Impedance Reconstruction Based Damping Control Strategy for Variable Speed Pumped Storage Unit

The complicated interactions between multi-time scale control loops of grid side converter (GSC) and machine-side converter (MSC) inevitably caused system instability under weak-grid conditions. The main reasons for causing the VSPS system instability are the numerous control loop and the overlap of their control bandwidths. Unlike a single grid-connected converter, the power transmission process and the impedance characteristic are not as easy to describe. For this reason, the overall impedance model, including hydro turbine, synchronous machine, GSC, and MSC, is built in this paper. The accuracy is verified by simulation, and the influences of parameters are further discussed. Besides, an impedance reconstruction based damping control strategy for VSPS unit is proposed. The strategy reshapes the damping characteristic by applying two auxiliary control loops, which eliminates the resonance oscillation and stabilizes the system. Simulation results show that the proposed damping control strategy is feasible under both weak and extremely weak grid conditions.