Research On Control Strategy For Low Voltage Hybrid Microgrid Converter

With the environmental degradation and the improvement of energy structure,a microgrid system working in islanding and grid-connected operation modes,has been constituted by distributed generation,a fresh mode of power supply,together with energy conversion device,grid load,monitor unit and protector.In the meantime,electric energy not merely the form of AC as the electrical load expands and new power electronic equipment is springing up.Although the existing power grid remaining in the operational mode of AC power is comparatively well-developed,AC/DC hybrid microgrid integrated merits of AC and DC both will become a new structural style as a solution to contradiction between power supply and demand,interconnection and interworking of intercontinental circuits.In this thesis,control strategies of DC/DC convertor in photovoltaic power generation system and AC/DC interface convertor are studied on the basis of low voltage AC/DC hybrid microgrid.The feasibility of on-site application and consequence for DC electric energy is analyzed firstly.As for the extra power loss arising from the increasing load and distance of transmission line in low-voltage DC microgrid,this thesis has put forward a control strategy for dynamic virtual resistance which can narrow the voltage fluctuation range of the DC bus significantly when current or load changes.With regard to DC/AC convertor,the LCL filter is determined to use on the basis of performance comparison between it and the L filter,the setting principle of filter parameters is given.In this thesis,a virtual resistance is introduced in the controlling unit of LCL filter to restrain resonances and the root-locus method is adopted to analyze influence of virtual resistance on system stability.The simulation compares the effects of underdamping,critical damping and active damping on the system,and proves that the active damping method can significantly reduce the waveform distortion.To enhance the inertia and damping characteristics of the system,a DC voltage-power droop control strategy with inertia and damping characteristic is proposed in this thesis at first through analogic reasoning and dimensional analysis;and then,on the basis of current control technique VSG,a bidirectional Virtual Inertia Control(VIC)method applicable to DC/AC converter is brought forward conclusively,meanwhile dominant poles are used to analyze influences of key parameters in the said control strategy on system stability.And then,a simulation model for low-voltage AC/DC microgrid is established with MATLAB/Simulink to simulate the microgrid employing VIC with islanding and grid-connected operations.The simulation results show that VIC is effective in controlling the voltage,current and frequency surges arising from load fluctuations,improving the power quality of hybrid microgrid.