As the power systems are becoming more complex, there is a need for careful design
of new devices for controlling the power flow in transmission systems, which should
be flexible enough to adapt to any momentary system conditions. The operation of an
AC power transmission line is generally constrained by limitations of one or more
network parameters and operating variables. By using Flexible AC Transmission System
(FACTS) devices such as Static Synchronous Compensator (STATCOM),
Thyristor-Controlled Series Capacitor (TCSC), Thyristor-Controlled Phase angle Regulator (TCPR) and
Unified Power Flow Controller (UPFC), the bus voltages, line impedances and phase angles
in the power system can be regulated rapidly and flexibly.
UPFC consists of two switching converters, as shown in
Figure 1. These converters are operated from a common
DC link provided by a DC storage capacitor. Converter 2
provides the power flow control of UPFC by injecting an
AC voltage Vpq with controllable
magnitude and phase angle in series with the transmission line via a series transformer. Converter 1 is
to absorb or supply the real power demand of converter 2 at the common
DC link. It can also absorb or generate controllable reactive power and provide shunt reactive power
compensation (Hingorani and Gyugyi, 2001).
The two voltage source model of UPFC is converted into two power injections in
polar form for power flow studies with approximate impedances, as shown in Figure 2.
The advantage of power injection representation is that it does not destroy the
symmetric characteristics of admittance matrix. The voltage sources can be represented by
the relationship between the voltages and amplitude modulation ratios and phase shift of
the UPFC. In this model, the shunt transformer impedance, the transmission line
impedance and the series transformer impedance are assumed to be constant and no power loss
is considered with the UPFC. |
