Transient Synchronization Stability Control Strategy for Virtual Synchronous Converter Based on Phase Difference Locking

With the increasing penetration of renewable energy sources, power systems require more grid-forming converters. Grid-forming converters with virtual synchronous generator control have transient stability problems similar to those of synchronous machines. However, the active power reference, frequency, and phase in virtual synchronous generators are artificially constructed and can be changed fast. This provides new approaches to improve the transient synchronization stability. Most existing virtual synchronous generator controls generate the internal voltage phase by integrating the frequency, resulting in limited control capability, which makes it hard to stop power angle divergence during deep voltage sags. This paper proposes a transient synchronization stability control strategy based on phase difference locking. Under deep voltage sags, the phase difference between the internal voltage and the terminal voltage is locked to prevent divergence of the power angle, while under shallow sags, the virtual synchronous generator control is retained to maintain active power support. Moreover, a smooth post-fault transition is ensured. The proposed strategy achieves stability and support functions in single converter and multi-node systems. In the single converter test, the maximum frequency deviation of the converter during the transient process decreased from 0.043 p.u. to 0.009 p.u. In the 39-bus test under deep voltage sag conditions, the maximum frequency deviation of the converters during the transient process was reduced from 0.214 p.u. and 0.109 p.u. to 0.016 p.u. and 0.027 p.u., respectively.