Research On Control Strategy Of Inverters Based On Virtual Impedance Droop Control In Low Voltage Grid

In recent years, an ongoing increase of grid-connected distributed and renewableelectricity generation technologies is clearly noticeable. From an environmentalperspective, the increase of renewable technologies is highly desirable, as they do notemit greenhouse gasses and thus not contribute to climate change. Nevertheless, theirincrease poses increasing challenges on the reliable operation of the grid, due to theirvariable nature, which is on top often hard to predict. This work proposes, develops andimplements techniques to cope with variable generation technologies both in islandedand interconnected electricity grids, while sustaining a high reliability and minimizingcosts. The focus is on the control of inverters, being the interface between thegeneration source and the low voltage grid.First, a profound review of the traditional voltage and frequency droop controlmethod and influence of different electric line impedance to droop control are analyzed.Depending on the type of control, inverters are classified, and the characteristics of eachtype are described. Control of each type is researched and developed, and theadvantages and disadvantages are summarized. The control requirement of inverters infuture low voltage grid is proposed. Then the dynamic phasor model is introduced toanalysis stability of invertor with droop control. The method is better than the traditionalone based on quasistatic model，and can acquire more accurate stable margin.Then, a virtual system model of invertor for use in the future low voltage grid isproposed, and a mixed voltage and current control strategy is suggested to achieve themodel. The control strategy is based on Linear Quadratic Gaussian optimal controltheory, and use the kalman state observer and the linear quadratic optimal controlmethod to implement the control of voltage and current instantaneously. The invertercan imitate a voltage source with finite output impedance, where the same robustnessunder different grid condition is acquired. A virtual impedance droop controller intime-domain is proposed, based on the mixed voltage and current controller, which cansupport the voltage and frequency of grid perfectly. Then the design of the controller isdescribed. Finally, the Matlab simulation experiments are made to verify the conclusionsderived in this paper and the effectiveness of control strategies. The simulation resultshow the proposed strategy has a lot of advantages, that it can support the voltage andfrequency of grid, can deal with the voltage harmonic, and can provide strong shortcircuit current.