Energy 89 (2015) 576e592

Contents lists available at ScienceDirect

Energy
journal homepage: www.elsevier.com/locate/energy

Design and simulation of a different innovation controller-based UPFC

(unified power flow controller) for the enhancement of power quality
M.R. Qader
University of Bahrain, Electrical and Electronics Engineering Department, P. O. Box 32038, Bahrain

a r t i c l e i n f o a b s t r a c t

Article history: The UPFC (unified power flow controller) is one of the modern power electronics devices that can be
Received 2 December 2014 used for the control of real and reactive power in a transmission line. The UPFC uses VSC (voltage sourced
Received in revised form converter) technology to inject a series voltage with the sending end ac source to achieve its control
5 May 2015
objective with high speed, making it suitable for maintaining the voltage and mechanical stability of a
Accepted 5 June 2015
network. There are frequent disturbances in a power system due to its dynamic nature. These distur-
Available online 19 July 2015
bances must be controlled so that they cannot lead the system to an unsteady condition. Recently
developed FACTS (flexible alternating current transmission system) provide steadfast solutions to avert
Keywords:
FACTS
these issues in power quality. Due to the improvements in these solutions, some critical issues have been
STATCOM come to sight pertaining to power quality, dependability and permanence. The most effective and po-
UPFC tential technologies among recently developed FACTS devices are STATCOM (static synchronous
Voltage sags compensator) and UPFC (unified power flow controller) that can significantly enhance the operations of
power systems and associated power quality problems. In order to control entire flow of load and voltage
sags/flickers; while eliminating harmonics simultaneously, this paper presents an inventive systematic
approach on the basis of optimal control and tracking with a PI (proportional integral) controller, the
desired steady state behavior, and a linear quadratic tracker. Moreover, a MATLAB/Simulink model is also
established in the paper for the UPFC in the environment of Simulink, once its principles are analyzed.
After monitoring the simulation results, it was concluded that UPFC based controller systems can effi-
ciently manage the load flow and voltage sags/flickers. Test results using different power system models
are presented throughout the thesis to illustrate the effectiveness of Unified Power Flow Controller.
© 2015 Elsevier Ltd. All rights reserved.

1. Introduction networks have caused instabilities and decreased the dependability

of the power supply, system fluxes, and power flow, and safe-
Disturbances in power quality is derived from multiple factors keeping issues have caused many blackouts in various places in the
including arcing devices, sensitive equipment, damage related to world. These issues and concerns are mainly caused by systematic
environment, large motor starting, power electronic devices, errors in planning and operation, excess load on the network, frail
embedded generation, network design and equipment's, etc. interconnections on the power system, or maintenance de-
Developing the energy quality (power quality) is very significant ficiencies. The solution to these issues to grant the preferred power
especially when the production routes become complex and flow with system stability and dependability is to install and set up
require accountability for the purpose of providing energy devoid new transmission lines.
of interruption and for harmonic deformation with tension regu- Nevertheless, installing new transmission lines has obstacles,
lation amid thin boundaries. Thus, several developing countries such as environmental problems and financial cost. Because of
having greater interconnected networks are sharing generation these concerns, power engineers try to find a method so that the
reserves for the purpose of relying more on the power system. On power flow can be increased using current transmission lines
the other hand, the growing complications on large interconnected devoid of any decline in stability and security of the system. There
are various definitions and meanings for the word power quality.
First of all, at the generator, the ability to produce power specifically
E-mail address: mredi@uob.edu.bh.

http://dx.doi.org/10.1016/j.energy.2015.06.012
0360-5442/© 2015 Elsevier Ltd. All rights reserved.
 M.R. Qader / Energy 89 (2015) 576e592 577

at 50 and 60 Hz provided a few amount of variation is defined as 2. Literature review

power quality. While at the distribution and transmission level,
power quality is the outstanding voltage contained by a range of Although a considerable amount of research has been done in
plus or minus five percent. According to Ref. [33] in “Electric Power the field of FACTS, very little literature exists with specific reference
Quality”, it can be defined as the measure, evaluation, and to UPFC. This is because UPFC is a relatively new FACTS device and
enhancement of bus voltage, generally a load bus voltage, in order power system problems associated with it have not been investi-
to preserve that voltage to be sinusoidal at rated voltage and fre- gated thoroughly. UPFC has the flexibility to incorporate any
quency [32]. Moreover, power quality can be determined not only operation functionality. For example, as explained, UPFC can be
by the supplier but also by the user's consumption of electricity. As made to operate as a SSSC (static synchronous series compensator)
electrical and electronic equipment such as computers and speed or a phase shifter based on the strategy used. Different control
drives are more developed and advanced, a decrease in power strategies for UPFC and their control systems for power flow control
quality will occur, which will cause the industrial and commercial have been discussed.
consumer to waste time and money [32,33].
Consequently, the control strategy that has been espoused for
these devices and the extent of input signals so as to damp power 2.1. Review
oscillations in an effectual, professional and vigorous manner is a
vital question here. In order to accomplish improved utilization of Given the integrated nature of the research, the relevant liter-
the transient constancy and damping of the power system by UPFC ature review has been divided into two sections. Accordingly, a
devices; discontinuous control or speed-based BBC (BangeBang section on review of control strategy and control systems for UPFC
Control) is adopted [23]. Lately, it has been understood that the BBC and a section on load flow and dynamic models for UPFC have been
(BangeBang Control) has various drawbacks; hence, it cannot be presented here.
the full area of deceleration when balancing the area of accelera-
tion; thus, this control method makes the most of the area of 2.1.1. Review on control strategy and control systems for UPFC
deceleration required to counterbalance the area of acceleration Very little work has been published in the area of UPFC control
subsequent to greater disturbances, thus providing a smaller sta- strategy for power flow control and control system design to ach-
bility limit [23]. Furthermore, the transient stability limit can be ieve the control strategy. Three different types of strategies for real
greatly enhanced with a mixture of control and discontinuous and reactive power flow control have been found in the literature
control techniques that reference [24] has shown lately and proven. and are described below.
First, in discontinuous mode, the overshoot of subsequent peaks
and settling times can be decreased; therefore, the devices based
on FACTS (flexible alternating current transmission system) tech- 2.1.2. SSSC (static synchronous series compensator strategy)
nology are mostly based on the concepts of power electronics; and This strategy is based on injecting the series voltage in quad-
have other stagnant controllers to improve the factor of controlla- rature with the transmission line current allowing it to function
bility, amplify the power transfer and offer control of a single or similar to that of a variable series capacitor. This fixes the phase
additional parameters of AC transmission system as stated by the angle of the series injected voltage to be in quadrature with the
standards and definition of IEEE [25]. transmission line current. By varying the magnitude of the series
The roving controller performance is determined by choosing injected voltage that is in quadrature with the transmission line
the correct and appropriate value of the controller gain. However, current, the real power flow can be controlled [13]. The reactive
in continuous mode, the damping torque given is relative to the power flow/transmission line side voltage is controlled by adjusting
controller gain. The proportional controller is normally used [26]. In the phase angle of the series injected voltage. This has been ach-
addition, for the power system dynamic stability to tune the PSS ieved by introducing a component of the series injected voltage to
(power system stabilizer) parameters, a diversity of methods have be in-phase with the transmission line current [19].
been suggested, such as the damping torque concept, pole place- Combining the quadrature component and the in-phase
ment, different optimizations, artificial intelligence techniques, and component, the magnitude and phase angle of the series injected
variable structure. The potential of these algorithms for ideal de- voltage are obtained.
signs of PSS has been proven by extremely good results, which were Concentrating on simultaneous control of real and reactive po-
obtained due to the heuristic methods for PSS tuning [26]. Thus, the wer flow/line side voltage using the above described strategy,
utilization of PSO (particle swarm optimization) technique has control systems based on linear control techniques have been used
been encouraged in order to improve the gain of controller. [19]. The control system design based on this strategy requires a
There has been extensive research work done on the UPFC supplementary controller to damp out the real power flow oscil-
including several research articles published recently; these articles lations when controlling the transmission line side voltage simul-
discussed the modeling of UPFC, its analysis, applications and taneously using a high gain PI controller [14,16]. The design of
control methods. Moreover, to study the steady state features coordination feedback between the series and the shunt inverter
through state space computations, devoid of taking into account control systems has not been considered in the control system
the potential impacts of the converters, and the dynamics of the design [17,18]. The need for coordination controller comes from the
generator; several mathematical models of UPFC are also devel- fact that the real power demand of the series inverter has to be
oped [6,7]. Hence, this paper provides a comprehensive presenta- supplied by the shunt inverter. If there is no coordination between
tion of UPFC model in practical circumstances; while the paper also the series and the shunt inverter operation, the DC link capacitor
discusses the control strategy and transient model of the UPFC. The voltage could collapse leading to the removal of the UPFC from the
control system presented in the paper is able to control the voltage power system. The strategy also has the problem that if the in-
flickers/sags; while eliminating the harmonics at the same time. phase injected voltage is out of action, the line side voltage could
The paper additionally presents a supplementary control system in be very high causing reactive power flow problems. Further the
order to balance the line current. The potential features and effi- problem of deterioration of the control system performance at
ciency of the control strategy being presented in the paper is operating points other than the one at which it is designed is a
demonstrated by the simulation results. point to be considered.
578 M.R. Qader / Energy 89 (2015) 576e592

