Research On Optimized Control Strategies For Modular Multilevel Converter Based Power Electronics Transformer

Transformers have been regarded as one of the key parts of the power system for their roles of voltage level transformation.Over the recent decades,with the rapid development of the industry and the economy,new requirements have been raised towards the modern power system,for example,the self-healing ability of power system,the goal of an uninterruptable power system,the high power quality and the economic affairs of the power system,etc.On the one hand,facing new requirements,traditional transformers cannot satisfy them all.On the other hand,compared with traditional transformers,Power Electronics Transformer(PET),which has drawn researchers’ attention based on high-frequency technology,can not only realize the basic function of voltage level transformation,but obtain other functions like dynamic reactive compensation,uninterruptable power supply,power system frequency transformation,power quality control and management,etc.Nowadays,the three-stage–structure based PET has been the main research object within the PET field,especially the threestage–structure based PET using Modular Multilevel Converter(MMC)as the input stage(usually called Modular Multilevel Converter Based Power Electronics Transformer,MMC Based PET)has been considered having excellent application prospects.Based on the background above,the thesis has conducted the study on the control strategy optimization of the input stage and the isolation stage in MMC based PET,which is essential for the efficient operation of power system.Firstly,the thesis has studied MMC-the input stage of MMC Based PET.The topology of MMC,the operation status of sub-modules,along with the MMC switch model have been analyzed.The thesis has also introduced and analyzed traditional modulation and control strategies of MMC,moreover,the thesis has proposed a new MMC modulation classification method based on the consideration of control goals’ priorities,introduced the concepts of centralized modulation strategies and distributed modulation strategies and designed a state machine based pulse distributor for integrated pulse dispositionm odulation strategies with necessary optimization.Secondly,the thesis has studied the topology,the operational principle,the phase-shifted control strategy and the ZVS condition of Dual Active Bridge(DAB)-the isolation stage of MMC Based PET.The control strategies using double closed loop(current loop and voltage loop)method and a controller which equalizes the input voltages have been put forward for single DAB and different DABs with ISOP parallel structure.At last,the effectiveness of all the proposed optimized control strategies has been valiadated in simulation and experimental work.Meanwhile,the thesis has also established the simulation of the whole MMC Based PET system under different working condition and validated the effectiveness of the proposed control strategies.