School of Electrical Engineering,Beijing JiaoTong University
As an effective means to support the adoption of high proportion of renewable energy, MMC-HVDC has become an important direction of power grid development. The pole-to-pole fault is the most serious fault in transmission lines. At present, state equations of DC system are usually written in the s domain, and then fault currents are solved based on the Inverse Laplace Transformation,while a practical engineering calculation method for fault current is needed urgently. In this paper, Zhangbei MMC-HVDC grid is taken as the research object, the fault characteristics and coupling mechanism of DC polo-to-pole fault are analyzed firstly. On this basis, the two ends of the fault lines near the valve side are regarded as two-ports respectively, the relationship between the fault currents and the voltages of the two-ports are analyzed. Secondly, based on the idea that the voltage of the two ports of the positive and negative lines does not change much, the annular MMC-HVDC grid is simplified to a two-terminal system or an open system. The practical calculation method of fault line current is obtained to calculate fault current directly, while it is no longer necessary to solve the high order Inverse Laplace Transformation. Finally, the feasibility and efficiency of the practical calculation method are verified by comparing with the electromagnetic transient simulation results.