Virtual-admittance (VA) based grid-forming controls have become increasingly important for inverter-based resources supporting modern power systems. However, researchers have uncovered a previously underexplored harmonic instability mechanism that could affect grid reliability when these controls are widely deployed.
The instability arises from intra-loop coupling between three control components: the virtual-admittance control itself, the inner-loop current controller, and voltage feedforward compensation. This coupling creates an s²-term in the inverter's equivalent output impedance, which manifests as negative resistance across harmonic frequencies. Critically, this negative-resistance property exists independently of control delays, distinguishing it from the digital delay-induced instabilities extensively studied in prior research.
Negative resistance at harmonic frequencies can trigger sustained oscillations and instability if the inverter interfaces with a grid having complementary characteristics. Traditional stability analysis focusing on control delays would miss this mechanism entirely, potentially leaving grid operators unaware of a latent vulnerability.
To address this challenge, the researchers propose a straightforward passivity-oriented damping control strategy. The method requires no modifications to existing current controllers or voltage feedforward implementations, and importantly, demands no knowledge of grid impedance—a parameter often difficult to characterize in real systems. By introducing selective damping, the approach effectively suppresses the negative resistance while maintaining control performance.
Experimental validation confirms both the theoretical analysis and the damping method's effectiveness across various operating conditions. This work has significant implications for grid-forming inverter design and deployment, particularly as penetration levels increase. It underscores the necessity of comprehensive harmonic stability assessment during control design, independent of delay analysis. Manufacturers and utilities should evaluate whether existing VA-based controls exhibit this coupling phenomenon and consider implementing similar damping strategies to ensure robust grid integration of renewable energy sources and energy storage systems.
