As power systems worldwide move toward high shares of wind and solar generation, synchronous generators are being displaced by converter-interfaced generation at all voltage levels. This transformation reduces system inertia and system strength , making frequency and voltage stability increasingly dependent on the control behaviour of inverter-based resources (IBRs). Grid-forming inverters (GFMIs) are therefore emerging as a key technology to provide virtual inertia, grid support and black-start capability in future converter-dominated power systems.

This paper addresses the stability and interoperability of GFMIs as a prerequisite for the secure integration of large-scale renewable energy and battery plants. As multiple units with different control concepts may be operated in parallel at common points of connection, it becomes essential to guarantee inherently stable behaviour and to avoid adverse interactions. The work addresses this challenge by discussing the implications for resonance and small-signal stability first for a single GFMI and second for the parallel operation of two or more GFMIs.

For a single GFMI connected to an ideal grid via real or virtual coupling impedance, a state-space model is derived, and eigenvalue analysis is used to determine stability boundaries as a function of power control parameters, coupling impedances and system strength at the point of connection. The analysis is then extended to the interoperability of multiple GFMIs in two representative scenarios: (i) islanded operation where two GFMIs jointly supply a local load, and (ii) parallel operation of two GFMIs at a common point of connection to a stiff grid behind an impedance characterizing system strength. Again, stability boundaries are determined as a function of power control parameters, coupling impedances and system strength. In all cases, small-signal analysis is complemented by EMT simulations and laboratory measurements.

The main outcomes are: (i) a GFMI Stability Criterion specifying the minimum required series impedance—real or virtual—for stable grid connection, and (ii) a GFMI Interoperability Criterion stating that each unit in a multi-GFMI system must independently fulfil the stability criterion for stable parallel operation. These results provide actionable guidance for controller design, parameter tuning and future grid connection requirements for IBRs employing grid-forming technology.