New SSA-FITSS method cracks stability analysis of black-box VSCs in power grids

Modern power systems increasingly rely on power electronic converters, but many are supplied as black-box models that hide internal controls, making conventional small-signal analysis (SSA) impractical. A team of eight researchers from multiple institutions presents SSA-FITSS, a unified multi-variable fitted state-space methodology that overcomes this limitation. The technique uses frequency-domain identification, adaptive pole-expansion, and reduced-order realization to construct accurate small-signal models of black-box Voltage Source Converters (VSCs). A key innovation is an automated state-interpretation strategy that assigns fitted states to representative control-loop categories based on dominant frequency ranges, providing approximate but meaningful physical interpretability without access to internal details. This enables extensive modal analysis, including eigenvalue sensitivities and participation factors, even in systems where converter internals are unknown.

The methodology was validated on a grid-following (GFL) VSC and then applied to the full New England 39-bus test system, which contained multiple black-box converters operating in both GFL and grid-forming (GFM) modes. Results show that SSA-FITSS models accurately reproduce converter and system dynamics, support full eigenvalue-based analysis, and reveal stability limits under varying synchronous generation and GFL penetration levels. The approach overcomes key limitations of existing identification-based techniques by enabling scalable, interpretable, and system-wide stability assessment. This work, submitted to IEEE Transactions on Power Systems, offers a practical tool for utilities and system operators to assess grid stability with proprietary converters, improving reliability in increasingly inverter-dominated power systems.