Abstract: Yongfu DC power transmission project is a national plan and a key project supervised by Yunnan Provincial government. It undertakes the important tasks of power transmission from west to east and hydropower consumption in Yunnan in China, and plays an important role in reducing the communication network transmission current in the province and improving the safety and stability of power grid operation. As the most important auxiliary equipment of the HVDC system, the valve cooling system occupies 90% of the cooling capacity of the converter valve heat loss to ensure the normal operation of the thyristor. This paper conducts in-depth research on the sewage discharge logic of the cooling system outside the valve, analyzes the operation risks, formulates and applies the hidden danger management scheme, eliminates the major hidden dangers, and effectively guarantees the safe and stable operation of HVDC.