09-02-2013, 03:20 PM
Improvement of Transient Stability Performance Using Adaptive Neuro-Fuzzy for FACTS Controllers
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INTRODUCTION
Transient and dynamic stability are
enhanced for secure operation of power systems
with increased power transfer. FACTS (Flexible
AC Transmission System) devices with a
suitable control strategy have the potential to
significantly improve the transient stability
margin. This allows increased utilization of
existing network closer to its thermal loading
capacity. The UPFC is capable of both
supplying and absorbing real and reactive power
and consists of two AC/DC converters. The
series branch of the UPFC injects a voltage of
variable magnitude and phase angle it can
exchange real power with the transmission line
and thus improve the power flow capability of
the line as well as its transient stability limit.
The shunt converter exchanges a current of
controllable magnitude and power factor angle
with the power system.
UNIFIED POWER FLOW CONTROLLER (UPFC)
The unified power flow controller
(UPFC), shown in Fig.1 consists of two
switching converters operated from a common
dc link provided by a dc storage capacitor. One
connected in series with the line, and the other
in parallel. The series converter of the UPFC
injects via series transformer, an ac voltage with
controllable magnitude and phase angle in series
with the transmission line. The shunt converter
supplies or absorbs the real power demanded by
the series converter through the common dc
link. The inverter connected in series provides
the main function of the UPFC by injecting an
ac voltage Vpq with controllable magnitude and
phase angle at the power frequency [10], in
series with the line via a transformer. The
transmission line current flows through the
series voltage source resulting in real and
reactive power exchange between it and the ac
system.
D-Q CONTROL STRATEGY FOR UPFC
Power electronic systems have the
capability of providing faster response
compared to traditional mechanically based
power system controls. Therefore to obtain the
maximum capability out of the UPFC, a
control system with an equally faster response
is required. It would be advantageous, if the
time-varying equations can be transformed to a
time invariant set.
CONVENTIONAL METHOD
A widely used conventional lead-lag PSS
is considered in this study. Its structure is shown
in Fig.5 It consists of a gain block with gain KP,
a signal washout block, and two-stage phase
compensation block with time constants T1P, T2P
and T3P, T4P. In this structure, TWP is the washout
time constant; Δω is the speed deviation and VS
is the stabilizing signal output of power system
stabilizer.