State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (North China Electric Power University), Beijing 102206, China

With the continuous expansion and development of the power grid, the structure of hydropower unit integration through the high voltage direct current (HVDC) transmission system appears in the power grid. In order to ensure the safe and stable operation of the power grid, it is necessary to explore whether the system has the risk of multi-frequency oscillation. First, the independent models of the hydropower unit and the HVDC system are established and the interface dynamic equation is deduced. Then, the state space model of the whole system is established and the correctness of the model is verified. Based on this model, the eigenvalue analysis method is adopted to identify the multi-frequency oscillation mode characteristics of the system and analyze the influence. The analysis results show that the integration system has ultra-low frequency, low frequency and sub-synchronous oscillation modes. The ultra-low frequency oscillation modes are not only strongly related to the hydropower unit governor, but also weakly related to the constant current controller of the rectifier station. The ultra-low frequency oscillation is prone to occur in the scenario of high proportion of hydropower unit. The low frequency oscillation modes are related to the phase-locked loop (PLL), which will not be excited if the PLL control parameters change within a reasonable range. The sub-synchronous oscillation modes are affected by multiple electrical and control links between the hydropower unit and the HVDC system. When the grid strength is weak at the receiving end, one of the sub-synchronous oscillation modes is excited. The modal damping can be improved by adjusting the excitation gain and the control parameters of the rectifier/inverter station.