In loss in synchronism in grid system and

In recent years power system has noticed increase in system
size, load, generation, transmission, distribution and voltage level. Disturbances
have also increased in system operation day by day. Voltage stability, rotor
angle and frequency are three main parameters for stable operation in power
system. When disturbances occur in power system, rotor angle stability remains
sysncronised with power system. It is of two type’s large disturbance transient
stability or rotor angle stability and second one is small signal stability or
small disturbance rotor angle stability. Among these small signals stability
has become a major problem in control of power system. Power grids are connected
through weak tie lines for which unwanted low frequency oscillations are
generated. Due to high positive damping these oscillations are non-observable, it
may causes loss in synchronism in grid system and many black out like  WSCC blackout (1996), scotland-england (1978),
ITALY blackout (2003), GHANA ivory coast (1985) , denmark-sweden (2003),
WESTERN AUSTRALIA (1982-1983) ,De-Ontario hydro-Hydro Qebec (1960,1985),USA and
CANADA (2011) and NEW and SR grid in INDIA (2014).

                         
 The range of low frequency oscillation
is 0.2 – 3.0Hz, Otherwise we have to damp these oscillations or else it may
cause in loss of synchronism. PSS have been used for many years for
damping purpose. But PSS
has a major draw backs it causes variation in the voltage profile and many
other disturbances. But in Morden power system FACTS based PSS is more suitable
with respect to traditional PSS .Due to presence of semiconductor devices in
FACTS devices it’s easy for operation and control in power system. SVC, TCPS,
SSSC, STATCOM are different kinds of FACTS devices. UPFC is more superior then
other FACTS devices due to three degree freedom and simultaneously it can control
 both reactive and active power. Hence it
can also used for reduction of low-frequency oscillations in power system, improvement
of transient stability and effective power flow control. For small signal
stability wang created a Heffron-philips model which can be installed with a
UPFC. The most importat task is to suitable optimization technique.