Damping Torque and Synchronous Torque Analysis of Power System Ultra-low Frequency Oscillation

Abstract

Recently, there are many cases of ultra-low frequency oscillations(ULFO) in power grids where hydropower units account for a large proportion, with the frequency below 0.1Hz. Analysis of existing studies assumes that the generator power is determined by the frequency characteristics of the load, while the electromagnetic connection between the generator and the system is ignored. In addition, the damping properties of electromagnetic power are also not clear enough in this model. For this issue, the damping characteristics of mechanical power and electromagnetic power of hydropower units are analyzed based on the single-machine and equivalent two-machine systems, respectively. Based on analysis of the actual phasor measurement unit(PMU) waveform of ULFO in Yunnan, it is derived that the phases of hydroelectric generator power angle fluctuation and the frequency fluctuation are nearly the same, and electromagnetic power oscillation is caused by power angle fluctuation. Theoretical analysis and simulation results show that in this ULFO case the hydropower units have sufficient synchronous torque and there is no risk of non-oscillation instability.