Research On Key Techniques Of MMC Based Power Electronic Transformer

With the rapid development of economy,the increasing demand for power supply leads to the increasing installed capacity of renewable energy.All of these require higher stability and power quality of the power system.The energy Internet is an effective scheme to further realize the efficient utilization of renewable energy and improve the power quality of the power grid.In the energy Internet,power electronic transformer is the executive equipment of energy management.Different from conventional power transformers,power electronic transformers should not only realize voltage transformation,isolation,and energy conversion but also have high flexibility to achieve different control objectives.Therefore,the development of power electronic transformer technology has very important economic and social value.This paper analyzes the principle of modular multilevel converter based four-port power electronic transformer and establishes the mathematical model.Focusing on its key technologies,the research has been made in the aspects of topology,DC fault protection,multi-module voltage measurement and reducing the complexity of software and hardware.The main work and achievements are as follows:1)Aiming at the DC short-circuit fault problem that may occur in the application of four-port MMC based power electronic transformer in medium or high voltage DC network,an improved scheme of topology structure is proposed.In order to improve the speed of fault removal and reduce the difficulty of the restart,an improved topology to enhance DC fault ride-through ability was proposed based on the advantages of existing technologies,and some fast pre-charging schemes for this topology under different voltage levels and a number of sub-modules was further proposed.2)An improved topology to reduce hardware complexity is proposed.In view of a large number of switching devices and their auxiliary circuits required for the basic topology of four-port MMC based power electronic transformer,on the one hand,the switching devices of the required dual active bridge are reduced by improving the sub-module structure.On the other hand,the sub-module of MMC and the switching devices of DAB are used to realize the DC fault ride-through function together to improve the utilization rate of switching devices and reduce the number of newly added switching devices.The improved structure not only reduces the hardware complexity but also further reduces the control complexity.3)Two multi-module voltage measurement technique suitable for different topologies are proposed.In order to reduce the problem of large number and complexity of hardware caused by the use of modular multilevel topology,the multi-module voltage measurement technology for different topologies was proposed by analyzing the variation rule of the arm AC-side voltage of MMC,reasonably configuring sensors and making full use of sampling values,so as to obtain more accurate voltage values of sub-modules with as few sensors as possible.For the limitation of the bandwidth and measuring range of the voltage sensor,a grouping method is proposed.4)The control strategy of MMC based power electronic transformer is optimized.The control strategies of MMC and multiple DAB output parallel system are analyzed and studied respectively.The characteristics of capacitor voltage balance control in MMC and DAB output parallel system are compared.The integrated control strategy in which the voltage balance control implemented in multi DAB output parallel system is proposed.This control strategy can reduce the amount of measurement required for control and reduce the complexity of control,providing a research basis for the power coordinated control of MMC based PET under different working conditions.Finally,the experimental prototype of the proposed MMC based power electronic transformer is designed,and the design idea and function realization scheme of the control system are introduced.Finally,the experimental prototype was used to verify the integrated control strategy.