Research On Seamless Switching Technology For Grid-connected/Islanded Microgrid Based On Virtual Synchronous Generator

Micro-sources are usually connected to the power grid through power electronic converters,and they have no inertia.Traditional control strategies have been difficult to meet the requirements of grid-connected control,often resulting in grid-connected/islanded instantaneous current shocks,and even lead to stability damage,and it is difficult to achieve seamless switching.Synchronous generators have the characteristics of large rotor inertia,strong self-regulation ability and frequency modulation characteristics,which can provide great help for grid-connected/islanded switching of microgrid.In this paper,a virtual synchronous generator(VSG)algorithm is introduced to support the grid-connected/islanded switching operation of microgrid by its primary regulation,secondary regulation and damping characteristics.The main research work is as follows:Firstly,the model of single micro-source is introduced.Then the control circuit of micro-source is explained and analyzed.The current inner-loop control and voltage outer-loop control are mainly introduced.Secondly,the virtual synchronous generator model is built.According to the active-frequency and reactive-voltage characteristics of the synchronous generator,the active-frequency control and reactive-voltage control of the virtual synchronous generator control strategy are realized.The virtual synchronous generator algorithm is introduced into the control of the inverter,and the overall control strategy for grid-connected/islanded switching is given.The influence of rotational inertia and damping on grid-connected/islanded switching is simulated and analyzed.For the traditional control mode of grid-connected/islanded switching of microgrid,it is necessary to switch between V/F mode and P/Q mode.The change of control mode often leads to the discontinuity of control.The virtual synchronous machine algorithm is introduced to ensure the continuity of control and avoid the switching of control modes when the microgrid converter is switched off or connected to the grid.A constant difference adjustment method is proposed,and the stable output of frequency and voltage of the inverters is achieved through frequency and voltage links,which realizes the smooth switching between grid-connected and islanded operation modes.In order to reduce the time of phase synchronization,the fast phase tracking adjustment is adopted to realize the fast switching between grid-connected and island operation modes.