Prefiltered Synchronization Structure for Grid-Connected Power Converters to Reduce the Stability Impact of PLL Dynamics

Grid-connected converters under weak grid conditions could present instabilityassociated 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 prefilter, based on a filtered quadrature signal generator (FQSG), is proposed in this article to improve the dynamics of the controller's performance. The small-signal impedance model, implemente as a multiple-input multiple-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.