Research on Applicability of Directional Element Mutation in UHVDC Hierarchical Connection Mode

Abstract

In order to solve the problem of voltage-supporting capability and evacuation difficulties caused by UHVDC from the structure of power grid, Chinese scholars first proposed a UHVDC hierarchical connection to AC grid. In this way, the electrical coupling by the AC-DC hybrid system is more complicated, while the traditional relay protection elements have been misoperated many times. Therefore, the research on the adaptability of the protection directional elements mutation has become an urgent problem to be solved. Respectively, a variety of sudden changes in the amount of directional components and direction of a comprehensive analysis of components, according to the principle of phase direction components and the sequence of the elements of the conditions. In AC / DC hybrid power system, due to the positive and negative sequence impedance of the inverting side equivalent AC system are not equal and the impedance angle and the line impedance angle are quite different, the positive and negative sequence directional components will be misjudged , Performance cannot be guaranteed; As for the zero-sequence principle of the directional element, as the inverter side of the zero sequence network is more stable, reliable performance, to correct operation. PSCAD simulation data verify the correctness of the analysis.