Research On Three-port Power Electronic Transformer Based On MMC

Aiming at the development requirement of DC distribution network,this paper proposes a high-power Power Electronic Transformer(PET)for AC/DC distribution network.The converter adopts a Modular Multilevel Converter(MMC)structure,with a push-pull isolation module added in the sub-module.Compared with the traditional MMC,a lowvoltage DC output port is added.The converter can realize the energy transmission between the AC grid and the medium/low voltage DC grid.Thisthesis introduces the topology of PET in detail at first,and then analyzes the energy transfer of PET converter under different working conditions,and the paper also introduces the topology and function of MMC and isolation.This paper analyzes the different working states of sub-modules during PET operation and the different working modes of LC series resonant soft switches.By establishing a mathematical model for the PET circuit,the DC component of the loop current can be used to transmit energy between the AC and DC sides.The AC component in the loop current is caused by the fluctuation of the sum of the upper and lower arms voltages.According to the principle of sub-module equalization control,sub-module voltage control and loop current suppression are performed.The simulation model of PET was built and verified by PSIM software.For the faults such as short circuit and open circuit of the switches inside the PET submodule,refer to the protection scheme of the line fault in the traditional distribution network,combined with the structural characteristics of the circuit,some internal and external faults that may occur in the actual operation of the PET system are introduced.The fault was analyzed and the corresponding protective measures were given.A fault protection simulation model for three-port PET parallel connection with single-phase MMC structure is built by PSIM software.Finally,based on the completion of simulation verification,a PET prototype of MMC structure was fabricated.Experiments were carried out for the isolation-stage push-pull resonance soft-switch and the MMC-level multi-level,which verified the consistency between the experimental results and the theoretical analysis.