Elimination of Static Angular Error and Stability Enhancement for Active Power-Synchronized Converter Under a Weak Grid

A phase-locked loop (PLL), as a synchronization unit commonly employed in grid-connected converters (GCCs), jeopardizes system stability under a weak grid. Therefore, synchronization units without a PLL, such as active power synchronization (APS), have received much attention. However, GCCs adopting APS suffer from two typical issues: static angular error and the risk of destabilization. In this paper, the mechanism for static angular error is analyzed, which is attributed to the presence of active power current, resulting in a non-zero q-axis voltage at the point of common coupling. To eliminate the static error in angular tracking, conventionally, a proportional–integral (PI) controller is integrated into APS, which arouses system instability. Thus, a closed-loop single-input single-output (SISO) model of a GCC is derived from angular perturbation to analyze the mechanism of this destabilization. It is revealed that the gain overshoot in the closed-loop SISO model resulting from the PI controller would lead to system instability under a weak grid. Consequently, according to the above mechanistic analysis, a composite direct damping controller is proposed, which achieves the elimination of static angular error while enhancing system stability. Finally, the effectiveness of the mathematical analysis and derived models are verified by experimental results.