19-03-2012, 01:58 PM
USING ACTIVE POWER FILTERS TO IMPROVE POWER QUALITY
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- INTRODUCTION
The proliferation of microelectronics processors in a
wide range of equipments, from home VCRs and digital
clocks to automated industrial assembly lines and hospital
diagnostics systems, has increased the vulnerability of
such equipment to power quality problems [1]. These
problems include a variety of electrical disturbances,
which may originate in several ways and have different
effects on various kinds of sensitive loads. What were
once considered minor variations in power, usually
unnoticed in the operation of conventional equipment,
may now bring whole factories to standstill. As a result of
this vulnerability, increasing numbers of industrial and
commercial facilities are trying to protect themselves by
investing in more sophisticate equipment to improve
power quality [2]. Moreover, the proliferation of nonlinear
loads with large rated power has increased the
contamination level in voltages and currents waveforms,
forcing to improve the compensation characteristics
required to satisfy more stringent harmonics standard [3],
[4].
SOLUTIONS TO POWER QUALITY PROBLEMS
There are two approaches to the mitigation of power
quality problems. The first approach is called load
conditioning, which ensures that the equipment is less
sensitive to power disturbances, allowing the operation
even under significant voltage distortion. The other
solution is to install line conditioning systems that
suppress or counteracts the power system disturbances.
Power Circuit Topology
Shunt active power filters are normally implemented
with pulse-width modulated voltage source inverters. In
this type of applications, the PWM-VSI operates as a
current controlled voltage source. Traditionally, 2 level
PWM-VSI have been used to implement such system.
However, in the past years multilevel PWM voltage source
inverters have been proposed to develop active power
filters for medium voltage applications. Also, active power
filters implemented with multiple VSI connected in
parallel to a dc bus but in series through a transformer or
in cascade has been proposed in the technical literature.
SERIES ACTIVE POWER FILTERS
It is well known that series active power filters
compensate current system distortion caused by non-linear
loads by imposing a high impedance path to the current
harmonics which forces the high frequency currents to flow
through the LC passive filter connected in parallel to the
load [5]. The high impedance imposed by the series active
power filter is created by generating a voltage of the same
frequency that the current harmonic component that needs
to be eliminated. Voltage unbalance is corrected by
compensating the fundamental frequency negative and zero
sequence voltage components of the system.