| With the implementation of energy-saving and emission-reduction strategies and the increase in the penetration rate of distributed power sources,the impact of distributed power supply access on the power grid is increasing.The use of microgrids to integrate distributed power is an effective solution.In the micro-grid system,the proportion of DC-type micro-power such as photovoltaic and energy storage is relatively high,and the number of DC-powered devices is increasing.Also,non-commercial AC power sources such as fans need to be transformed by AC-DC-AC to access AC.Grid.Therefore,compared with AC microgrids,DC microgrids have the advantages of simple control,low cost,low loss,high reliability,and easy access to DC units,which have attracted widespread attention from scholars at home and abroad.At present,the related technology of DC microgrid is still in its infancy,and it is necessary to study modeling and operation of DC microgrid in-depth.Therefore,this article takes DC microgrid as the research object,focuses on its modeling problem,and develops it in three steps.In the first step,equipment-level modeling is performed,ie sub-units within the microgrid are modeled.For example,photovoltaic and energy storage.Therefore,this thesis first established a simulation model of photovoltaic power generation unit and energy storage unit,and analyzed the external characteristics of photovoltaic and its control method through simulation.The second step is to perform system-level modeling,that is,the overall modeling of the microgrid.Firstly,the structure of DC microgrid is analyzed and its operation control method is studied.The droop control is used to solve the power sharing problem of parallel units in the system.Based on the use of renewable energy sources as much as possible,three types of DC microgrid operation modes are divided based on the bus voltage level,and the system can switch between multiple operation modes according to the DC bus voltage level.Second,when peer-to-peer control is used,when multiple units participate in the bus voltage control,the DC droop control cannot take into account voltage stability and multi-source current sharing.Therefore,based on the analysis of DC droop control principle,two methods of droop control secondary improvement are discussed,including quadratic control based on droop translation and secondary control based on adaptive droop control,respectively from the droop curves.The droop curve was adjusted in both the distance and slope.The third step is to model the external characteristics of the DC microgrid as a whole.To study the modeling of DC microgrids under grid-connected operation mode,taking into account the fact that DC microgrid systems have strong non-linearity and it is difficult to establish mathematical models,refer to the idea of black-box modeling and consider DC microgrids as a whole,based on the detailed model,the neural network nonlinear modeling method is used to establish the external characteristic model of the direct microgrid,and the particle network optimization algorithm is used to optimize the parameters of the neural network. |
PDF Full Text Request