2.2. PS (phase shifter strategy) control inputs to the series inverter. Further the problem of dete-
rioration of the control system performance at operating points
This strategy is based on injecting the series voltage in quad- other than the one at which it is designed is a point to be consid-
rature with the UPFC buses (the bus voltage to which the shunt ered. The shunt inverter control system is also based on the DeQ
inverter is connected) [15]. By doing so, the phase angle of the axis strategy and controls the shunt reactive power and the shunt
transmission line side bus can be adjusted for a specified real power inverter real power. The control of DC link capacitor voltage which
flow. The reactive power flow is controlled by having a component is very essential for the proper operation o f the UPFC, is done by
of the series injected voltage to be in-phase with the UPFC bus another control loop that adjusts the shunt inverter real power
[5,25]. This is similar to that of a tap-changer strategy. This allows reference. This further complicates the control system. Further,
the phase angle of the series injected voltage to vary from its they have neglected the dynamics of the DC link capacitor while
quadrature position, thereby changing the reactive power flow/line designing their control system. By doing so, the control system
side voltage. Complete control system design for real and reactive design may not provide the best PI control gains.
power flow/line side voltage control that uses the above strategy A control system based on DeQ axis theory has been published
has not been well documented. in the literature by Round et al. [29]. The strategy that has been
Though the individual effect of quadrature series voltage in- used is that the D-axis voltage component controls the trans-
jection, in-phase series voltage injection and shunt compensation mission line reactive power and the Q-axis voltage component of
on transient stability have been studied, the effect of combined the series injected voltage controls the transmission line real po-
operation has not been researched [15]. The effect of combined wer. This is in contrast with the strategy used by Papic et al. [21]
operation on transient stability has been later studied by Limi- where the transmission line real power flow was controlled by
ycheron et al. [20]. Here, three control inputs, namely the series the D-axis voltage and the transmission line reactive power flow
quadrature injected voltage, in-phase series injected voltage and was controlled by the Q-axis voltage. In this case, the UPFC is
shunt compensation has been coordinated to improve transient assumed to be located at the receiving end. Based on the receiving
stability. To achieve this coordination, fuzzy logic has been used. end real, reactive powers and receiving end DeQ axis voltages,
The model chosen for UPFC to show the effect of coordination on current references of the series inverter are generated. Two PI
transient stability is not an accurate one. The shunt inverter has controllers are used to generate the required DeQ axis control
been modeled as a variable shunt capacitor in parallel with a cur- voltages for the series inverter to obtain desired real and reactive
rent source. The variable shunt capacitor represents the shunt power flow in the transmission line. For the shunt inverter, based
inverter compensation capability and the parallel current source on the sending end real power, reactive power references and
representing the real power capability to charge/discharge the DC sending end DeQ axis voltages, the sending end DeQ axis current
link capacitor. By doing so, they have neglected the model of the references are then generated.
shunt inverter transformer and assumed that the real and reactive Knowledge of the sending and receiving end current references
power flow through the shunt transformer are separated. Further, are used to generate the current references for the shunt inverter.
their coordination strategy has only been carried out on single By doing so, the shunt reactive power and the DC link capacitor
machine infinite bus power system. Further, no coordination exists voltage are controlled. Remote end signal measurement is required
between the shunt and series inverter control system in terms of for this type of control system to operate. This would necessitate
real power exchange between the series and shunt inverters thus remote sensing units to be installed at the sending end. Further,
casting serious doubts about the validity of such a coordination coordination between the series and the shunt inverter control
scheme. system has not been considered by the authors [29].
Other types of control systems have been designed based on the
2.3. DeQ axis control strategy above control strategies which have neglected the DC capacitor
voltage control systems [32].
In this strategy, the DeQ axis current in the transmission line is Control systems for the shunt inverter have been designed
individually controlled allowing for independent control of real and based on LQ (linear-quadratic) control, but have not shown as to
reactive power flow [26e29]. The DeQ axis could be with respect to how it can be applied to the series inverter control system [31]. The
UPFC bus voltage or the remote end bus voltage. In this strategy, the problem with LQ control is that it requires the measurement of all
series injected voltage is split into two components. One is in-phase states used to design the controller [31].
with the D-axis and the other in-phase with the Q-axis. In all the above strategies discussed for UPFC, the series inverter
Similarly, the transmission line current is split into D and Q axis controls the real power flow in a transmission line by an output
currents. The D-axis voltage controls the transmission line real feedback control system. The problem in the design of an output
power by varying the D-axis current in the transmission line and feedback PI (proportional-integral) control system for UPFC is the
the Q-axis voltage controls the transmission line reactive power by presence of low margin of stability associated with the series
varying the Q-axis current in the transmission line. Thus the in- inductance of the transmission line [14]. Intelligent controllers with
phase series injected voltage component (D-axis) that controls specific reference to fuzzy controllers have been investigated in this
the transmission line real power flow varies the line side voltage thesis to overcome the problem. Further, the above control strate-
and the Q-axis component of the series injected voltage that con- gies suffer either in their complexity of the control system or non-
trols the reactive power varies the phase angle of the UPFC bus. To inclusion of real power coordination controller between the series
achieve this type of strategy, the control system employs cascaded and the shunt inverter control systems or both.
linear controllers. PI (proportional-integral) controllers have been A very fascinating capability of the UPFC has been reported in
used to implement the DeQ axis control strategy for the series reference [22].
inverter [9,26]. The coordination between the series and the shunt Any change in the transmission line reactive power flow is
inverter control system has been considered [9,26]. The problem balanced by an equal and opposite change in the reactive power
with this strategy for the series inverter is the complexity of the output of the shunt inverter of the UPFC when the shunt inverter is
control system. Two control loops are required to regulate the real controlling the voltage of the bus to which it is connected. This
and reactive power flow. The outer loop to set the reference for the means that any request for change in transmission line reactive
inner loop. The inner loop tracks the reference thus providing the power by the series inverter of a UPFC is actually supplied by the
 M.R. Qader / Energy 89 (2015) 576e592 579

