Frequency-constrained unit commitment with grid-forming hybrid energy storage systems considering complementary frequency support and battery degradation

High penetration of renewable energy is reducing synchronous inertia and making post-contingency frequency security harder to maintain in low-inertia power systems. Consequently, day-ahead scheduling must incorporate the post-disturbance frequency support available from committed resources, a challenge effectively addressed by frequency-constrained unit commitment (FCUC). Grid-forming hybrid energy storage systems (GFM-HESS), which combine the fast response of supercapacitors (SC) with the sustained support of batteries, provide a promising resource for FCUC. While existing FCUC studies have incorporated energy storage into frequency-security constraints, they still cannot explicitly represent the SC-to-battery support transfer in GFM-HESS or the resulting implications for reserve allocation and frequency security. In addition, battery degradation associated with frequency-support duty remains difficult to represent in FCUC because detailed real-time battery trajectories are not explicitly resolved. This paper develops an FCUC model with GFM-HESS. The model distinguishes the fast SC branch from the sustained battery branch, incorporates the SC-to-battery support transfer through mode-dependent frequency-nadir constraints, and introduces a tractable degradation mapping for battery wear associated with frequency-support duty. Case studies on a modified IEEE 39-bus system show that the proposed model coordinates the complementary frequency-support capabilities of the SC and battery while maintaining frequency security. The resulting schedules improve the system-level economic performance of GFM-HESS relative to the benchmark architectures, while independent time-domain and Rainflow evaluations support the scheduling use of the proposed degradation mapping.