Functional-Rotation-Based Active Dampers in AC Microgrids With Multiple Parallel Interface Inverters

LCL filters are commonly used for attenuating high-frequency switching components induced by pulsewidth modulation, while it should be also noticed that its inevitable resonance peak may impose stability issues. Furthermore, with increasing penetration of distributed energy resources (DERs), multiple LCL -filtered inverter interfaces may further deteriorate system stability due to the coupled filters and grid impedance. In order to resolve the multiresonance issues in ac microgrids with parallel-connected LCL-filtered inverters, a bilevel framework with functional-rotation-based active dampers is proposed and implemented to mitigate the impacts of multiple resonance peaks simultaneously. A generic model of ac microgrids with multiple grid-connected LCL-filtered interface inverters is established as a multi-input-multioutput (MIMO) system. By using this MIMO model, the functional-rotation-based active damper is designed and implemented. In the first layer of the bilevel framework, rather than relying on any additional passive or active dampers integrated in each inverter or connected at the point of common coupling, the active damping function is embedded into a designated DER interface inverter while the other inverters do not participate in damping the resonance peaks. The interface inverter that is designated for dampening multiple resonance peaks is selected in the second layer of the bilevel framework, where the online status of each individual DER is monitored and coordinated. It should be noted that the selection of interface inverter is switched among multiple inverters, so a functional-rotation-based active damping scheme is thereby achieved. Experimental results are presented to verify the effectiveness of the proposed bilevel active damping framework.