Research on frequency oscillation of HVDC sending system in islanded mode of hydro power units

When large hydro power plants in sending-end of HVDC operate in islanded mode, water pressure pulsation caused by vortex strip in turbine draft tube will result in frequency vibration in islanded system. This paper designs a resonance function to simulate the actual water pressure in turbine draft tube and set up a new hydraulic turbine model considering the draft tube water pressure pulsation as well as the elastic water column, which is a big improvement to the original linear turbine model. At the same time, it gives the pole figure of the new hydraulic pulse function. Then, the accuracy of the turbine model is verified by using time domain method and the effect of the hydro turbine parameters as well as the governor parameters on frequency oscillation of HVDC sending system in islanded mode is also analyzed. Simulation results illustrate that hydraulic startup time T_w, governor parameter K_t, K_D have greater influence on frequency oscillation in sending-end of islanded DC system. Finally, to avoid reforming the structure of hydraulic turbine, DC frequency is adopted to restrain from frequency oscillation. The simulation results show that the improved DC frequency limiter after the optimization of its parameters cooperated with governor which is also optimized can control the sending frequency at 49.8Hz ~ 50.2Hz.