PLL Synchronization Stability of Grid-Connected Multiconverter Systems

Phase-locked loop (PLL) synchronization instability of grid-connected converters under grid faults is a serious concern, in particular for multiconverter plants/stations connected to a weak grid. The multiconverter interaction can lead a large number of converters to lose stability successively. The PLL synchronization instability of a single-converter system has been well studied previously, but it remains unclear how the multiconverter interaction affects the synchronization stability. This article is aimed to fill this gap, in which multiconverter modeling and interaction analysis, stability level assessment, and stability improvement are addressed. It is found that the interaction is detrimental to the near-end converters, but it is beneficial to the far-end ones. From the most prone-to-instability converter, a simple method is proposed to efficiently assess the synchronization stability level of the entire system. In the aspect of improving synchronization stability, the existing methods are evaluated comparatively, and a simple feedforward-compensated PLL is proposed to address the disadvantage of the existing methods. Among them, recommendations are provided by considering multiple aspects of performance and different application requirements. The performance of the methods is verified by hardware-in-the-loop experiments. The results of this study provide a simple engineering method for grid-connected multiconverter synchronization stability assessment and practical guidelines for the selection of stability improvement methods.