On Control Strategy Of Microgrid Inverter Based On Droop Control

In recent years,with the increase in power demand,environmental and energy issues have become increasingly serious,and microgrid technology based on renewable resources has emerged at the historic moment.Distributed energy is connected to the microgrid through the power electronics.Inverters based on droop control are widely used because of simple communication and "plug and play".However,the traditional droop control is also affected by factors such as line impedance mismatch and local load fluctuations,and there are many shortcomings.Therefore,it is necessary to formulate reasonable control plans to ensure that the microgrid can operate stably in the working mode.In this thesis,the three-phase voltage source parallel inverter based on droop control is taken as an object to study the power coupling phenomenon of island mode droop control,the power distribution of different parallel micro-source inverters and grid-connected pre-synchronization control.First,the traditional droop control characteristics are analyzed,the droop controller is designed,and the power distribution conditions between distributed power sources are derived.On this basis,for the problem of inaccurate power distribution caused by the difference in line impedance in the low-voltage microgrid,the virtual impedance is used to complete the decoupling of the droop control power,and the frequency domain method Bode plot is used to compare and analyze the change of the equivalent output impedance of the system before and after adding the virtual impedance.The appropriate virtual impedance value is selected,and the equivalent output impedance property of micro-source inverter is corrected to make it approximately inductive at 50 Hz,which satisfies the condition that the power is distributed in proportion to the capacity.Secondly,in order to solve the voltage drop caused by reactive load and virtual impedance,the secondary control of voltage feedback and frequency compensation is adopted to maintain the bus voltage amplitude and frequency near the rated value.Among them,in order to prevent the introduction of the communication line when measuring the bus voltage,the bus voltage should be calculated according to the field measurement signal,so that the reactive power is only related to the droop gain n,and the accuracy of reactive power distribution is further improved.Using Matlab/Simulink,under the condition of two micro-power islands of the same capacity and different capacities connected in parallel,the traditional droop control,droop control based on virtual impedance and improved droop control are simulated to verify the effectiveness of the control strategy.Finally,in order to achieve smooth switching from island to grid,an improved pre-synchronization control technique is adopted.By integrating the amplitude adjustment and angular frequency adjustment,the disturbance is reduced,the control structure is simplified,and on the basis of the three-phase phase-locked loop(SPLL),by controlling the q-axis component,the phase lock of the grid voltage is completed.After switching to the grid-connected mode,by switching into the grid-connected power scheduling link,stable operation of the grid-connected is achieved,and it is verified by Matlab/Simulink simulation:through the improved pre-synchronization algorithm,the voltage phase,amplitude and frequency of the microgrid and the large power grid are synchronized to achieve smooth switching from the island to the grid-connected mode;in the grid-connected mode,the load transfer of the microgrid is completed,and by changing the power reference value at the microgrid bus(PCC),the load power loss is compensated by the output power of the inverter,thereby effectively suppressing the power flow fluctuation at the PCC.