Hybrid micro-grids require harmony operation of renewable energy resources based voltage source inverters (VSIs) in grid-tied as well as islanded modes. In this paper, the modeling, analysis, and control strategy of VSIs based autonomous micro-grid are developed. An integrated control system for autonomous micro-grid is carried out including two control levels. The primary control level is comprised of the current and voltage inner control loops, the virtual inductor loops, and the droop control loops. This control level is necessary to regulate voltage and frequency, and also to achieve accurate power sharing among the paralleled distributed generators (DGs). However, the secondary control level is employed to eliminate the voltage magnitude and angular frequency deviations produced by primary control level. The parameters of secondary controllers are optimized using particle swarm optimization technique to achieve good dynamic and steady state performance for micro-grid voltage and frequency. Two micro-grid structures are modeled and simulated in MATLAB environment to accomplish this study. The first structure consists of only one distributed generation unit, while the second contains four DGs. Each structure is tested with and without secondary control level under the load variations to confirm the robustness of the control system. The results of the traditional and optimized secondary controllers are compared. |
