01-01-2013, 02:25 PM
A New Control Strategy for the Unified Power Flow Controller
A New Control Strategy for the Unified Power.pdf (Size: 254.84 KB / Downloads: 34)
Introduction
The promising concept of the Flexible AC
Transmission System (FACTS) makes it possible to
achieve fast and reliable power system control by
means of power electronic devices. The Unified
Power Flow Controller (UPFC), which is the most
versatile FACS device, has the capabilities of
controlling power flow in the transmission line,
improving the transient stability, mitigating system
oscillations and providing voltage support [1][2][5].
It can control all three basic power transfer
parameters (line impedance, voltage magnitude and
phase angle) independently or simultaneously in any
appropriate combinations. Previous efforts have
focused mainly on controlling system steady state
power flows and improving system stability. This
paper will present the dynamic control of the UPFC.
Firstly, a dynamic model of the UPFC is derived
based on the synchronous dq-frame in this article.
Secondly, the PI based control strategy for the shunt
part and the series part of UPFC are described
respectively in the same synchronous frame and used
to provide real and reactive power flow control along
the transmission line at its series output end, while
regulating the magnitude of the voltage at its shunt
input end and maintaining the DC-link capacitor
voltage constant. Finally digital simulation results
obtained from two power systems are presented to
illustrate the contributions of the new control
approach.
The UPFC Control System
In general, the UPFC has three control parameters:
magnitude and angle of series injected voltage and
shunt reactive current [4]. We can achieve real and
reactive power flow control independently by
injecting series voltage with appropriate magnitude
and angle. In the synchronous rotating dq frame, the
injected voltage can be split into Ed and Eq . By
controlling Ed and Eq properly, different active and
reactive power flow objectives can be accomplished.
It is well known that shunt reactive current can
provide reactive power support and shunt active
current provides the DC-link capacitor voltage
regulation.
Conclusions
This paper investigates the new control strategy for
UPFC dynamic analysis. By implementing a
synchronously rotating d-q frame, a dynamic model
and control system for UPFC has been derived. With
the new control approach, UPFC can perform
independent control of transmittable real and reactive
power at series output while regulating the shunt
input voltage and maintaining the DC-link Capacitor
voltage constant. It can also damp the power
oscillations and improve the transient stability of the
system by appropriate modulation of the controller
references.