Research On Three-port DC/DC Converter Applied To Modular DC Grid-connected PV System

With the growth of human society's demand for energy and the exhaustion of traditional fossil energy,as one of the main forms of new energy,photovoltaic power generation has been widely concerned and developed.Multi-level,easy-to-expand modular grid-connected photovoltaic systems can achieve both distributed generation and independent maximum power point tracking,which is an ideal form of large-scale photovoltaic power generation system.In this thesis,an efficient three-port interleaved boost LLC resonant converter is used as the basic converter unit of modular DC gridconnected photovoltaic system.The bi-directional port of the converters are connected through a low voltage DC bus to solve the problem of unbalanced output power and voltage oscillation of each module in the system.Firstly,the operational principles of three-port interleaved boost LLC resonant converter are analyzed,and the time-domain expressions of main voltage and current parameters are listed.On this basis,the gain characteristics of the converter are derived.The theoretical constraints of soft-switching for semiconductor devices in converter are analyzed,and the operating range of soft-switching in practice is deduced.The interleaved parallel boost structure of the converter is compared with the conventional boost converter,and the ripple characteristics of the input current of the converter are studied.Secondly,the modular DC grid-connected photovoltaic system based on three-port DC/DC converter is introduced.The operational principles of the system are analyzed,and the method of single module accessing and exiting the system is expounded.The three-port interleaved boost LLC resonant converter is used as the basic converter unit of the system.The control strategies of the system in MPPT mode and constant power mode are studied.The output voltage sharing error of the control strategy used in the system is analyzed.Thirdly,according to the characteristics of three-port interleaved boost LLC resonant converter,the design flow of device parameters of the converter is given,and the parameters of each device of the converter are designed according to the flow.The loss calculation methods of semiconductor devices,magnetic components and capacitors in converter are given,and the loss models of the main devices are established,which lays a foundation for the optimal design of the converter.Finally,the simulation and experiment platform of modular DC grid-connected photovoltaic system based on three-port interleaved boost LLC resonant converters is built.The operational principles and characteristics of interleaved boost LLC resonant converter are verified by simulation and experiment.The rationality and effectiveness of modular DC grid-connected photovoltaic system and its voltage sharing control strategy are also verified.