Study On Loss And Efficiency Optimization For Bi-directional DC/DC Converter In The Microgrid System

As the key component of microgrid’s electric power storage system, the bi-directional DC/DC converter is of great significance to the grid’s stability. The converter’s power losses and efficiency are closely related to the whole system’s well-running and economic benefits. Therefore, the study on losses and efficiency of bi-directional DC/DC converters in the microgird system is of much practical use. This dissertation mainly studies the optimization of a bi-directional DC/DC prototype’s losses and efficiency centering on three different kinds of practical loss problems: power device losses, magnetic element losses and filter capacitor losses. The main contents of this dissertation are as follows:The topology of the bi-directional DC/DC converter is designed for the microgrid system. The parameters of the transformer, flyback winding, filter capacitors and power devices are calculated to meet the performance indexes, using common calculation methods from the standpoint of function implementation. These circuit parameters are the object and suction variables of subsequent efficiency optimization.Switching processes of fast recovery diode (FRD) and IGBT are analyzed to deduce the approximate expression and main factors of losses. Loss experiments of the bi-directional hard-switching DC/DC converter are made to analyze power device losses. The dissertation adopts two loss optimization schemes:the lossy snubber circuit and soft-switching, and compares the two scheme’s experimental results in detail. Based on soft-switching conditons and equivalent circuits of ZVZCS in charging mode and ZVS in discharging mode, a iteration optimization strategy of the soft-switching auxiliary circuit is presented to determine the optimization results. The experimental results indicate the effectiveness of optimization strategy.The main reasons of magnetic core loss and winding loss are analyzed to deduce the magnetic loss expression on the base of the woking waveforms of the transformer and flyback winding. Magnetic loss experiments are made. The optimization method is studied for the loss of the transformer and flyback winding. By considering all factors, including heat, efficiency, cost and volume, etc, the dissertation selects the best scheme, significantly enhancing the converter’s performances.The capacitor equivalent loss model is analyzed to deduce the approximate expression of two filter capacitors in the bi-directional DC/DC conveter. Filter capacitor loss experiments of bi-directional DC/DC converter are made. Three optimization methods are presented for the loss of filter capacitors. Considering such aspects as loss, efficiency, ripple voltage, cost, and volume, this dissertation presents the best capacitor optimization scheme and its experimental results, realizing capacitor’s low loss, high efficiency, low ripple voltage, low cost and small volume.In summary, studies on optimization methods have been made for the losses of power devices, magnetic elements and filter capacitors in the bi-directional DC/DC conveter. The experimental comparions are made to prove the effectiveness of these optimization methods. The studies on loss and efficiency optimiztion can serve as ideas and foundations,in order to further reduce thermal loss, enhance efficiency and increase the power density.