Topology Derivation And Modulation Optimization Of Multi-Material-Devices Based Hybrid Multilevel Converter

Large-capacity power electronics converters are the foundation and core of the conversion and efficient utilization of electric energy in national key development areas such as electrified transportation,AC-DC transmission and distribution,large data centers,and national defense and military equipment.Multi-level converters,which have the advantages of low voltage stress,good waveform quality,and high power density,are one of the optimal and most attractive solutions for realizing medium-voltage high-power AC-DC power conversion and are widely used in the industry.However,as common-used silicon(Si)power devices gradually approach their theoretical limits,it is difficult to further improve the performance of silicon-devices-based multi-level converters.High-performance wide bandgap devices represented by silicon carbide(SiC)MOSFETs are restricted by high prices and therefore have not yet achieved large-scale commercial applications.In response to the above issues,based on the multi-material-devices based hybrid multi-level converters that adopt SiC and Si devices,this dissertation investigates the topology derivation and modulation strategies for this kind of hybrid converters.Then,a series of SiC and Si hybrid multi-level topologies and their optimized modulation methods are proposed,which as a result provide high-performance and low-cost solutions for large-capacity and high-performance power conversion occasions.Firstly,the generalized construction method and modulation strategy for SiC and Si hybrid multi-level converters are proposed.The proposed topology construction method includes the definition of the basic cells,the criteria that enable the hybrid structure,and special steps that generate the hybrid topologies.Based on this,a series of SiC and Si hybrid multi-level topologies are derived.By selecting the redundant switching states in hybrid multi-level topologies,a generalized modulation strategy is also proposed,which can concentrate all high-frequency switching events on the SiC devices.The generalized construction method and modulation strategy for SiC and Si hybrid multi-level converters are verified by simulations and experiments and consequently guide the proposal of new hybrid multi-level topologies.Secondly,a SiC MOSFET and Si diode hybrid five-level unidirectional rectifier is proposed.It requires only four SiC MOSFETs with relatively low blocking voltage in each phase.Meanwhile,by adding snubber capacitors,all the Si devices are with low-speed switching,and the voltage stresses of fast SiC MOSFETs are minimized.In this dissertation,the operational analysis,control strategy,and carrier-based modulation scheme for this circuit are illustrated.The capacitor voltage balancing and unity power factor are both realized.The simulation and experiment verify the advantages and effectiveness of the proposed rectifier.Finally,a multi-step soft-switching modulation strategy for SiC and Si hybrid four-level converters is proposed.In this dissertation,SiC and Si hybrid four-level inverter and rectifier are derived according to the proposed construction method for hybrid multi-level topologies.When the proposed multi-step soft-switching modulation strategy is applied to these two hybrid topologies,the operational analysis are done in detail.The proposed modulation strategy realizes both the soft switching of all silicon devices and the balance of the neutral-point voltage.The effectiveness of the multi-step soft-switching modulation strategy is verified by the simulation.