Research On Hybrid MMC Capacitor Voltage Control Strategy

The hybrid MMC contains both HBSM and FBSM,which has both the fault ride-through capability of FBSM-MMC and the economy of HBSM-MMC,thus having broad application prospects.MMC distributes the energy in the capacitors of each submodule.In order to maintain the normal and smooth operation of the system,it is necessary to control the balance of the capacitor voltage among the submodules.Due to the difference in topology and operating characteristics of the full-bridge submodule and the half-bridge submodule,the control strategy of the capacitor voltage balance of the hybrid MMC is different from the traditional HBSM-MMC.The question of capacitor voltage balance of hybrid MMC is explored from the following aspects.First of all,this paper introduces the topology and basic working principle of MMC,constructs its mathematical model in dq rotating coordinate system,and briefly describes the design of the inner current loop and outer power loop controllers of MMC,and completes the overall control frame.Then,the article analysis the mechanism of the difference in capacitance voltage between the full-bridge submodule and the half-bridge submodule during hybrid MMC overmodulation operation.When the hybrid MMC is operated in an over-modulated condition,the switching rules will change due to the difference in structure between the full-bridge submodules and the half-bridge sub-modules,which in turn will bring about the problem of capacitance voltage balance between the FBSMs and the HBSMs.This article analyzes the mechanism in capacitance voltage ripple difference between the hybrid MMC with boosted modulation index,and introduces a control method based on third harmonic voltage injection.To get the best performance of the control method,the suitable third harmonic voltage injection coefficient h should be found.So,after that,the waveform of the reference voltage of the bridge arm after harmonic voltage injection is discussed.According to the different waveforms,the Fundamental modulation ratio m and the third harmonic voltage injection coefficient h are divided into different sections,and the variation of the capacitor voltage difference between the valve sections in each section is discussed,with the effect classification node of the third harmonic voltage injection to suppress the capacitor voltage difference between the valve sections being found.Next,the expression of the maximum energy difference of the capacitor voltage between FBSMs and HBSMs under different working conditions is analyzed.Then,according to the expression,the relationship between the improved injection index and the modulation ratio that can suppress the difference of the capacitor voltage fluctuation is explored.After that,the architecture of the hybrid MMC submodule capacitor voltage balance control is introduced.The capacitor voltage balance is divided into Five levels of balance: the relative balance of submodules in the same valve section,the relative balance between the valve sections in the same bridge arm,and the relative voltage between the upper and lower bridge arms,the relative balance of the voltage between the three-phase sub-modules and the total capacitance voltage balance.The voltage balance between the bridge arms is achieved through the control of the total energy and energy difference of the upper and lower bridge arms of each phase.The control method of capacitor voltage balance between valve segments based on average voltage control and holding factor is proposed,and the system-wide control strategy of capacitor voltage balance is established.The traditional hybrid MMC sorting algorithm is improved,so that the software calculation and hardware switching loss of the sub-module capacitor voltage balance control in the bridge arm are greatly reduced.Finally,this paper builds a hybrid MMC model in MATLAB / SIMULINK.Through the MATLAB / SIMULIK simulation and experiment results,the reliability of the relevant theoretical analysis and control strategy is verified.