Research On Operation And Control Strategy Of AC/DC Hybrid Microgrid

The world is setting off a new round of energy transformation characterized by clean energy,which is an important way to solve the problem of large-scale renewable energy access.With the diversification of distributed micro-power supplies and load forms,the AC/DC hybrid microgrid that takes into account the advantages of AC microgrid and DC microgrid will become a new form of power supply network.In different operating modes,how to AC/DC hybrid microgrid power control and voltage frequency control are the key to the development of AC/DC hybrid microgrid.Therefore,this thesis takes the AC/DC hybrid microgrid as the research object,and mainly studies the following aspects:1)The typical topologies of three AC-DC hybrid microgrids are summarized.The advantages,disadvantages and applicability of each structure are discussed.The common micro-power supplies in hybrid micro-grids are modeled to study its working principle and output characteristics.The basic structure and working principle of the AC-DC hybrid micro-grid interconnected converter are analyzed.The mathematical model of the voltage source-type interconnected converter dq coordinate system is established.The DC-side filter capacitor and AC side of the interconnected converter are designed.The LCL filter circuit parameters improve the stability and filtering effect of the interconnected converter interface.2)The power balance relationship between the AC/DC hybrid microgrid in the grid-connected operation mode and the island operation mode is analyzed.This thesis focuses on the control strategy of energy storage unit in hybrid microgrid.It is difficult to meet the requirements of traditional single battery when the power is abrupt.The hybrid energy storage system and grid-connected control strategy are studied.The hybrid energy storage is based on PU drooping.The characteristic control strategy automatically adjusts the output power of the energy storage system.At the same time,the supercapacitor responds quickly to the high-frequency component of the load power change,which reduces the impact on the battery and bus voltage when the load suddenly changes.The distributed power supply is verified by simulation.The control strategy during fluctuations can effectively stabilize the bus voltage.3)In view of the influence of the line impedance on the traditional droop control,the distribution of the load power of the hybrid micro-grid in the operation of the micro-power supply and the sub-network will be deviated.In this thesis,a modified droop power control strategy for hybrid microgrid in peer-to-peer control mode is established.The mathematical relationship between voltage,frequency and switching power is established.A unitized hybrid droop control system based on common bus voltage is proposed to improve the microgrid.The power allocation problem is to dynamically adjust the power transmitted by the interconnected converter by detecting the common bus voltage and the AC side frequency of the DC subnet,and correct the traditional droop control strategy of each micro power supply.The established method is applicable to different line impedances between the micro power supply and the load.In addition,in order to improve the bus voltage quality of the AC subnet,the method studied can also control the reactive power converter to perform reactive power compensation on the AC subnet.Finally,the hybrid microgrid model is built in MATLAB/Simulink.The hybrid microgrid with traditional droop control and improved control strategy is compared and analyzed.The simulation results show that the hybrid microgrid can operate stably in the grid-connected mode and in the island mode.The accurate distribution of the load power between the units improves the power quality and verifies the effectiveness of the system coordinated control.