16-08-2012, 11:36 AM
A New Approach to Harmonic Compensation in Power Systems-A Combined System of Shunt Passive and Series Active Filters
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ABSTRACT
A new approach to compensate for harmonics in power
systems is proposed. It is a combined system of a shunt passive filter and
a small rated series active filter. The compensation principle is descibed,
and some interesting filtering characteristics are discussed in detail
theoretically. Excellent practicability and validity to compensate for
harmonics in power systems are demonstrated experimentally.
INTRODUCTION
Background
HA RMONIC interference problems generated by bulk thyristor converters become increasingly serious as they
are widely used in industrial applications and
transmission/distribution systems. So far, shunt passive filters
have hitherto been used to suppress harmonics in power
systems. However, shunt passive filters have many problems
to discourage their applications. As shown in Fig. 1, a shunt
passive filter exhibits lower impedance at a tuned harmonic
frequency than the source impedance to reduce the harmonic
currents flowing into the source. In principle, filtering characteristics
of the shunt passive filter are determined by the
impedance ratio of the source and the shunt passive filter.
Therefore the shunt passive filter has the following problems.
SYSTEMCO NFIGURATION
Fig. 3 shows a system configuration of the proposed
approach to harmonic compensation, which is an experimental
circuit developed in this paper. Fig. 4 shows a detailed
circuit of a series active filter on a per-phase base. A passive
filter consisting of a 5th- and 7th-tuned LC filter and a
high-pass filter is shunted with a three-phase six-pulse thyristor
converter of rating 20 kVA, which is considered a typical
harmonic-producing load. The circuit constants of the shunt
passive filter of rating 10 kVA are shown in Table I. Three
single-phase voltage-source PWM inverters of rating 0.45
kVA are inserted in series with the source impedance through
three single-phase current transformers (CT’s; turns ratio =
1:20), thus forming a series active filter.
COMPENSATIPORNIN CIPLE
Assuming for simplicity’s sake that the voltage-source
PWM inverter is an ideal controllable voltage source uc, Fig.
3 is represented on a per-phase base by Fig. 5. The threephase
thyristor converter is also assumed to be a current
source i, due to. the presence of sufficient inductance on the
dc side. Here, Z, is the equivalent impedance of the shunt
passive filter, the constants of which are shown in Table I,
and Z, is the source impedance.
CONCLUSION
In this paper, the combined system of a shunt passive filter
and a small rated series active filter was proposed, giving
both practical and economical points of view. The compensation
principle applied to this system was quite different from
conventional shunt and series active power filters, thus succeeding
in better filtering characteristics and lower initial and
running costs. The required rating of the series active filter is
mainly determined by the quality factor of the shunt passive
filter. The quality factor of the shunt passive filter used in this
experiment was 14. Since it is 50-80 for a practical shunt
passive filter in power systems, the required rating of the
series active filter will be reduced to less than 1%, as
compared with the rating of a three-phase six-pulse thyristor
converter or cycloconverter. It is believed that the combined
system is the most suitable to harmonic compensation for
large rated thyristor converters and cycloconverters.