Decoupled digital control of a three-phase four-leg inverter to feed balanced and unbalanced loads

This paper presents a digital control strategy for three-phase four-leg inverters to meet the requirements of international power quality standards for UPS, when balanced and unbalanced loads are feeding. The proposed control strategy is developed in the discrete-time dq0-reference frame. It has an inner current loop to control the inductors currents and an outer voltage control loop. The analysis of this paper demonstrates that using conventional controllers, the voltages of d and q axes can be decoupled, into the whole operating range, which allows the outer voltage controllers to be designed as independent SISO systems, meeting the requirements of the transient and permanent regimes imposed by Standards. To improve the performance when the inverter feeds unbalanced loads, this paper proposes to use a Proportional-Resonant controller only in the inner loop of the axis 0, achieving a simple control strategy and, consequently with a low computational cost. Simulation and experimental results allow validating the proposed control strategy.