Evaluating State-of-Charge Constraints for Pumped Storage Hydropower in ISO-Scale Unit Commitment
Unit commitment (UC) minimizes system production cost while satisfying network, reserve, and generator operating constraints. Pumped storage hydropower (PSH) is of particular interest because it provides both renewable generation and grid-scale energy storage, but practical day-ahead UC implementations often rely on simplified PSH representations. We evaluate the operational and computational impact of introducing a state-of-charge (SOC) PSH formulation into an ISO-scale UC model with full transmission security constraints. The key distinction from current ISO practice is the replacement of decoupled daily energy constraints with explicit SOC coupling that links charging and discharging decisions across time. We show that this formulation, together with a symmetry-based aggregation for identical PSH units, can be integrated without impacting solve times on large security-constrained instances. We also evaluate three practical strategies for mitigating end-of-horizon effects: a Naive method requiring no future information, a Fixed method that enforces a terminal SOC target estimated from a multi-day LP relaxation, and a Dual Pricing method that additionally prices deviations from that target. Across normal and extreme operating weeks, the Fixed and Dual Pricing methods substantially improve PSH profitability, by up to 193% in extreme conditions, with modest reductions in total system cost. Importantly, the simpler Fixed method captures nearly all the benefit of Dual Pricing, a directly actionable finding for ISO practitioners.