DC-side Fault Characteristics Analysis And Fault Identification Of DC System Based On MMC

Modular multilevel converter based HVDC(MMC-HVDC)is an available method to achieve efficient access to large-scale new energy generation.With the increase in voltage level and transmission capacity,the overhead lines in flexible DC transmission systems have a lot of advantages so that it will be the main way of large-scale power transmission in the future.The usage of overhead transmission lines will lead to an increase in the probability of faults in the MMC-based HVDC system.Therefore,it is urgent to break through the related technology of MMC-based HVDC system line protection for overhead lines applications,which is the key to ensure the reliable operation of the flexible DC system.In this paper,the transmission line fault characteristics analysis and fault current calculation of MMC-based HVDC system are taken as the main entry point.The approximate calculation method of fault current of transmission line in MMC system is proposed,which is the theoretical basis of system-related parameter setting.Based on the fault transient characteristics of the MMC-MTDC system,the new principle of a fast and accurate flexible DC lines protection is studied.The proposed new method can meet the performance requirements of system line main protection in terms of speed and selectivity.(1)Theoretical analysis of DC line fault characteristics of MM C-HVDC system under different main wiring modes is taken.The approximate calculation method of fault current of pole-to-ground fault under symmetrical monopole HVDC configuration with AC side grounding mode is proposed.The pole-to-pole fault is the most serious fault of DC system.The mechanism of DC line fault current in the event of pole-to-pole fault is analyzed in detail.A detailed analytical formula of fault current and MMC arms current before and after the lock MMC converter station is proposed.The relevant simulation verified the correctness of the theoretical analysis and the accuracy of the short-circuit current calculation method.(2)Theoretical analysis and approximate calculation of fault current of pole-to-pole fault in MMC-MTDC system is taken.Firstly,the fault mechanism of fault current under MMC-MTDC DC line pole-to-pole fault is analyzed.The reasonable assumption and simplification of the equivalent circuit of MMC-MTDC system before the MMC converter station is locked is put forward and the approximation calculation of fault current of line pole-to-pole fault is proposed.The method can provide a basis for the selection and setting of transmission line protection settings and DC circuit breakers related parameters.The relevant simulation model is built on the simulation platform to verify the accuracy of the short-circuit current calculation method.(3)A transmission line faults identification scheme for multi-terminal flexible DC systems based on single-ended unipolar current is proposed.The scheme realizes the rapidly positioning of the transmission line fault by detecting the extreme value of the current dynamic deviation value and the calculation of the current difference value.The difference between DC bus fault and the line fault is distinguished by the bus short-time energy protection.A fault detecting and identifying unit is designed according to the proposed protection scheme.The simulation model is built on the simulation platform.It is verified that the protection schem e can identify faults rapidly and reliably under various conditions by simulation case,without double-ended data communication and complicated data analysis and processing.The result shows that this scheme can meet the requirements of the multi-terminal flexible DC transmission system.