Impact analysis of UPFC on directional protection based on the power-frequency variation

This paper presents quantitative analysis and simulation results of the influence of UPFC on directional protection based on the power-frequency variation. First, with a domestic 500 kV UPFC project as the basis, the principle and equivalent model of UPFC and its protection system are proposed. Next the principle of directional protection based on the power-frequency variation is introduced. Then quantitative analysis of the characteristics of power-frequency variation impedance calculated by the relay located at the UPFC side of line is carried out, which provides basis for quantitative estimation of the influence of UPFC on directional protection based on the power-frequency variation in the actual work. During forward faults, directional protection could operate reliably. During reverse faults, an expression is proposed on the relationship of power-frequency variation impedance, UPFC series injected voltage and the power-frequency variation relay voltage. When the amplitude of power-frequency variation relay voltage is less than UPFC series injected voltage amplitude, misoperation of the directional protection may occur. RTDS Simulation tests verify the validity of the theoretical analysis.