As the typical representative of FACTS controllers, UPFC rapidly adjusts active and reactive power flow. Protections of UPFC would trip to isolate the UPFC from system quickly when fault occurs in the power grid. It would form power variation source located at the installation of UPFC, which has influence on impedance directional protection based on variation. In this paper, taking a domestic 500 kV UPFC project as the basis, the principle and protection system of UPFC are proposed. The principle of impedance directional protection based on variation is introduced. Quantitative analysis of the characteristics of power-frequency variation impedance calculated by the relay located at the UPFC side of line is carried out. During forward faults, power-frequency variation impedance is approximately equal to the inverse of the sum of system impedance and line impedance, directional protection could operate reliably. During reverse faults, once fault measuring voltage drops below 0.917 p.u., directional protection would not operate reliably; the real part of voltage variation compensation factor λ below -1 is the sufficient and necessary condition of protection malfunction. Furthermore, quantitative analysis results and λ of adjacent line protections are given. RTDS simulation tests verify the validity and accuracy of the theoretical analysis. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900604).