Research On Microgrid Power Sharing And Grid-connected Switching Control Technology Based On Droop Control

With the widespread rise of renewable energy in the world,in recent years,distributed generation(DG)technology has developed rapidly.As a carrier of distributed power,microgrid can effectively coordinate different types of renewable energy.At the same time,microgrid is an important way for distributed renewable energy to access power systems.The microgrid can be operated in the mode of gridconnected or island.In the operation of the grid-connected,each DG unit is required to output the maximum power.Under the operation of the island,the system load should be shared proportionally.Therefore,the current microgrid widely adopts the droop control strategy simulating the external characteristics of the traditional power system synchronous generator,but this strategy can only achieve the active power sharing of the microgrid during the island operation and the reactive power distribution will be biased.In addition,the smooth switching between the two modes of operation,such as islanding and grid-connected,is also important for enhancing the reliability of power system,improving power quality,protecting local loads and power equipment.In response to the above questions,the contents of this paper are as follows:This paper first establishes the structure of AC microgrid system and studies the basic principle of droop control of microgrid.The power regulation process of microgrid grid-connected and island operation under droop control is analyzed and the droop controller structure of inverter is designed,including power control loop,voltage-current closed loop,virtual impedance and other control loops.The microgrid system simulation model is built on Matlab/Simulink,and the operation results of gridconnected and island mode under droop control are analyzed.By analyzing the power distribution principle of the microgrid droop control,it is found that the island microgrid is difficult to reasonably distribute the reactive power of the system according to the droop coefficient,due to factors such as feeder impedance mismatch.Therefore,a power grid power sharing strategy is proposed in this paper.On the basis of droop control,adaptive virtual impedance control is introduced to compensate for the mismatch of feeder voltage drop and achieves accurate reactive power sharing.An adaptive virtual impedance controller is designed and the sensitivity of the controller parameters to the operating point is analyzed.Due to its low sensitivity characteristics,this strategy does not require high communication reliability.Even in a communication interruption,an almost accurate reactive power sharing is achieved.The feasibility and effectiveness of the proposed control strategy are verified by Simulink simulations.Finally,this paper studies the control method of grid-connected switching of microgrid based on droop control.The microgrid pre-synchronization control technology is researched,and the pre-synchronization control unit is designed to realize the smooth switching of the microgrid from the island to the grid-connected operation mode.However,the output power of the microgrid inverter under the situation of grid-connected is determined by the grid voltage level at the moment,which may cause overcurrent or underload of the inverter.Therefore,the droop control method of grid-connected is improved,so the rated power can be output when the inverter is connected to the grid.However,the integral control loop in the droop control under the grid-connected mode will affect the power sharing and power quality of the island microgrid.Therefore,the microgrid needs to cut off the integral control loop before the off-grid operation,and finally realize the smooth switching from the gridconnected operation to the island operation mode of the microgrid.Simulink simulation results verify the effectiveness of the above control techniques.