15-01-2013, 12:22 PM
The Unified Power Quality Conditioner: The Integration of Series- and Shunt-Active Filters
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Abstract
This paper deals with unified power quality conditioners
(UPQC’s), which aim at the integration of series-active
and shunt-active filters. The main purpose of a UPQC is to compensate
for voltage flicker/imbalance, reactive power, negativesequence
current, and harmonics. In other words, the UPQC
has the capability of improving power quality at the point of
installation on power distribution systems or industrial power
systems.
This paper discusses the control strategy of the UPQC, with a
focus on the flow of instantaneous active and reactive powers inside
the UPQC. Experimental results obtained from a laboratory
model of 20 kVA, along with a theoretical analysis, are shown to
verify the viability and effectiveness of the UPQC.
INTRODUCTION
ASPECIALLY designed 12-pulse thyristor rectifier of 5–8
MVA is required to generate a strong magnetic field
with high stability as a low-voltage high-current dc power
supply for super-conductive material tests, proton synchrotron
accelerators, and so on. The thyristor rectifier has to be
equipped with a filter consisting of reactors and capacitors on
its dc terminals to prevent current ripples from flowing into the
electromagnet. The filter can easily eliminate high-frequency
current ripples accompanying ac/dc power conversion. It is,
however, difficult to reduce low-frequency current ripples
caused by a supply voltage flicker with a frequency range
from 1 to 20 Hz. It is pointed out that such a voltage flicker
appearing at the point of common coupling (PCC) results from
large capacity arc furnaces and/or cycloconverters installed on
the same or upstream power system.
GENERAL UPQC
Fig. 1 shows a basic system configuration of a general
UPQC consisting of the combination of a series-active and
shunt-active filter [1]. The general UPQC will be installed at
substations by electric power utilities in the near future. The
main purpose of the series-active filter is harmonic isolation
between a subtransmission system and a distribution system.
In addition, the series-active filter has the capability of voltageflicker/
imbalance compensation as well as voltage regulation
and harmonic compensation at the utility-consumer point of
common coupling (PCC). The main purpose of the shuntactive
filter is to absorb current harmonics, compensate for
reactive power and negative-sequence current, and regulate
the dc-link voltage between both active filters.
EXPERIMENTAL SYSTEM
Fig. 2 shows an experimental system configuration of a
specific UPQC. The aim of the specific UPQC is not only to
compensate for the current harmonics produced by a 12-pulse
thyristor rectifier of 20 kVA, but also to eliminate the voltage
flicker/imbalance contained in the receiving terminal voltage
from the load terminal voltage . The receiving terminal
in Fig. 2 is often corresponding to the utility-consumer
point of common coupling in high-power applications. The
UPQC consists of a 1.5-kVA series-active filter and a 0.5-
kVA shunt-active filter. The dc links of both active filters
are connected to a common dc capacitor of 2000 F. The
12-pulse thyristor bridge rectifier is considered a voltageflicker/
imbalance-sensitive load identical to a dc power supply
for super-conductive material tests.
COMPENSATING CHARACTERISTICS
A. Control Circuit
Fig. 7 shows a control circuit of the series-active filter
based on the combined control method of (3). The control
circuit consists of two – transformation circuits and
, which take the detected three-phase supply current
and the detected three-phase receiving terminal voltage ,
respectively.
Two first-order high-pass filters (HPF’s) with cutoff frequencies
of 1.6 Hz in are used for extraction of
current harmonics , while two more HPF’s with cutoff
frequencies of 0.8 Hz in are used for extraction of
voltage flicker/imbalance . The control gain is set to 2
. The control circuit is implemented in a DSP(TMS320C20).
CONCLUSION
This paper has dealt with UPQC’s, the aim of which is
not only to compensate for current harmonics produced by
nonlinear loads, but also to eliminate voltage flicker/imbalance
appearing at the receiving terminal from the load terminal.
Theoretical comparison among three types of control methods
for the series-active filter has clarified that the combination of
current and voltage-detecting methods is suitable for voltageflicker/
imbalance elimination and harmonic compensation. The
flow of instantaneous active and reactive powers has shown
that installation of the shunt-active filter is effective in performing
dc-voltage regulation.
Although the specific UPQC dealt with in this paper provides
no power factor correction in order to minimize the
required rating of the shunt-active filter, the general UPQC
is capable of improving “power quality” as well as improving
power factor.