Simulation Research On The DC Side Short Circuit Fault Overvoltage Of Zhangbei±500kV Flexible DC Power Grid

Flexible DC transmission technology can realize multi-power supply,multi-drop power receiving and island access of new energy,which is a very effective new model to solve the problem of large-scale new energy transmission and consumption.In order to achieve large-scale and long-distance consumption of intermittent clean energy and serve the construction of low-carbon green Olympic zone,State Grid Corporation will construct Zhangbei renewable energy flexible DC power grid demonstration project.The unique true bipolar four-terminal ring structure of the project makes the fault development process particularly complicated,and the first application of 500kV dc circuit breaker to fault quick clearance,resulting in the system overvoltage characteristics are not yet clear.Therefore,the research on the generation mechanism and influencing factors of the system overvoltage has important engineering value for the overvoltage insulation coordination of the flexible dc power network.In this thesis,the generation mechanism,influencing factors and inhibiting measures of DC busbar overvoltage under the dc transmission line fault condition of Zhangbei project are developed.According to whether the transmission line corresponding to the busbar is in fault,it is divided into the healthy pole bus and the fault pole bus for analysis.Firstly,based on the modeling of MMC converter valve,hybrid HVDC circuit breaker,converter transformer,current-limiting reactor and other key equipment,the PSCAD platform is used to build an accurate flexible DC grid simulation model,which can meet the simulation requirements of fault overvoltage electromagnetic transients.Then it is explained that there is a particular phenomenon that the overvoltage of healthy pole bus is higher than that of fault pole bus in the monopolar grounding fault of Zhangbei project.According to the three aspects of electromagnetic coupling,converter output voltage determined by MMC control system and neutral point potential uplift due to metal loop network,the mechanism of the overvoltage of healthy pole bus is studied.The influence of the grounding resistance,current limiting reactor and dc circuit breaker threshold current on the overvoltage of the healthy bus is evaluated.Then,the overvoltage of fault pole bus in the case of monopolar grounding fault and double-pole short circuit fault is studied.The causes of overvoltage are analyzed from the perspective of equivalent circuit,and the influencing factors of the overvoltage of the fault bus are studied.Finally,the restraints of bus overvoltage are put forward from the perspective of insulation co-ordination of bus arrester and reasonable configuration of system grounding pole.In this thesis,the generation mechanism of dc bus overvoltage in Zhangbei project is studied in depth,and the influence degree of system parameters on bus overvoltage is analyzed,and the suppression measures of bus overvoltage are put forward,which provides important theoretical guidance for insulation coordination in Zhangbei project.