Research On Topology And Control Strategy Of Power Electronic Transformer Based On MMC

In recent years,the installed capacity of distributed energy into the power grid has become larger and larger.Therefore,concepts such as micro-grid,smart grid and energy internet have been widely mentioned and deeply studied.However,due to the randomness and intermittence of the new energy power,its integration into power grid puts forward higher requirements for the power regulation of the power grid.The traditional power electronic transformer only has traditional functions such as voltage regulation,and cannot adjust the power quality.The MMC-based power electronic transformer is suitable for MV and HV high-power systems,and is of great significance to the construction of smart grids.Therefore,MMC-PET is taken as the research object in this paper,and its topology structure and control strategy are deeply studied.The main research contents of this paper are as follows:Firstly,the working principle of MMC-type power electronic transformer is analyzed,and the mathematical models of its input,intermediate and output stages are established to obtain its internal characteristics.In this paper,the soft-switching technology has been adding to solve the switching loss problem caused by cascade structure of multi-modules of intermediate stage of power electronic transformer,and the energy transmission process of bidirectional DC-DC converters after the soft switching technology added has been analyzed in detail.Secondly,in this paper,three-level independent control method has been adopted.By analyzing the mathematical model that using the Input stage modular multilevel converter as rectifier for MMC-PET,a control strategy based on internal characteristics is proposed,which is composed of Input stage current control,circulating current restraining,capacitor voltage balance control,etc to realize the adjustment of the primary voltage regulation of the HV DC bus bar and the system power quality.In addition,the pre-charge control strategy established by the initial capacitor voltage of the input stage MMC sub-module is designed for the startup phase of power electronic transformer.The carrier phase-shift modulation strategy in the MMC-PET input stage is designed to reduce the switching loss and capacitor voltage fluctuation caused by high switching frequency,reducing the actual switching frequency of submodule fully controlled devices by increasing the equivalent switching frequency.In addition,the way that combine the input voltage error and single phase shift control is proposed in this paper in order to solve the system instability and power loss caused by the circulating current between modules when MMC-PET intermediate-level multi-module cascade.By introducing the error of the primary side input voltage of the MMC-PET intermediate-stage bidirectional DC-DC converter into the phase shift angle of the single phase shift control,the phase shift angle is adjusted twice to realize the voltage regulation of the LV DC bus bar voltage and the suppression of circulation between bidirectional dc-dc modules.Due to the diversification of MMC-PET application conditions,the way of third harmonic injection is used to improve its ability in carrying unbalanced loads.Finally,the simulation model of MMC-based power electronic transformer is built in Matlab/Simulink platform to simulate the steady-state,voltage surge,voltage drop,voltage flicker,and Short circuit fault of the power electronic transformer network side voltage,verifying its validity of the topology and control strategy and the advantages in system power quality regulation.It has certain theoretical significance for the application of MMC-PET in new energy field.