Research On Optimal Control Strategy Of Cascade Power Electronic Transforme

With the construction of new power systems,a high proportion of new energy sources and distributed power sources are connected to the grid,and the nonlinear characteristics of their power electronic devices have brought new challenges to the power quality management of the grid.Compared with traditional transformers,power electronic transformers have become important equipment in the new generation of power systems because of their advantages such as realizing power conversion,isolating fault transmission,and controlling the direction of power flow.This paper takes the cascaded power electronic transformer with input series output parallel structure as the research object,deeply analyzes the working principle and power transmission characteristics of each part of it,establishes a mathematical model and introduces the corresponding control strategy,and optimizes the dynamic performance and power balance.etc.Firstly,the cascaded H-bridge rectifier is analyzed and studied,its mathematical model is derived according to the working state and the basic modulation method is explained,and the double closed-loop control strategy based on the dq coordinate axis and the DC current control strategy are introduced,comparing their dynamic performance.Considering that the power quality of grid-side current is greatly affected by grid fluctuations,a cascaded CDSOGI is designed as a single-phase frequency adaptive phase-locked loop based on SOGI filters,which improves the detection of grid phase angle and frequency accuracy,with better power grid harmonic distortion suppression ability.Secondly,the topology,working principle and transmission characteristics of multimodule parallel output DAB are analyzed,and the power feedforward PI control strategy and power feedback PI control strategy are introduced,as well as the power balance method under these two strategies.In order to solve the optimization problem of power balance effect and dynamic response speed,the model predictive control strategy is designed by using the characteristics of DAB modularization,the prediction model is built and the optimal cost function is determined,and the power balance control is realized while the system dynamic performance is improved.Then the three-phase LCL grid-connected inverter is studied,the mathematical model of the inverter in the two-phase rotating and two-phase stationary coordinate systems is analyzed,and the corresponding PI controller and quasi-PR controller are designed to realize zero steady-state control in two coordinate systems,the dynamic response of the two control strategies and the harmonic content of the grid-connected current are compared,and the quasiPR controller is verified to have better suppression of high-order harmonics of the gridconnected current ability.Finally,a two-module parallel DAB experimental platform is built,the hardware circuit and software module design of the experimental device are given,and the proposed model predictive control strategy is verified by physical objects.