Damping ultra-low frequency oscillations by PID parameter tunings of hydropower governors in hydro-generation-dominant power system

Studies have revealed that the negative damping produced by the hydro governor is one of the major factors of ultra-low frequency oscillation (ULFO). If the setting and design of the hydro governor parameters are unrealistic, the negative damping of the system would be aggravated. In this paper, the damping torque coefficient is derived from the hydropower governing system, and the critical frequency is defined in the first time, to identify the positive- or negative-damping generated by the hydro governor-turbine system. To suppress the ULFO, a novel PID parameter optimization strategy, based on an improved particle swarm optimization (PSO), is presented. Considering both the rotation speed deviation of the hydro turbine and the damping torque variation of the governing system, this method has effectively mutated the damping produced by the hydro-governing system from negative to positive during the ULFO process and is proven by using a single-machine infinite bus power system. Furthermore, a 3-machine 9-bus system and an actual system are selected to verify the validity of the theoretical analysis. The PID parameter optimization strategy is also presented in this paper. The results do confirm that the negative damping produced by the hydro governor is the major factor of the ULFO. Moreover, by using the PSO optimization technique presented in this paper, after PID parameters of the hydro governor are adjusted, the ULFO occurred in the system would effectively be suppressed.