To improve the dynamic recovery characteristic of Ultra High Voltage DC (UHVDC) transmission system, this paper focuses on effects of nonlinear optimal control and PI control in electromagnetics transient process. Control systems in UHVDC based on state feedback exact linearization method and PI controllers are respectively established. The rationality and validity of UHVDC transmission system on resisting disturbances are verified by simulation results using PSCAD/EMTDC software. Compared with PI controllers, nonlinear optimal control can keep constant current and voltage more accurately, shorten system recovery time about 50%, realize the decoupling control, are more useful for system stability of UHVDC and ac networks.

±1100kV UHVDC,PI,state feedback,nonlinear optimal control,dynamic recovery characteristic