Tamil Nadu is at the forefront of India's renewable-energy transition, with installed renewable capacity reaching approximately 29.8 GW by July 2026, including about 14.1GW of solar and 12.3GW of wind. The increasing penetration of must-run inverter-based renewable generation is progressively reshaping system operation through thermal-generation backing-down, reduced synchronous-machine commitment, changing system strength, greater renewable forecasting dependence, and increasingly demanding real-time balancing requirements. These challenges now coexist with record system demand: Tamil Nadu reached an all-time peak demand of 21,724 MW on 14 July 2026 and record daily energy consumption of 475.447 MU on 17 July 2026. During high-renewable periods, wind and solar together have supplied a substantial share of daily energy, creating operating conditions characterised by high inverter-based resource penetration, steep net-load ramps, renewable variability and reduced synchronous support.

As this transition progresses, maintaining frequency stability, system strength, voltage regulation and adequate reactive-power support becomes increasingly important, particularly in renewable-rich and electrically weak corridors. Grid-forming (GFM) PV-BESS can provide capabilities including fast frequency response, virtual inertia, voltage formation and dynamic reactive-power support; however, the principal operational challenge is moving beyond the question of whether GFM technology can provide these functions towards determining how much GFM capability is required, where it is required, when it should be available, and whether sufficient capability exists in real time.

This paper presents a practical, system-oriented framework for assessing GFM capability for frequency control, using the Tamil Nadu power system as a representative high-renewable case. The proposed approach combines demand and renewable forecasts, synchronous-generation commitment, frequency-security indicators, short-circuit ratio (SCR), voltage and reactive-power conditions, renewable forecast uncertainty, network contingencies and system ramping requirements to identify temporal and locational GFM needs. Available GFM capability from PV-BESS resources is assessed considering battery state of charge, active- and reactive-power headroom, converter availability and local grid strength.

Particular attention is given to multi-inverter coordination, GFM–GFL interoperability, protection adaptation under converter-limited fault-current conditions, and SOC-constrained provision of frequency-support services. The study further proposes a real-time GFM sufficiency assessment that compares required and available capability as system conditions evolve.

Finally, the paper advances the concept of GFM capability as a future ancillary system service, enabling grid-forming resources to be forecast, scheduled, coordinated and monitored through day-ahead assessment, intraday reassessment and real-time sufficiency monitoring. The framework provides a practical pathway for integrating GFM-enabled PV-BESS into grid codes, system operation and future ancillary-service arrangements for secure operation of renewable-dominant, low-inertia power systems.

Keywords— Grid-Forming, PV-BESS, Frequency Control, GFM Adequacy, Ancillary Services, SCR, Renewable Integration, Tamil Nadu.