The Westek WestSine active filters are based on IGBT technology and utilise a somewhat similar method to that used in inverters and converters, to generate an anti-phase signal to the harmonic current components produced by the load. The filters are connected in parallel with the circuit they ’protect’ and draw leading phase angle current in order to provide DC link power for the converter. The latter consists of insulated gate bi-polar transistors (IGBT) operating in switching mode to generate the required harmonics balancing signals.

Wide-band current transformers in two of the phases measure the load current, and an electronic notch filter rejects the fundamental frequency component and passes the other components including non-harmonic (including transients) as well as harmonics and inter-harmonics to controlled attenuators.

The output from the notch filter-fed attenuators match the output signals to the IGBT switching unit so as to generate anti-phase current according to the rating of the downstream load. The control logic for the IGBT’s modulate their output to achieve this precise matching of the non-linear current components and harmonics ranging from the 2nd to the 51st order.

The Westek WestSine filters respond to harmonic load currents as well as other non-linear components, and in this way act in a distinctively different way to passive filters. Important advantages include the non-interference with the supply circuit impedance parameters, as there are no series elements in the filtering operation. In other words, resonance frequencies cannot arise. An additional advantage of the Westek WestSine active filters is their leading displacement power factor, which therefore counterbalances the usually lagging power factor of many loads.

Westek's WestSine active filters may be connected in parallel and in three-wire execution are available in current ranges from 25 to 300 amps and voltages (4-wire models range from 25 to 100 amps) and voltages to 690 volts. The three-wire models are suitable for non-triplen harmonics (eg: the third harmonic) situations, whereas the four-wire filters will remove triplens as well as all other harmonics.