Research On The Balance Of Capacitor Voltages In MMC With Full Bridge Sub-modules

With the development of DC transmission technology,modular multilevel converter(MMC)has been more and more widely used due to many advantages such as good expansibility and fault tolerance.When a short-circuit fault occurs on the DC side,MMC with half bridge sub-modules used in the existing transmission projects cannot cut off the fault current autonomously.Therefore,MMC with full bridge sub-modules with DC short-circuit fault ride-through capability has received more and more attention.The performance of MMC is closely related to the capacitance voltage of each sub-module.If the capacitor voltage is significantly unbalanced,the safety and reliability of the MMC will be greatly reduced.Therefore,it is of great significance to study the influence of non-ideal parameters on capacitor voltages,and put forward a control strategy and an equalization method of capacitor voltages suitable for MMC with full bridge sub-modules for improving the reliability of MMC system.In this paper,the single-phase MMC with independent loading is studied.The sub-module is full bridge structure.The working principle of MMC is analyzed and a mathematical model is established.This paper analyzes the difference of capacitor voltages of different sub-modules under ideal conditions and non-ideal conditions,explains the dynamic balance process of capacitor voltages under ideal conditions,and reveals the influence of on-resistance of switching devices on capacitor voltages under non-ideal conditions through theoretical derivation.The effects of the internal loss distribution of sub-modules on the balance of capacitor voltages is analyzed by simulations under different operating modes.Based on the carrier phase shift modulation(CPSM),the working states of the sub-modules with different modulation wave overlap are compared and analyzed.The analytical expressions of the left and right bridge arm modulation waves of the full bridge sub-module are derived.And a closed loop modulation strategy which can optimize the loss distribution in the sub-module and increase the range of output voltage is proposed.On this basis,the relationship between the overlapping degree of modulation waves and the voltage modulation ratio is studied,and the selection principle of the offset of modulation waves is analyzed.The balance control method of capacitor voltages for this closed-loop modulation strategy is illustrated,and the effectiveness of the proposed scheme is verified by simulations.Finally,a single-phase MMC with six full bridge sub-modules is designed and built,and experiments are carried out.The experimental results show that the proposed closed-loop modulation strategy and corresponding balance control strategy of capacitor voltages for MMC with full bridge sub-modules are effective.