Research On Hybrid Modular Multilevel Converter And Its Control Technology

Modular multilevel converter(MMC),as a new multilevel topology,has received extensive attention in the field of variable frequency drive in recent years because of its advantages of modularity,easy expansion,good output waveform and so on.The main challenge is that under low-frequency and high-torque operation requirements,the capacitor voltage of the sub-modules fluctuates significantly.In order to suppress the capacitor voltage fluctuation of MMC,a hybrid MMC topology with a series switch on the DC-side is proposed,which can limit the capacitor voltage fluctuation by reducing the DC-side equivalent voltage.However,there are still many problems in the existing research of the hybrid MMC.Firstly,the existing researches only analyze the application of constanttorque loads,but MMC is more suitable for pump-load applications.Secondly,the DC-side series switches are all IGBTs with high cost.Finally,when operating at very low and zero frequencies,the capacitor voltage fluctuation of the hybrid MMC is still too large.The work aims to solve the above problems and expand the hybrid MMC.This article firstly analyzes the capacitor voltage fluctuation of the hybrid MMC under pump-loads,and then gives the design method of sub-module capacitance.At the same time,a thyristor with lower cost is used for the series switch.In order to ensure the reliable turn-on and turn-off of the thyristor,the coordinated control strategy of the thyristor and MMC is designed.Secondly,for the problem of large capacitor voltage fluctuation at very low frequencies,this article uses the symmetry of the three-phase power and the precharge circuit to achieve stable operation at very low frequencies.Based on this,corresponding control strategies are proposed,including capacitor voltage balance control,charging control,and operating mode switching control.Finally,this article expands the hybrid MMC to a back-to-back structure to realize four-quadrant operation of the motor,which is suitable for industrial applications such as pumped storage.The two MMCs are connected by an antiparallel thyristor,which can realize the bidirectional energy transfer,and inherit the ability of suppressing the capacitor voltage fluctuation.For the back-to-back topology,this article presents the coordinated control strategies,including DCside voltage control,capacitor voltage balance control and so on.The analysis of the capacitor voltage fluctuation and the proposed topologies with corresponding control strategies have been verified by simulation or experimental results.