Improved VSG Controlled SST in a Low-Voltage AC Distribution Network

More and more distributed generators and single-phase loads are connected to the distribution network dispersedly, which aggravates three-phase voltage imbalance. To solve this problem, this paper applies a grid-side converter and a source-load-side converter to form a non-isolated solid-state transformer (SST), and two converters cascade in their DC sides. First, the control principle of a virtual synchronous generator (VSG) is briefly described, and the three-phase voltage unbalanced mechanism is analyzed. Then, two improved VSG control strategies are proposed. In the grid-side converter, DC droop control is introduced to active power control loop. A positive-sequence grid voltage feedforward control is proposed to realize the grid-side current balance when grid voltage is unbalanced, and DSOGI is used to lock the phase and extract the positive-sequence grid voltage. In the source-load-side converter, the quasi-proportional resonant control loop is cascaded to VSG power loop to suppress the negative and zero-sequence voltage components, thus the three-phase voltage balance can be ensured, especially when the DGs and loads are unbalanced. Finally, simulations and experimental results are presented to evaluate the effectiveness of the proposed techniques.