Research On Output Voltage Quality Improvement Strategy Of Microgrid Inverter Under Complex Load

Microgrid technology can effectively improve the distribution network's acceptance of distributed energy generation.And microgrid is an effective complement to the traditional grid.However,the capacity of the microgrid is relatively small,especially the off-grid microgrid,which exhibits severe weak grid characteristics.This results in the distribution bus voltage of the microgrid being susceptible to load,especially when the proportion of complex loads such as non-linear loads,fluctuating loads and three-phase unbalanced loads is high.This thesis focuses on how to ensure the output voltage quality of low-voltage microgrid inverter under complex load conditions.The microgrid interface inverter uses a T-type three-level inverter topology.Firstly,the working principle of the interface inverter is analyzed,and the mathematical model of the inverter is established.The model analysis shows that as long as the output impedance of the interface inverter system within a certain frequency band can be effectively reduced,the adverse effects of complex load current on the bus voltage of the microgrid can be effectively suppressed.For this reason,the multi-resonance control is introduced in the voltage-current double closed-loop control of the interface inverter,which ensures that the interface inverter has a lower output impedance at the fundamental frequency and the main harmonic frequency.The characteristics of the resonant controller are analyzed.The key parameters of the multi-resonant controller are designed and calculated,and the phase compensation angle design method for the low-order harmonics of the multi-resonant controller is given.For the output voltage and frequency control of the interface innverter,the Q-U and P-f droop characteristics are introduced,which enhanced the load adaptability of the interface inverter.The simulation model of interface inverter system based on Matlab/Simulink software is established.The simulation results show that under various complicated conditions,the control strategy used in this thesis can effectively reduce the influence of load three-phase unbalanced current and harmonic current on the output voltage of the interface inverter.The output voltage and frequency of the interface inverter can be automatically adjusted to follow the change in load power in accordance with the droop characteristics.Established HIL real-time simulation experiment platform based on RT-Box.The experimental results verify the effectiveness of the controller and control strategy designed in this thesis.