| In recent years,the electric vehicle(EV)industry of China has made remarkable achievements and has become a significant force leading the transformation of the automobile industry all over the world.As Inductive Power Transfer(IPT)has the pros of flexible energy transmission,high-level automation,low operation and maintenance costs,and strong environmental adaptability,it has a good application prospect in the field of EVs.Although IPT technology has been successfully applied in consumer electronics,implantable medical devices,and unmanned underwater vehicles,there are still some problems when applying it in EVs as distributed mobile energy storage units.On the one hand,the uncertainty of the car position during parking makes the coupling coefficient of the receiver and the transmitter vary in a broad range,which requires the charging/discharging system to have a strong anti-offset capability.On the other hand,the charging/discharging system also needs to meet the demand for wide voltage ranges during charging and discharging EV's power batteries.Thus,ensuring efficient,stable,and reliable operation under a wide range of offsets and outputs is an imminent issue to be solved.This dissertation takes a 6.6 k W bidirectional wireless EV charging/discharging system as the research object.Starting from the topology structure and control method of each conversion unit,this dissertation aims at achieving high-efficiency,wide-range bidirectional power transmission and carries out in-depth research and discussions on the optimization of the efficiency and power density of bidirectional AC-DC converters,the implementation of wireless DC transformers,and the architecture and control strategy of the bidirectional charging/discharging system through theoretical analysis,numerical simulation and experimental verification.The detailed research contents include the following four aspects.1)Magnetic powder core inductors have the advantages of high operating frequency,small sizes,and low costs,so they are widely used as the AC filter inductor in Power Factor Correction(PFC)converters.However,their inductance changes nonlinearly with the current,which seriously affects the dynamic and steady-state characteristics of converters.In this regard,this dissertation starts with the operating principle of totem pole PFC converters and takes into account the soft saturation characteristics of magnetic powder core inductors.By analyzing the impact of nonlinear time-varying inductance on traditional PI control,a totem-pole PFC converter control strategy considering the inductor's soft saturation characteristics is proposed.By combining offline calculation and online correction,the object model in the controller is updated in real-time,which can effectively eliminate the adverse effects of the varying inductance and ensure the efficient and stable operation of the system.Finally,the proposed control method is verified with an experimental platform.After adopting the enhanced control method,the measured full-load input current THD decreased from 4.516% to 3.015%,with the transmission efficiency in the load range of 10% to 100% over 95.5%.The peak transmission efficiency is 99.26% when the output power is 1939 W.2)Targeting the decrease of power density and conversion efficiency when enlarging the capacity of the bidirectional totem-pole PFC converter,this dissertation gives full play to the characteristics of different core materials.It makes use of magnetic powder core inductors and ferrite inductors,respectively,to deal with the DC and AC components of the PFC input current.The topology of a two-phase interleaved totem-pole PFC converter with a magnetic powder core AC filter inductor and a ferrite coupling inductor is proposed.The current ripple characteristics of the combined nonlinear inductor are analyzed in detail,with the parameter design method for the inductors given.This dissertation then explores the zero-crossing distortion issue of the input current of the totem-pole PFC converter and reveals the reason for current zero-crossing distortion.A digital implementation method of totem-pole PFC without input current distortion is proposed,which ensures the stable control and efficient operation of the system.The performance superiority of the proposed combined filter inductor and the efficacy of the inductor current zero-crossing distortion elimination method are verified on the prototype of a 7.7 k W two-phase interleaved totem-pole PFC converter.Under the same level of the input current ripple,the AC filter inductance in the proposed topology is 25% of that in traditional interleaved structures.The total volume of the inductor cores is reduced to 67% of conventional inductors.Experiment results show that the transmission efficiency is 99.29%when the power converter works on the inverter mode and the output power is 3895 W.The transmission efficiency is 98.62% in the fully-loaded rectifier mode,with the fully-loaded input current THD equal to 1.87%.3)In order to eliminate the influence of load fluctuations on the output voltage gain of current-source bidirectional wireless IPT systems,this dissertation proposes a wireless DC transformer structure based on the LCC-S high-frequency compensation network,whose port voltage is fully decoupled with the load.Combined with the proposed bidirectional zeroreactive-power real-time control strategy,synchronous control of the switches in the primary and secondary sides is achieved,which has the potential for natural two-way flowing of energy.Through the established equivalent circuit model,the influence of resonance parameters on the port current characteristics is analyzed.The circuit parameters of the high-frequency compensation network are also designed and optimized to meet the high-efficiency power transfer requirements of the system.The effectiveness of the proposed circuit structure and its control scheme is verified on a 6.6 k W wireless IPT platform.The transmission efficiency is over 90% at a transmission distance of 140 mm and a load range of 20% to 100%.The peak transmission efficiency of the system reaches 96.64% when the output power is 7046 W.4)Existing bidirectional wireless charging/discharging systems have problems such as high dependence on real-time wireless communication for coordinated control of the vehicle and the ground terminal,as well as the low reliability and robustness of the control system,limiting the further promotion of IPT technology in the field of wireless charging of EVs.In this regard,this dissertation proposes a three-stage bidirectional wireless charging/discharging system architecture based on the wireless DC transformer.By realizing wide-range voltage matching and transmission power regulation with the vehicle-side DC converter,along with the enhanced low-frequency ripple suppression capability of the current loop contributed by a PIR controller that automatically adapt to the ripple frequency,the proposed system can achieve efficient and stable bidirectional wireless charging/discharging control without control-level data communication between the primary and secondary sides.The feasibility of the proposed system architecture and the effectiveness of the cooperative control strategy are verified on a6.6 k W bidirectional wireless charging/discharging platform.As the distance between the primary and secondary coils is 140 mm,the measured transmission efficiency of the G2 V mode is 96.14% when the AC voltage is 239.90 V,and the DC output is 420.64 V/3617 W.The transmission efficiency of the main power loop of the V2 G mode is 96.21% when the DC input is 420.14 V,and the AC output is 238.97 V/3615 W. |
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