The DC bus and sub-module should be fully discharged to ensure that the voltage is below the safety threshold after the shutdown process of modular multilevel converter based on HVDC（MMC-HVDC）. In view of the limited research on the discharge process

we established a simulation model of MMC-HVDC flexible transmission project in China during the discharge process

and analyzed the discharge characteristics. The mathematical description formula of discharge time and residual voltage was deduced based on the discharge simulation model

and the setting method of discharge time was also proposed. Finally

a designed example was given and the verification tests were conducted at Xiamen HVDC flexible project. The results show that the simulation results are nearly identical to the experimental data

which indicates that the DC bus discharges via the equivalent grounding resistance of potential transformer; and the discharge process of sub-module can be divided into two cases according to different working states of the energy-absorbing source: the capacitor discharges via the control circuit and shunt resistance when the source is working

and the capacitor discharges only by the shunt resistance when the source is turned off. The discharge time setting method has been applied to Xiamen HVDC flexible project

which has ensured the fully discharge of the DC bus and sub-module

thus further confirms the validity of the designed setting method of discharge time.