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Advances in Power Electronics
Volume 2016 (2016), Article ID 9123747, 14 pages
Research Article

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

1Faculty of Electrical Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia
2Department of Electrical Engineering, Faculty of Engineering, Arak University, Arak, Iran
3Young Researchers and Elite Club, Islamic Azad University, Saveh Branch, Saveh, Iran

Received 6 March 2016; Revised 14 June 2016; Accepted 30 June 2016

Academic Editor: Don Mahinda Vilathgamuwa

Copyright © 2016 Mohammad Reza Miveh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


This paper proposes an improved multiloop control strategy for a three-phase four-leg voltage source inverter (VSI) operating with highly unbalanced loads in an autonomous distribution network. The main objective is to balance the output voltages of the four-leg inverter under unbalanced load conditions. The proposed control strategy consists of a proportional-integral (PI) voltage controller and a proportional current loop in each phase. The voltage controller and the current control loop are, respectively, used to regulate the instantaneous output voltage and generate the pulse width modulation (PWM) voltage command with zero steady-state tracking 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 far from being straightforward. In order to cope with this challenge, the stationary reference frame equivalent of the voltage controller in the rotating frame is derived. Subsequently, a systematic design based on a frequency response approach is provided. Simulation results are also carried out using the DIgSILENT PowerFactory software to verify the effectiveness of the suggested control strategy.