Active Filters

Power conversion by virtue of its basic role produces harmonics due to the
slicing of either voltages or currents. To a large extent the pollution in the
utility supply and the deterioration of the power quality has been generated
or created by non-linear converters. It is therefore ironic that power converters
should now be used to clean up the pollution that they helped to create in
the first place.
In a utility system, it is desirable to prevent harmonic currents (which result
in EMI and resonance problems) and limit reactive power flows (which
result in transmission losses).
Traditionally, shunt passive filters, comprised of tuned LC elements and
capacitor banks, were used to filter the harmonics and to compensate for
reactive current due to non-linear loads. However, in practical applications
these methods have many disadvantages.