Research On Cascaded Three-phase-bridge Converter And Its Application In Power Electronic Transformer

Cascaded multilevel converters have been one of the research forcuses in the fieldof power electronics, and they are widely used for power transmission, motor drive andso on. At present, single-phase cascaded H-bridge (CHB) converter obtaines the mostattention and applications due to its good performance, and its modulation strategy andcontrol methods have been very mature. However, when used in three-phase high-voltage system, CHB converters should be star or delta-connected to obtain three legs,and this will result in employing more switches and DC capacitors and adopting morecomplex control method.Based on the thought of cascaded multilevel, a novle multilevel topology, named ofcascaded three-phase-bridge (C3PB) converter, is proposed in this paper. This convertercan be directly used in three-phase high-voltage system. It not only can operate as aninverter, but also it can operate as a rectifier as the active front-end.Firstly, the structure thought of C3PB converter is given; the currents and voltagesrelationship and the transmitted power of each module are analyzed and the topologyextension method is given. Secondly, its switch modulating strategy is discussed.Considering the implementation scheme and harmonic components, CPS-PWM isemployed and the theoretical analysis is verified by simulation. Furthermore, theapplication of C3PB converter in power electronic transformer (PET) is researched. Athree-stage PET system is designed, with three-level C3PB rectifier as the input stage,dual active bridge (DAB) converter as the isolation stage, and three-phase-bridgevoltage source inverter as the output stage. Their control methods are annlyzedseparately. Simulation results show that PET can realize the functions of10kV/380Vvoltage transmission, unit power factor and stable three-phase output voltage under fivedifferent working conditions. The five different working conditions include three-phasevoltage drop and unbalance, input voltage distortion, output load mutation, poweringnonlinear load and inductive load interface. Finally, an experimental prototype oftwo-level C3PB inverter is built with DSP+FPGA as the control core. The converterfeatures and CPS-PWM are verified by hardware experiment.In brief, this paper proposes a novel topology of C3PB converter and its levelextension method, switch modulating strategy, control method and application in PETsystem are researched. The correctness of theoretical analysis is demonstrated by simulation and experiment.