LAUSR

Isochronous controlled battery energy storage system (ESS) can maintain constant frequency in an islanded ac microgrid, but its power and state of charge (SoC) may go beyond limit due to intermittent renewable energy resources and impact load change. This paper introduces a control strategy integrated with bang-bang and backlash mechanism to coordinate distributed energy resources (DERs). When overloading will take place, ESS will change from isochronous control to droop control, with droop gain varying based on bang-bang control, by which the overloaded power will be spontaneously transferred from ESS to other DERs. Moreover, frequency will be automatically restored when impact load disappears, which eliminates the effort of secondary frequency control. When SoC of ESS comes to limit, the diesel generator will change from droop control to isochronous control, whereas ESS will adopt virtual inertia and grid-supporting droop control to help stabilize frequency. A backlash mechanism is also employed to latch the control mode near SoC limit. Comprehensive tests on real-time simulator demonstrated the effectiveness of the proposed control strategy.