Research On Overvoltage Characteristics And Insulation Coordination For LCC And MMC Hybrid-HVDC System

High Voltage Direct Current based on Line Commutated Converter(LCC-HVDC) has a lot of advantages such as lower cost and longer distance, however, it presents some drawbacks such as commutation failure of inverter, prone to interrupt the transmission of power. High Voltage Direct Current based on Modular Multilevel Converter(MMC-HVDC) can solve the problem, but it cannot solve the problem of DC line fault. Therefore, it is not suitable for overhead lines transmission over a long distance. Hybrid-HVDC composed of the MMC and LCC associates the merits of both, can settle both the problems of commutation failure and DC line fault, so it has a wide range of applications. But, there is few study of Hybrid HVDC so far and mainly focuses on the topology structure design and electromagnetic transient simulation modeling. Overvoltage producted by the system may destroy the electrical equipment insulation and affect the safety of system, so it is necessary to study the overvoltage characteristics and insulation coordination of Hybrid-HVDC. This paper focuses on Hybrid-HVDC, and researches the overvoltage characteristics and insulation coordination deeply.First, the working principle of the hybrid system was analysed, and then the system mathematical model was presented, the operating characteristics of the system was studied by the way of volt-ampere characteristic, the coordinated control strategies under steady-state and fault-state were proposed. Based on the theoretical analysis, the simulation model of the system was built, especially the pole control model, and the validity of the control strategies and simulation model was verified by simulation results.Secondly, the electromagnetism transient process and overvoltage characteristics caused by different fault conditions were researched while comparisons with LCC-HVDC and MMC-HVDC were made. By analyzing the equivalent circuit of the three types of system, come to the conclusion that the main differences of the electromagnetism transient process and overvoltage characteristics between the three systems were due to the control modes and topological structures, the relationship between fault types and control modes and topology structures were also studied. Using the models of the three systems, typical switching overvoltage faults were simulated, results showed that overvoltage characteristics of the system were consistent with theoretical analysis.Finally, combined the above analysis of overvoltage characteristics with the basic principle of arrester configuration for conventional DC system, the insulation coordination of Hybrid-HVDC was studied. Overvoltage and insulation level of main equipments in the converter station under different arrester configurations were calculated. Results of the research could offer important foundation for the actual project.