| Rural power grid is characteristic of complicated lines,changing loads and poor regularity of electricity consumption.The design philosophy and pattern proposed by the agriculture network can solve a series of problems in current isolated and grid-connected operation of rural power grid well.DC bus plays the key role in energy conversion and transmission of rural nano grid.DC-DC converter is an important component of DC bus energy transmission and conversion and its structural parameter design is crucial.In this study,LLC-type resonant DC-DC converter is used as the research object,aiming to optimize the theory and engineering design method of converter and exploring the design strategy of DC-DC converter which adapts to future development and application of rural nano grid.Research conclusions can provide strong guarantee to high-efficiency stable.energy transmission and conversion in DC bus of rural nano grid.In this study,380V DC bus is used as the input of converter.Basic analysis method of converter characteristics,structural design and parameter calculation of directional and bidirectional converters,resonant parameter optimization design,parasitic oscillation optimization,and other relevant engineering practice design optimization techniques are explored.Relevant design and calculation method is proposed from theoretical perspective.Besides,a simulation and prototype experiment is designed to verify the proposed methods.Detailed research contents are introduced as follows:(1)A comparative study on analytical methods of characteristics of resonant converter.Characteristic analysis of resonant converter is the premise for structural parameter optimization design.At present,there are many analytical methods of the converter.However,further studies are needed on comparison of application scenes,advantages and disadvantages of different methods.In this study,a comparative study on two common analytical methods of Fundamental Harmonic Approximation and operation mode analysis was carried out.Advantages and disadvantages of these two analytical methods in different scenes are proposed,which provids follow-up researchers and designers some references in accurate selection of appropriate method.(2)Research on 4-11-type CLLC two-way DC-DC converter.In application scenes of bidirectional energy transmission,the 4-11-type CLLC bidirectional DC-DC converter has relatively high reverse gains.However,the design is relatively complicated due to the involvement of many magnetic elements and the asymmetry between primary and secondary side.Based on the topology theory,this study compares the bidirectional 4-type CLLC resonant converter and 4-11 type CLLC resonant converter with the traditional 4-type LLC resonant converter.The working principle of reverse working of the 4-11 type CLLC resonant converter is analyzed.The state equation of the converter in different stages is written according to basic characteristics in the working principle and working state of the converter.Combining with constraints of bidirectional working compatibility and design demands of the converter,a simple resonance parameter design method for the 4-11 type CLLC resonant converter is proposed and its validity is verified by an experiment.(3)Studies on optimization design method of converter based on phase-angle analytical method.Resonant circuit conduction loss of the converter occupies the dominant role in operation energy loss of the converter.In this study,the parameter optimization design of resonant element of the converter to decrease resonant network conduction loss is discussed.Phase angle of the resonant network is used as the dividing sequence and the working process of the converter was analyzed.The roles of magnetic inductance/resonance inductance(m)and characteristic impedance of the converter(Zr)in balancing the relationship between voltage gain and tank circuit loss are analyzed.Relevant states are analyzed.An optimization design model is established and parameter optimization design method of resonant element is proposed by using effective value of resonant current as the identification and optimization design objective function of conduction loss,and using ZVS working condition of the primary switch of the converter as the constraint.(4)The optimization design system that can make comprehensive evaluation of converter performances is studied.Many parameters,including characteristics of soft switching,working frequency range,DC gain,electric energy loss and reactive circulation,have to be considered in design of converters.With considerations to simplicity and intuition of evaluation indexes as well as accuracy and convenience of optimization design,an optimization design system which uses the power factor angle(?)of the primary resonant network of the converter as an evaluation index is proposed.Next,the parameter design method of the converter based on this optimization design system is put forward.This method establishes a model by decreasing ? as the optimization design goal and by using primary ZVS working area within the design loads and working frequency range.It is characteristic of comprehensive evaluation and simple solving.Moreover,the peak gain characteristic analytical method of the converter based on the proposed optimization design system is proposed.On this basis,a normalized comparative analysis and simulation experimental verification are designed.The proposed design method is compared with classical design method of engineering practices and leading design method in the industry in term of power factor angle,resonant network current and reactive power.It proves the validity of the proposed optimization design method by using ? as the evaluation index.Compared with two comparative methods,the proposed design method can realize comprehensive,simple and visual evaluation of working area,resonant network current,reactive power of the converter.It is a parameter optimization design method that can balance the relationship between energy transmission efficiency and safety stable operation.(5)Experimental prototypes are designed and parasitic parameter detection and optimization problems are studied.In high-frequency resonant converter design,many problems of the equipment(e.g.stray capacitance)can cause great interferences to experimental prototype design in order to increase frequency by decreasing volume of the device.Based on the experimental prototype design,parasitic parameter detection and optimization methods are discussed from the theoretical and engineering application perspectives.Influences of parasitic parameters of converter in real operation environment are simulated by building RC test circuit for switching tube under a simple experiment environment.This method can help designers to correct designs based on theoretical calculation of parameters of components like switching tube in the manual and thereby realize the goal of providing practical and accurate design of switching tube.A new optocoupler feedback circuit is proposed for optimizing parasitic parameters of the feedback circuit of the converter,thus realizing the goal of reducing parasitic vibration and stable.feedback and operation.Finally,the effectiveness of the method proposed in this paper is verified by the prototypes. |
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