22-11-2012, 06:01 PM
Modelling of Unified Power Flow Controller into Power Systems using P-Spice
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Abstract
Flexible AC Transmission Systems (FACTS)
use power electronic components to enhance
controllability and capability of electrical power system.
FACTS devices are able of opportunely modify voltage,
phase angle and/or impedance and then the power flows at
particular points in power systems [1-2].
One of the more intriguing and potentially most versatile
class of FACTS device is the Unified Power Flow
Controller (UPFC).
Two are the main purposes of this paper. The first one is to
illustrate a UPFC model with two separate control systems
for the shunt and the series inverters realising an
appropriate co-ordination between them. The second
purpose is to implement this UPFC model using the PSpice
(Simulation Program with Integrated Circuit
Emphasis) as simulation program.
INTRODUCTION
The deregulation and competitive environment in the
contemporary power networks will imply a new scenario in
terms of load and power flows condition and so causing
problems of line transmission capacity. But, nowadays
some problems exist to change the present structure of
transmission system. So, the need for new power flow
controllers capable of increasing transmission capacity and
controlling power flows through predefined transmission
corridors will certainly increase.
For this reason, as well known in recent years a new
class of controllers, Flexible AC Transmission System
(FACTS) controllers has rapidly met with favour. Indeed,
the two main objectives of FACTS technology are to
control power flow and increase the transmission capacity
over an existing transmission corridor [1].
UPFC CHARACTERISTICS
The basic components of the UPFC are two voltage
source inverters (VSI's) sharing a common dc storage
capacitor, and connected to the system through coupling
transformers. One VSI is connected in shunt to the
transmission system via a shunt transformer, while the
other one is connected in series through a series
transformer. A basic UPFC functional scheme is shown in
Fig.1.
The series inverter is controlled to inject a symmetrical
three phase voltage system, vse, of controllable magnitude
and phase angle in series with the line to control active and
reactive power flows on the transmission line. So, this
inverter will exchange active and reactive power with the
line. The reactive power is electronically provided by the
series inverter, and the active power is transmitted to the dc
terminals. The shunt inverter is operated in such a way as
to demand this dc terminal power (positive or negative)
from the line keeping the voltage across the storage
capacitor Vdc constant. So, the net real power absorbed
from the line by the UPFC is equal only to the losses of the
two inverters and their transformers. The remaining
capacity of the shunt inverter can be used to exchange
reactive power with the line so to provide a voltage
regulation at the connection point.
CONCLUSIONS
This paper after a brief summary of the principal
characteristics of a UPFC, illustrates in detail the proposed
UPFC model with two separate control systems for the
series and shunt inverters and a control for their coordination.
Finally, the implementation of this model using
P-Spice as simulation program is shown and some
simulation results are illustrated to validate the
implemented P-Spice UPFC model. The results are obtained
for a PWM-based control technique, but it's very simple to
modify the inverter control technique such as phase
control. The next aim of authors is to modify the illustrated
UPFC P-Spice model in such a way to be able to perform a
damping oscillation operating function.