Research On Control Strategy Of Wind/pv/ Storage DC Microgrid

Microgrid can overcome the weakness of distributed generation access to large power grids,improve power supply reliability,optimize power structure,and solve the problem of electricity consumption in mountain areas and islands.Therefore,microgrid technology has become the focus of current researches of many units and institutions at home and abroad.Compared with the AC microgrid,the DC microgrid does not need to consider the phase and frequency of the bus voltage.In addition,the DC microgrid control is simple and reliable,and there is no need to consider the reactive power loss and eddy current loss in the AC grid.Therefore,the research on DC microgrid has attracted more and more attention from domestic and foreign scholars.Firstly,based on the mathematical model of wind turbine and generator,the decoupling model under dq coordinate system is established in this thesis.The maximum power point tracking of wind energy can be realized by controlling the pitch angle,rotation speed and machine-side converter.According to the engineering mathematical model of photovoltaic cell,the output characteristics of photovoltaic cells under different illumination intensity and temperature are simulated and analyzed.The maximum power point tracking of photovoltaic cells is realized by using conductance increment method.At the same time,according to the off-grid operation of the DC microgrid,the PV constant voltage control strategy and the load switching control strategy are designed as needed to ensure that the system is able to cope with the various conditions that may occur in off-grid mode.Secondly,the affine nonlinear model of the bidirectional DC-DC converter of the DC microgrid energy storage system in off-grid mode is established.Using this model,the internal dynamic stability of the system with DC bus voltage as output and output current of lithium ion batteries as output is analyzed respectively.An input-output feedback linearization control strategy that uses the DC bus voltage error to dynamically adjust the Lithium ion battery current is proposed.By analyzing the topology of the grid-connected converter,the mathematical model in the dq coordinate system is established,and a double closed-loop control strategy with outer-loop voltage control and inner-loop current sliding mode control is proposed for grid-connected converters.Finally,the coordinated control of DC microgrid is studied.In order to ensure the internal energy balance of the system and maintain the stability of the bus voltage,the coordinated control strategy of the system is designed.The switching conditions and coordinated control methods of the DC microgrid in the grid-connected mode and off-grid mode are analyzed to ensure the safe and stable operation of DC microgrid under various operating conditions.