Research On Connected/off Grid Operation And Smooth Switching Control Of Microgrid Inverters

With the large-scale integration of new energy,microgrid systems have been promoted due to their flexible power generation and efficient operation.As the interface for energy exchange between microgrid power generation systems and the power grid,inverters have a critical impact on the stable operation of microgrid systems.The microgrid system operates in parallel and off grid modes,and there are issues such as bus voltage oscillation and output current jump during the switching between the two modes.In response to the above issues,this article conducts research on the control strategies for smooth switching of inverters under different operating modes of microgrids in the following three aspects.(1)Establish a mathematical model for the DC side photovoltaic power generation system of the microgrid.Firstly,by analyzing the output characteristics of photovoltaic cells,maximum power tracking control is adopted to ensure that the DC side voltage of the microgrid system can be stably output in connected/off grid mode.Then,conventional P/Q and V/f control is used for the three-phase three-level inverter to ensure stable operation of the microgrid in both grid connected and off grid modes.Finally,the transient performance of the microgrid system during connected/off grid mode switching was analyzed.(2)To reduce the impact of disturbances on the operational performance of microgrids and reduce the oscillation of inverter output current during connected/off grid switching.Firstly,referring to active disturbance suppression technology,active disturbance rejection control strategy is adopted for the control of the main inverter.Then,taking the d-axis current loop as an example,the design concept and process of the active disturbance rejection controller were elaborated.Finally,in the case of load switching and connected/off grid switching,simulation has verified that the microgrid system has better output current impact quality under the active disturbance rejection control strategy.(3)To solve the problem of the output current response on the inverter side not being able to balance speed and reduce overshoot.Firstly,combining the idea of disturbance observer to improve the conventional linear active disturbance rejection controller,a cascaded active disturbance rejection controller is proposed.Then,the transfer function of cascaded active disturbance rejection control system was derived,and the dynamic performance of cascaded active disturbance rejection control and conventional active disturbance rejection control was compared.Finally,the better dynamic performance of the cascaded active disturbance rejection control system was verified through simulation under the conditions of harmonic injection into the power grid voltage,high impedance series connection on the power grid side,reference current jump in the current loop,and connected/off grid switching of the inverter.