Research On Internal Fault Of Converter Station And AC Asymmetric Fault Protection Strategy Based On MMC

With the development of power electronics technology,flexible DC transmission systems stand out in the field of long-distance large-capacity transmission engineering by high control freedom,power supply to passive systems,and small footprint.HVDC technology based on modular multi-level converter has been widely used in power grid because of its large transmission capacity,less harmonic content,low switching loss and wide application range.In the MMC-HVDC system,the protection strategy of the internal and asymmetric faults of the converter station is directly related to the reliability and safety of the transmission system.Therefore,it is of great engineering application value and reality to study the failure analysis and protection strategy of MMC-HVDC.The meaning is as follows:(1)Starting from the MMC topology structure,the working state of the sub-module and the relationship between the variables of the upper and lower bridge arms in each phase are combined.Then the three-phase mathematical model of MMC is established in the static coordinate system.For the design of the MMC-HVDC control system,the MMC mathematical model in the synchronous rotating coordinate system is obtained by Park transformation.(2)After completing the establishment of the system mathematical model,the analysis of the main circuit parameters related design of the MMC-HVDC system is carried out.The method of determining the number of bridge arm sub-modules is studied.The value formula of the submodule capacitance is inferred.The constraints of the parameters of the bridge arm reactor and the selection methods of different operating states are analyzed.Then the vector control system used in the MMC-HVDC system is designed to complete the design of the rectifier side and inverter side vector controllers,and the simulation verification is completed in the PSCAD/EMTDC electromagnetic transient simulation platform to verify the derived system master.The correctness of the circuit parameters and the designed vector control system provides a platform and theoretical basis for the simulation verification of the system behavior and protection strategy in the fault state of the converter station.(3)Analyze the causes of the AC system faults of the converter station of the MMC-HVDC system and the internal faults of the converter.Firstly,the sub-module fault andthe bridge arm reactor fault inside the converter station are studied.The sub-module redundancy protection and the bridge arm over-current protection scheme are designed for the fault,and the correctness of the scheme is verified.(4)The low-frequency transient mathematical model and power model of MMC under three-phase asymmetry are established.The phase sequence decomposition link is designed for the three-phase asymmetric state of MMC,and the positive sequence current and negative sequence current are used as the control quantity.The inner loop current decoupling control system is designed,and then the control strategy of suppressing the negative sequence current is designed by suppressing the negative sequence current.The controller design of the MMC-HVDC system is completed.Finally,the simulation verification is completed in the PSCAD/EMTDC electromagnetic transient simulation platform,and the effectiveness of suppressing the negative sequence current control strategy is verified.