| Compared with traditional AC distribution system,DC distribution network has prominent advantages in power supply capacity,power quality,line loss,new energy and DC load adaptation,so it has become the focus of widespread attention under the development of energy internet.Especially with the increasing penetration of new energy represented by photovoltaic power generation,it is difficult for the traditional AC distribution network to achieve efficient and stable accommodation.It will become an important way of energy convergence to connect photovoltaic energy to the DC distribution system in the development of the future smart grid.Therefore,the study of high efficiency,low cost,reliable and stable grid-connected photovoltaic DC system has far-reaching significance for building a strong regional intelligent distribution system.Under the application background of DC distribution system,this paper proposes two kinds of photovoltaic power integration system which all based on DC/DC converter.By designing appropriate topology and control strategy,the photovoltaic grid-connected systems have the characteristics of high efficiency,low loss and high reliability.In this paper,a novel scheme of photovoltaic power integration system based on modular LLC resonant converter is proposed under the application scenario of large capacity photovoltaic power integrate into medium voltage DC distribution network.The detailed topology structure of the grid-connected system is designed,and the sub-modules of the modular converter are analyzed and selected concretely.According to the characteristics of photovoltaic cells,the operation parameters of the system and the structure parameters of the sub-module are designed,the characteristics of the modular LLC resonant converter are studied and the operation curves are drawn.In the electromagnetic transient simulation of the system,a photovoltaic equivalent simulation module is built.Aiming at accelerating the simulation speed of modular converter,an optimized equivalent simulation model is adopted.Two operating conditions and four working stages of the simulation model are studied.Thanks to the elimination of a large number of switching devices,the optimized simulation model reduces the order of nodal admittance of the system and improves the simulation speed greatly while ensuring the simulation accuracy.This avoids the difficulty of debugging the control system afterwards.Through the study of the operation characteristics of the sub-module,a control system consisting of voltage command generation part and frequency control part is designed.While maintaining the voltage and current sharing of the sub-module,it can achieve the maximum power tracking of the photovoltaic generating unit.The feasibility of the proposed system and the effectiveness of the control system are verified by simulation.In order to apply in the scenario of distributed photovoltaic access to low-voltage DC distribution network,this paper proposes a new scheme for photovoltaic power integrating into low-voltage DC grid based on partial power converter,which can effectively achieve distributed maximum power tracking under partial shading condition.Due to the application of partial power converter,it only needs to deal with the differential power,which greatly reduces the operating loss of the system and saves the capacity cost of the equipment.This paper studies the characteristics of photovoltaic arrays under partial shading condition and designs the structure of voltage compensator and current compensator in the system,which can compensate the unbalance between adjacent components or between components and DC buses to achieve power matching.In this paper,the operation characteristics of partial power converters are studied,and the control strategy of the system is proposed.Finally,the simulation results verify the system can realize module-level maximum power tracking at the cost of low operating loss and improve the efficiency of photovoltaic power integration system. |
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