Research On Protection Of LCC-VSC Hybrid DC Transmission Line

In order to build a global energy internet and realize global grid interconnection,direct current transmission technology plays an important role.The traditional high-voltage direct current(LCC-HVDC)transmission system has commutation failure,and high-voltage direct current transmission based on the voltage source converter(VSC-HVDC)can complete the power control independently,without the problem of commutation failure,but the switching device loss and the converter station investment high.In order to combine the advantages of the two DC transmission technologies,the hybrid high-voltage direct current(Hybrid-HVDC)transmission system has become an effective solution.Due to the diverse topology of Hybrid-HVDC,this paper selects a specific object for analysis and research,namely LCC-VSC bipolar hybrid double-ended DC transmission system.In order to ensure the safe and reliable operation of the system,research on the protection of DC transmission lines was carried out for the system.The work done in this article is as follows:(1)This paper studies the control strategies of LCC-HVDC and VSC-HVDC,as well as the operating principle and starting method of LCC-VSC hybrid double-ended HVDC transmission system.PSCAD was used to model the hybrid HVDC transmission system at both ends,and the working state of the steady-state operation and fault transient operation of the hybrid DC transmission system was simulated and verified.The results show the feasibility of the established hybrid DC transmission model,and Prepare for the research of DC line relay protection.(2)Based on the model,the pilot protection of bipolar hybrid double-ended DC transmission lines based on model identification is studied.Using the line protection principle of model identification principle,the fault model within and outside the line is established by analyzing the additional network of fault components,and the simpler fault model is selected as the benchmark.Only the comparison with the benchmark model can distinguish the internal and external faults of the line.(3)Based on the model,the longitudinal protection of bipolar hybrid double-ended DC transmission lines based on model identification and current polarity is studied.Combining the model identification protection and current polarity protection principles,the model error direction discriminating element and the current polarity direction discriminating element are established,and the two direction components act when a forward fault occurs.Only when the discriminating elements in both directions are activated,it is determined as a fault in the line area,otherwise it is a fault outside the line area.Compared with the traveling wave protection,the above two line protection methods are not affected by the distributed capacitor current,and the resistance to transition resistance is improved.It can not only supplement the main protection,but also accelerate the backup protection action.