New stability criterion lets grid-forming and grid-following converters use separate rules, cutting conservatism

As converter-interfaced renewable resources proliferate, power systems are becoming more heterogeneous, with both grid-forming and grid-following converters operating together. Existing decentralized stability criteria—using either small-gain or small-phase conditions—force all converters to meet the same type of requirement at a given frequency. This creates unnecessary conservatism because grid-following converters excel on gain bounds while grid-forming converters favor phase characteristics. The new paper by Diego Cifelli and Adolfo Anta (arXiv:2608.03641) breaks this limitation by partitioning converters into subsets, allowing one group to be certified via small-gain bounds and the complementary group via small-phase bounds simultaneously.

The key innovation is a network-dependent quadratic constraint that determines the admissible trade-off between gain and phase margins across the partition. Because the constraint remains local at each converter, the stability certificate stays scalable and technology-aware—operators can apply different local requirements without global reconfiguration. The authors prove convexity and boundedness properties for the admissible set, then provide a practical procedure to select the gain and phase bounds. They validate the approach on a two-converter system and the IEEE 39-bus benchmark, demonstrating it outperforms traditional decentralized small-gain and small-phase criteria. This offers a more flexible, less conservative path for stability assessment in modern grids with high renewable penetration.