INTRODUCTION
Flexible Alternating Current Transmission System or FACTS uses power
electronic based systems and others static equipment to provide control
of one or more ac transmission system parameters to enhance controllability
and increase power transfer capability. It was first introduced in 1980s
by Narain G. Hingorani (Moore and Ashmole, 1995). Traditionally, power
flow control is gained with the use of a phase shifter and mechanically
changing tap setting of a transformer. However this method is not flexible
enough to cope with the increasing needs. Following the trend of deregulating
the electric power industry, a demand for flexible power load flow is
becoming a technical need feasibly achievable by the innovative power
electronics (Wang and Fang, 1999) thus the use of FACTS devices. This
technology is based on the used of high voltage and high current power
electronics devices in association with communication links and local
automatic controllers.
UPFC concept was first proposed with objectives of controlling, simultaneously
or selectively, all the parameters affecting power flow in the transmission
line i.e., voltage, impedance and phase angle. It can also independently
control both real and reactive power flow in the transmission line (Hingorani
and Gyugyi, 2000) beside than that, it has the capabilities of improving
transient stability, mitigating system oscillations and providing voltage
support (Dong et al., 2002).
Since the introduction of the UPFC, many studies and investigation of
its performance have been carried out either by simulation and hardware
model. Zheng et al. (2000) reported the simulation model of UPFC
with 12pulse converters using Matlab and Simulink software has been developed.
Static and dynamic characteristics of the developed UPFC simulation model
in a power system are investigated under normal operating condition (i.e.,
no disturbance) and under severe disturbance. The developed model reflects
precisely the operation characteristics of the practical devices. It shows
that the UPFC can control the voltage and power flow of the system effectively.
Toufan and Annakkage (1998) investigate performance of a UPFC constructed
by a backtoback connection of a Hysteresis Current Forced (HCF) converter
and a Pulse Width Modulated (PWM) inverter. The model has been developed
at a component level and simulated using PSCAD/EMTDC software. From the
investigation, the UPFC model can maintain an almost constant dc bus voltage
and has the ability to pass the real power bidirectional. It has also
been shown that using quadrate or in phase voltage injection, the UPFC
can enhance the dynamic stability of the power systems effectively.
A current injection model of the UPFC is developed for transient stability
(Meng and So, 2000). The effect of UPFC can be represented by an equivalent
circuit with a shunt current source and a series connected voltage source.
The series voltage source can be solved into inphase and quadrature components
with respect to the line current and the current injection model is obtained
by replacing the voltage source with the current source. The controller
of the UPFC is based on optimal control strategies in a SingleMachine
InfiniteBus (SMIB) system. The study proposed that the controller coordinates
input signals to control the two components of the UPFC series voltage
and the shunt compensation of the UPFC, in order to maintain the system
bus voltage. The eigenvalue analysis and nonlinear results show that the
proposed model and control method can significantly improve system dynamic
performance.
New control approach combining the traditional control technique with
an artificial intelligence technique such as Genetic Algorithm (GA) has
been studied by Faried and Eldamaty (2004). The GA based UPFC is designed
using eigenvalue shifting technique and the effectiveness of the new controller
is demonstrated through timedomain simulations using Matlab. From the
results, it shows that this new control approach give the UPFC more flexibility
and increase capabilities in damping the power system oscillations when
compared to the fixed power injection UPFC.
This research concentrates on the designing and developing a simulation
model of a three phase UPFC using SimPowerSystem Blockest of Matlab/Simulink
software. The shunt converter is developed using the traditional 6pulse
threephase bridge topology, while the series converter is developed using
multilevel, 3level topology. The system is modeled as an open loop system.
Results obtained from the simulation model are then compared with the
theory of operation of the UPFC. Faults are set to the system to observe
the operation of STATCOM and phase shift, φ of the SSSC is varied
to observe the operation of SSSC. A good working simulation model has
been obtained for the UPFC.
UNIFIED POWER FLOW CONTROLLER
UPFC consists of two switching power converters connected to each other
backtoback through a DC link capacitor as shown in Fig.
1. The converters are connected to the AC system by a shunt and series
transformers. This arrangement functions as an ideal ACtoAC power converter
in which real power can freely flow in either direction between the two
inverters.
The shunt inverter or STATCOM is connected to the system via a shunt
transformer. It injects an almost sinusoidal controlled current of variable
magnitude at the point of connection. STATCOM compensates reactive power
flow in the transmission line and at the same time keeps the DC voltage
constant across the DC link capacity or (i.e., regulates the active power
flow between shunt and series converters).
The series converter or SSSC operates by adding a series voltage, ΔV
of a variable magnitude and phase angle and thus forcing the power to
flow to a desired value. This voltage addition has a controllable magnitude
of range, δV (0≤δV≤δV_{max}) and phase angle,
φ (0°≤φ≤360°) and can be considered as asynchronous
AC voltage source (Wang and Peng, 2004). It is connected to the system
through a series transformer. Figure 2 shows the vector
diagram of the voltages in the power system.

