Research On Modeling And Control Of Medium Voltage Direct Current Transmission MMC Inverter System

Medium voltage direct current transmission and distribution system has been widely used in ships,railways,wind farms,photovoltaic power stations and other occasions in recent years.The high-power voltage source converter is a basic component of the medium-voltage DC(MVDC)distribution network.The modular multi-level converter(MMC)has the advantages of modular structure,high reliability and cost-effectiveness,and it is a highly respected topology to realize DC-AC conversion.The objective of this paper is to study a DC-DC converter based on MMC topology,and to realize a MVDC transmission system with high voltage ratio,high performance control,transformerless isolation and small size.The MVDC transmission MMC inverter system described in this paper is based on the structure of “MMC+ three-phase uncontrolled diode rectifier”.The research is focused on the modeling and control strategy of the MMC inverter system.In this paper,the working principle of MMC is discussed in detail.The staircase wave PWM and level shifted-SPWM are studied and compared by means of theoretical research and simulation experiments.Combined with the concept of medium frequency square wave modulation,this paper presents a control strategy for medium voltage direct current transmission MMC inverter system.The intermediate frequency square wave modulation strategy can reduce the design value of the passive components of the MMC inverter system by increasing the fundamental frequency,thereby reducing the volume of the device.In addition,a “cyclic modulation + bubble sorting” capacitor voltage balancing strategy of submodules can reduce the switching loss of the IGBT to some extent.Through the simulation experiment of MATLAB/SIMULINK,the harmonic analysis of the output voltage and current waveforms is carried out,and the fluctuation of the capacitor voltages of the sub-modules is observed.The effect of the control strategy is verified by simulation.This paper also illustrates the overall architecture,hardware platform and software flow diagrams of a 6kV-300 kW MVDC step-down prototype.The feasibility and effectiveness of the control strategy applicable to the system were successfully verified on the test platform.