Research On Topology And Control Of High Efficiency Back-to-Back Conversion

Three-phase back-to-back converters are widely used in industrial applications such as uninterruptible power supplies and variable frequency motor drives.The back-to-back(BTB)converter supplies power to the load through a two-stage conversion structure consisting of the front-end rectifier and the load-side inverter.Its energy conversion efficiency is one of the important specifications of the system.Due to the hard switching operation,traditional BTB converter has a large switching loss,so the switching frequency is limited,resulting in larger passive components and audio noise.This thesis mainly studies the high efficiency conversion technology of back-to-back converters.In this thesis,the hybrid module is first introduced to the two-level and three-level BTB converters,and the efficiency improvement of the hybrid module on the two-level and three-level topology is analyzed.According to the commutation characterastics of different working modes of T-type three-level converter,different combinations of hybrid modules and silicon modules are designed,and the system efficiency of BTB converters with hybrid modules and silicon modules at different switching frequencies are evaluated.Then a unified control scheme that can be applied to both PWM rectifier and inverter in a three-phase four-wire BTB converter is discussed.This control scheme can shorten the control system development cycle and simplify the debugging and maintence of the program code.In addition to reducing the power loss with power device,soft-switching technology is also an effective way to increase the efficiency of the back-to-back converter.DC-side resonant converters has simple auxiliary circuit,which is more suitable for multi-phase converters.This thesis presents a three-phase four-wire zero-voltage-switching(ZVS)BTB converter topology.In this topology,the rectifier side and the inverter side can realize zero-voltage turn-on of all switching devices by sharing one auxiliary resonant branch,which effectively reduces the switching loss of the switching device and improves system efficiency.This thesis first analyzes the working principle of ZVS BTB converter circuit and its Edge Aligned pulse width modulation(EA-PWM)scheme.Using this modulation scheme,the auxiliary switch only needs to act once in each switching cycle to achieve ZVS turn-on of all switches,which can effectively reduce the loss of the auxiliary circuit and reduce its control complexity.At the same time,the modulation strategy is suitable for different power factors,unbalanced load,non-linear load and other complicated working conditions.In addition,the loss model of the three-phase four-wire ZVS BTB converter is established,and the efficiency of the 50 kVA experimental prototype is optimized based on the loss model,including power device selection,resonant parameter optimization,filter inductor optimization and key current commutation loop busbar design.Finally,the soft switching operation and basic circuit functions of the circuit are verified on a 50 kVA prototype.The experimental results show that the soft switching technology can significantly improve the conversion efficiency of the BTB converter.Compared with traditional silicon devices,SiC MOSFETs have better switching performance.At the same time,it has larger turn-on loss than turn-off loss.Therefore,it can be used in ZVS-on soft switching circuits to eliminate most of the switching losses,which can make better use of the advantages of soft switching circuits,and further improve system efficiency and power density.In order to evaluate the efficiency improvement effect of SiC devices on three-phase four-wire zero-voltage-switching back-to-back converters,the three-phase four-wire zero-voltage-switching back-to-back converters using SiC MOSFETs at different switching frequencies were compared with traditional hard-switching converter.A 10 kW three-phase four-wire zero-voltage-switching back-to-back converter prototype based on SiC MOSFET device was built and compared with hard-switching back-to-back converter in terms of switching device voltage stress and efficiency.Finally,the extended application of the soft-switching technique is discussed.