| The reserves of traditional non renewable energy such as coal mine and oil are limited,and the environmental problems caused by their consumption cannot be ignored.In order to solve the contradiction between the growth of energy demand and the shortage of traditional fossil energy and environmental problems,distributed new energy power generation,mainly wind power generation and photovoltaic power generation,is widely used.The direct current(DC)microgrid is easier to integrate distributed generation units such as photovoltaic and wind power,and has the advantages of higher efficiency,less loss,and no need to consider frequency and reactive power issues.It has also received widespread attention.In this thesis,the research object is a multi source DC microgrid with multiple converters in parallel.The control strategies and system stability analysis methods for improving system stability are studied,and the power oscillation mechanism of the DC microgrid is revealed.The main research contents are as follows:(1)Firstly,this thesis compares and analyzes the implementation principles of traditional droop control strategies and improved droop control strategies.Based on the hierarchical structure of the improved droop control strategy,the underlying controller uses a passive integral controller,and a hierarchical control strategy based on passive integral is proposed.Aiming at a multi source DC microgrid system composed of multiple converters in parallel,the basic principle of consistency control is analyzed.Based on a hierarchical control structure,combined with improved droop control and current consistency control,a droop control strategy for DC microgrid based on current consistency is proposed.For the proposed control strategies,simulation and comparative analysis were conducted through the PLECS environment to verify the effectiveness of the proposed control strategies.(2)Secondly,on the basis of the proposed droop control strategy of DC microgrid based on current consistency,the local linearization small signal modeling and analysis of the multi converter parallel DC system near the steady-state operating point are carried out to obtain the transfer function relationship between the small-signal modeling and the small disturbance,analyze the zero pole distribution map,and obtain the relationship between the control parameters and the system stability.A large signal modeling stability analysis is conducted for a multi converter parallel DC microgrid system with combined loads.The relationship between system parameter boundary conditions,system asymptotic stability region,and microgrid parameters is obtained.The PLECS simulation results demonstrate the effectiveness and accuracy of the stability analysis method.(3)Finally,this thesis builds a low-power hardware experimental platform with four converters in parallel,and describes in detail the design principles of the power supply circuit,BOOST main topology circuit,MOS drive isolation circuit,voltage sampling circuit,and current sampling circuit.The PCB boards of the above circuits have been debugged,and sensor data has been fitted and calibrated,Finally,a control program was written on the hardware platform control board to achieve the control of each converter in the DC microgrid.Comparative analysis experiments were conducted on three control strategies,namely,uncoordinated control,traditional droop control strategy,and current consistency based droop control strategy.The experimental results show the limitations of non coordinated control and traditional droop control strategies.Under the control of current consistent droop control strategies,the DC bus voltage is equal to the voltage reference value given by the control strategy,without steady-state error,and the output current of each converter in the system also achieves the goal of uniform distribution.The experimental results coincide with the theoretical part of this thesis,verifying the effectiveness and correctness of the proposed control strategy. |
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