Active Power Factor Controller

Passive filters are used to provide a low impedance path for harmonic currents so that they flow in the filter and not the supply. The filter may be designed for a single harmonic or for a broad band depending on requirements. Sometimes it is necessary to design a more complex filter to increase the series impedance at harmonic frequencies and so reduce the proportion of current that flows back onto the supply.

Simple series band stop filters are sometimes proposed, either in the phase or in the neutral. A series filter is intended to block harmonic currents rather than provide a controlled path for them so there is a large harmonic voltage drop across it. This harmonic voltage appears across the supply on the load side.

Since the supply voltage is heavily distorted it is no longer within the standards for which equipment was designed and warranted. Some equipment is relatively insensitive to this distortion, but some is very sensitive. Series filters can be useful in certain circumstances, but should be carefully applied; they cannot be recommended as a general purpose solution.