Fig. 1: 
Threephase diagram of UPFC 

Fig. 2: 
Vector diagram of system voltages 
SYSTEM SIMULATION
The main interest in this study is to design and develop a simulation
model of a three phase UPFC using SimPowerSystem Blockest of Matlab/Simulink
software. The designing of the UPFC takes two steps. The shunt converter
or STATCOM is designed first and then followed by the designing of the
series converter or SSSC.
The STATCOM has been developed using the traditional 6pulse, threephase
bridge topology, Which consists of two converters; rectifier and inverter,
as shown in Fig. 3. The rectifier is constructed using
diode and the inverter using thyristors. A capacitor connects the dc side
of the rectifier and inverter and a shunt transformer connects both AC
sides of the converters to the AC supply. Under normal operation, the
rectifier will charge up the capacitor and when there is a voltage drop
or sag occurs to the system such as a fault, the inverter operates and
the capacitor gets discharged.
As the charge in the capacitor depletes, the rectifier will operate and
charge back the capacitor.
SSSC is developed based on the threelevel multilevel structure using
Insulated Gate Bipolar Transistors (IGBTs) as the power switches. Figure
3 shows the simulation model of the SSSC. By controlling the phase
shift, φ, the angle of the output voltage of the SSSC, ΔV can
be controlled and by varying the value of dc link capacitor voltage, V_{dc},
the magnitude of the output voltage of the SSSC, ΔV can be varied.
This two parameters; phase shift, φ and DC link capacitor voltage,
V_{dc}, provide the control of series voltage with variable magnitude
and phase angle for the SSSC. By combining the STATCOM and the SSSC as
in Fig. 4, a complete system of UPFC is constructed.
RESULTS AND DISCUSSION
The operation of the STATCOM is studied by applying two separate three
phase solid faults at two separate times on the same transmission lines.
The first fault occurs at time 0.5 to 0.7 sec for duration of 0.2 sec
and the second fault occurs at time 1.2 to 1.3 sec for duration of 0.1
sec.
As shown from the Fig. 5, when a fault occurs, the
voltage of the AC supply line drops and returns to normal after the fault
is removed. The drop of sag value depends on fault and system impedances.
A slight transient is noticed after the removal of fault.

Fig. 3: 
Simulation model of STATCOM 

Fig. 4: 
Simulation model of SSSC 

Fig. 5: 
AC supply line voltage 
Figure 6 shows the AC current waveform of the inverter
and rectifier. As shown, the current is of pulsed type due to the reason
of the switching OFF and ON of the power devices when the converter operates.
When faults occurred, the ac voltage of the supply lines will dropped,
so, in order to mitigate the voltage drop, the inverter starts to operate.
By using the DC link capacitor as power supply, the inverter discharges
the capacitor to inject current to the AC supply. As the capacitor discharges
and its voltage decreases, the rectifier starts to charge the capacitor
back and injecting a reactive power into the AC system.
Figure 7 shows that during the duration of fault, the
real power is absorbed by the STATCOM and the reactive power is generated
by the STATCOM.
The SSSC operates by changing the phase shift, φ in order to change
the angle of the output voltage of the SSSC, ΔV. Figure
8 shows the SSSC AC output line voltage for phase shift, φ equal
to zero. When this voltage is added to the ac supply line voltage, a new
V_{2} is achieved. This way, the active power and reactive power
can be controlled.
Figure 8 shows the linetoneutral voltage and linetoline
voltage of the threelevel neutralpointclamped SSSC. It differs from
the conventional twolevel inverter as it is now has three voltage level
i.e., +200V, 0V and 200V, as compared to the twolevel where it only
has two voltage level i.e., +200V and 200V. This zero voltage value is
obtained by switching ON two power switches that are connected to the
neutral point of the phase leg.
Figure 9 shows the voltage of V_{1}, V_{2}
and ΔV. In this case the voltage V_{1} and V_{2}
are of different magnitude and inphase to each other as the phase shift
is zero.

Fig. 6: 
AC line current of the rectifier (Top) AC line current
of the inverter (Bottom) 

Fig. 7: 
Real power (top) and reactive power (bottom) of the
STATCOM 
The phase shift, φ is varied, but with the same magnitude of δV.
As the phase shift, φ is varied, the voltage of the SSSC is also shifting.
When this output voltage, ΔV is added to V_{1}, a new V_{2}
is obtained. If phase shift, φ leads, voltage, ΔV also leads and the
resultant voltage, V_{2} will has a leading phase shift and vice versa.
This phase shift φ, also affects the amount of real power and reactive
power transferred from the SSSC. This show by specifying certain value of phase
shift φ, the control of power can be achieved.

Fig. 8: 
SSSC linetoneutral voltage (top), linetoline voltage
(middle) and line current (bottom) 

Fig. 9: 
Voltage ΔV, V_{1} and V_{2} 
CONCLUSION
Designing and developing a simulation model of a three phase UPFC using
SimPower System Blockest is presented in this paper. The STATCOM is modeled
based on the traditional 6pulse threephase bridge and the SSSC is modeled
based on threelevel multilevel structure. Simulation is carried out using
a simple on and off switching of the switching devices. The control of
power in the transmission system can be achieved by controlling the phase
shift, φ of the SSSC. Simulations of normal and abnormal conditions
have been carried out and the results obtained agreed with the theory
of operation of the UPFC.