Fast Power Control for VSCs to Enhance the Synchronization Stability in Ultra-Weak Grids

In order to effectively transmit the renewable power to the remote load center, more and more utility-scale renewable generation systems have to be operated under the ultra weak gird condition, i.e., the Short-Circuit Ratio (SCR) of power transmission lines impedance is close to 1. As a result, the steady-state operating point of load angle is close to the stability limit 90°. Even a small grid disturbance can result in small-signal power instability, and trigger off the whole renewable generation system, resulting in catastrophic cascading failure in the power system. In this paper, a novel fast power control method is proposed for the interfacing grid-connected Voltage-Source Converters (VSCs). With this method, the power control bandwidth can be increased far beyond the fundamental frequency, which provides superior dynamic power control performance. More importantly, the load angle can be safely limited within the stable region against the grid disturbance by quickly reducing the input power. As a result, the synchronization stability of VSC is greatly improved. The control method is verified by both analytical models and experimental results.