Topology Selection And Control Strategy Of Modular Multilevel AC-AC Converter For Feeders Connection

With the depletion of traditional energy sources,the replacement of new clean energy has become an inevitable trend.At present,clean energy is connected to the distribution network in the form of distributed energy,and a series of new challenges,such as power quality,system stability and relay protection,are brought to the distribution network.Soft normally open point(SNOP),as a new type of power electronic device installed at the end of the feeder line,is used to replace the traditional contact switch.It can accurately control the active power and reactive power of the flow,and solve the problems of power flow control,distributed energy consumption and reliability of power supply,which is of great significance to the future distribution network construction.This paper focuses on the study of the topology of AC-AC flexible multi state switch and its control strategy.The research work is as follows:Firstly,the flexible multi state switch based on voltage source type back to back modular multilevel converter is analyzed from its main circuit topology,working principle,mathematical model,control strategy and access position,and the simulation analysis of the flexible multi state switch with this topology(AC-DC-AC)is completed.Secondly,the topology selection of the AC-AC modular multilevel converter is studied.The topology structure,working principle and mathematical model of modular multilevel matrix converter,Hexverter,Y modular multilevel converter are summarized,and the number of modules,number of devices and control operation characteristics of the three topologies are summarized and the selection of three topologically suitable workplaces is analyzed.After analysis,the Y modular multilevel converter is selected as the main research object in the application scenario of distribution network feeder interconnection.Then,the power control strategy of the Y modular multilevel converter is deeply studied.The dynamic mathematical model is established in the d-q coordinate.The double closed loop control strategy,including the outer loop voltage,power control and inner loop current control,is put forward.And the implementation methods of carrier superposition modulation,module capacitor voltage equalization and module capacitor precharge are given.MATLAB/Simulink is used to build a simulation model for the application scenario of medium voltage distribution network feeder interconnection,and the detailed construction process of each core simulation module is given.Finally,combined with the actual situation of the flexible multi state switch operating in the power grid system,simulation analysis is made on two scenarios of sudden increase in feeder load and sudden increase of distributed generation output.The simulation waveform of the steady-state and dynamic running of the system is analyzed,and the bridge arm voltage,the module capacitance,the current are analyzed in detail,and the applicability of Y modular multilevel converter for medium voltage feeder interconnection scenarios is summarized.