Selective Harmonic Elimination by Program- mable Pulse Width Modulation in Inverters

The objective of the electric utility is to deliver sinusoidal voltage at fairly constant magnitude throughout their system. This objective is complicated by the fact that there are loads on the system that produce harmonics currents. These currents result in distorted voltages and currents that can adversely impact the system performance in different ways. When harmonic currents flow through the impedances of the power system they cause corresponding voltage drops and introduce harmonics onto the voltage waveform. If harmonics do cause a problem, it is through the cumulative effect on the power system of numerous harmonic-generating loads. Problems may be experienced within the plant that generates the harmonics, or in other premises fed from the same supply. The main possible effects are: power factor less than unityi.e current higher than necessary for given power, increased heating of power plant , over-stress of power factor correction plant due to local resonance, noisy bus bars, electrical protection gear etc., over heating of neutral conductors(single –phase loads only). Harmonics must be reduced in order to reduce the size of filters. PWM technique is one of the energy processing methods used in power electronics. PWM applies a pulse train of fixed amplitude and frequency, only the width of the pulse is varied in proportion to the input voltage so that output voltage is constant but with less wastage of power at the output stage by eliminating harmonics.