Pre-filtered synchronization structure for grid-connected power converters to reduce the stability impact of PLL dynamics

Grid-connected converters under weak grid conditions could present instability associated with the synchronization elements and not directly with the current control loop. The impedance-based stability criterion is an effective tool to address the problem. To solve the negative effect of the synchronous reference frame phase-locked loop (SRF-PLL) on grid current control under weak grid conditions, a synchronization structure with pre-filter, based on a filtered quadrature signal generator (FQSG), is proposed in this paper to improve the dynamics of the controller’s performance. The small-signal impedance model, implemented as a multi-input multi-output system in the dq-frame, characterizes the effect of the FQSG-PLL dynamics on the current control loop. From it, the impedance-based analysis shows the reduction of the stability issues of the converter. The analysis results were verified by time-domain simulations and by experimental tests in a full-scale 2 MW converter test-bench.