28-11-2012, 02:55 PM
Interline Unified Power Quality Conditioner
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INTRODUCTION
VOLTAGE-SOURCE CONVERTER (VSC)-based custom
power devices are increasingly being used in custom
power applications for improving the power quality (PQ) of
power distribution systems. Devices such as distribution static
compensator (DSTATCOM) and dynamic voltage restorer
(DVR) have already been discussed extensively in [1]. A
DSTATCOM can compensate for distortion and unbalance in
a load such that a balanced sinusoidal current flows through
the feeder [2]. It can also regulate the voltage of a distribution
bus [3], [4]. A DVR can compensate for voltage sag/swell
and distortion in the supply side voltage such that the voltage
across a sensitive/critical load terminal is perfectly regulated
[5], [6]. A unified power-quality conditioner (UPQC) can
perform the functions of both DSTATCOM and DVR [7], [8].
The UPQC consists of two voltage-source converters (VSCs)
that are connected to a common dc bus. One of the VSCs is
connected in series with a distribution feeder, while the other
one is connected in shunt with the same feeder. The dc links of
both VSCs are supplied through a common dc capacitor.
SYSTEM DESCRIPTION
An IUPQC connected to a distribution system is shown in
Fig. 4. In this figure, the feeder impedances are denoted by the
pairs and . It can be seen that the two
feeders supply the loads L-1 and L-2. The load L-1 is assumed to
have two separate components—an unbalanced part (L-11) and
a non-linear part (L-12). The currents drawn by these two loads
are denoted by and , respectively. We further assume
that the load L-2 is a sensitive load that requires uninterrupted
and regulated voltage. The shunt VSC (VSC-1) is connected to
bus B-1 at the end of Feeder-1, while the series VSC (VSC-2)
is connected at bus B-2 at the end of Feeder-2. The voltages of
buses B-1 and B-2 and across the sensitive load terminal are denoted
by , , and , respectively. The aim of the IUPQC
is two-fold:
• to protect the sensitive load L-2 from the disturbances occurring
in the system by regulating the voltage ;
• to regulate the bus B-1 voltage against sag/swell and or
disturbances in the system.
In order to attain these aims, the shunt VSC-1 is operated as a
voltage controller while the series VSC-2 regulates the voltage
across the sensitive load.
CONCLUSIONS
The paper illustrates the operation and control of an interline
unified power quality conditioner (IUPQC). The device is connected
between two feeders coming from different substations.
An unbalanced and non-linear load L-1 is supplied by Feeder-1
while a sensitive load L-2 is supplied through Feeder-2. The
main aim of the IUPQC is to regulate the voltage at the terminals
of Feeder-1 and to protect the sensitive load from disturbances
occurring upstream. The performance of the IUPQC has been
evaluated under various disturbance conditions such as voltage
sag in either feeder, fault in one of the feeders and load change.
It has been shown that in case of a voltage sag, the phase angle
of the bus voltage in which the shunt VSC is connected plays
an important role as it gives the measure of the real power required
by the load. The IUPQC can mitigate a voltage sag of
about 0.6 p.u. (9 kV to 5.5 kV) in Feeder-1 and 0.33 p.u. (i.e.,
9 kV to 3 kV) in Feeder-2 for long duration. The IUPQC discussed
in the paper is capable of handling system in which the
loads are unbalanced and distorted. Extensive case studies have
been included to show that an IUPQC might be used as a versatile
device for improving the power quality in an interconnected
distribution system.