09-02-2013, 12:34 PM
USING ACTIVE POWER FILTERS TO IMPROVE POWER QUALITY
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Abstract.
This paper describes different power quality
problems in distribution systems and their solutions with
power electronics based equipment. Shunt, hybrid and
series active power filters are described showing their
compensation characteristics and principles of operation.
Different power circuits topologies and control scheme for
each type of active power filter are analyzed. The
compensation characteristics of each topology with the
respective control scheme are proved by simulation and
experimentally.
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.
A flexible and versatile solution to voltage quality
problems is offered by active power filters. Currently they
are based on PWM converters and connect to low and
medium voltage distribution system in shunt or in series.
Series active power filters must operate in conjunction
with shunt passive filters in order to compensate load
current harmonics. Shunt active power filters operate as a
controllable current source and series active power filters
operates as a controllable voltage source. Both schemes
are implemented preferable with voltage source PWM
inverters, with a dc bus having a reactive element such as
a capacitor. Active power filters can perform one or more
of the functions required to compensate power systems
and improving power quality. As it will be illustrated in
this paper, their performance depend on the power rating
and the speed of response. The selection of the type of
active power filter to improve power quality depends on
the source of the problem as can be seen in Table 1.
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.
HYBRID ACTIVE POWER FILTER
Active power filters can be used with passive filters
improving compensation characteristics of the passive filter,
and avoiding the possibility of the generation of series or
parallel resonance. One example of this combination is the
series active power filter shown in section V. In this
scheme, (Fig. 5), if the passive filters are not connected, the
series active power filter can compensate only voltage
regulation, and voltage unbalance. If passive filters are not
used in Fig. 5, the topology can not compensate current
harmonic components.
Another possibility to combine the compensation
characteristics of passive and active power filters is by
connecting the active passive filter in series with the passive
one, as shown in Fig. 13. In this way, the compensation
characteristics of the passive filter is significantly improved,
since the active scheme generated voltage harmonic
components across the terminal of the primary windings of
the series transformer, forcing current harmonics generated
by the load to circulate through the passive filter instead of
the power distribution system.
CONCLUSION
In this paper the use and advantages of applying active
power filters to compensation power distribution systems has
been presented. The principles of operation of shunt, series,
and hybrid active power filters has been presented. Alos, a
brief description of the state of the art in the active power
filter market has been described. The shunt active power filter
performance under fault power distribution system was
discussed.