Research On Control Method For Grid-Connected/Islanding Switching Of Hybrid AC/DC Microgrid

As a good method to integrate renewable energy such as distributed generation,hybrid AC/DC microgrid has become a research hotspot because it can provide direct power supply interface for different load,reduce the stage of power transformation and improve energy utilization efficiency.An important characteristic of hybrid AC/DC microgrid is that it can operate in grid-connected mode or islanding mode based on different conditions.However,when unplanned islanding happens,the voltage and current in the microgrid will experience remarkable fluctuations,affecting the safety of the system.In this paper,the hybrid AC/DC microgrid is chosen to be the research subject,and the control method for grid-connected/islanding switching of hybrid AC/DC microgrid is investigated.The main work is done as follows:Firstly,the common topologies and control strategies like droop control and master-slave control etc.of hybrid AC/DC microgrid are introduced.The characteristics and application situations of each topology and control method are analyzed.The characteristics of wind power,photovoltaic power and energy storage devices are introduced respectively.The mathematical model is established,and the commonly used control strategies are given.Secondly,the islanding mode of hybrid AC/DC microgrid is introduced.Several stages of the transition procedures from grid connection mode to islanding mode in the case of unplanned islanding and the affecting factors are analyzed.It is concluded that to realize smooth mode switching,fast islanding detection algorithm and stable mode switching control methods are of great importance.Based on the importance of the islanding detection algorithm for unplanned islanding,an islanding detection algorithm for DC microgrid with triangular wave disturbance is proposed.The proposed method takes the voltage change rate of DC bus as the criterion to determine the occurrence of islanding.As to the switching control method,the control methods of DC distributed units and bidirectional DC/AC converters in hybrid AC/DC microgrid are presented and the transfer functions of them are derived respectively.Based on the master-slave control,a grid-connected/islanding switching control method for hybrid AC/DC microgrid is proposed by introducing state tracking control between DC constant power and constant voltage control method.The proposed control method can reduce the voltage and current fluctuation of DC bus by improving the overshoot and rapidity of the reference value of current loop.Finally,based on MATLAB/Simulink,simulation models for grid-connected wind,photovoltaic power generation devices and the charge and discharge control of energy storage unit are presented.Secondly,the proposed islanding detection algorithm is implemented based on MATLAB to verify the correctness by detecting the DC bus voltage data when islanding occurs in a DC microgrid with triangular wave disturbance.Then,a simulation model of hybrid AC/DC microgrid is built in MATLAB/Simulink,and simulations are carried out in grid-connected,islanding mode and the switching of two modes.