Research On Control Strategy Of Half-bridge Three-level Bidirectional DC-DC Converter

With the proposed goal of achieving "carbon peaking and carbon neutrality",fossil fuels have been gradually replaced by renewable energy sources such as wind and solar energy.However,renewable energy,especially solar energy,has the disadvantage of being uncertain and intermittent.DC microgrids can absorb electric energy on the spot to ensure a stable output of clean energy and have been rapidly developing in recent years.The bidirectional DC-DC converter,as a key link in energy transmission of DC microgrids,plays a crucial role in the performance of DC microgrids.The half-bridge three-level bidirectional DC-DC converter studied in this thesis has the characteristics of high voltage resistance,high efficiency,and bidirectional energy flow.Therefore,studying the optimization strategy of the converter and improving its efficiency are of significant value for its practical application.This thesis first analyzes the working principle and characteristics of a half-bridge three-level bidirectional DC-DC converter under phase shift control,and calculates the transmission power,soft-switching range,and backflow power of the converter.Optimize the return power of dual phase shift control.The optimization strategy adopted can enable the converter to achieve soft-switching within the full power range and effectively reduce the return power.However,this method still has the problem of high backflow power under light load conditions and potential voltage disturbances during operation.To address the issues of voltage disturbances,a PWM-phase-shift control with two degrees of freedom was studied.The control method divides the operation status into four working modes,and the transmission power,soft-switching conditions,and backflow power are calculated for each mode.The Karush Kuhn Tucker algorithm is used to optimize the backflow power,and the resulting control strategy achieves the minimum backflow power of the converter under soft-switching conditions.However,this method still has the problem of high backflow power under light load conditions.To address the issue of high backflow power under light load conditions with a control strategy with two degrees of freedom,a dual-PWM-phase-shift control strategy with three degrees of freedom is analyzed.According to the operating characteristics,the operating state is divided into 12 modes.Analyze the transmission power,softswitching conditions,and backflow power of six forward transmission modes,and optimize the backflow power based on this.There are four modes that enable the converter to achieve minimum backflow power while meeting soft-switching conditions.Using graphical methods to comprehensively solve the optimal four modes,a minimum backflow power control algorithm for the full power range is ultimately proposed.Compared with the control method with two degrees of freedom,this control method has zero backflow power under light load conditions,reducing both backflow power and current stress.The simulation results verify the correctness of the theoretical analysis.A hardware platform was designed and built for experimental verification of the converter modulation strategy.The experiment showed that both PWM-phase-shift control and dual-PWM-phase-shift control can achieve minimum backflow power under soft-switching conditions,while dual-PWM-phase-shift control can achieve zero backflow power under light load conditions,significantly improving the efficiency of the converter.The efficiency of the three control strategies was compared under the same operating conditions.