Control of a high power PWM current source rectifier

This paper describes the design of a digital control for PWM current source rectifiers (CSRs) in the range of 1 - 10MVA. A cascaded control structure is proposed: the inner loop controls the grid currents and the outer loop controls the dc-link current. The proposed grid-current control combines state feedback for active damping of the LC-filter with a servo compensator to increase robustness and guarantee zero steady-state error. It is implemented in the stationary reference frame, which avoids cross coupling as introduced by rotating reference frame transformation. The servo compensator contains a digital resonator and an integrator that eliminates offset currents. It is efficiently implemented as digital filter. The time varying reference for the instantaneous grid current is generated by a disturbance observer for grid voltage. This closed-loop technique inherently compensates the voltage drop across the filter inductance. DC-link current is controlled by a PI-controller. The power factor of the CSR is dynamically adjustable. The proposed control design is verified using a CSR prototype with 11 kVA rated output power. Experimental results are given.