Comparative Analysis of Reserve Formulations: Leveraging Flexibility From Energy-Limited Resources

As power systems integrate higher shares of variable renewable energy, balancing supply and demand across different timescales becomes increasingly challenging. Historically, reserves procured per time interval have helped power system operators maintain that balance. Per-interval procurement can compromise reliability when reserves are sourced from energylimited resources. This article compares, for the first time, two solutions: (a) a set of constraints, termed envelopes, which uses a per-interval estimate of reserve energy intensity and calculates the cumulative impact of reserve activation on stored energy as the sum of per-interval impacts and (b) a product existing in the literature, called energy reserves, which procures reserves for periods, with durations ranging from a single interval to the model horizon. For the IEEE RTS-GMLC 2019 system, the latter approach, at an increased computational burden, produces more cost-effective day-ahead schedules by allowing resources, including storage, to determine the energy intensity of the reserves provided.