Variable structure control of four pole voltage source inverter for active filtering of nonlinear loads in 3-phase 4-wire systems

This paper presents the analysis of four leg voltage source inverter (VSI) used as an active filter (AF) for reactive power compensation, load balancing, harmonic elimination and neutral current compensation in three-phase four-wire nonlinear loads. Variable structure control (VSC) of the average DC bus capacitor of the AF is employed. A hysteresis rule based carrierless pulse width modulation (PWM) current control scheme is employed to generate the gating signals to the switching devices of the active filter. A set of three single-phase diode bridge rectifiers feeding a resistive-capacitive load is used for nonlinear loads on three-phase four-wire power systems. The merits of the proposed method of control are: (1) less complex and reliable active filter configuration; (2) fast controller action; (3) negligible transients in the DC bus voltage of the active filter at the time of instant load changes; (4) gradual convergence of controlled supply currents to new steady state values at the occurrence of abrupt load changes; (5) total harmonic distortion (THD) of the compensated load has been brought well below the standard harmonic specification IEEE-519 limit; (6) neutral current is reduced below 1%; and (7) simultaneous balancing of unbalanced loads. Simulated results confirm the validity of the proposed approach for three-phase (12.5 kW)/two-phase (8.3 kW)/single-phase (4.2 kW) nonlinear loads. The suggested method is expected to be widely used in future for active power filter designs produced for load compensation.