shunt inverter of the UPFC. Reference [8,22] states “In essence, it angle are controlled at the same time or particularly by the UPFC
can “manufacture” inductive and capacitive MVARS using the shunt because of the unconstrained series voltage injection. Thus, it be-
inverter and “export” this reactive power into a particular trans- comes possible for UPFC to control the flow of reactive power in the
mission line (i.e., the one with the series insertion transformer) transmission line. Moreover, the active power produced by the
without changing the local bus voltage and without changing the series converter must be equalized by the active power being
reactive power on any o f the other lines leaving the substation”. drawn by the shunt converter; the reason behind this is the minor
In light of this fascinating capability of a UPFC, there is a need to and low energy storage capacity of the DC capacitor. The versatility
investigate the mechanism by which changes in transmission line and malleability of power flow control is greater when the shunt or
reactive power flow is related to the shunt inverter reactive power series converter's reactive power is selected independently. In
flow. Further, the effect of step changes in transmission line reactive addition, a coupling transformer is also used to link and join the
power flow on UPFC bus voltage needs to be studied. system and the device. It is likely that controllable shunt reactive
All the strategies published in literature have concentrated on compensation might provide independently by the UPFC. The is-
the use of series inverter of a UPFC to control the transmission line sues with the utilization of FACTS devices are congestion, cascading
reactive power flow. In view of the fascinating capability where the line tripping, and line outages. The last generation FACTS device is
shunt inverter responds to a reactive power request from the series the UPFC, which has the ability to control each and every of the
inverter control system, there is need to look into the possibility of a entire three parameters of line power flow concurrently. These
reactive power coordination controller in addition to a real power parameters include line voltage, line independence, and phase
coordination controller. angle. The UPFC also merges the characteristics of the STATCOM
The above fascinating capability leads to another point that is to and SSSC, which are two “old” FACTS devices, into a new device in
be considered. As mentioned earlier, all the strategies published in the form of UPFC.
the literature focuses on the use of series inverter for reactive po- The system configuration is shown in Fig. 1. The purpose of using
wer control. Changes in transmission line reactive power are re- shunt inverter is to accomplish regulation of voltage; while the real
flected as an equivalent change in the shunt inverter reactive power power flow being exchanged is balanced by inserting a suitable
flow. Thus the cause and the effect are on two portions of the UPFC. reactive flow through the connection. It can additionally be utilized
The cause being the series inverter control system and the effect for the purpose of controlling the flow of reactive and real line
seen on the shunt inverter reactive power flow. Thus all strategies power; which can be achieved by inserting a suitable voltage in
discussed in the literature would fall under the category of indirect series with the transmission line, provided the magnitude and
control with respect to reactive power flow. Thus there is a need to phase of the voltage is controllable. In contrast, the purpose of
look into other strategies that provide direct control of trans- using series converter is the insertion of an equal and symmetrical
mission line reactive power flow/line side voltage and that which three-phase voltage system in series with the transmission line;
includes all the necessary coordination between the series and the having a magnitude and phase angle that is controllable so as to
shunt inverter for the proper operation of the UPFC. Further, the control the flow of active and reactive powers in the transmission
control system should utilize only local measurements for its line. Another feature of series inverter is that it can provide reactive
control system. A new control strategy needs to be proposed that power electronically. The active power is conveyed to the DC ter-
utilizes the shunt inverter to directly control the transmission line minals as the reactive power is provided electronically. By keeping
reactive power flow. the voltage across the storage capacitor Vdc constant; the shunt
An advantage with such a strategy would be that it one can inverter makes this dc terminal power, whether it is negative or
replace a part o f the shunt inverter reactive power capability with positive, from the line. Hence, the losses of both two inverters and
switched shunt capacitors that are inexpensive. By doing so, a transformers are equal to the real power absorbed. The reactive
lower MVA rating of the shunt inverter and its transformer could be power will be traded with the line for voltage regulation by the
used thereby reducing the cost of the UPFC. remaining reactive power.
The series voltage VB must be added with certain amplitude, VB
3. Methods and phase shift to Vs Subsequently, a new line voltage Vr will be
obtained through the result of addition, which is likely to have a
The concurrent control of the UPFC with multiple power system different phase shift and magnitude. In addition, the phase shift
variables has many complications; furthermore, this lack of control between Vr and VE varies as the angle alters or changes. Since, it
or the difficulty in control rises because the controllers and vari- would become possible for active power to get moved to series
ables relate to each other. First of all, the UPFC is recognized as the converter from the shunt converter by means of the DC bus; thus,
most powerful and influential FACTS device and the most broad, the FACTS topology is likely to provide considerably greater flexi-
multipurpose and multivariable FACTS controller. Moreover, the bility as compared to the SSSC so as to control the reactive and
STATCOM and SSSC, when grouped, produce the UPFC; while the
STATCOM and SSSC are basically two voltage source converters
with series and shunt converter. The purpose of SSSC or series
converter is to adjoin the magnitude of controlled voltage with
phase angle. It is important to understand that this addition is done
in series with the line; while the role of STATCOM or shunt con-
verter is to supply reactive power to the AC system. In addition, it
will also supply the required DC power to both inverters. The SSSC
and STATCOM are both made up of a power electric converter and a
transformer; in addition, a common DC capacitor is also shared by
these two series and shunt converters.
A DC link is sued to group both STATCOM and SSSC; the purpose
is to allow bidirectional flow or real power in the middle of output
terminals of shunt in the STATCOM; while the series output ter- Fig. 1. Equivalent circuit diagram of the system having two converters (series and
minals of the SSSC. The transmission line voltage, impedance and parallel).
580 M.R. Qader / Energy 89 (2015) 576e592

active power of the line. Moreover, the injected voltage VB in the operating conditions. Thus, it is paramount to design a vigorous
SSSC is forced to remain with the line current (I) in the quadrature. controller having ability to become accustomed to the probable
In contrast with the SSSC, now there can be an angle between the alterations in the operating conditions of the system so as to
injected voltage Vs and the line current (I). Consequently, the locus maintain the damping efficiency at an acceptable rate covering a
presented by the end of vector Vs ðVr ¼ Vs þ VB Þ is a circle. For this to widespread extent of operating points. As contenders for applica-
happen, it is necessary that the magnitude of the injected voltage tion in power system control, many controllers have been consid-
Vs has been kept stable; while the phase angle concerning Vs varies ered, especially in recent times. These controller considerations can
from 0 to 360 . Moreover, there must be a variation in the phase be found in extensive reviews and discussions in Ref. [28].
shift amid the voltages Vr and VE at the two line ends. Moreover, the controller proposed has many different advan-
Next, it becomes possible to control equally the reactive and tages and benefits versus the conventional control systems. Thus,
active powers that are conveyed at one line end; while the shunt suggestions have been made including several algorithms for
converter is operating as the STATCOM. On the whole, it can be control adaptation so as to design vigorous power system con-
stated that the DC bus voltage can be controlled through the shunt trollers [29]. The most fascinating control properties from the
converter for the reason that a dual voltage regulation loop is also benefits of power quality control is that many controllers entail the
used by it; it includes an internal current control loop with an capability of memorizing and generalizing the factors of flexibility,
external loop that regulates the AC and DC voltages. On the other quickness, fault tolerance and heftiness; and it is possible to access
hand, the AC voltage has been controlled at the terminals. The the FACTS devices as there has been rapid expansion and quick
process of controlling the series branch is different in the SSSC as response of power electronics. In addition, the damping there are
compared to that in UPFC. First of all, in SSSC, it uses two degrees of many different devices that can be utilized to attain the damping of
freedom for the series converter for the purpose of controlling the low frequency electromechanical oscillations. Furthermore, in both
reactive power and DC voltages. In contrast, the two degrees of steady and transient states, these devices are generally an influ-
freedom in UPFC are utilized for the purpose of controlling the ential factor for improving system operations. The results for this
reactive and active powers [4]. Furthermore, the series converter is research regarding the development of a damping controller with
additionally capable of operating in any of the modes; including the robust nature are shown in Ref. [28]; on the other hand, the asso-
power flow control mode and the manual voltage injection mode. ciated research in UPFC regarding the development of robust
The power flow control mode is also known as the “automatic damping controllers, the latest and adaptable FACTS technologies
mode”. [29], has not been explored properly. Thus, a multi-layer feed for-
Two VSIs (voltage source inverters) are considered as the ward neural network, having a technique for error back-
fundamental components of the UPFC. These VSIs share a common propagation training, is used for the purpose of designing a sim-
DC storage capacitor and connect to the power system by means of ple adaptive damping controller anticipated for the UPFC.
a coupling transformer. Out of these two VSIs, one is connected in In a fixed range of system's operating conditions, the suggested
series by a series transformer; while the other is connected to the controller possess some similarities with a finely-tuned PI controller.
transmission system in series by a shunt transformer. A path for Moreover, the design of the PI controller is based on a simple
active power exchange among the converters is being created by approach of damping control; borrowed from the time-domain
the coupling of DC terminals of the two static VAR compensators. In evaluation of the system's transient energy function [30]. For
this way, the shunt converter can provide the active power, which is transient simulations that are performed, the thorough models of
being supplied by the series converter to the line [21]. Conse- UPFC and controller are assembled and merged to make a complete
quently, a diverse variety of control choices is accessible in com- power system analysis software package; while these models
parison with the STATCOM or DSSSC (distributed static provide a simple illustration of their control variables. The reason
synchronous series compensator) [14,16]. The APFs (active power behind this merging is the intended testing of the efficiency of the
filters) of shunt are utilized for the compensation of issues related suggested control scheme. It has been observed from the results of
to the current. For instance, reactive power compensation, load simulation that the damping controller provides an equivalent
unbalance compensation and current harmonic filtering. On the performance with the suggested controller design; provided the
other hand, the purpose of using APFs in series is to compensate operating conditions of the system undergoes a change. The results
issues related to voltage; it includes voltage sags, voltage har- also showed that; when the system operating conditions are
monics, voltage flickering issues and voltage swells. In UPFC, Both changed; the damping controller additionally enhances the
of these shunt and series APFs are aimed to be integrated by means damping efficiency and effectiveness versus the PI controller that
of a common DC link capacitor. has fixed parameters.
The utilization of VSI (voltage source inverter) has become
3.1. UPFC design according to different control schemes practicable at both the distribution and transmission extents. The
reason behind this utilization is the recent advancements in the
Between intervals, the operating conditions of power systems in power system handling abilities of static switches. This paper
practical operation change either in a real-time coordinating mode demonstrates an innovative scheme for optimal control for UPFC so
caused by wheeling transactions that are deregulated power sys- as to control the flow of power, to incorporate improvement in
tems or in an arrangement caused by system load changes for stability of system. In addition, a testing of optimal control with
generations of economical dispatch that are basically regulated three tracking strategies is also performed. It includes tracking in
power systems. Moreover, the variation and deviation of network accordance with the PI controller, steady state behavior and
configurations can be caused by major random disturbances; these continuous linear quadratic tracker. The reason for performing
disturbances may include faults in three-phase or tripping issues in testing of these three strategies on a system is to control load flow
generator that consequently causes the tripping of transmission and faults of short circuit.
lines. Therefore, it can be observed that UPFC controllers having a
group of fixed control parameters would not hold accurate to any 3.2. System model
further extent; provided a drastic change in system operating
conditions is observed. However, the set of fixed control parame- The UPFC is that kind of FACTS device voltage source that has
ters can give decent dynamic performance under the specific many unique features; one of which is that it is can control the
 M.R. Qader / Energy 89 (2015) 576e592 581

