Research On Simplified Calculation And Protection Strategy Of Fault Current In Multi-terminal Flexible DC Grid

Flexible DC transmission based on Modular Multilevel Converter（MMC）has the advantages of large transmission capacity and flexible operation mode.It is suitable for new energy generation network access and has broad prospects.The DC power grid composed of them has the characteristics of "low inertia and weak damping".After a DC fault occurs,the energy storage elements in the system are rapidly discharged,and the fault current rises rapidly and has a large amplitude.There is an overcurrent risk in the system.Reliable fault protection requires accurate analysis and calculation of the electrical component of the fault,and design of a reliable rapid protection scheme.The current acquisition method of fault current mainly based on detailed electromagnetic transient simulation is difficult to meet the needs of large-scale DC power grids.Therefore,this paper studies the simplified calculation method of fault current in multi-terminal flexible DC grid,analyzes the fault propagation mechanism and electrical quantity characteristics after fault,and designs a new protection strategy based on line transient voltage characteristics.The specific research contents include:（1）Establish the equivalent circuit of the DC power grid under different faults,and analyze the main influencing factors of the MMC fault current.The dynamic process of MMC discharge in bipolar short-circuit fault and unipolar grounding fault is analyzed,and the equivalent model of the converter after the fault is constructed,as well as the equivalent method of the circuit under different faults such as bipolar short-circuit and unipolar grounding fault.The influence of various parameters on the fault current is analyzed,and the correctness of the equivalent model of the MMC and DC network after the fault is verified by simulation.（2）A simplified calculation method of fault current in multi-terminal flexible DC grids with different topologies is proposed.According to the characteristics of the parameters of the DC power grid,the frequency response characteristics between the dynamic components of the fault circuit of the power grid after the fault are analyzed,and the multi-terminal DC power grids with different topologies are decomposed and equivalent according to the energy conservation principle,and the equivalent simplified method of the fault network is proposed,and the DC power grid Simplified calculation method of fault current.Simulation systems with various topologies are built in the RT-lab simulation platform,and it is verified that the proposed calculation method can accurately reflect the changing trend of fault current,and the proposed current analytical method has good generality.（3）A DC line protection strategy based on boundary transient voltage characteristics is designed.On the basis of simplified calculation of fault current,the difference of terminal voltage of current-limiting reactor under different faults is analyzed,and the fault discrimination method is proposed.The integral method is used to construct protection criterion to avoid interference such as measurement error and noise.The principle of protection setting threshold setting is designed.Corresponding border protection scheme.The effectiveness of the proposed protection scheme is verified in the simulation platform.The proposed protection can correctly determine the fault category and faults inside and outside the area,and has a good ability to withstand transition resistance.It only uses local information,and has no need for local communication and faults It has the advantages of rapid detection and strong anti-interference ability.