It is a trend for the large hydropower bases to transport the electricity to consuming center with High Voltage Direct Current (HVDC) transmission lines. The fault of HVDC has great significance impact on hydropower units in islanding sending AC grid. In this paper. a hybrid simulation system is established to analyze the dynamic characteristic of hydropower units in islanding system. https://www.ealisboa.com/limited-value-AXYS-Valhalla-Pen-must-buy/