08-16-2017, 10:30 PM
The flexible ac trans mission system (FACTS) technology is the application of power electronics in
transmission systems. The main purpose of this technology is to control and regulate the electric variables in the
power systems. This is achieved by using converters as a controllable interface between two power system
terminals. The resulting converter representations can be useful for a variety of configurations. Basically, the
family of FACTS devices based on voltage source converters (VSCs) consists of a series compensator, a shunt
compensator, and a shunt/series compensator. The static Compensator (STATCOM) is a shunt connected device
that is able to provide reactive power support at a network location far away from the generators. Through this
reactive power injection, the STATCOM can regulate the voltage at the connection node. The static
synchronous series compensator (SSC) is a series device which injects a voltage in series with the transmission
line.
Ideally, this injected voltage is in quadrature with the line current, such that the SSC behaves like an
inductor or a capacitor for the purpose of increasing or decreasing the overall reactive voltage drop across the
line, and thereby, controlling the transmitted power. In this operating mode, the SSC does not interchange any
real power with the system in steady-state. The unified power-flow controller (UPFC) is the most versatile
device of the family of FACTS devices, since it is able to control the active and the reactive power, respectively,
as well as the voltage at the connection node.
The Unified Power Flow Controller (UPFC) is comprised of a STATCOM and a SSC, coupled via a
common DC page link to allow bi-directional flow of active power between the series output terminals of the SSC
and the shunt output terminals of the STATCOM. Each converter can independently generate (or) absorb
reactive power at its own AC terminal. The two converters are operated from a DC page link provided by a DC
storage capacitor.
The UPFC is not widely applied in practice, due to their high cost and the susceptibility to failures.
Generally, the reliability can be improved by reducing the number of components; however, this is not possible
due to the complex topology of the UPFC. To reduce the failure rate of the components, selecting components
with higher ratings than necessary or employing redundancy at the component or system levels. Un fortunately,
these solutions increase the initial investment necessary, negating any cost related advantages. Accordingly, new
approaches are needed in order to increase reliability and reduce cost of the UPFC.
The same as the UPFC, the DPFC is able to control all system parameters like line impedance,
transmission angle and bus voltage. The DPFC eliminates the common dc page link between the shunt and series
converters. The active power exchange between the shunt and the series converter is through the transmission
line at the third-harmonic frequency. The series converter of the DPFC employs the distributed FACTS (DFACTS)
concept. Comparing with the UPFC, the DPFC have two major advantages: 1) Low cost because of the
low voltage isolation and the low component rating of the series converter and 2) High reliability because of the
redundancy of the series converters and high control capability. DPFC can also be used to improve the power
quality and system stability such as power oscillation damping, Voltage sag restoration or balancing asymmetry.