| The instantaneous power on the AC side of a single-phase power converter will cause low-frequency ripple voltage and ripple current on the DC-link bus,which will affect the performance or life of the DC-side equipment.The low-frequency ripple is usually suppressed by using a parallel capacitor on the DC-link bus: a passive capacitor module solution consisting of large-capacity aluminum electrolytic capacitors and film capacitors;the active capacitor solution whose capacitor voltage is controlled by the active switches.The passive capacitor solution undoubtedly limits the power density of the converter.The active capacitor solution improves the utilization rate of the capacitor,which means the required capacitance can be greatly reduced,thereby improving the power density.However,in addition to smoothing the DC-link bus voltage as an energy buffer,the DC-link capacitor also serves as a energy storage component during hold-up time,provide the necessary energy for the system,and even provide some reactive power to the grid to support grid recovery.Therefore,the DClink capacitor also needs to meet the hold-up time performance,which undoubtedly poses a certain challenge to the power density of the power converter.This thesis will research and design three kinds of DC-link capacitors,compare and analyze the low frequency ripple suppression ability,hold-up time performance and power density,and an experimental verification will be carried out.Firstly,this thesis designs DC-link capacitors that suppress the ripple voltage.The circuit topology and mathematical modeling are adopted to analyze the double-line frequency ripple generation mechanism of the DC-link in single-phase PWM rectifier.The passive capacitor,current-fed active capacitor and voltage-fed active capacitor are selected for parameters design and control methods design.In PLECS,a single-phase PWM rectifier and three kinds of DC-link capacitors are built for circuit simulation.The ripple voltage and ripple current suppression capability of three DC-link capacitors are evaluated.Then,the energy storage capacitors are redesigned in three DC-link capacitors and the control part are added for some applications that require hold-up time.To reduce power loss and improve power density of active switches,the wide band-gap Ga N is selected.The power loss of Ga N in two active capacitors are calculated and the thermal resistance of Ga N and other components are considered for selecting heatsink.Based on the component selection,the cost and power density of three DC-link capacitors are benchmarked.In PLECS,three DC-link capacitors are simulated and the DC-link voltage under the hold-up time are analyzed and compared with theoretical analysis.Finally,the experimental verification and analysis of the first two DC-link capacitors is carried out by building an experimental platform.A 5.5k W single-phase PWM rectifier,passive capacitor and current-fed active capacitor hardware are built.Experiments verify that the two DC-link capacitors can achieve ripple voltage suppression capability and meet the hold-up time requirement.Based on the realization of the functions,the power density of the manufactured current-fed active capacitor hardware is higher than that of the passive capacitor,which has a significant effect on the power density of the converter. |
