One issue to be aware of with active filters, is that they are like the output of a VFD in that there is a PWM bridge connected to the supply. Because it is a switching waveform, there is switching current in the output waveform.
A switching filter is connected between the output bridge and the supply to attenuate the switching frequency curents, but depending on the attenuation of the switching filter and the impedance of the supply, there can be significant levels of the switching voltage appearing on the supply.
The switching frequency is above the 31st harmonic, so these voltages do not appear when checked with a power quality meter, but we hve many experiences where power factor correction capacitors and lighting circuits are badly affected by these switching voltages.
Where the supply impedance is very low, this is not an issue, but if the reactive impedance is high, then it can become a problem.
Phase shifting transformers are useful at reducing 5th and 7th harmonics by cancellation, but some higher order harmonics are actually additive rather than subtractive, so not a perfect solution. Additionally, because it is a subtractive method, it requires identical harmonic loads on each transformer to give best results.
A good broad band filter such as the Lineator is an efficient solution.
Mark Empson
Advanced Motor Control Ltd