Enhancing Transient Stability of PLL-Synchronized Converters by Introducing Voltage Normalization Control

This paper presents a novel phase-locked loop (PLL) structure for improving the transient stability of grid-connected converters by introducing voltage normalization control (VNC) in the conventional PLL. First, the underlying mechanism of losing synchronization during a grid fault is analyzed, and it is revealed that the key factor is the significant decrease of voltage magnitude at the point of common coupling (PCC). In order to avoid the decrease of damping ratio due to the voltage dips, a voltage normalization control method is introduced by controlling the d-axis voltage magnitude to the rated value even during grid faults. To this end, the transient stability can be improved during grid faults. Phase portraits when using a conventional PLL and the PLL+VNC are both visualized for validating the effectiveness of the proposed structure. The performance of adding the additional voltage normalization control is analyzed through both small-signal and large-signal models. Finally, the experimental results are presented to verify the effectiveness of improved PLL structure for enhancing the transient stability.