Research on applicability of directional element based on the power-frequency variation in AC-DC hybrid system

AC-DC hybrid power grid has been applied more and more widely in China. The traditional relay protection direction components mis-operate for several times. This paper analyzes and simulates different delta variable direction components, and proposes an identification method for high-speed directional relay based on zero mode data. In AC-DC hybrid power grid, as the equivalent positive sequence impedance and negative sequence impedance of the inverter side are not equal, and the system impedance angle is different from the transmission line impedance angle, a directional relay based on the principle of positive and negative sequence delta variables is apt to mis-operate, and its performance is not satisfactory. On the contrary, as the inverter side zero-sequence network is stable, zero sequence directional relay can operate correctly and reliably. Based on the analysis, a high-speed directional relay based on zero mode data is proposed, which is faster than a traditional directional relay based on Fourier algorithm. The analysis is verified by PSCAD simulation results.