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ABSTRACT
Flexible alternating current transmission systems (FACTS)
technology opens up new opportunities for controlling power and
enhancing the usable capacity of present, as well as new and
upgraded lines. The Unified Power Flow Controller (UPFC) is a
second generation FACTS device which enables independent
control of active and reactive power besides improving reliability
and quality of the supply. This paper describes the real and
reactive power flow control through a transmission line by
placing the UPFC at the sending end of an electrical power
transmission system. The power flow control performance of the
UPFC is compared with that of the other FACTS device called
Static Synchronous Series Compensator (SSSC). Simulations are
carried out in Matlab/Simulink environment to validate the
performance of the UPFC.
INTRODUCTION
In today’s highly complex and interconnected power systems,
there is a great need to improve electric power utilization while
still maintaining reliability and security. While power flows in
some of the transmission lines are well below their normal
limits, other lines are overloaded, which has an overall effect on
deteriorating voltage profiles and decreasing system stability and
security. Because of all that, it becomes more important to
control the power flow along the transmission lines to meet the
needs of power transfer. On the other hand, the fast development
of solid-state technology has introduced a series of power
electronic devices that made FACTS a promising pattern of
future power systems.
is clear that the power flow is a function of transmission line
impedance, the magnitude of the sending end and receiving end
voltages and the phase angle between voltages. By controlling
one or a combination of the power flow arrangements, it is
possible to control the active, as well as, the reactive power flow
in the transmission line. With FACTS technology, such as Static
Var Compensator (SVC), Static Synchronous Compensator
(STATCOM), Static Synchronous Series Compensator (SSSC)
and Unified Power Flow Controller (UPFC) etc., bus voltages,
line impedances and phase angles in the power system can be
regulated rapidly and flexibly. These FACTS controllers are
based on voltage source converters. Thus, FACTS can facilitate
the power flow control, enhance the power transfer capability,
decrease the generation cost, and improve the security and
stability of the power system [1]-[2].
A Static Synchronous Series Compensator (SSSC) is a member
of FACTS family which is connected in series with a power
system. It consists of a solid state voltage source converter which
generates a controllable alternating current voltage at
fundamental frequency. When the injected voltage is kept in
quadrature with the line current, it can emulate as inductive or
capacitive reactance so as to influence the power flow through
the transmission line [3]-[5]. While the primary purpose of a
SSSC is to control power flow in steady state, it can also improve
transient stability of a power system [6].
The UPFC is a member of the FACTS family with very attractive
features [7]. The UPFC is able to control, simultaneously or
selectively, all the parameters affecting power flow in the
transmission line (voltage, impedance, and phase angle) [8]. It is
recognized as the most sophisticated power flow controller
currently, and probably the most expensive one. The UPFC,
which consists of a series and a shunt converter connected by a
common dc link capacitor, can simultaneously perform the
function of transmission line real/reactive power flow control in
addition to UPFC bus voltage/shunt reactive power control [9]-
[10]. The shunt converter of the UPFC controls the UPFC bus
voltage/shunt reactive power and the dc link capacitor voltage.
The series converter of the UPFC controls the transmission line
real/active power flows by injecting a series voltage of adjustable
magnitude and phase angle [11]-[13], on the other hand, the
series part known as SSSC can be controlled without restrictions.
The phase angle of series voltage can be chosen independently
from line current between 0 and 2π, and its magnitude is variable
between zero and a defined maximum value. The parallel part
known as STATCOM injects an almost sinusoidal current of
variable magnitude at the point of connection. In this paper, a
single machine infinite bus system with and without UPFC has been simulated in Matlab/Simulink environment. The real and
reactive power flow control performance of the UPFC has been
evaluated and is compared with that of SSSC. The simulation
results show the effectiveness of UPFC on real and reactive
power flow control through a transmission line.
2. FACTS CONTROLLERS
The basic principles of the following FACTS controllers, which
are used in the single machine infinite bus system under study,
are discussed briefly.
2.1 Unified Power Flow Controller (UPFC)
The UPFC is made out of two voltage-source converters (VSCs)
with semiconductor devices having turn-off capability, sharing a
common dc capacitor and connected to a power system through
coupling transformers. The basic structure of UPFC is shown in
Fig.1. The shunt converter is primarily used to provide the real
power demand of the series converter at the common dc link
terminal from the ac power system. It can also generate or absorb
reactive power at its ac terminal, which is independent of the
active power transfer to (or from) the dc terminal. Therefore,
with proper control, it can also fulfill the function of an
independent advanced static VAR compensator providing
reactive power compensation for the transmission line and thus
executing indirect voltage regulation at the input terminal of the
UPFC.
The series converter is used to generate a voltage at the
fundamental frequency with variable amplitude and phase angle,
which is added to the ac transmission line by the series
connected boosting transformer. The inverter output voltage
injected in series with the line can be used for direct voltage
control, series compensation, phase shifting, and their
combinations. This voltage source can internally generate or
absorb all the reactive power required by the different type of
controls applied and transfers active power at its dc terminal.
The reactive power is generated/absorbed independently by each
converter and does not flow through the dc link [14]-[15]. The dc
link provides a path to exchange active power between the
converters. The series converter injects a voltage in series with
the system voltage through a series transformer. The power flow
through the line can be regulated by controlling the magnitude
and angle of the series-injected voltage. The injected voltage and
line current determine the active and reactive power injected by
the series converter. The converter has a capability of
electronically generating or absorbing the reactive power.
However, both the series and shunt converters can independently
exchange reactive power with the ac system. However, the
injected active power must be supplied by the dc link, in turn
taken from the ac system through the shunt converter. When the
losses of the converters and the associated transformers are
neglected, the overall active power exchange between the UPFC
and the ac system becomes zero