Research On Short Circuit Fault Analysis And Short Circuit Current Calculation Method Of Flexible DC Grid

In recent years,large-scale new energy interconnection has brought great challenges to the stable operation of traditional AC power grids.Modular Multilevel Converter Based on High Voltage Direct Current(MMC-HVDC).It is a new type of transmission technology with flexible operation mode,which provides an effective solution for the consumption of large-scale renewable energy and the interconnection of regional power grids.However,the flexible DC power grid system has low damping.After the DC line fault,the MMC sub-module capacitors discharge quickly,and the fault propagation range is wide.It has a great impact on the DC power grid and the power electronic devices inside the MMC sub-module,which seriously threatens the power supply of the MMC-HVDC system Safety.This paper focuses on two issues:the establishment of equivalent model after DC fault in MMC-HVDC system and the calculation of short circuit current of each branch in the system.First,the MMC mathematical model is established to analyze the influence of the internal current path of the converter station and the working state of the submodule on the transient circuit after the fault.Based on the analysis of the capacitor discharge path of the submodule,the equivalent of the single-ended MMC converter station is constructed Circuit model.Secondly,the system parameters such as the equivalent capacitance value of the converter station are derived,and the impact of the fault occurrence time and the selection of the system modulation ratio on the initial equivalent capacitance parameter value after the fault is quantitatively analyzed.Based on the equivalent model of the single-ended converter station,the current path formed by the converter stations to the fault point when the system is short-circuited is analyzed,and the network equivalent model after the failure of the MMC-HVDC system is established,and the equivalent is verified by simulation The accuracy of the model and the calculation accuracy of the equivalent capacitance parameters.Finally,based on the relationship between the converter stations in the equivalent model and the system equivalent model,decouple the converter stations of the DC power grid,consider the approximate solution of each branch current after the fault as the initial value,and take the remote revision into account The equivalent resistance and equivalent inductance of the multi-terminal MMC-HVDC line affected by the station can be used to solve the fault current value of each branch in the multi-terminal MMC-HVDC system through multiple iterations.A four-terminal flexible DC power grid model is built on the MATLAB simulation platform,and the calculated fault current is compared with the detailed electromagnetic transient simulation results.The results show that the fault current solving method proposed in this paper can accurately and effectively calculate the current value of each branch after a multi-terminal MMC-HVDC short circuit fault.The research work in this paper realizes the establishment of the equivalent model of MMC-HVDC system,parameter selection and offline solution calculation of each branch current after fault.Compared with the solution results of the time domain method,it is proved that the method for fault current analysis of DC multi-terminal power grid proposed in this paper can improve the calculation accuracy,meet the calculation efficiency requirements during offline calculation,and can be used for the selection of system equipment,It is of practical significance to lay the foundation for protection setting and short-circuit hazard analysis.