An Improved Control Strategy for a Four-Leg Grid-FormingPower Converter under Unbalanced Load Conditions

This paper proposes an improved multiloop control strategy for a three-phase four-leg voltage source inverter (VSI) operatingwith highly unbalanced loads in an autonomous distribution network. The main objective is to balance the output voltages of thefour-leg inverter under unbalanced load conditions. The proposed control strategy consists of a proportional-integral (PI) voltagecontroller and a proportional current loop in each phase. The voltage controller and the current control loop are, respectively, used toregulate the instantaneous output voltage and generate the pulse width modulation (PWM) voltage command with zero steady-statetracking error and fast transient response. A voltage decoupling feedforward path is also used to enhance the system robustness.Since the outer voltage loop operates in the synchronous reference frame, tuning and stability analysis of the PI controller is farfrom being straightforward. In order to cope with this challenge, the stationary reference frame equivalent of the voltage controllerin the rotating frame is derived. Subsequently, a systematic design based on a frequency response approach is provided. Simulationresults are also carried out using the DIgSILENT PowerFactory software to verify the effectiveness of the suggested control strategy