| Modular multilevel converter based high voltage direct current（MMC-HVDC）project accounts for an increasing proportion in HVDC system of China year by year.The system protection scheme research of flexible HVDC project are inseparable from the fast electromagnetic transient simulation model.Existing models can not meet the requirements of both accuracy and rapidity,so it is necessary to establish an electromagnetic transient simulation model that can meet the above requirements.Since the grounding fault in MMC-HVDC converter station will have serious impact on the components in the station,and the existing grounding protection in the converter station is unclear,therefore,it is also of great significance to put forward a reliable and effective grounding protection scheme for converter station.First,the MMC traditional thevenin equivalent model under pre-charging and DC fault condition has low precision,and the MMC traditional thevenin equivalent model can’t handle the internal fault of the bridge arm,In order to solve these problems,this paper analyzes the features of bridge arm current under the MMC pre-charging and DC fault condition,according to the analysis results,further refines the thevenin arm model parameter assignment process of two kinds of working conditions,and then proposes an improved thevenin equivalent model,a flexible and adjustable fault node is added in the bridge arm to realize the internal fault handling of the bridge arm.In PSCAD,a two-terminal MMC-HVDC system is built,the converter model adopts the detailed model,the traditional thevenin model,and the improved thevenin model,and the simulation speed and accuracy of the three has been compared.The results show that the improved thevenin equivalent model has higher accuracy than the traditional thevenin equivalent model,and can simulate the bridge arm internal faults,providing model support for the subsequent research of MMC grounding protection scheme and MMC-HVDC converter station grounding backup protection.Secondly,the MMC ground fault protection scheme is studied.Aiming at the problem that the DC differential protection of MMC will malfunction when the valve side of the converter transformer has a grounding fault,the reason for the DC differential protection of MMC malfunction is obtained by analyzing the characteristics of the current of converter during the ground fault of the converter transformer valve side,and then combined with the analysis of the current characteristics under the normal working condition and the internal ground fault condition of MMC,a new method of MMC ground fault protection is proposed,which can replace the DC differential protection as the main ground fault protection of MMC.The two-terminal MMCHVDC system built earlier is used to verify the protection method,and the results show that the proposed ground fault protection method is reliable and effective.Finally,aiming at the problem that the ground fault protection of MMC-HVDC converter station is susceptible to the current transformer operation status,the station domain protection framework of MMC-HVDC converter station is constructed by introducing the idea of wide area protection,and the corresponding minimum differential loop is divided according to the ground fault protection area of the converter,converter transformer and the bus,and then a method of station domain ground fault protection based on the minimum differential loop is proposed.On this basis,a fusion differential loop is formed to realize the backup protection of ground fault protection in the converter station when the current transformer is not working normally.Finally,The two-terminal MMC-HVDC system built earlier is used to verify the protection method.The simulation results show that the proposed station domain protection method can effectively isolate the grounding fault when the current transformer is in abnormal operation state,and can be used as the grounding backup protection of converter station. |
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