Research On Voltage-Regulated Dual-Active-Bridge DCX Converter

With the large-scale connection of new energy power generation,new energy transportation,and energy storage systems to the distribution network,it will be one of the development trends to integrate the DC hybrid distribution network into the existing AC distribution network.To achieve the energy balance between different DC links in the AC/DC hybrid distribution network,a bidirectional DC-DC converter is usually necessary to regulate the bidirectional power flow,which is also known as a DC transformer(DCX).A dual-active-bridge DCX converter is one of the bidirectional DC-DC converters,which is widely used as an energy management unit between different DC buses due to its simple topology and efficient operation.There are two main types of dual-active-bridge DCX converters widely used,namely resonant dual-active-bridge DCX converters and phase-shifted dual-active-bridge DCX converters.The traditional resonant dual-active-bridge DCX converter has high conversion efficiency when the switching frequency is equal to the resonant frequency,but its output voltage is uncontrollable,which is not conducive to flexible energy regulation under the operating condition of DC bus voltage fluctuation.The traditional phase-shifting dual-active-bridge DCX converter can achieve voltage regulation and flexible power control,its zero-voltage-switching(ZVS)range is limited,and it will produce a large circulation current at heavy load,which increases the additional power loss and reduces the conversion efficiency of the converter.This thesis mainly focuses on voltage-regulated dual-active-bridge DCX converters,including resonant dual-active-bridge DCX converters and phase-shifted dual-active-bridge DCX converters,and conducts indepth research on its new topology and modulation strategy.The main research includes the following four parts:(1)Aiming at the problem of limited ZVS range of traditional phaseshifted dual-active-bridge DCX converters,a phase-shifted dual-active bridge DCX converter based on the magnetizing inductance and its optimization design method are proposed,which realizes the full-range ZVS and reduces switching losses.By optimizing the leakage inductance and transformer,the topology can realize the full-range ZVS within a certain voltage fluctuation range.In addition,a new standard dual UI core integration structure is proposed to integrate the leakage inductor and the transformer,which reduces the volume of magnetic components.The experimental results show that the proposed converter improves the conversion efficiency under light load conditions,and verifies the feasibility of the proposed new magnetic integration structure.(2)Aiming at the problem of heavy load reactive circulation of traditional phase-shifted dual-active-bridge DCX converter,a modulated couple inductor technology is proposed,and a combined phase-shifted dual-active bridge DCX converter based on this technology and its optimized modulation are proposed to achieve full-range ZVS and reduce the current stress.By controlling the operating phase between the two modules,the equivalent inductance of the modulated coupled inductor can be controllable under different working conditions.According to the basic principle of the modulated coupled inductor,the minimum current composite control law is proposed to realize modulation switching.Finally,taking the full-range ZVS and minimum RMS current as the optimization goals,an optimization design method for converter parameters is proposed.The experimental results show that the overall conversion efficiency of the proposed method is improved.(3)Aiming at the problem that the voltage gain of the traditional resonant dual-active bridge DCX converter is uncontrollable when the switching frequency is equal to the resonant frequency,resulting in the difficulty of achieving flexible energy regulation under the condition of DC bus voltage fluctuation,a voltage-regulated resonant dual-active bridge DCX converter and its co-duty cycle modulation strategy are proposed to achieve controllable voltage gain.Based on the idea of multiplexing switching devices,a new voltage-regulated resonant topology is obtained.The basic operating mechanism of the proposed converter under the coduty cycle modulation is studied.Finally,the design method of the converter is proposed.The experimental results verify the voltage regulation ability of the proposed converter under a certain voltage fluctuation range.(4)Based on the characteristics of two types of dual active bridge DCX converters,a resonant + phase-shifted dual-active-bridge DCX converter and its modulation strategy are proposed,and the general topology and design of the two types of dual active bridge DCX converters can be obtained.The proposed topology can control the equivalent inductance of the modulated coupled inductor by changing the modulation strategy,which changes the structure of the resonant tank in different operating modes.According to the basic principle of the modulated coupled inductor,a parameter decoupling design method for the two operating modes of the converter is finally proposed.The experimental results show that the proposed converter can operate stably in both resonant mode and phase-shifted mode,and the operation mode can be flexibly selected according to the actual operating conditions.