parameters involved in the transmission system. There are two For branch B,
voltage source inverters present in the UPFC that are connected in
2 3
back-to-back fashion by means of a common DC link. This RB
arrangement provides an ideal scenario for AC-to-AC conversion   6 u 7   
ibd 6 LB 7 ibd 1 vtd  vrd
such that the flow of real power becomes possible amid the AC p ¼6 7 i þ (3)
ibq 4 R 5 bq Ls vtq  vrq
sides of two inverters in either way. The system has two inverters u  B
LB
that have different functions; thus, they are named as “exciter”
(Inverter 1) and “booster” (Inverter 2); while there is a possibility to For branch E,
control the reactive power independently on the two AC sides of 2 3
the inverters. RE
  6 u 7   
Two voltage sources are used to represent the UPFC; these iEd 6 LE 7 iEd 1 vtd  vEd
p ¼6 7 i þ (4)
voltage sources represent basic elements of output voltage wave- iEq 4 R 5 Eq LE vtq  vEq
forms for the two converters and impedances in the form of leakage u  E
LE
reactance of the two coupling transformers [21]. The dynamic
model of the UPFC is shown in Fig. 1 below. This model is based on It is vital to understand that the elimination of two states is
the fundamental principle of UPFC and network concepts of DSSSC possible in following way:
and STATCOM [22].
The system model shown in the below Fig. 1 is a generalized isd ¼ iLd þ iEd þ ibd
(5)
model. It has two converters connected in series and shunt. These isq ¼ iLq þ iEq þ ibq
two converters are capable of eliminating the potential current
The equations for finding voltage of capacitor are as follows:
harmonics of the system; while removing the flickers and sags in
the system voltage. dVdc
Rs ; Ls are used to represent the resistance and inductance in the Cdc Vdc ¼ pE þ pB (6)
dt
first transmission line. Where the RB ; LB are showing the resistance
and inductance of the second transmission line. Resistance and
3 
inductance of the shunt converter branch is represented by RE ; LE ; pB ¼ v i þ vBq ibq (7)
2 Bd sd
while Vs þ Vsh shows the distorted voltage at sending end; to show
the distorted voltage at receiving end, Vr þ Vrh is used IL þ ILh de-
3 
notes the distorted load current; while the voltage injected by the pE ¼ vEd iEd þ vEq iEq (8)
series converter is represented by VB. The voltage injected by the 2
shunt converter is represented byVE ; while the voltage of capacitor     
is denoted byVdc. PB 3 vBd vBq isd
¼ (9)
The reason for choosing these system parameters is the qB 2 vBq vBd isq
simplification; plus the derivation of general conclusions.
    
pE 3 vEd vEq iEd
Vs ¼ 220V; qs ¼ 0; Vr ¼ 120V; qr ¼ p=4; RB ¼ 1U; ¼ (10)
qE 2 vEq vEd iEq
LB ¼ 0:1H; RE ¼ 1U;
The system can be expressed in the following state space form
once the linearization and mathematical manipulation is
LE ¼ 0:1H; RS ¼ 1U; Ls ¼ 0:1H; Vdco ¼ 220V; completed.
Cdc ¼ 1F; u0 ¼ 2p*50 rad=sec;
x_ ¼ Ax þ Bu þ Ed (11)
Vsh ¼ 50V; uoh ¼ uo =10rad=sec; ILh ¼ 1A; 2 Dv 3
uoh ¼ 3uo rad=sec; Vrh ¼ 20V; sd
2 3 6 Dvsq 7
Disd 2 3 6 7
DvBd 6 Dvrd 7
uoh ¼ uo =10rad=sec 6 Disq 7 6 7
6 7 6 DvBq 7 6 Dvrq 7
x¼6 7
6 Dibd 7
6
u¼4 7 d¼6 7 (12)
For branch S, the dynamic equation to derive the state space 4 Dibq 5 DvEd 5 6 DiLd 7
6 7
DvEq 6 DiLq 7
equation of the system is as follows: Didc 6 7
4 Di_ 5
Ld
2 R 3
s Di_Lq
 0 0
2 3 6 6
Ls 72 3
7 2 3
isa 6 7 isa vsa þ vBa  vta In addition, the output equation is,
6 Rs 74 5 1 4
p4 isb 5 ¼ 6 0  0 7 isb þ vsb þ vBb  vtb 5
6 Ls 7 Ls y ¼ Cx þ Du þ Fd (13)
isc 6 7 isc vsc þ vBc  vtc
4 Rs 5
0 0  The system presented and explained earlier was simulated in
Ls order to observe the benefits of the suggested adaptive controller.
(1) The simulation was performed in three different cases; a) tracking
isd ; isq ; vdc b) tracking iBd ; iBq ; vdc c) with and without capacitor
2 3
Rs feedback. The three cases simulated for the four different controller
  6 u 7   
isd 6 Ls 7 isd 1 vsd  vBd  vtd cases a) system responses when no control applied b) system
p ¼6 7 þ (2)
isq 4 Rs 5 isq Ls vsq  vBq  vtq response to steady state behavior c) system response to linear
u  quadratic tracker d) system response to PI controller. The optimal
Ls
control with two converter systems can track four outputs for only
one occasion: if the capacitor is maintained constant, the system
582 M.R. Qader / Energy 89 (2015) 576e592

can track three outputs only. However, it is likely that the steady
state behavior and the number of tracking outputs will be affected
if the capacitor voltage is maintained at a certain reference value.
The tracking will be according to. isd ; isq ; iBd ; iBq ; vdc where;

isd ; isq : Current for the first transmission line in the dynamic
mode
iBd ; iBq : Current for the second transmission line in the dynamic
mode
vdc : Capacitor voltage
vt : Mid bus terminal voltage

Therefore, the optimal control with different tracking strategies

was studied. This simulation study was performed with the MAT-
LAB/Simulink program.

