Research On Modulation Method And Control Strategy Of MMC In Medium Voltage

With the gradual increase in the penetration rate of various distributed renewable energy sources,and the continuous increase in power loads in cities,the distribution network faces multiple challenges.Compared with the AC distribution network,the medium-voltage DC distribution network has gradually increased its proportion in the distribution network due to the advantages of high transmission efficiency,flexible and convenient control,and easy acceptance of distributed energy.With the characteristics of easy expansion and high output voltage quality,modular multilevel converter(MMC)has absolute advantages in the field of HVDC.However,it is used in medium voltage DC distribution,the number of sub-modules is small.If the traditional modulation method of MMC is used,it will cause problems such as low voltage quality,high current distortion rate,and complicated control.This paper combines the characteristics of MMC in the medium-voltage DC distribution network,and improves the voltage quality of the medium-voltage MMC from the perspective of modulation method and control strategy.The main contents are as follows:(1)The basic topology,working characteristics and principle of MMC are explained,and the equivalent mathematical model of MMC is deduced;the stationlevel control of MMC converter is studied,and the structural design of MMC inner and outer double-loop controller is analyzed in detail.The principle and voltage equalization method of traditional NLM and CPS-PWM modulation are studied,and the characteristics and applicable occasions of each modulation are analyzed.(2)A double multiplication hybrid modulation(DMHM)suitable for medium voltage MMC is proposed by the hybrid modulation method of superimposing PWM wave on the output ladder wave voltage.The DMHM multiplies the output level by superimposing the output 2N-level staircase wave and 2-level PWM wave,and at the same time uses the PWM wave to improve the effect of approximating a sine wave.The pressure equalization strategy under DMHM only needs to be adjusted on the basis of the sorting algorithm,and the pressure equalization effect is good and the control is simple.(3)The model predictive control is applied to the MMC in the medium voltage DC distribution network,and combined with the frequency fixed control,a fixed frequency model predictive control(FF-MPC)based on frequency fixed is proposed.FF-MPC performs hierarchical control of each control variable through independent index functions,which greatly reduces the number of switching state combinations to be calculated by model predictive control.(4)Comparing and analyzing various modulation methods in Matlab/Simulink,the results show that DMHM has higher output waveform quality,simple and effective voltage equalization strategy,and lower switching frequency and loss of sub-modules.Comparing FF-MPC with traditional MPC and traditional modulation methods,it verifies that the tracking of reference current is more accurate,the output waveform quality is higher,the switching frequency is lower,and the control is more streamlined under FF-MPC.