Research On Optimal Control Strategies Of The Dual Active Bridge DC-DC Converter In Auxiliary Power Supply System Of Electric Locomotive

As an important part of the high-power electric locomotive,the auxiliary power supply system provides AC power for various auxiliary equipment in the locomotive.Traditional isolation schemes with the power frequency transformer cannot meet the requirement of lightweight for the auxiliary converter of the locomotive.Developing high-frequency and isolated converters is the significant development tendency of the auxiliary power supply system.In this dissertation,the dual active bridge(DAB)DC-DC converter is selected as the voltage regulation converter in the intermediate DC link of the auxiliary converter.This paper first introduces the power transmission principle of DAB converter,and analyzes the converter soft switching range based on the single phase shift under different voltage ratios.The small signal model of DAB converter is derived based on the reduced order model.To deal with the slow dynamic response of the single voltage close-loop control with the proportional-integral(PI)controller,two methods with fast dynamic response are researched in this paper,including the load current feedforward control and the direct power control.In the load current feedforward control,the load current is introduced in the feedforward loop,and it is used as a disturbance to compensate the changes in the phase shift duty ratio caused by the sudden change of transmission power.In the direct power control,the phase shift duty cycle is calculated by using the sampled values in every switching period,which improves the dynamic response.In addition,to reduce the hardware complexity and cost,an extended state observer(ESO)is used to replace the current sensor in current feedforward and direct power control.The paper analyses the cause and influence of double grid frequency in input voltage of auxiliary converter.Aiming at this problem,three optimization ideas are proposed: The input voltage feedforward link is added in the control loop to offset input voltage fluctuation;The proportional-resonant(PR)controller is utilized to increase the gain in the frequency of the voltage ripple;The direct power control is used to calculate the phase shift duty ratio in real time,and compare the suppression effect of the three methods on the output voltage ripple.At last,the simulation and DAB converter experiment platform are used to verify that:(1)current feedforward and direct power control can improve the dynamic response speed of the converter,and the direct power control is better;(2)Direct power control has the best suppression effect among the three optimization methods to suppress the secondary pulsation of output voltage;(3)The introduction of ESO will not affect the control performance.It can be concluded that the direct power control method with ESO can achieve fast dynamic response and suppress the double grid frequency ripple of the output voltage simultaneously.