4. Results

4.1. System responses when no control applied Fig. 3. System responses when no UPFC control is applied (short circuit receiving end
bus, Vr ¼ 0 at t ¼ 1 s, for a duration of 1 s).
The software used to perform simulations is “MATLAB/Simu-
link”. Proposed controller's performance was evaluated under components, and the harmonics, voltage sag, and voltage flickers
various operation conditions, including model parameter un- are shown. In Fig. 4, the current harmonics are shown under a
certainties and disturbances acting on the power system. After- condition where no control is applied. It is very clear that the cur-
wards, a comparison was performed among a conventional PI rent harmonics are very high in the system and that the UPFC is not
controller, the results obtained from simulation of proposed controlling the system current harmonics. Additionally, the second
controller, steady state, and linear quadratic tracker controller. First, quality problem voltage flickers clearly show the voltage Vt while
a short circuit is applied (assuming that the electrical network re- the capacitor voltage Vdc s constant at 220 V.
ceives a disruption) for the voltage sags at the receiving end bus for In Figs. 5 and 6, the voltage for the system responses is shown;
Vr ¼ 0 at t ¼ 1 sec and the fault is cleared at t ¼ 2 sec. For the under the supply side voltage flickers Vs ¼ Vs þ Vsh while the
current harmonics, ILh s added. For the voltage flickers, the system receiving end voltage is Vr ¼ Vr þ Vrh . The voltages added to the
is disrupted by adding Vsh and Vrh o the sending and receiving end, system for testing purposes are denoted by Vsh and Vrh. It can be
respectively, followed by implementing the UPFC with and without observed from the figure that with voltage addition at the supply
capacitor feedback into the network. Then, the influence of the
side, the voltage flickers are high for isd and isq. However, at the
system performance is displayed in Figs. 3e6.
receiving end after application of voltage Vrh , the voltage flickers
From the results shown, it is very clear that for the system under
are higher for ibd and ibq. Although there are some voltage flickers
no UPFC control, the system quality is poor. In Fig. 2, it is very clear
for the other components, the components that are closer have a
that the system is not damped for the harmonics and flickers in the
larger effect.
five compounds of uncontrolled qualities ðisd ; isq ; ibd ; ibq and Vdc Þ,
while the voltage Vdc constant. In Fig. 3, for the five uncontrolled
4.2. System response to steady state behavior
components for the currents and voltages when the fault is applied
at the receiving end point, it is very clear from the currents and
In order to analyze the steady state behavior of controller for
voltages that the UPFC is not controlling the three quality
controlling the UPFC, and the performance of the UPFC for the

Fig. 2. System responses under normal conditions (no UPFC control applied). Fig. 4. System responses with current harmonics ILh (no UPFC control applied).
 M.R. Qader / Energy 89 (2015) 576e592 583

Fig. 5. System responses when no UPFC control is applied (voltage flicker supply side Fig. 7. System responses when a steady state control scheme is applied without
Vs ¼ Vs þ Vsh ). capacitor feedback.

Fig. 6. System responses when no UPFC control is applied (voltage flicker receiving
end bus side Vr ¼ Vr þ Vrh ). Fig. 8. System responses when a steady state control scheme (with capacitor feed-
back) is applied.

system response; the system simulations were performed in

MATLAB/Simulink for a similar disruption of the system without a    
controller. The steady state behavior controller is used to improve  CA1 B þ D u ¼ y   CA1 E þ F d (16)
the power quality by reducing the voltage sags/flickers and current
harmonics. A steady state controller is used without the capacitor
feedback and with a closed loop with the capacitor feedback. The
results are shown in Figs. 7e16 for the system voltages and cur-  1  1  
u¼  CA1 B þ D y  CA1 B þ D  CA1 E þ F d
rents. In addition, a comparison was exposed between the open
loop and closed loop based UPFC system. There was no capacitor (17)
feedback for open loop; however, closed loop did have a capacitor
feedback. Therefore, by referring to the steady state equations Hence, in steady state, for the purpose of achieving reference
in references [22], the following system controller was established: output:
In steady state,
 1  1  
x ¼ A1 ðBu þ EdÞ (14) ur ¼  CA1 B þ D yr   CA1 B þ D  CA1 E þ F d
(18)
 
y ¼ C  A1 ðBu þ EdÞ þ Du þ Fd (15) Thus, current harmonics and voltage flickers can be eliminated
as:
584 M.R. Qader / Energy 89 (2015) 576e592

Fig. 12. System responses when UPFC control is applied with capacitor feedback
Fig. 9. System when steady state control is applied without capacitor feedback (under (under current harmonics ILh ).
a short circuit at the receiving end bus).

Fig. 13. System responses when UPFC control is applied without capacitor feedback
Fig. 10. System responses when steady state control is applied with capacitor feedback (under the voltage flicker supply side Vs ¼ Vs þ Vsh ).
(under a short circuit at the receiving end bus).

Fig. 11. System responses when UPFC control is applied without capacitor feedback Fig. 14. System responses when UPFC control is applied with capacitor feedback
(under current harmonics ILh ). (under the voltage flicker supply side Vs ¼ Vs þ Vsh ).
 M.R. Qader / Energy 89 (2015) 576e592 585

Accordingly, the reference calculated values are:

isdr ¼ 2:1212A; isqr ¼ 1:2121A; ibdr ¼ 1:5713A;

ibqr ¼ 2:3570A; Vdcr ¼ 220V

It is noted from the results of simulation that the voltage flickers

can be eliminated due to the UPFC control strategy. Moreover, the
UPFC control strategy is capable to eradicate the current harmonics
and voltage sags as well; while maintaining the desired load flow.
In Figs. 7 and 8; the system responses are shown; in the presence of
a steady state control scheme with and without capacitor feedback.
It can be clearly observed that the system quality current harmonics
and voltage flickers are damped because of the high-quality
controller designed. As a result, it controls the quality current
harmonics and voltage flickers very well. It is also apparent from
the results that the system without capacitor feedback for the dc
voltage Vdc is not constant through the system with capacitor
feedback. The voltage is constant at 220 V.
Figs. 9 and 10 show the voltage control when steady state
Fig. 15. System when UPFC control is applied without capacitor feedback (responses
under the voltage flicker receiving end bus side Vr ¼ Vr þ Vrh ).
control is applied with and without capacitor feedback under a
short circuit at the receiving end bus. The voltage sag is nearly null;
and the capacitor voltage Vdc that is controlled in Fig. 10 with
capacitor feedback is constant.
 1  
Fig. 12 shows the system responses when UPFC control is
urh ¼   CA1 B þ D  CA1 E þ F dh (19)
applied with and without capacitor feedback under current har-
monics ILh . Fig. 12 shows that the voltage is constant at 220 V when
Below is the control rule that was applied:
the capacitor feedback is applied. Similarly, Fig. 14 shows system
2 3 2 32 ði  i Þ 3 2 3
responses when UPFC control is applied with and without capacitor
v*Bd k*11 k*12 k*13 k*14 k*15  sd sdr  VBdr feedback under the voltage flicker supply side Vs ¼ Vs þ Vsh . It can
6 v* 7 6 * 6
* 76 isq  isqr 7 6 V
7
6 Bq 7 6 k21 k*22 k*23 k*24 k25 76 7 be seen that the voltage flickers are controlled in both figures.
6 * 7¼6 * 7 ðiBd  iBdr Þ 7 6 Bqr 7
7 þ 4 VEdr 5
4 vEd 5 4 k31 k*32 k*33 k*34 k*35 56
4 iBq  iiBqr 5 However, the capacitor voltage Vdc is constant at 220 V in the
v*Eq k*41 k*42 k*43 k*44 k*45 ðV  V Þ
VBqr system with feedback, while it is uncontrolled when there is no
dc dcr
feedback in the system. Similarly, for the voltage flickers applied to
(20) the receiving end Vr ¼ Vr þ Vrh with and without capacitor feed-
The values chosen are shown below: back as shown in 16, the system responds to the voltage flickers
very well, but the capacitor feedback for Vdc is constant at 220 V.
Qisd ¼ 100; Qisq ¼ 100; Qibq ¼ 100; Qibq ¼ 100; Qvdc ¼ 100000; R ¼ 0:1
Psr ¼ 700; Qsr ¼ 400; Prr ¼ 500; Qrr ¼ 100 4.3. System response to the linear quadratic tracker

It can be noted that the lowest amount of control energy is

In order to observe system response to the performance of the
needed because the lower value for the weighting matrix Q is
UPFC controller so as to control the flickers and sags in voltage and
acceptable for achieving the required tracking.
the current harmonics; and system response to linear quadratic
tracking; the simulations were performed in a similar manner as
system response to steady state behavior. Therefore, following
control scheme is established by referring to the linear quadratic
control equations and after determining the solution for the linear
quadratic tracker problem [22].

