Abstract
This paper proposes a new connection for a unified
power quality conditioner (UPQC) to improve the power quality
of two feeders in a distribution system. A UPQC consists of a series
voltage-source converter (VSC) and a shunt VSC both joined
together by a common dc bus. It is demonstrated how this device
is connected between two independent feeders to regulate the bus
voltage of one of the feeders while regulating the voltage across a
sensitive load in the other feeder. Since the UPQC is connected between
two different feeders (lines), this connection of the UPQC
will be called an interline UPQC (IUPQC). The structure, control
and capability of the IUPQC are discussed in this paper. The efficacy
of the proposed configuration has been verified through simulation
studies using PSCAD/EMTDC.
Index Terms—Distribution system, power quality, sensitive load,
voltage sag, voltage-source converter (VSC).
I. 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.
It is also possible to connect two VSCs to two different
feeders in a distribution system. In [9], a configuration called
IDVR has been discussed in which two DVRs are connected
in series with two separate adjacent feeders. The dc buses
of the DVRs are connected together. The IDVR absorbs real
Manuscript received May 11, 2005; revised March 22, 2006. This work was
supported by the research fund of the Central Power Research Institute (CPRI),
Bangalore, India under Research Project “Development of active power line
conditioners for power distribution systems.” Paper no. TPWRD-00278-2005.
A. K. Jindal and A. Joshi are with the Department of Electrical Engineering,
Indian Institute of Technology, Kanpur 208 016, India (e-mail:
jindal[at]iitk.ac.in; ajoshi[at]iitk.ac.in).
A. Ghosh is with the School of Engineering Systems, Queensland University
of Technology, Brisbane 4001, Australia (e-mail: a.ghosh[at]qut.edu.au).
Digital Object Identifier 10.1109/TPWRD.2006.881581
Fig. 1. Single-line diagram of an IUPQC-connected distribution system.
power from one feeder and maintains the dc link voltage to
mitigate 40% (about 0.6 p.u.) voltage sag in the other feeder
with balanced loads connected in the distribution system. It is
also possible to connect two shunt VSCs to different feeders
through a common dc link. This can also perform the functions
of the two DVRs mentioned above, albeit with higher device
rating.
This paper presents a new connection for a UPQC called interline
UPQC (IUPQC). The single-line diagram of an IUPQC
connected distribution system is shown in Fig. 1. Two feeders,
Feeder-1 and Feeder-2, which are connected to two different
substations, supply the system loads L-1 and L-2. The supply
voltages are denoted by and . It is assumed that the
IUPQC is connected to two buses B-1 and B-2, the voltages
of which are denoted by and , respectively. Further two
feeder currents are denoted by and while the load currents
are denoted by and . The load L-2 voltage is denoted
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