Design Of Power Electronic Transformer And Its Application In Photovoltaic Grid-connected System

The development of renewable energy has become an inevitable means of alleviating the world’s resource constraints,and photovoltaic development has become a hot topic of research in the development of renewable energy pools in this new era.However,because of its instability,volatility and intermittency,the question of how to integrate photovoltaic energy into the grid efficiently and effectively while improving power efficiency has become a major focus of discussion.As a highly flexible electrical device,the Power Electronic Transformer(PET)is the core of the entire grid-connected PV system,combining DC and AC interfaces and featuring an intelligent control unit that can improve power quality by flexibly controlling the primary and secondary sides,effectively solving many of the problems that exist in the grid.This effectively solves many of the problems in the grid.In view of this,this paper provides an in-depth study of the design of power electronic transformers and their application in grid-connected photovoltaic systems,with the following aspects being accomplished:1.The overall topology of the PET system is determined,the topological principles and mathematical modelling of each link of the system are analysed,and the control strategies of the converters at all levels of the system are established: the output characteristics of the PV cells are analysed at the input link,and the MPPT control strategy based on the perturbation observation method is adopted to make the PV cells always work near the maximum power point;the double active full-bridge DAB converter at the isolation level adopts the double phase-shift Double phase-shift(DPS)control strategy can reduce the inductor current stress and return power,so that the device can achieve soft switching;the inverter output link adopts double closedloop PI control strategy,and the closed-loop system ensures the output power quality and safe grid connection.2.Establish the inverter optimisation control strategy.In view of the fact that the control effect of the control strategy depends on the selection of the controller parameters,for the traditional PI controller parameters adjustment is time-consuming and lack of accuracy,this paper proposes a fuzzy PI control strategy to redesign and adaptively adjust the controller parameters.The combination of fuzzy PI controller and double closed-loop greatly improves the stability of the control system,reduces the impact of resonance problems to a certain extent,and further improves the effect of grid-connected PV.3.The simulation model of the whole PV grid-connected system based on PET is built in MATLAB,and several operating conditions such as steady-state operation,comparison of total harmonic distortion rate of current,input voltage disturbance and grid-connected current disturbance are simulated and analysed respectively.The experimental results prove that the output power quality of the PET system designed in this paper is excellent and meets the grid-connected conditions;the proposed fuzzy PI control strategy can effectively improve the dynamic and static performance of the grid-connected PV system and optimise the output of the system during steady-state operation;when disturbances occur,it can enhance the anti-disturbance capability of the system and reduce the influence of the interaction between the various levels of converters on the stability of the system during operation.4.In order to further verify the feasibility of the system and the effectiveness of the control strategy,the hardware circuit of the system is designed and the DSP chip is used for software control.At the same time,a prototype of the system was built in the laboratory,and the feasibility and effectiveness of the PET system were finally verified through experiments.