Communication-based secondary control in microgrids with voltage-based droop control

Smart microgrids are becoming an important concept to cope with the increasing demand of electricity and to integrate the large amounts of intermittent power sources in the electrical network. Microgrids are able to operate in grid-connected and islanded mode and present themselves to the utility network as controllable entities. For the primary control of the inverter-interfaced DG units, voltage-based droop controllers have been developed. With this fully distributed controller, an optimized integration of renewable energy is obtained, by possibly increasing the penetration depth of renewable DG units and mitigating over-voltage conditions. The main reason is the usage of a constant-power band with a width dependent on the nature of the energy source. This paper investigates the usage of secondary control in a microgrid with primary voltage droops. The secondary control operates at a slower rate than the voltage-based primary control and is communication-based, opposed to the primary controller. The secondary controller regulates the voltage at pilot points in the microgrid, the voltage at the point of common coupling or the power exchange between microgrid and utility network. The set-points for secondary control can be determined based on an optimisation scheme such as for losses, power quality and economic revenue. Dependent on these set-points, the microgrid secondary controller changes the power set-points of the DG units.