Research On Key Technology Of Soft Open Point In Smart Distribution Network

With the development of dual carbon strategy in China,the permeability of distributed energy in distribution network increases gradually,which leads to problems such as uneven load of feeders and off-limit voltage,and seriously affects distribution reliability and power quality.Traditional distribution network mainly realizes power flow control between power nodes through mechanical interconnection switch,which is difficult to meet the demand of fast power flow control brought by high proportion of distributed energy grid-connection.Soft Open Point(SOP)is a new type of power electronic equipment to replace the traditional mechanical interconnection switch and connect each distribution network node.It has the advantages of reducing current impact,balancing feeder load,realizing multi-node interconnection,flexible power flow control and fast fault isolation.It is of great significance to improve the reliability of distribution network with high proportion of distributed energy.Due to the limited voltage withstand capacity of switching tubes,SOP used in medium voltage distribution network is often used in modular topology,which leads to a large number of modules in SOP topology,high cost,large volume and low efficiency.Therefore,aiming at the above problems,this paper carries out a series of in-depth studies on the topologies and controls of SOP devices.Firstly,the working principles of the existing SOPs topology are introduced in detail,and the Cascaded H-bridge(CHB)type SOP topology,which is the most widely used,is deeply studied.Based on the analysis of the working principle of three-phase CHB-SOP topological AC stage and DC-DC isolation stage,the mathematical model is established,and the control strategy of CHB-SOP is deduced.The simulation of three-phase CHB-SOP is carried out based on Matlab/Simulink to verify the feasibility of the control strategy.Secondly,in view of the problems of large number of modules,low power density and high cost in CHB-SOP topology,a new shared module soft open point(SMSOP)topology is deduced based on the idea of module sharing.Due to the sharing of some modules,a large number of full-bridge modules and DC-DC isolation modules are saved,which effectively reduces the volume and cost.This paper starts from analyzing the working principle of singlephase SMSOP topology,establishes its mathematical model,studies the parameter design method,deduces the voltage,power control and module voltage equalization strategy,and extends the design method and control strategy to the three-phase topology.The corresponding principle prototype platform is built,and the topology feasibility verification is completed.Finally,a square wave modulation method is proposed to reduce the switching loss of the shared module,the principle of square wave modulation is analyzed,and its control strategy is deduced.Based on square wave modulation,aiming at the shortcoming of square wave modulation easy to overmodulate,a quasi-square wave modulation is proposed,and the zerolevel time is introduced to reduce the overmodulation phenomenon,and the control strategy of quasi-square wave modulation is designed.However,the operating range of quasi-square-wave modulation still has certain limitations.Therefore,on the basis of quasi-square-wave modulation,quasi-sine modulation is proposed.The switching loss of the module is verified,and the feasibility of the above method is verified by simulation and four modulation methods including SPWM are compared and analyzed.