Application Of Photovoltaic-energy Storage Based On Bidirectional LLC Resonant Converter

With more appearances of problems in photovoltaic grid-connected systems such as power-off and power-limited,power grid impact,etc.,photovoltaic energy-storage has caused much attention because energy-storage of battery can solve the above problems well.A bidirectional DC to DC converter(DC/DC)must be used in the energy-storage of battery.It has become a research orientation to make the energy-storage of battery achieve high frequency,high power density,adapt to the characteristics of the photovoltaic system and miniaturization.A serial resonant circuit with two inductors and a capacitor(LLC)can achieve soft switching by resonance and improve the conversion efficiency and power density.It is a good attempt to apply bidirectional LLC to the photovoltaic energy-storage systems.In this paper,the advantages and disadvantages of the current bidirectional DC/DC topologies including non-isolated and isolated are analyzed and summarized.It is concluded that the current single topologies cannot meet all the three elements of high efficiency,adapt to photovoltaic characteristics and electrical-isolated.Therefore,this paper designs a serial structure of bidirectional LLC and Buck-Boost converter.This paper introduces the serial bidirectional DC/DC structure.The basic working principles and processes of the circuit has been analyzed,including forward charging and reverse discharging.The control model of the system has been established and the closed-loop control method has been designed.According to the actual project requirements,the selections and designs of the main components and loss calculation have been done.According to the characteristics of this structure,main points of the switches for bidirectional LLC are analyzed and made.The bidirectional LLC' soft-switch processes were simulated with the simulation tool.According to the classical control theory,the models of the control system have been established by small signal method.The controller' parameters under charging and discharging conditions have been designed respectively.The controller' parameters have been verified by Matlab.According to the theoretical guidance,a 2.5k W bidirectional DC/DC prototype has been developed,and relevant efficiencies and working waveforms have been obtained through tests.Datas and waveforms show that this serial structure of bidirectional DC/DC achieves high frequency and high power density.And its good closed-loop control behaviors can meet the requirements of photovoltaic energy storage systems.