2 3 2 32 i 3
2 3
v*Bd k*11 k*12 k*13 k*14 k*15 sd VBdr
6 v*Bq 7 6 * 76 isq 7
6 7 6 k21 k*22 k*23 k*24 k*25 76 7 6 VBqr 7
6 7¼6 * 76 iBd 7þ6 7 (21)
4 v*Ed 5 4 k31 k*32 k*33 k*34 k*35 56
4 iBq
7 4 VEdr 5
5
v*Eq k*41 k*42 k*43 k*44 k*45 V
VEqr
dc

Below are the chosen values:

Qisd ¼ 100000; Qisq ¼ 100000; QiBd ¼ 100000;

QiBq ¼ 100000; QVdc ¼ 1000000;

R ¼ 0:1
It can be noted that for obtaining the required tracking level,
Fig. 16. System responses when UPFC control is applied with capacitor feedback high value of the weighting matrix Q is needed.
(under the voltage flicker receiving end bus side Vr ¼ Vr þ Vrh ). Accordingly, the reference calculated values are,
586 M.R. Qader / Energy 89 (2015) 576e592

isdr ¼ 2:1212A; isqr ¼ 1:2121A; ibdr ¼ 1:5713A;

ibqr ¼ 2:3570A; Vdcr ¼ 220V

The simulation results for the linear quadratic tracker are shown
as follows.
The performance of the UPFC while using the system response
to linear quadratic tracking is shown in Fig. 17 onwards; for the
system mitigation of current harmonics and voltage sags and
flickers. The responses of the system with and without capacitor
feedback are shown in Figs. 17 and 18, while; the short circuit is
applied at receiving end bus. It is very obvious that the voltage sag
without the capacitor feedback is very poor, and the capacitor
voltage is not constant. The system with capacitor feedback has
improved voltage sag, and the capacitor voltage is constant. For
current harmonics control, it is apparent from Figs. 19 and 20 that
the harmonics can be controlled and that tremendous results are
obtained without capacitor feedback. However, for the system with
capacitor feedback and constant dc capacitor voltage Vdc , the har-
monics are worse on the sending end side Isd and are improved on Fig. 18. System responses when linear quadratic control is applied with capacitor
Isq , while both the receiving side Ibd and Ibd have high current feedback (under a short circuit at the receiving end bus).

harmonics.
By controlling the voltage flickers when the disturbance Vsh is
applied to the sending end, it is very clear that the voltage flickers
are always nil without capacitor feedback; however, the voltage
flickers are high with capacitor feedback as shown in Figs. 21 and
22. As a result, it is notable that the linear quadratic tracker is un-
able to remove current harmonics and voltage flickers at the supply
side efficiently; when the capacitor feedback is used. However,
when the disturbance voltage flickers are added to the receiving
end Vrn (the results are shown in Figs. 23 and 24), it is evident that
there are voltage flickers with and without capacitor feedback, but
they are enhanced with capacitor feedback.

4.4. System response to the PI controller

Moreover, dissimilar controllers are used to control the opera-

tion of the UPFC. Some of these different PI controllers include
decoupling PI controllers, hybrid PI controllers, and cross-coupling
PI (proportional integral) controllers. The real and reactive power
flows decrease because cross-coupling PI controllers are used [31],
Fig. 19. System responses when linear quadratic UPFC control is applied without
and so, the decoupling PI regulation methods can lessen the
capacitor feedback (under current harmonics ILh ).

Fig. 17. System responses when linear quadratic control is applied without capacitor Fig. 20. System responses when linear quadratic UPFC control is applied with capac-
feedback (under a short circuit at the receiving end bus). itor feedback (under current harmonics ILh ).
 M.R. Qader / Energy 89 (2015) 576e592 587

Fig. 21. System responses when linear quadratic UPFC control is applied without Fig. 24. System responses under the voltage flicker receiving end bus side
capacitor feedback (under the voltage flicker supply side Vs ¼ Vs þ Vsh ). Vr ¼ Vr þ Vrh when linear quadratic UPFC control is applied with capacitor feedback.

harmonics in the measurement. For the purpose of damping os-

cillations in power systems, a mixture of cross-coupling PI
controller and direct coupling controllers, known as hybrid con-
trollers, were recommended [31]. The robust control theory pro-
vides the basis for a different complex method for UPFC regulation.
This method is used particularly because PI controllers do not work
properly under a wide operating region. The UPFC in this technique
requires a mathematical model. So, to answer the optimization
equations, online communication is performed. However, the
controller is responsible for the most complex control and regula-
tion technique used lately; this controller has various benefits and
advantages in contrast with known controllers. Although the PI
controller has many benefits, the controller constant used cannot
adjust to the system operations to enhance the operation.
In a similar manner to the above controller (steady state and
linear quadratic tracker), the PI controller is established and applied
to the system. So, when the PI gains are set at random; the control
scheme develops into the below mentioned form.

Fig. 22. System responses when linear quadratic UPFC control is applied with capac- 2 3 2 32 ði  i Þ 3
itor feedback (under the voltage flicker supply side Vs ¼ Vs þ Vsh ).
v*Bd k* k*12 k*13 k*14 k*15  sd sdr 
6 v*Bq 7  6 11 76 isq  isqr 7
6 7 k 6 k* k*22 k*23 k*24 k*25 76 7
6 7¼ kp þ i 6 21 76 ðiBd  iBdr Þ 7
4 v*Ed 5 s 4 k*31 k*32 k*33 k*34 k*35 56
4 iBq  iiBqr 5
7
v*Eq k*41 k*42 k*43 k*44 *
k45 ðV  V Þ
dc dcr
(22)
Below are the chosen values

Qisd ¼ 10000; Qisq ¼ 10000; QiBd ¼ 10000; QiBq ¼ 10000;

QVdc ¼ 1000000;

R ¼ 0:1; Kp ¼ 10; KI ¼ 10
It can be noted that for obtaining the required tracking level,
high value of the weighting matrix Q is needed.
Accordingly, the reference calculated values are,

isdr ¼ 2:1212A; isqr ¼ 1:2121A; ibdr ¼ 1:5713A;

ibqr ¼ 2:3570A; Vdcr ¼ 220V

The simulation results for the PI controller are shown as follows.

Fig. 23. System responses under the voltage flicker receiving end bus side Vr ¼ Vr þ Vrh The simulation results shows that voltage flickers, current har-
when linear quadratic UPFC control is applied without capacitor feedback. monics and voltage sags can be eliminated through the UPFC PI
588 M.R. Qader / Energy 89 (2015) 576e592

Fig. 27. System responses under current harmonics ILh when PI UPFC control is applied
Fig. 25. System responses under a short circuit at the receiving end bus when PI
without capacitor feedback.
control is applied without capacitor feedback.

control strategy; maintaining the desired load flow. The results are 5. Discussion
shown in Fig. 25 onwards for the controlled power quality com-
ponents, current harmonics and voltage sags/flickers. The results There are many adverse effects on the power quality problem,
for voltage sags, when a short circuit is applied to the receiving end power system and customers. One of these effects is the in-
with and without capacitor feedback are shown in Figs. 25 and 26. efficiency and vulnerability of generating equipment, electrical
It can be noted that the system response with capacitor feedback is equipment and transmission lines. Another adverse effect may be
recovered for the voltage sags. Similarly, Figs. 27 and 28 shows the noise caused by the telecommunication system when it undergoes
system response under current harmonics; with and without any type of disruption. This disruption could cause message drop-
capacitor feedback. It can be observed that the system without ping. Third, harmonic production results in parallel and series
capacitor feedback can control the system harmonics very well resonance; therefore, the capacitor and transmission lines can be
because it is nearly zero. When we apply the system disturbance damaged because of overheating or a decrease in endurance.
Vsn or voltage flickers at the sending end with and without Additionally, because of mechanical vibration or even overvoltage,
capacitor feedback, the results are superb in both cases as shown in the transformer may be overheated and may have more false
Figs. 29 and 30. On the other hand, Figs. 31 and 32 shows the results tripping, which can result in the false reading of electric testing
when the disturbance voltage Vrn is applied to the receiving end. instruments. In the commercial, industrial and residential appli-
The results show that the voltage flickers with and without cations, power quality is a vital concern. The growing requirement
capacitor feedback cannot be controlled very well. Table 1 shows for better quality, dependable electrical power and a rising amount
the results for different control strategies for the UPFC with and of distorting loads have derived to an amplified alertness of power
without capacitor feedback. Table 2 shows the results of the cor- quality, equally by utilities and consumers.
responding system response under fault conditions for different
control strategies.

