Page 265 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 253

6.8 Active filters (AFs)

The solid-state power electronic converters can be used as part of an apparatus to
control electric loads such as adjustable speed electric motor drives, to create regu-
lated power supplies, etc. The same equipment however generates harmonics and the
currents drawn from the AC mains are highly reactive. The harmonics injected back
into the AC system create serious problems and the `pollution' of the supply net-
works has become a major concern for all utilities and power engineers.
There are ways to rectify the problems associated with `polluted' power networks.
Filtering or power conditioning which may include other more sophisticated func-
tions for the equipment used is therefore not only required in electric power sys-
tems but is also considered a mature technology as far as passive elements are
concerned.
A combination of inductive±capacitive networks has been used successfully in
most cases to filter harmonics and capacitor banks have been employed to improve
the power factor of a plant. Such conventional solutions have fixed levels of perform-
ance, are usually bulky and create resonance phenomena.
The requirements for harmonics and reactive power compensation along with the
continuous development of power electronics have resulted in dynamic and adjust-
able solutions for the pollution of the AC networks. Such equipment is based on
power electronic converters. The converters interacting with the network to filter
harmonics or compensate for reactive/real power they are known as active filters or
power conditioners or power quality equipment.
In this section we present the various converter-based topologies used as active
filters or power conditioners in schematic form only. It is beyond the scope of this
book to provide any more detailed information on this subject, but rather show the
potential of the VSC technology for power system applications.
Converter based active filtering topologies are used to provide compensation for:

. harmonics
. reactive power
. neutral currents
. unbalanced loads.
The applications include different cases such as:

. single-phase
. three-phase with floating neutral (three wires only)
. three-phase with neutral (four wire).
A number of topologies are used as active filters in series or shunt connection along
with a combination of them as well in series/shunt configuration. The series topo-
logies are normally used to deal with:
. voltage harmonics
. spikes
. sags
. notches