| In recent years,the penetration rate of distributed power sources and power electronic equipment has been increasing,and the DC distribution grids has received extensive attention due to its advantages in terms of improving new energy penetration rate and DC load access.Modular Multilevel Converter(MMC)is a key interface device between the DC grid and the AC grid.It has many advantages such as modular design,low switching frequency and good harmonic characteristics.MMC has been successfully applied in flexible DC transmission systems.However,when the application of MMC is extended to the medium and low voltage DC distribution grids,the traditional modulation method will bring about problems such as low harmonics and serious current distortion.At the same time,the DC distribution grids has a complicated structure and a high probability of fault perturbation,which puts higher requirements on the DC fault ride-through capability.Therefore,this paper studies the modulation method of medium voltage MMC applicable to DC distribution grids and the control of MMC topology with DC fault ride-through capability.The main research contents are as follows:(1)Analyze the working principle and control strategy of MMC,establish the MMC harmonic analysis theory,and study the modulation mechanism and harmonic characteristics of NLM and CPS-PWM.When the number of levels is small,the NLM output voltage has a higher content of lower harmonics and a higher degree of current distortion.In the medium voltage field,CPS-PWM has better output voltage harmonic characteristics,but it is necessary to configure a voltage equalization and current sharing control link for each sub-module,and the control complexity is increased.(2)Combined with the modulation characteristics of NLM and CPS-PWM,the hybrid modulation strategy of medium voltage MMC is studied.A recent Nearest Half Level Modulation(NHLM)method for medium voltage distribution grids is proposed,and the capacitor voltage equalization control strategy is improved.The NHLM doubles the equivalent level of the MMC output voltage,thereby reducing the lower harmonic content.Compared to CPS-PWM modulation,the switching frequency is reduced,and the process and voltage equalization control are simplified.Deriving the harmonic expression of the NHLM output voltage waveform,providing a theoretical basis for harmonic analysis in its application to medium voltage distribution grids.(3)Based on the research of sub-module hybrid MMC DC fault ride-through capability,a diode-clamp hybrid MMC topology is proposed and its working principle,fault transient characteristics and DC fault ride-through mechanism are analyzed.The topology has the advantages of simplifying the pre-charging process and equalizing the energy distribution in the sub-modules during the DC fault clearing.Combined with the proposed DC fault traversing control strategy,the DC fault isolation and the system quick restart are realized.(4)Matlab/Simulink simulation is built to verify the correctness of the harmonic expression of the NHLM output voltage waveform,and compare the output voltage and current harmonics of the four modulation strategies.Based on the DSP+FPGA architecture controller,a three-phase 7-level MMC physical experiment system is built to verify the applicability of the NHLM modulation strategy.Finally,the Matlab/Simulink simulation is used to verify the fault ride-through capability of the diode-clamp hybrid MMC topology under DC bipolar short-circuit faults. |
