Control Strategy Of Four-switch Buck-boost Converter

Currently,carbon emission and energy shortage have become serious problems around the world.Building a new power system with new energy as the main body is an important measure to achieve "carbon peak" and "carbon neutrality".With the rapid development of new energy power generation technology and the direct current applied in new power supply and demand,DC microgrid is one of the important networking methods in the future.As the interface between the energy storage system and the DC microgrid,bidirectional DC-DC converters play an important role in the performance of the entire microgrid system.Among the current bidirectional DC-DC converters,fourswitch buck-boost converter(FSBB)has the characteristics of low voltage stress,the same polarity of input and output,and can work in both boost and buck modes.Compared with other non-isolated converters,it is more suitable for a wide range of voltage and gains more and more attention in energy storage interface converters and DC microgrids.Therefore,in this paper,the working principle and characteristics of three modes,the modulation strategy and the control strategy under different control requirements of FSBB are analyzed and studied,and then verified through simulation and experiment.Firstly,the working principle and modulation strategy of FSBB are reserched.The operation of FSBB in three modes(buck mode,boost mode and buck-boost mode)is analyzed and introduced,and inductance current ripple and switching loss under different input and output voltage relationships of three modes are also comparatively analyzed.Due to the shortcomings of traditional modulation strategies,a single-carrier three-mode modulation strategy is proposed.The proposed strategy can realize smooth switching between three modes by only one modulation quantity.On this basis,combined with actual engineering project with quadruple design,the carrier phase-shift method is adopted to improve the performance of the converter.Finally,the theoretical analysis and the proposed modulation strategy are verified through simulation.Secondly,in order to meet the control requirements of the DC microgrid for FSBB,according to different control objectives,the average equivalent models of FSBB in buck mode,boost mode and buck-boost mode are established,then the current and voltage control equations are given.On this basis,current closed-loop strategy,voltage and current double closed-loop control strategy,droop control strategy and fault ridethrough strategy are given.Based on the equivalent model,the feed quantity is introduced in the voltage and current double closed loop to further optimize the control strategy of three modes.Then,the effectiveness of control strategies is verified by simulation.Finally,on the basis of theoretical research,a prototype of the quadruple FSBB is built.The design of the inductance and capacitance parameters of the main circuit is given.Based on the prototype,the modulation strategy,current closed-loop,voltage and current double closed-loop,droop control strategies and fault ride-through are experimentally verified.The experimental results show that the proposed modulation strategy can achieve smooth switching between three modes;current closed-loop and voltage current double closed-loop can track current and voltage commands with good dynamic and steady-state performance;the droop control can output power according to the preset curve when the DC bus voltage changes,and provide support for the bus voltage;FSBB can effectively limit the inrush fault current when the output side is short-circuited,realize fault ride-through and provide protection for the entire system.The experimental results further verify the effectiveness of theoretical analysis and modulation and control strategies.