Research On Stability Of Cascade System Of Dual Active Full Bridge DC-DC Converter And Three Phase Inverter

Developing renewable energy is an inevitable way to solve the problem of energy shortage.It has become one of the research hotspots to collect and utilize renewable energy by means of AC-DC micro grid.AC-DC microgrid usually needs different cascaded power electronic converters to achieve flexible power conversion and voltage level conversion,but the interaction between cascaded converters will adversely affect the stability of the system and reduce the power quality and safety.The dual active bridge(DAB)converter has the advantages of flexible voltage conversion,simple hardware structure,reliable and safe performance.It can be cascaded with three-phase inverter for medium and high power applications,and is widely used in AC-DC converter.It is necessary to improve the stability of the cascaded system.For this purpose,this paper studies the following contents.Firstly,the research status of the stability of cascade system is summarized,including identification method and improvement method.The application scenarios of inverter control strategy and modeling method are described.The topology,working principle and mathematical model of DAB converter and three-phase inverter are analyzed,which is the basis of impedance modeling of the converter.Secondly,the inverter adopts the widely used PQ control.According to the topology and control loop,the small signal model method is used to model the DAB converter and inverter,and the expressions of input and output impedance are derived.Based on this,the impedance interaction mechanism and influencing factors between DAB converter and inverter are analyzed,and an improved coordinated optimal control is adopted.The improved strategy can not only suppress the grid connected impact of the inverter,but also change the negative impedance characteristic of the inverter at low frequency,reshape the impedance,and effectively improve the stability of the cascade system when the power mutation and direction reversal occur.Moreover,the virtual synchronous generator(VSG)control has made many improvements to PQ control,which can make up for the problems of grid inertia and damping reduction caused by a large number of power electronic devices.For VSG control,this paper considers the influence of topology,control loop and coordinate transformation on impedance modeling,and deduces VSG input impedance model.At the same time,the VSG grid connected small signal model is established to analyze the factors affecting the grid connected stability,and then the factors affecting the impedance are analyzed by using the Bode diagram and Nyquist curve.The active damping compensation strategy is adopted to adjust the inverter impedance,which effectively improves the stability margin of the cascaded system.The simulation verifies the effectiveness of the strategy.Finally,the experimental platform of DAB converter and three-phase inverter is developed and built to further verify the active damping compensation strategy.The results prove the correctness and effectiveness of the theory.