Comparative Study of Symmetrical Controlled Grid-Connected Inverters

The asymmetrical dynamics of the conventional synchronous reference frame phase-locked loop (PLL) enhance the complexity of the system and endanger the stability of the grid-connected voltage source converters. In this article, the admittance modeling of three different symmetrical control strategies, i.e., voltage oriented control with symmetrical PLL, voltage modulated direct power control, and proportional-resonant control, is proposed and uniformed based on the same power tracking property. The voltage feedforward and decoupling of the controllers are both considered. Moreover, due to the symmetrical structure of the three control strategies, their admittance models can directly be modeled under \alpha \beta reference frame as single-input single-output systems. In addition, to reveal the similarity between different control methods and emphasize their differences, the admittance models are constructed by using the same framework. The admittance modeling is verified using frequency-scan, and the control performance is tested using MATLAB/Simulink Simscape Power System and experimental prototype. Finally, the stability criterion based on the proposed admittance modeling is verified by simulation results carried out in real-time simulation platform RTLAB from Opal-RT.