Variable Structure Control of Four Pole Voltage Source Inverter for Active Filtering of Nonlinear Loads in 3-phase 4-wtoe 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 non-linear loads. A variable structure control (VSC) over the average dc bus capacitor of the AF is employed for the control. A hysteresis rule based carrierless pulse width modulation (PWM) current control is employed to generate the gating signals to the switching devices of active filter. A set of three singlephase diode bridge rectifier feeding resistive-capacitive load is used for non-linear loads on the three-phase fourwire system. 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 dc bus voltage of the active filter at die 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) Simultaneously balances the unbalanced load. Simulated results confirm die validity of die proposed approach for the three-phase (12.5 kW) / two-phase (8.3 kW) /single-phase (4.2 kW) nonlinear loadings. Suggested method is expected to be widely used in future for active filter designs produced for load compensation. method. Moran et al [5] suggested a mk> point capacitor inverter in series of the load for harmonic current elimination and Peng et al [6] have proposed a generalized instantaneous reactive power theory applicable to threephase four-wire system. Haddad et al have [7] discussed the impact of various parameters on the performance of AF, employing a capacitor mid-point inverter. In three-phase four-wire systems usually reported active filter configurations consist either of a mid point capacitor inverter (die main drawbacks of such filter are the need of two identical, large capacitors and shifting of neutral) or three single-phase VSI, which essentially results in use of three single-phase transformers and complex control circuitry. So sometimes it becomes essential to find an alternative, winch is free from constraints posed by above two configurations. Keeping in mind die relative advantages and disadvantages of filter configurations, a four pole voltage source inverter is chosen as; an active filter with variable structure voltage controller on dc bus of die active filter. The objective of diis paper is to present the analysis of a new control mediod of active filter to compensate non-linear unbalanced load completely, without deviating from sinusoidal nature of supply current even at die time of load disturbances. II. SYSTEM CONFIGURATION AND CONTROL SCHEME