Survey of harmonic reduction techniques applicable as ancillary service of dispersed energy generators (DG)

Fundamental ways to reduce harmonics are: reduction of resonances, damping of harmonics and compensation of harmonics. Harmonic compensation is difficult to achieve, but can reduce harmonics to zero. However, if a rest distortion is allowed, harmonic compensation could be replaced for only harmonic damping. Harmonic damping by a resistive harmonic behavior of a power electronic converter of a dispersed generator (DG), is an attractive way of harmonic reduction. This damping can have effect on a total range of harmonics and cannot result in instabilities in the power system. The damping can be provided as an ancillary service of a power electronic converter in general. The resistance is virtual, therefore the energy involved is limited to losses in the converter. The effort to be taken, to implement this kind of ancillary service, is an extension in the control system of the converter, therefore costs can be kept minimal. There is however a contradiction with the needed measure for background harmonics, therefore harmonic reduction by damping must be limited to avoid wrong compensation for background harmonics, resulting in excessive currents through the distribution transformer, cables and/or lines. A combination with a series active filter on substation level, and harmonic damping dispersed over the distribution network, can avoid the wrong compensation for background harmonics, and be therefore an optimal solution for harmonic mitigation