Research Of Power Electronic Transformer Based On Modular Multilevel Matrix Converters

Transformers are widely used in various fields, such as distribution transformer in grid, on-board traction transformer in electric locomotives, and so on. And in some areas, there are lots of transformers, that performance of the transformer directly affects the performance, reliability and economy of the whole system. However, along with the proposed smart grid, development of distributed new energy sources, speed-up of railway, etc. Traditional transformer cannot meet to the requirements of power field. In this background, a new transformer-power electronic transformer(PET), achieving energy transfer and voltage conversion through power electronic conversion technique, gets more and more attention. PET does not has only the basic functions of traditional transformer, but also has the functions of grid interconnection, distributed energy grid-connection, reactive power compensation, etc.So it can be applied in many fields, such as, distubuded energy grid, locomotive traction and interconnection of power grids, etc.This paper analyzes the progress of the research on PET home and abroad and the features of the exsting PET topologies, most existiong PET topologies can not orient to the medium-and high-voltage occasions, and contain a number of high-frequency transformers, thereby, the PET has low efficiency, high cost, and does not solve the problem of heavy volume and weight. Achieving miniaturization and lightening of PET is the primary goal of PET design, in order to achieve this goal, and with the development of smart grid and new energy power generation technology, this paper has a research on a new topology of PET based on Modular Multilevel Matrix Converters(M3C). The new PET is available in medium/high voltage and high capacity occasions, for instance in distribution network, high-speed EMU unit, since M3C which is the core of the PET is highly modular and flexible, and can export arbitrary waveform. Compared with the existing topologies, the proposed topology requires only one MF transformer, therefore, the proposed topology is superior to the existing circuits in terms of volume and weight. And, sub-module of M3C is H-bridge, so it can output DC or AC voltage in power frequency or medium frequency for PET, that the PET can realize bidirectional flow of energy, applied to the situation of distributed energy grid-connection, grid interconnection, etc.This paper describes the proposed circuit topology, through establishing the mathematical small-signal model of the topology, and presents a set of PET control strategy giving consideration to capacitor voltages balance control of M3C. M3C adopts the multi-hierarchy method to balance the capacitor voltages, based on arm current direct control. Full-bridge on the secondary of the MF transformer adopts the output voltage phase-shift single and double loop control strategy. And this paper shortly introduces the control strategy of other PET based on M3C, active compoment of input or output current in the arm current are the controlled variable, and M3C adopts multi-hierarchy method. According to the model of the PET built in Matlab/Simulink platform, the software simulation results verify the accuracy of the proposed strategy, and give the simulation experiment based on RT-lab real-time simulation platform. The simulation results verify that the proposed control scheme is feasible and effective, and has excellent dynamic and static characteristics.