Unified Power Flow Controller (UPFC) injects controlled voltage into the transmission line through series transformer. UPFC is equivalent to series connection of capacitance or inductance to improve transmission efficiency of power network. Series compensation (SC) adjusts line impedance through capacitance similar to UPFC. SC is widely used in EHV transmission line and the influence of SC system on the protection is analyzed in detail. As a new power electronic device, the impact of UPFC on the surrounding system and protection hasn’t been deeply studied. In this paper, the characteristics and effects of UPFC on protection and SC are analyzed. The equivalent characteristics of UPFC under various operating conditions and the solution of line protection are studied. The research improves operation reliability of UPFC system.