A unified controller for a microgrid based on adaptive virtual impedance and conductance

A unified controller for a voltage source inverter (VSI) is one of the most important technologies for a microgrid to switch smoothly between grid-connected mode and islanding mode. Generally, as the control objectives of grid-connected and islanding modes are different, the controller adopts different control algorithms for the two modes, which demands to monitor the operating status of the microgrid all the time and therefore this may lead to poor dynamic performances during mode switching and decrease the reliability of the system. The proposed controller of VSI includes power compensator embedded in droop control algorithm, which could regulate its control objectives autonomously without identifying the operation modes in advance, thus realizing unified control of the microgrid. Besides, by adding adaptive virtual impedance and conductance, the active and reactive power control for VSIs could be decoupled and the VSIs could share loads proportionally to their capacity under all types of line impedance. Furthermore, the parameters of the unified controller are optimized with small signal models under both grid-connected mode and islanding mode. Simulation and experiment results verify the proposed control method.