Intra-loop coupling causes negative resistance in grid-forming inverters

A team of researchers including Jaekeun Lee from multiple institutions has published a paper in arXiv (2606.28705) addressing harmonic instability in virtual-admittance (VA) based grid-forming control for inverters. They discovered that intra-loop coupling among VA control, inner-loop current control, and voltage feedforward control produces an s²-term in the equivalent output impedance. This term results in a negative-resistance property in the harmonic range, which is fundamentally different from previously studied instability mechanisms that depend on digital control delay. The negative resistance is independent of control delay, making it a distinct and previously unrecognized source of harmonic instability.

To mitigate this issue, the authors propose a simple passivity-oriented damping control method. The method fully retains the well-established current controller and voltage feedforward, and does not require grid impedance information. Experimental tests verified the theoretical findings and the effectiveness of the damping approach. This work is significant because it identifies a new instability mechanism in grid-forming converters, which are critical for renewable energy integration and microgrid stability. The proposed solution is practical and easy to implement, offering a path to more reliable power electronics.