Abstract: The safety and reliability operation of UHVDC projects, which are characterized by large transmission capacity and long working distance, has vital practical significance. This paper establishes a fault-tree model of UHVDC system with parallel converters, which can help to calculate the forced energy unavailability, monopole forced outage times and bipole forced outage times of UHVDC projects effectively. Then, this paper proposes several sensitivity indices which show the impact of every equipment on the reliability of UHVDC system, thus to effectively find the weak points of the project. Finally, based on the statistical data of the reliability of HVDC projects of SGCC （State Grid Corporation of China） in the past 10 years, this paper calculates and compares the reliability indices for the main wiring schemes of three different kinds of UHVDC system with parallel converters. Moreover, this paper completes the sensitivity analysis on the reliability index of system affected by equipments, and finds the key links which affect the forced energy unavailability, monopole forced outage times and bipole forced outage times. The results show that this research can determine the topology of high-reliability UHVDC system with parallel converters, and provide important reference information for the reliability improvement of UHVDC projects.

Key words: parallel converter, UHVDC transmission, reliability, fault tree, sensitivity