DSP based control method of active filter: elimination of switching ripples

This paper deals with a DSP based implementation of a three-phase shunt active filter (AF) to compensate harmonics and reactive power of nonlinear loads. A three-phase diode bridge rectifier with R-L and R-C loads is taken as the nonlinear load. It is observed that the AF systems suffer from the problems of switching ripples and these ripples contribute considerably to the total harmonic distortion (THD) in the supply current. This investigation proposes a new, cost free and simple approach for the elimination of switching ripples. A PWM current controlled voltage source inverter (CC-VSI) is used as the AF. In current controlled VSI two types of PWM techniques, namely, carrierless (hysteresis) and carrier based techniques are used. Two types of current control techniques, namely, direct current control and indirect current control are designed to obtain PWM switching signals for the devices of the AF. A laboratory prototype model of an AF is tested with both the direct and indirect techniques. The capability of the indirect current control technique towards the elimination of switching ripples is established. Experimental results for steady state and transient operating conditions of the AF are given and discussed in detail.