Analysis of ultra-low frequency oscillation in yunnan asynchronous sending system

Yunnan power grid has a large-scale hydropower station whereby the power generated at the station is been transmitted via high-voltage direct current (HVDC) transmission lines under asynchronous operation mode. In the system performance test of Yunnan asynchronous operation, 0.05 Hz ultra-low frequency oscillation occurred in the grid. We analyzed the damping characteristics of different types of units and figured out that the negative damping characteristic of the hydropower units are mainly responsible for the ultra-low frequency oscillation. There is a tendency for ultra-low frequency oscillation in a regional power grid with high ratio of hydropower penetration which transmits electricity using HVDC. The water hammer effect of hydraulic turbine influences the damping characteristic of the hydropower unit while the governor control gain influences the strength of the damping. By setting reasonable governor control gain and optimizing the dead-zone of HVDC frequency limit controller the ultra-low frequency oscillation of the Yunnan asynchronous system can be effectively curbed. Simulation results and system performance test verified the effectiveness of the method.