Simulation And Design Of Capacitance Voltage Balance Strategy For MMC

MMC(Modular Multi-level Converter)is a new type of VSC(Voltage Source Converter)topology.Structure of VSC from the original two-level,three-level converter,to modular multi-level converter which is popular in recent years,because it has advantage in good output power quality,adaption to HVDC applications,redundancy characteristics,and becomes the focus in the flexible HVDC.In this paper,my main research on the application of MMC in the inverter was about the modular multi-level inverter control technology involved in the modulation technology,capacitance voltage balance strategy and analysis of its structure,method of working,mathematical model,modulation and control algorithm.In the end,its computer simulation and experiments was done.Firstly,the structure and basic working principle of the MMC were introduced.A mathematical model of the modular multi-level converter was built.By a slight equivalent changing,three-phase equivalent circuit diagram and single phase equivalent circuit diagram had been given.The formula based on switch function in this paper had been established,it illustrates the MMC multi-level converter operating mechanism more clearly.Secondly,according to the structural characteristics of MMC,MATLAB/Simulink as a mathematical tool was utilized to establish the simulation model based on the carrier voltage balance algorithm in carrier phase modulation technology.Using Averaging controlling and Balance controlling achieved the balance of capacity voltage.The simulation results show that the equivalent switching frequency was proportional to the number of sub-modules,the capacitor voltage remained stable and the fluctuation coefficient and the output voltage harmonic component decreased.Finally,a DSP-based simulation control platform was built according to the simulation data and the existing conditions,it included the main circuit,the driving circuit,the voltage sampling circuit and current sampling circuit.Through the design and debugging of the hardware and software,the waveform of the upper arm capacitance voltage was given.The experimental waveform showed that the fluctuation coefficient of the capacity voltage was within the reasonable range,which also proved that the control strategy can guarantee the normal operation of the system.