Research On Combined Operation Of LLC Resonant Converters

As a clean and renewable energy source,wind energy has received increasing attention.Due to the proximity of the power load center,offshore wind farms have been greatly developed.Compared with the traditional AC-accumulated AC transmission and AC-accumulated DC transmission offshore wind farm construction scheme,the full DC wind farm has the advantages of small size,low cost and easy maintenance due to the use of high-power DC/DC converter instead of the original power frequency transformer for voltage conversion,which represents the future development direction of offshore wind farms.As a core component of offshore DC wind farm construction,DC/DC converters must meet the requirements of high gain,high power and high efficiency.In this paper,the LLC resonant converter is used as the basic unit,and the high-gain conversion of the DC fan output voltage and the high-power energy transmission of the whole machine are realized by the series-parallel combination of the converter unit.Therefore,this topic focuses on the equalization and current sharing characteristics of different combinations of converter units and the corresponding control strategies to achieve high efficiency and average power energy transmission for the basic unit of the converter.In this paper,the current sharing and voltage Equalizing characteristics of the LLC resonant converter with the input parallel output parallel(IPOP)and the input series output parallel(ISOP)in the LLC DC converter mode and DC transformer mode are analyzed in detail.In combination with the application requirements of the subject,the equalization and current sharing characteristics of the LLC input parallel output series(IPOS)combination method in DC transformer mode are analyzed,and the output current imbalance of different combinations is analyzed by the difference of the resonant tank parameters.Compared with other working states,when the LLC works at the resonant frequency point,the resonant tank current rms value and the MOSFET turn-off current are both small.Therefore,the conduction loss and the turn-off loss of the MOSFET can be reduced,and a high power transmission efficiency can be obtained,so a frequency tracking control strategy needs to be introduced.This paper analyzes the existing frequency tracking control methods,such as current harmonic detection method,pulse width phase-locked loop method and resonant period calculation method.On the basis of clarifying the limitations of the existing control methods,a new frequency control algorithm for sampling the output rectifier diode voltage is proposed.The new frequency control algorithm analyzes the working principle of the new frequency tracking control algorithm in detail,and gives the control block diagram and flow chart.Finally,combined with the circuit design indicators,the resonant tank parameters of the main power circuit are designed in detail,and the design flow of the magnetic components is given.At the same time,the software simulation platform and hardware test platform are built according to the design parameters.The correctness of the equalization and current sharing characteristics analysis and the new frequency tracking control strategy are verified by simulation and actual measurement.