The AIM AHF-series active filters available from Westek Electronics 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 AHF 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 class D 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 feedback mechanism is hardwired and therefore responds, in real time, within 1/100th of a half-cycle to changes in line current. This important feature cannot be stressed enough because without this short response time active filters generally can under many circumstances causes more harm than good.
For example, software-controlled, digital systems tend to have longer response times and this can have deleterious effects in that corrective anti-phase current in fact no longer is anti-phase and therefore has to draw an artificial harmonic component from the supply side.
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 AHF-series 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 AHF-series active filters is their leading displacement power factor, which therefore counterbalances the usually lagging power factor of many loads.
AHF-series 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 (e.g. the third harmonic) situations, whereas the four-wire AIM filters will remove triplens as well as all other harmonics.