State-space current control for four-leg grid-connected PWM rectifiers with active power filtering function

This paper presents a control strategy for four-leg grid-connected PWM rectifiers that have an active power filtering function. The four-leg voltage source converter topology gives ability to control an input current in the neutral leg. That is important for the active power filtering function when there is an unbalanced load. The control strategy is based on a voltage oriented method that has a cascaded DC voltage and current controllers designed in a rotating reference frame. The DC voltage controller is a PI-type, whereas the input current is shaped by a full-state feedback controller. The novelty of the proposed solution is the control structure that has modified the DC voltage error signal by adding a discontinuity element and reduction the number of current signals by half. The discontinuity element is needed to provide a constant reference current signal during active power filtering operation. The paper describes in detail the design procedure for the voltage and current controllers. Numerical results for active rectification and active power filtering operations are presented and discussed. Numerical results have been obtained by using a discrete control model.