| Distributed generation of microgrid technology has been widely used with the depletion of traditional fossil fuels.DC microgrid has become a central issue for scholars due to its advantages of simple control,fewer power conversion steps and high system efficiency.DC microgrid consists of distributed microsources,energy storage,load and grid-connected inverters.Devices are interconnected through power electronic converters due to the different working principle and characteristics.Large number of power electronic converters can lead to the interaction between the converter subsystems,which is not conducive to stability.This subject applies the impedance theory to study the system-level stability of DC microgrid.This paper consists of the following three parts:1)Cascade is the most basic connection between submodules in DC microgrid.The negative damping effect of a constant power load is the main reason for the instability of the cascaded system.In response to this problem,the idea of active damping is used to propose an improved control strategy based on virtual impedance.By introducing an additional capacitive current control loop,the output impedance of the source converter is changed,so that the impedance ratio of the cascaded system meets the Middlebrook criterion.And verify the control effect through Matlab / Simulink simulation.2)The existing impedance theory is not suitable for direct application in the study of the stability of the microgrid directly,because the microgrid is not simply composed of cascaded systems.Therefore,the criterion of stability criterion for parallel microgrid is studied.The factors that affect the stability of microgrid can be divided into voltage disturbance and current disturbance.According to the current disturbance,a new parallel stability criterion is deduced by Nyquist criterion.Frequency domain analysis and simulation verification.3)The grid connected inverter is an important part of the DC microgrid,and the stability of the inverter is particularly worthy of attention.The voltage type algorithm for grid connected inverter under the condition of weak grid is studied,and a grid connected inverter model based on virtual synchronous generator control is established.The selective harmonic compensation strategy is proposed to restrain the negative influence of the background harmonics on the output power quality of the grid connected inverter.Verification of control effect through Matlab /Simulink simulation. |
