The FACTS-devices contain these
elements as well but use additional power electronic thyristor-based fast
operating devices for control and switching operations. The additional power
electronic valves to switch the elements in smaller steps within a cycle of the
alternating current. They have low losses because of their low switching
frequency of once a cycle in the converters or the usage of the Thyristor to
simply bridge impedances in the valves. The benefits of FACTS devices
can be given below,

·       Increased
transmission system reliability and availability.

·       Increased
transient grid stability and reduction of loop flows.

·       Increased
quality of supply for sensitive industries.

·       Flicker mitigation.

·     
Interconnection
of renewable and distributed generation and storages.

·       Voltage
control.

·       Increase utilization of lowest cost
generation.

·       DS enhancement. This FACTS
additional function includes the TS improvement, POD and VS control.

·       Increase the loading capability of
lines to their thermal capabilities, including short term and seasonal demands.

·       Elimination or deferral of the need
for new transmission lines.

·        Added flexibility in siting new generation.

·       Upgrade of transmission lines.

·       Increased system security.

·       Reduce RP flows, thus allowing the
lines to carry more AP.

·       Loop flow control.

·       Control
of power flow as ordered. This is the main function of FACTS devices

Many countries, increasing the
energy transfer capacity and controlling the load flow of transmission lines
are of vital importance, especially in de-regulated markets, where the
locations of generation and the bulk load centers can change rapidly. Frequently,
adding new transmission lines to meet increasing electricity demand is limited
by economic and environmental constraints. These FACTS controllers are based on
voltage source converters. FACTS can easier the power control, increases the
power transfer capability, decrease the generation cost, and improve the
stability of the power system 2.

The unified power
flow controller enhances the power flow due to the separate controlling
capability of active and reactive power. In this proposed work two bus systems
is simulated with UPFC. IEEE 14 bus systems are also simulated with and without
UPFC. The real and reactive power is investigated.

1.5 Advantages of Facts Technology

·       Rapid response

·       Dynamic control of power flow in selected transmission lines within the network
to enable optimal power flow conditions.

·       Damping of the power swings from local and inter-area oscillations

·       Suppression of sub synchronous oscillations

·       Decreases DC offset voltages

·       Reduction of short circuit current

·       Frequent variation in output

·       Smoothly adjustable output

1.6 Relative Importance of Controllable Parameters

·      Control of the line impedance X can provide a powerful means of current
control.

·      When the angle is not large, which is often the case, control of X or
the angle substantially provides the control of active power.

·      Control of angle, which in turn controls the voltage, provides a
powerful means of controlling the current and hence active power flow is
controlled when the angle is not large.

·      Injecting a voltage in series with the line and with any phase angle
with respect to the driving voltage can control the magnitude and the phase of
the line current.

·      When the angle is not large, controlling the magnitude of one or the
other line voltages can be a very cost-effective means for the control of
reactive power flow through the interconnection.

·      Shunt controller control the voltage Regulation and the line impedance
control with a series controller and also control both the real and reactive
power flow between the two systems.

1.7 Background

Now a day’s power system is facing
new challenges due to deregulation of the electricity. During last two decades
various types of high power semiconductor devices and control technologies are
introduced. Overall FACTS devices are studied in Gygui. UPFC is the most
powerful device. It is proposed firstly by Gygui, 1991. The significant impact
of the AC transmission via increasing use of thyristor controlled static var compensator
1. The principle operation of UPFC and its advantages is studied in paper of
Raju pandey and A.K.Kori. It can be compare its performance with various types
of FACTS controller. When fault occurs in this system there is said to be drop
in voltage so the UPFC is used to improve the transient stability.

In UPFC, the shunt converter is
used for voltage regulation at the connection of point, injecting reactive
power flow into the line and to balance the real power flow exchanged between
the series inverter and the transmission line. Thus, the UPFC functions of
active and reactive series compensation, reactive shunt compensation. Therefore
the UPFC can control the flow of real and reactive power in the transmission
line. The UPFC will enhance the power transfer capability and also improve the
stability of the power system. For the power system stability studies, UPFC
model is used to improve the dynamic performance of the system. UPFC implement
at appropriate place in transmission line is done by injecting the voltage at
an angle of 90 degree, to regulate power flow is described by K.N.Babu, 2015.

1.8 Motivation

In view of above background, it is
clear that the role of FACTS devices and UPFC is very important for power
system stability and to control the real and reactive power flow. In FACTS
devise the member is unified power flow controller and this controller is very
attractive features. FACTS device
can independently control parameters, so it is the combination of the
properties of a STATCOM and static synchronous series compensator (SSSC) 1. It
can be control all the three system variables namely line reactance, magnitude
and phase angle difference of voltage across the line. This research motivates
to do further research and experiment on UPFC.