Unified power flow controller (UPFC) can realize flexible control of transmission line power flow, and improve system operation mode.However, the access of UPFC changes the uniform distribution of line voltage and brings some problems to relay protection.Based on the fault control and protection logic of UPFC, the transient variation characteristics of UPFC injection voltage are studied firstly.Then, the influence mechanism and degree of injection voltage variation on fault components based directional elements are studied.It is pointed out that when the protection locates between the fault point and the UPFC access point, the sensitivity of the direction element using positive sequence fault component and power frequency fault component can be greatly reduced.When a fault occures in UPFC, the direction elements may misjudge the fault direction.Based on the practical model and parameters of 500 kV UPFC project in southern Suzhou grid, RTDS simulation model contained UPFC control and protection logic is built.The theoretical analysis conclusions are fully verified by RTDS tests.