In recent years, new types of Flexible AC Transmission System (FACTS) devices have
been investigated that may be used to increase power system operation flexibility
and controllability, to enhance system stability and to achieve better utilization
of existing power systems. The evolution of power electronic devices, along with
the development and control, has helped in the design and implementation of structural
controllers known as FACTS, which are emerging as feasible technology for the improvement
of system’s dynamic behavior. The benefits arising from FACTS devices are widely
appreciated. The concept of FACTS was introduced as a family of power electronic
equipments which have emerged for controlling and optimizing flow of electrical
power in the transmission line (Song and Johns, 1999; and Hingorani, 2001). The
concept of Unified Power Flow Controller (UPFC), its performance and steady-state
characteristics have been widely reported in the literature (Gyugi, 1992; and Gyugi
et al., 1995). The UPFC has been researched broadly, and many research articles
dealing with UPFC modeling,analysis, control and application have been published
in the recent years. Mathematical models were developed for UPFC to determine steady-state
operational characteristics using state space equations without considering the
effects of converters and the dynamics of the generator (Papic, 2000a and 2000b).
The performance of UPFC was analyzed by designing a series converter using conventional
and advanced controllers (Round et al., 1996; and Yu et al., 1996). The mathematical
model of UPFC using general Pulse Width Modulation (PWM) and space vector approach
was used to perform the power flow studies, Eigen analysis and transient stability
investigations (Nabavi-Niaki and Iravani, 1996). A nonlinear dynamic small signal
model of network with UPFC was established for transient studies. The model evaluated
the compensation effects of UPFC, optimized the location of UPFC and its control
design (Smith et al., 1997). An equivalent two-bus power network was developed based
on sets of equations for a system including the UPFC was proposed. This provided
a useful tool to rate and evaluate the performance of UPFC on power systems (Keri
et al., 1999).
The UPFC was modeled as voltage source model and PWM switching level model. The
voltage source model of UPFC was constructed with equivalent voltage source and
impedances using MATLAB. The switching level model of UPFC was designed and simulated
in EMTP. The equivalent impedance of voltage source model was found from the dynamic
responses of UPFC switching level model. The results show that switching level model
was more accurate than voltage source model (Dong-Jun et al., 2003). The optimal
location and equivalent impedance of UPFC are found by voltage source model and
switching level model by varying the amplitude and phase angle of injected voltage
(Rajeswari et al., 2009). In laboratory implementation of FACTS devices, UPFC was
set up by PWM controllers, providing more effective control of real and reactive
power flow (Dong et al., 2004).
A new configuration of UPFC has been proposed here. Two inverters are connected
back-to- back through a common DC link. It greatly reduces the VA rating of the
inverters (Jin Wang and Fang, 2003). The new conduction mode of 120o inverter is
implemented in the brushless DC motor and the performances are analyzed (Sudhoff
and Krause, 1990). The conduction mode of 150° is implemented and it is analyzed
for the three-phase inverter performance (Mohamed et al., 2006). The double band
hysteresis current controller was designed for Static Synchronous Compensator (STATCOM)
to compensate the reactive power in the distribution network. The current error
and switching frequency are reduced (Kim et al., 2004).
This paper presents a study of voltage profile, real and reactive power flow in
the transmission lines by implementing the 150° conduction angle for individual
switches in the inverters.
The rest of the paper is organized as follows: the following section explains the
proposed model of UPFC, the next section describes the modeling of UPFC, and the
succeeding section provides the simulation results and analysis.
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