Research On Operation Characteristics Of Backup Operation Of UHV DC Transmission Line

The HVDC transmission project in China will reach about 50 by 2020 which accounted for more than 60% of UHVDC transmission projects. Due to the high voltage of UHVDC transmission lines and the complex environment which UHVDC transmission lines need to pass through, the UHVDC transmission line has much probability to suffer grounding fault which seriously affects the safety and stability of HVDC transmission system.While the electric parameters and the protection for UHVDC transmission lines are affected by UHVDC control system, first of all, the basic control principle, control strategy of UHVDC control system was analyzed. Then the simulation model based on the PSCAD/EMTDC with actual parameters of Yunguang ±800kV UHVDC system was established. Then the influence of control system on the back-up protection for UHVDC transmission lines and the selection of UHVDC control model under the different operating as well as the different fault types were analyzed.Differential Under-Voltage Protection and Line Longitudinal Differential Protection are the most common back-up protection for UHVDC transmission lines. First of all, the influences of operating condition,fault position and fault distance on the two back-up protections above was analyzed. Then based on the analysis results, the protection setting for the two protections was proposed. Then the influences of transition resistance and UHVDC control system on the operating reliability of the two protections was analyzed.In order to solve the problem fundamentally that the back-up protection for UHVDC transmission lines has low correct action rate. A new method based on the harmonic characteristics of AC/DC systems as well as transient response characteristics of UHVDC control system,and the protection setting method were presented in this thesis. The correctness and effectiveness of the new method were verified by the simulation results. The influences of transition resistance and fault distance on the operating reliability of the new method was analyzed, simulation results show:the new principle has better ability to sand high resistance compared with Differential Under-Voltage Protection,and its action time increased by about 63% compared with Line Longitudinal Differential Protection.