Multi-Loop Model Based Internal Fault Analysis of Variable Speed Pumped Hydro Machines

Doubly-fed motors are extensively employed as high-capacity variable speed pumped hydro (VSPH) machines. The Fengning Pumped Storage Power Station in Hebei province constructed the first two VSPH machines in China. However, the rotor protection for the Fengning VSPH unit has not been completed to date, and traditional protection methods exhibit inherent deficiencies. To design an appropriate rotor protection scheme for the VSPH machines, it is vital to ascertain the internal fault characteristics through an accurate simulation model. Nonetheless, the commonly used dq0 model fails to account for all harmonics. To address this limitation, this paper proposes a multi-loop model to analyze the internal fault characteristics of VSPH machine windings. Experimental validation conducted on a 12kW VSPH prototype demonstrates the model's ability to accurately predict voltage and current waveforms under different internal fault conditions. Subsequently, the internal fault characteristics of the investigated VSPH prototype are determined by performing harmonic analysis on the stator and rotor currents. The observation and analysis results indicate that the utilization of fractional order currents in stator branches can effectively safeguard the rotor windings of VSPH machines.