Page 374 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 355

Fig. 8.75 Typical waveforms of diode rectifiers: (a) voltage V bus ; and (b) current I S .

harmonic pollution and to increase the line power factor. However, in practical
applications passive filters have the demerits of fixed compensation, being relatively
bulky, and resonating with other system elements. The increased severity of har-
monic pollution in power networks has attracted the attention of power electronics
and power system engineers to develop dynamic and adjustable solutions to the
power quality problems giving rise to the active power filters technology.
Active filtering provides compensation for harmonics, reactive power, and/or
neutral current in AC networks. It is also used to eliminate voltage harmonics, to
regulate terminal voltage, to suppress voltage flicker, and to improve voltage balance
in three-phase systems. This wide range of objectives is achieved either individually
or in combination, depending upon the requirements and control strategy and
configuration that have to be selected appropriately.
Active filters are based on sophisticated power electronics and can be much more
expensive than passive filters depending upon the application. However, they have
the distinctive advantage that they do not resonate with the system. They can be used
in very difficult circumstances where passive filters cannot operate successfully due to
parallel resonance complications. They can also address more than one harmonic at a
time and combat other power quality problems such as flicker. They are particularly
useful for large, distorting loads fed from relatively weak points on the power system.
The basic idea behind the active power filters is to replace the portion of the sine
waveform that is missing in the current and/or voltage in a non-linear load. An
electronic control monitors the line voltage and/or current, switching the power
electronics very precisely to track the load current or voltage and force it to be
sinusoidal. Many circuit topologies for series and/or shunt active filtering exist and
have been presented in Chapter 6.