Research On Full Bridge Modular Multilevel Converter Control Strategy

At present,Modular Multilevel Converter(MMC)has attracted more and more attention because of its advantages such as high equivalent switching frequency and strong scalability,and its application in the field of flexible DC transmission has now become Research hotspots.At present,the most widely used in flexible DC transmission system engineering is an MMC converter based on a half-bridge submodule.When a short-circuit fault occurs on the DC bus,the half-bridge MMC cannot actively defend against DC faults,so it has DC short-circuit fault ride-through The ability of full-bridge MMC is getting more and more attention.This paper mainly studies the control strategy and capacitance voltage fluctuation of the full-bridge MMC converter,and performs simulation verification.The main contents include:(1)The topology of the full-bridge MMC and the working principle of the sub-modules are introduced,and the operating mechanism of the full-bridge MMC to reduce DC voltage and reverse polarity is analyzed.The modulation methods applied to the fullbridge MMC are compared,and the carrier phase-shift modulation method using dualmodulated waves is given.The harmonic analysis is performed and the feasibility of the modulation method is verified by simulation.(2)The control strategy of the full-bridge MMC is given: the equivalent mathematical model of the full-bridge MMC system is established and the basic control strategy of the system is designed using its mathematical model.Aiming at the characteristic that the full-bridge sub-module can output negative level,the realization method of full-bridge MMC buck and reverse polarity operation is given,and the DC command can be arbitrarily changed to achieve double-loop control.Adopt the energy sharing control and balance control method of the sub-module capacitor voltage to complete the control goal of capacitor voltage balance.(3)The control strategy of the full-bridge MMC is given: the equivalent mathematical model of the full-bridge MMC system is established and the basic control strategy of the system is designed using its mathematical model.Aiming at the characteristic that the full-bridge sub-module can output negative level,the realization method of full-bridge MMC buck and reverse polarity operation is given,and the DC command can be arbitrarily changed to achieve double-loop control.Adopt the energy sharing control and balance control method of the sub-module capacitor voltage to complete the control goal of capacitor voltage balance.