Dual-mode Transition Control Strategy For Inverter-Based Microgrid Operating In Grid-Connected And Standalone Mode

At present,various distributed power generation units mostly consume renewable energy through direct grid connection.Due to the insufficient peak shaving capacity of traditional power grids and the decline in the stability of traditional power grids,the power of renewable energy is not compatible with Traditional grid.In order to solve the above problems,the microgrid embedded with various distributed generation units,energy storage devices and loads is gradually applied.When the traditional large power grid allows,the microgrid is merged into the large grid operation,and when the island or the large grid fails,the microgrid is switched to off-grid mode.The dual-mode operation of the microgrid improves the utilization rate of renewable energy,To the greatest extent,it overcomes the contradiction that the large-scale integration of renewable energy power causes the stability of the traditional power grid.However,when the microgrid is switched between the two modes,it needs to be shut down,which cannot guarantee the uninterrupted power supply to important loads.In view of the above problems,this paper has conducted an in-depth study on the smooth switching operation control strategy between the grid-mode and off-grid for micro-grid inverter,and carried out a series of simulation and experimental verification.Experimental results show that droop control strategy is more suitable for micro-grid inverter off / grid dual mode switching operation compared to PQ/Vf control strategy.In addition,in order to enhance the frequency stability of the inverter in off-grid mode and achieve accurate control of the inverter’s grid-connected power,the traditional droop control strategy is improved from the aspects of increasing system inertia and virtual impedance technology.Simulation has proved that the improved droop control strategy Effectiveness.The specific research contents are as follows:First,the development status of micro-grid and the smooth switching operation is summarized.There are two common smooth switching operation control strategy of micro-grid,including PQ/Vf control strategy and droop control strategy.Aiming at the PQ/Vf dual-mode switching control strategy,the dual-mode inverter mathematical model is established,the dual mode controller structure of PQ/Vf is designed,and the transient process of the PQ/Vf switching strategy is analyzed according to the equivalent circuit at the time of switching.It is concluded that the PQ/Vf switching strategy is prone to overshoot voltage during grid-conected and off-grid switching.In order to achieve dual-mode smooth switching operation of the inverter,the droop control strategy is selected,which overcomes the shortcomings of the abrupt modulation ratio when working mode of the inverter is switched in the PQ/Vf control strategy,and avoids the overshoot voltage.In addition,the pre-synchronization control link has been added to suppress the inrush current generated during the switching,and the smooth switching of grid-conected and off-grid is realized.Furthermore,in order to improve the steady-state characteristics of the grid-connected mode of the inverter,this paper adds virtual impedance technology on the basis of traditional droop control,eliminates the transmission power coupling caused by line impedance,and realizes accurate control of grid-connected power in steady state.In order to increase the inertia and damping of the inverter in off-grid mode,the frequency mathematical model of the synchronous generator and the droop control inverter is compared and analyzed.The frequency mathematical model of the synchronous generator is introduced into the droop controller to make the inverter To obtain the inertia similar to synchronous generators,and in order to eliminate the problem of inverter grid-connected power oscillation caused by the introduction of the synchronous machine frequency mathematical model.This paper optimizes and improves the structure and parameters of the droop controller,which not only retains the inertial control link of the inverter but also eliminates the second-order oscillation of the grid-connected power.Compared with the PQ/Vf switching strategy,the droop control strategy is more suitable for the dual-mode smooth switching operation of the microgrid inverter,but the inverter with droop control will cause transmission power Coupling due to line impedance parameters in the grid-connected mode,accurate control of grid-connected power cannot be achieve.Other more,the inverter lacks inertia in off-grid mode.Therefore,while adopting the droop control strategy to control the inverter’s two-mode smooth switching operation,the virtual impedance technology is used to eliminate the grid-connected power coupling,and the synchronous generator frequency mathematical model is introduced to increase the inverter’s inertia.By improving the structure and optimizing the parameter,the grid-connected power oscillation caused by the introduction of the synchronous generator mathematical model is eliminated.Finally,an experimental system platform was built,the inverter off-grid experiment,and grid-connected experiment are completed.The experiment verified the effectiveness of the proposed strategy.