  This paper presents decentralized, passivity-based stability protocol for
inverter-based resources (IBRs) in AC microgrids and a non-intrusive approach
that enforces the protocol. By "non-intrusive" we mean that the approach does
not require reprogramming IBRs' controllers to enforce the stability protocol.
Implementing the approach only requires very minimal information of IBR
dynamics, and sharing such information with the non-IBR-manufacturer parties
does not cause any concerns on intellectual property privacy. Enforcing the
protocol allows for plug-and-play operation of IBRs, while maintaining
microgrid stability. The proposed method is tested by simulating two networked
microgrids with tie lines and two IBRs modeled in the electromagnetic transient
(EMT) time scale. Simulations show that oscillations with increasing amplitudes
can occur, when two stable AC microgrids are networked. Simulations also
suggest that the proposed approach can mitigate such a system-level symptom by
changing less than 2 percent of energy produced by IBRs.
