into transmission system
will increase and provide more assistance in controlling the power flow through the transmission lines effectively.
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