Research On Coordination Control Strategy Of Hybrid Micro Grid Based On Load Transfer

With ever-increasing demands for fossil energy and environmental protection,photovoltaic energy,wind energy,and other types of distributed renewable energy have undergone rapid development.Against such a background,microgrid emerges,enabling large-scale distributed energy to connect to medium and low voltage distribution systems conveniently by virtue of its advantages such as great reliability,economical efficiency,and controllability.But the operating control problem,resulting from the access of a large number of random loads to the microgrids,has long been a study hotspot in the field of microgrids.In this thesis,summaries and analyses are made of the current operating status of microgrids as well as the current situations of studies on coordinating control strategies.With an AC/DC hybrid microgrid equipped with a hybrid energy storage system being taken as the study object,a study is conducted on the coordinating control strategies of the microgrid system in different operating modes,and in the meanwhile,another study explores the coordinating control of AC load transfer and energy storage unit in isolated grid operating mode.Firstly,the structural features of the existing three typical microgrids are analyzed.An AC/DC hybrid microgrid is selected to model a system,which builds equivalent circuits and mathematical models of three micro-sources,namely,photovoltaic array,battery,and super capacitor.The working model and coordinating control framework of the system are analyzed briefly.Based on the micro-source models of the system,a model is built for the power converters that connect micro-sources and buses in the system.The control strategies for physical layer are formulated.The basic structure and control mode of SSTS solid-state transfer switch for use in load transfer are taken as the focuses to be analyzed,and meanwhile,simulation is conducted on the switch in the condition that the phases of the two buses are not synchronized,which verifies the reliability of the switch in load transfer.In this thesis,improvements are made to the coordinating control strategies of the system in the grid-connected operating mode.And load transfer is introduced into the microgrid energy management system in the isolated grid operating mode,and coordinating control strategies are developed for microgrids and controllable loads,realizing the optimal matching between the microgrld and the load.On this basis,a control strategy is formulated for the switches among different system modes so as to reduce the transient impact caused by mode switch and ensure smooth switches among different system operating modes.At last,the simulation verifies the control effects of the proposed control strategies,indicating that the microgrid system in different operating modes can be controlled effectively.