Fig. 26. System responses under a short circuit at the receiving end bus when PI Fig. 28. System responses under current harmonics ILh when PI UPFC control is
control is applied without capacitor feedback. applied with capacitor feedback.
 M.R. Qader / Energy 89 (2015) 576e592 589

Fig. 29. System responses under the voltage flicker supply side Vs ¼ Vs þ Vsh when PI Fig. 32. System responses under the voltage flicker receiving end bus side
UPFC control is applied without capacitor feedback. Vr ¼ Vr þ Vrh when PI UPFC control is applied with capacitor feedback.

The disturbances in power quality are mainly caused by ampli-

fied utilization of non-linear loads; for instance, variable speed
drives, power electronic equipments, and electronic control gears.
Poor power quality can affect the safe, reliable and efficient oper-
ation of the equipment. Various aspects of power quality are
voltage sag, voltage flickers/swells, voltage fluctuations, voltage
unbalance, and harmonics [1,2]. Using power electronic devices
such as FACTS is one of the major aspects to enhance power quality.
The reason is that the FACTS devices are based on power electronic
concept; plus they entail other static controllers that can readily
improve the factors such as controllability and increased power
transfer.
Moreover, they can offer control of one or more AC transmission
system parameters, which are stated in the standards and defini-
tions of IEEE. It has an ability to autonomously control multiple
parameters; hence, it can be defined as the amalgamation of fea-
tures of an STATCOM and SSSC (static synchronous series
compensator) [1]. Previously, many authors have presented the
UPFC models [10]. Similarly, the UPFC model defined in Ref. [11] is
Fig. 30. System responses under the voltage flicker supply side Vs ¼ Vs þ Vsh when PI consisted of a voltage source (controllable), which is connected
UPFC control is applied with capacitor feedback. with two current sources added in the shunt and a transmission
line in series. While in Ref. [12], the model is comprised of two ideal
voltage sources (synchronous), which are connected in series with
a shunt and the transmission line. In Ref. [13], the UPFC model's DC
link is considered, though, convertors losses and the losses of
coupling transformers are overlooked. These devices present a
substitute way to alleviate power system oscillations.
Therefore, an imperative understanding is the choice of the
input signals and the implemented control approach for these de-
vices to reduce power oscillations in an effectual and vigorous way.
Considerable work has been recognized in this domain [2,4].
Several methods that refer to the detection of the series portion to
enhance the transient constancy of the system on the basis of
“optimal parameters” [2], “injection model” and “state variables”
[3] have been researched. For effective implementation of FACTS
devices, multiple topologies pertaining to power converters have
been suggested; for instance, multiple converters with 24 pulses,
48 pulses and multi-level inverters [10e12]. Moreover, the benefits
and drawbacks of converters having high power have also been
studies [13]. In order to amplify the power flow control with the
UPFC in power transmission systems, several algorithms have also
Fig. 31. System responses under the voltage flicker receiving end bus side
been discussed [19]. Several case study analyses have been carried
Vr ¼ Vr þ Vrh when PI UPFC control is applied without capacitor feedback. out on the fundamental bus network. A technique has been
590 M.R. Qader / Energy 89 (2015) 576e592

Table 1
The results for different control strategies for the UPFC with and without capacitor feedback.

Reference No Required steady Linear quadratic PI controller Required steady Linear quadratic PI controller
value for load UPFC state design with tracker (no capacitor (no capacitor state design with tracker (with (with capacitor
flow control control no capacitor feedback feedback) feedback) capacitor feedback capacitor feedback) feedback)

isd(A) 2.12 1 2.121 2.119 2.136 1.633 1.702 1.694

isq(A) 1.212 3.67 1.212 1.219 1.218 1.324 1.233 1.322
ibd(A) 1.57 1.835 1.571 1.578 1.554 1.603 1.722 1.766
ibq(A) 2.357 0.5414 2.357 2.345 2.354 2.476 2.509 2.466
Vdc(V) 220 220 220.8 220.8 220.9 220 220 220.1
Vt(V) 105.5 164.9 164.5 164.9 168.3 168.6 167.2
Ps(W) 700 330.1 700 699.2 704.8 538.8 561.7 558.9
QE(Va) 400 1211 400 402.2 402.1 436.8 406.8 436.3
Pr(W) 500 302.4 500 498.8 497.3 519.1 541.1 538.7
Qr(Va) 100 164.6 100 98.2 101.8 111.1 97.66 89.14
PB þ PE 0 0 179 179.4 186.5 0 0.03177 0.471
VBdr 19.3267 0 19.3267 2.6126  103 806.6862 19.3267 3.5947  105 3.3852  105
VBqr 27.5833 0 27.5833 2.2836  103 698.7101 27.5833 1.6424  105 2.0148  105
VEdr 196.4403 0 196.4403 328.8844 238.2984 196.4403 5.5468  105 5.7150  105
VEqr 55.1189 0 55.1189 1.3285  103 417.8874 55.1189 1.4072  105 4.3934  104

proposed in Ref. [20] with the aim of controlling the reactive and the growth pertaining to operational manners to utilize trans-
real power by utilizing two-leg three-phase converters in the mission systems with their tremendous thermal ability. What
transmission line on the basis of UPFC. Moreover, in Ref. [21], it has affected the power industry is the fast development in power
been discovered by the authors that the performance of a system electronics; moreover, this is an outcome of the supple AC trans-
improves when the UPFC is linked to a bus having a low voltage mission systems, otherwise known as FACTS features, which have
profile [20]. A simulation study for the UPFC on the basis of IEEE 14- become possible because of the development of power electronic
bus test system [21]; is carried out in this paper. Moreover, the devices. FACTS devices become able to provide fast control of the
paper investigated the performance of UPFC to control power flow reactive and active power supposedly by means of a transmission
over the transmission line. line [11,12]. The UPFC is one of the several devices based on FACTS
To discover all of the monetary and technologically possible technologies that include various appealing features, for example,
ways of increasing the limit of constancy, financial factors, such as the UPFC can significantly control numerous factors; basically it is a
the income earned by the deliverance of added power, and the combination of STATCOM and SSSC. Moreover, these devices can
great price of long lines provide massive motivations to do so. offer a substitute way of mitigating power system oscillations
However, currently, many studies have been gathered because of [11,12].

Table 2
Showing the results of the corresponding system response under fault conditions for different control strategies.

