Research On Hierarchical Control And Smooth Switching Of Microgrid

With the rapid development of economy and technology,smart grid has gradually entered people’s field of vision.Among them,micro grid is widely used in practice because of its low line loss rate,high energy utilization rate and high-power quality.The system control method of microgrid and the smooth switching between islanding mode and grid connected mode of microgrid are key technical problems in microgrid.Microgrid with droop control has many advantages,such as high reliability,control without interconnection,convenient operation and so on,which makes droop control widely used in the actual microgrid.Although droop control is widely used in microgrid,in the actual line,due to the impedance of each line has a certain deviation,the application of traditional droop control will lead to the reactive power cannot be reasonably distributed,so that the control effect cannot achieve the desired effect.At the same time,when the droop control is used to regulate the islanded microgrid,there will be a certain difference between the auxiliary voltage and frequency of the system and the large grid,which will lead to the generation of impulse current in the grid connected operation and cannot achieve smooth switching.In view of the above problems in microgrid,this paper mainly studies the following aspectsFirstly,several main types of distributed generation are briefly introduced,and the main circuit topology and filter of microgrid are selected and designed.Then,the mathematical model of inverter is deduced,and the traditional three-phase coordinate system is pushed to Axis synchronous rotation coordinate system,that is,the AC flow is transformed into direct flow.At the same time,the mathematical model of current voltage double loop controller is analyzed.Secondly,based on the traditional droop control,this paper improves it.According to the load capacity and the inverter capacity,the required rated power is obtained.Then the output power is controlled by each inverter.According to the given reactive power,the virtual impedance value is adaptively adjusted to compensate the voltage drop caused by different line impedance and realize the balanced distribution of reactive power.At the same time,in order to improve the power quality of the microgrid,a two-layer control of the microgrid is added to compensate the frequency and voltage amplitude of the system to ensure the stable operation of the system.In the simulation model software,a microgrid system with four DG units in parallel is established,and the simulation experiment is carried out to verify whether the proposed hierarchical control strategy is correct.Finally,due to the difference in voltage amplitude,phase and frequency between the islanded microgrid and the large grid,if the microgrid is directly connected to the grid,it will produce a large impact current.In order to solve the harm caused by the impact of grid connection,the pre-synchronization control is introduced into the microgrid.Based on the traditional pre-synchronization control,this paper proposes a pre-synchronization control method without PLL to track the phase automatically.This method does not need to lock the grid voltage,but it can still achieve the effect of smooth switching,simplify the control system and improve the stability of the system.The pre-synchronization control strategy is simulated in a single DG system and multiple DG systems,and the simulation results verify the effectiveness of the above control method.