From Nominal Flexibility to Firm Reserve: A Readiness Assessment of Thermostatically Controlled Loads for Primary Frequency Response

Thermostatically controlled loads (TCLs) can respond rapidly to frequency disturbances, but fast modulation alone does not establish firm primary-frequency reserve. This study develops a readiness framework that links evidence synthesis, probabilistic reserve sizing, post-event recovery and feeder-level compatibility. A focused review of 74 journal studies informed a two-stage simulation framework. Stage A combined deterministic stress testing, 300-realization Monte Carlo sizing and an independent capacity-hold verification, while Stage B embedded this independently simulated profile in an 8500-node benchmark distribution feeder. The realistic aggregation remained frequency-responsive, but operational constraints produced substantial nominal-to-firm derating. For the modeled upward-reserve case, under the assumed population distributions and operating conditions, an empirical declaration of 0.885 MW achieved approximately 95% joint compliance, while only 56.35% of event-state nominal upward flexibility was firm in the independent verification. Recovery remained distinct from delivery readiness, and feeder analysis showed that an aggregate-compliant profile could still create local voltage and loading violations. The results show that credible TCL reserve requires reliability-conditioned derating, explicit recovery assessment and feeder-specific qualification rather than reliance on nominal flexibility or response speed alone.