No UPFC control Design according to Linear quadratic PI controller Design according to Linear quadratic PI controller
desired steady state tracker (no capacitor (no capacitor desired steady state tracker (with (with capacitor
(no capacitor feedback) feedback) feedback) (with capacitor feedback) capacitor feedback) feedback)

isd(A) During: 0.05134 During: 2.123 During: 2.122 During: 2.135 During: 0.1481 During: 0.2915 During: 0.192
Post: 1.057 Post: 2.121 Post: 2.119 Post: 2.145 Post: 1.633 Post: 1.7 Post: 1.8
isq (A) During: 4.73 During: 2.173 During: 1.243 During: 1.22 During: 2.627 During: 1.331 During: 1.678
Post: 3.407 Post: 1.212 Post: 1.219 Post: 1.223 Post: 1.324 Post: 1.233 Post: 1.309
ibd (A) During: 0.1889 During: 0.8561 During: 1.683 During: 1.583 During: 0.9846 During: 2.44 During: 2.511
Post: 1.759 Post: 1.571 Post: 1.574 Post: 1.522 Post: 1.603 Post: 1.743 Post: 1.705
ibq (A) During: 2.247 During: 4.49 During: 2.47 During: 2.378 During: 4.97 During: 3.188 During: 2.678
Post: 0.7758 Post: 2.357 Post: 2.345 Post: 2.351 Post: 2.476 Post: 2.51 Post: 2.439
Vdc (V) During: 220 During: 223.8 During: 222.4 During: 222.4 During: 220 During: 220.5 During: 220.2
Post: 220 Post: 220.4 Post: 220.4 Post: 220.4 Post: 220 Post: 220 Post: 220.1
Vt (V) During: 73.38 During: 170.3 During: 93.95 During: 89.78 During: 159.2 During: 126.2 During: 120
Post: 105.5 Post: 164.9 Post: 164.5 Post: 164.9 Post: 168.3 Post: 168.6 Post: 167.2
Ps (W) During: During: During: During: During: During: During:
Qs (Va) Ps ¼ 16.94 Ps ¼ 700.7 Ps ¼ 706.9 Ps ¼ 704.4 Ps ¼ 48.89 Ps ¼ 96.2 Ps ¼ 63.35
Qs ¼ 1516 Qs ¼ 717.1 Qs ¼ 410 Qs ¼ 402.7 Qs ¼ 866.8 Qs ¼ 439.1 Qs ¼ 553.6
Post: Post: Post: Post: Post: Post: Post:
Ps ¼ 348.8 Ps ¼ 700 Ps ¼ 699.2 Ps ¼ 707.9 Ps ¼ 538.8 Ps ¼ 561.2 Ps ¼ 593.9
Qs ¼ 1124 Qs ¼ 400 Qs ¼ 402.2 Qs ¼ 403.5 Qs ¼ 436.8 Qs ¼ 406.8 Qs ¼ 432
Pr(W) During: During: During: During: During: During: During:
Qr(Va) Pr ¼ 0 Pr ¼ 680.5 Pr ¼ 0 Pr ¼ 0 Pr ¼ 0 Pr ¼ 0 Pr ¼ 0
Qr ¼ 0 Qr ¼ 462.5 Qr ¼ 0 Qr ¼ 0 Qr ¼ 0 Qr ¼ 0 Qr ¼ 0
Post: Post: Post: Post: Post: Post: Post:
Pr ¼ 322.6 Pr ¼ 500 Pr ¼ 498.8 Pr ¼ 495.5 Pr ¼ 519.1 Pr ¼ 541.2 Pr ¼ 527.4
Qr ¼ -125.1 Qr ¼ 100 Qr ¼ 98.2 Qr ¼ 102.9 Qr ¼ 111.1 Qr ¼ 97.66 Qr ¼ 93.46
PB þ PE During: 0 During: 655.5 During: 684.3 During: 683.2 During: 0.003 During: 68.52 During: 10.16
Post: 0 Post: 179 Post: 179.4 Post: 191.4 Post: 0.004 Post: 0.679 Post: 98.04
Vr (V) During: 0 During: 0 During: 0 During: 0 During: 0 During: 0 During: 0
Post: 120:p/4 Post: 120:p/4 Post: 120:p/4 Post: 120:p/4 Post: 120:p/4 Post: 120:p/4 Post: 120:p/4
 M.R. Qader / Energy 89 (2015) 576e592 591

5.1. Conclusions and policy implications the operations of power systems and associated power quality
problems.
The paper provided a detailed discussion and justification of One more implication is the growth of power systems and
an innovative and moderately submissive transient stability and market conditions given by unregulated schemes have demon-
steady state model for the UPFC. Additionally, the paper pro- strated the technical and operational constraints that SEP power
posed a comprehensive illustration so as to clarify appropriate systems have to supply the demand strict safety, power quality and
management of restrictions in the controller. The justification reliability. A unified power flow UPFC (unified power flow
was drawn on the basis of different results obtained from the controller) controller is one of the devices in the FACTS (flexible AC
simulations performed through the MATLAB/Simulink software. transmission systems) technology that provides greater flexibility
The objective of the simulations was based on several conditions in terms of interaction with the operating variables and control of a
in controller operations of the test system for the controller. SEP and may be optimal to additional control requirements solu-
Furthermore, various controller structures and strategies for the tion modern SEP. This paper presents a mathematical and con-
UPFC were also discussed thoroughly in the paper on the basis of ceptual formulation for the inclusion of a UPFC device in the load
qualitative and quantitative comparison of their performances. In flow analysis in a SEP and a methodology for inclusion in a software
addition, advantages and disadvantages of these controllers were load flow in steady state. To validate the efficiency of the models
also determined through the comparison. The paper proposed a and methods proposed, a power system suitable test for the ex-
unique and novel controller for series and shunt converters of pected effects of the inclusion of UPFC in the system is selected.
the UPFC, and analyzed it in detail. It is evident from the control Based on the basic architecture of a UPFC, its ability to control
system of UPFC presented in the paper that it has the capability the transmission power and the different modes of operation,
of controlling the flow of paper. Additionally the proposed UPFC depending on the interaction you want with the SEP, world litera-
control system also showed evidently that stability of the system ture has proposed different models for these devices. One of them
can be enhanced through elimination of voltage sags, current is the “model of two current sources,” presented by Dussa n Povh
harmonics and voltage flickers all at once. Furthermore, fast (Povh, 2000a). This model proposes a circuit diagram with two
dynamic response, strength and efficiency of the presented current sources connected in parallel and a voltage source in series
control scheme is indicated in the paper by the results of with the line. This model is not very realistic because neglected
simulations. losses coupling transformers and converters, but for simplicity can
A number of implications come into view from this study; one of be useful for optimum location of analysis devices. Another model
them implies that additional knowledge regarding the system state known as “two-port model” is presented in Orfanogianni and
is equivalent to further assurance; while making operational de- Bacher (2003), a methodology based on identifying the optimal
cisions and issues related to the planning, which can lead towards location of FACTS devices to increase the maximum power transfer,
an improved system of risk management in power system distur- and is a model that does not consider the impedance transformer
bances. Risk diminishing in forecasting of operational state of the excitation and coupling and that does not include the ability to
system is an imperative component; particularly in power system control the UPFC voltage.
requirements and decisions related to business and operations. The so-called “impedance UPFC model” is the most complete
Predicting the precise state of power system has momentous sig- version of a model for UPFC. Consider two coupling transformers
nificance; particularly in bidding approaches, management of risks, and voltage sources with their respective series impedance and
and effective decision-making in power systems. Therefore, it is includes more variables than previous models, making it difficult to
vital to consider this matter, specifically in those power systems incorporate into tools of power flows. However, this higher level of
that have greater saturation of renewable energies. modulating allows a more flexible and realistic representation,
It emphasizes the significance of making an allowance for cer- achieving greater controllability and closer to the actual operation
tainty index in FACTS devices; including issues related to power of a UPFC in a power system (Cerda and Palma, 2004) results. The
system optimization as well as traditional objective tasks; for mathematical model penthouse steady state was developed in
instance, the loss of power, profile of voltage and system stability Nabavi-Niaki and Iravani (1996), and the process of incorporation of
etc. The significance of this research study in reducing power sys- UPFC on power flow using Newton's method outlined in this article.
tem disturbances, risk management and several operational de-
cisions of FACTS technologies is quite exceptional and influential for
the domain of power systems. Acknowledgments
Another implication is related to the issue of frequent distur-
bances in a power system due to its dynamic nature. The ultimate The author greatly acknowledges the help of H. S. Hassan and N.
implication emerged from this study is that UPFC based controller J. Jassim (University of Bahrain), who provided practical details and
systems can efficiently manage the load flow and voltage sags/ discussion.
flickers; while eliminating the harmonics at the same time. More-
over, a prominent implication of this study concerns with a huge References
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