| Full-bridge-half-bridge 1:1 hybrid MMC(hybrid MMC for short)is widely used in the topology of the converter station of the flexible DC transmission system due to its advantages of low construction cost,high operating efficiency,and strong DC short-circuit fault ride-through capability.Attention,and has initially become the first choice for the flexible-to-straight converter station.In scenarios such as extreme weather and forest fires,considering the insulation problem of the transmission line,the DC side voltage of the hybrid MMC is generally reduced to maintain operating reliability;when the transmission power is fixed,the DC side voltage of the hybrid MMC can also be reduced.Increase the transmission current on the DC side to realize the DC ice melting function.When the hybrid MMC operates under low DC voltage conditions,the difference in the work of the full-bridge sub-module and the half-bridge sub-module will cause the sub-module capacitor voltage to be unbalanced.In view of the above problems,this article focuses on the full-bridge-half-bridge The 1:1 hybrid MMC is researched,and the main contents are as follows:(1)The basic control principle of hybrid MMC is analyzed,and the mathematical model of hybrid MMC is established in the dq coordinate system.On this basis,the design process of the existing voltage and current double closed-loop control system and circulating current controller is summarized.Finally,A full-bridge-half-bridge 1:1 hybrid MMC double-ended simulation model was built in Matlab/Simulink to verify the correctness of the equalizing sequencing algorithm and the effectiveness of the circulating current suppressor.(2)Aiming at the unbalanced problem of the capacitor voltage of the two sub-modules under the low DC voltage operating condition of the hybrid MMC,the FBSM and HBSM capacitor voltages are derived by analyzing the charge and discharge process of the FBSM and HBSM capacitor voltages in a fundamental frequency cycle.The mechanism of the imbalance problem,and then calculate the boundary conditions of the imbalance problem,and summarize the influence of the modulation ratio and power factor on the sub-module capacitor voltage imbalance problem.Finally,simulations verify the correctness of the analysis of the sub-module capacitor voltage imbalance under low DC voltage operating conditions.The advantages and disadvantages of the existing hybrid MMC capacitor voltage equalization control methods are briefly described.First,the improvement measures based on the third harmonic voltage injection are proposed to improve the sub-module capacitor voltage imbalance problem by changing the waveform of the output voltage of the bridge arm.After that,on the basis of the improvement of the existing double-frequency circulating current injection,the correction is further calculated.The improvement effect of hybrid MMC operating area.Quantitatively and qualitatively analyze the influence of the third harmonic voltage and the second harmonic current injection amplitude and the initial phase on the sub-module capacitor voltage.Through the analysis of the loss after injection,the comprehensive expansion effect on the operating area is obtained: based on double The improvement measures of high-frequency circulation injection have good use value.(4)A three-phase MMC physics experiment platform based on the full bridge sub-module was built.The experiment platform is based on a modular design idea.The circuit with different functions of the converter is designed in blocks,which is convenient for debugging and later upgrades.The hardware circuit part and software part of the test platform are introduced in detail,the debugging of each part of the circuit is completed,and the preliminary debugging waveform is obtained. |
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