Research On Control Strategy Of Micro-grid Inverter Output Impedance

Micro-grid technology can realize coordination and mutual support between distributed generation and power grid, which is the most effective way of giving full play to the advantages of distributed generation. This paper takes the inverter power supply in island microgrid as the research object, and focuses on study the control strategy of the inverter equivalent output impedance. As most of the load in power grid is resistive-inductive and most inverters have the induction output impedance at low frequencies, when the microgrid subjects to a large number of nonlinear resistive-inductive load, the inverter with resistive or inductive equivalent output impedance will cause its output voltage fluctuations, which is not conducive to the stability of micro-grid voltage. In this dissertation, mainly design for capacitive inverter, and analyze the stability of capacitive inverter system and compared with resistance or sensibility inverter.Firstly, analyze the difference of droop characteristic on inverters with different equivalent output impedance and the relationship between the equivalent output impedance and circulation, power allocation, based on the parallel micro-sources model. Establishment of inverter equivalent output impedance mathematical model, virtual impedance is adopted to correct inverter equivalent output impedance.The design and control method of capacitive inverter is deeply researched. A strategy of inverter control, including the LCCL filter configuration, sampling current selection and virtual impedance controller design, is proposed to make its output impedance capacitive by providing a controllable degree of freedom for its output im-pedance model. The characteristics of the proposed capacitive inverter are analyzed.Furthermore, study the droop control principle, establish the small signal model of droop control system and analyze the influence of parameters on the stability of the system, which provide the evidence for parameters selection. The reason of power distribution deviation is analyzed and an improved droop control method is employed.Finally, the system based on single inverter is established by Matlab/Simulink simulation and the DSP control experiment platform respectively. The proposed control strategy can effectively improve the quality of voltage in microgrid, with fast response speed and strong stability of voltage by comparing with the resistive or inductive inverter on voltage quality and dynamic performance.