Analysis and design of interfacing inverter output virtual impedance in a low voltage microgrid

This paper presents the control strategies and output (virtual) impedance design approach for parallel operated distributed generation (DG) units in a low voltage (LV) microgrid system. To achieve communicationless control among DG interfacing converters, virtual impedances are placed between the interfacing converter outputs and the grid to improve system stability and mitigate coupling between real and reactive power. In order to properly design the virtual impedance, small-signal models of a microgrid in different operation modes are developed using complex number matrix, which is flexible and can be easily extended when more DG units are added. Based on the developed microgrid models, system damping and stability are evaluated. Together with the DG power capacity and power decoupling requirements, the desired DG output virtual impedance can be designed. To realize the virtual impedance, a robust implementation method that avoids using of any high/low pass filters is presented. Moreover, to further improve the power control performance during transients, a transient power decoupling algorithm and an adaptive transient impedance control scheme are proposed. Experimental results are provided to validate the impedance design approach, the virtual impedance implementation method and the proposed transient power control strategies.