Impact of Transmission Line Capacity Variability on Pumped Storage Scheduling Strategies: Analysis of Static vs. Time-Varying Congestion Scenarios

Pumped storage hydropower (PSH), with its advantages of fast response and large-scale energy storage, has become a key means of enhancing power system flexibility. However, the time-varying nature of transmission line capacity may constrain the effective utilization of its regulating capability. Most existing studies treat transmission capacity as a fixed boundary, failing to adequately account for the impact of its dynamic variations on scheduling strategies. To address this gap, this paper constructs three typical transmission capacity scenarios: fixed high (no congestion), fixed low (persistent severe congestion), and time varying (capacity reduced during daytime and restored at night). A power system dispatch optimization model incorporating wind power, solar power, and pumped storage is established, with the objective of minimizing total system operating cost. Under different capacity scenarios, the pumping/generating behavior of PSH, unit output structure, controlled line operation status, and economic indicators are compared and analyzed. Furthermore, sensitivity analyses are conducted from three dimensions—congestion severity, congestion time window, and PSH installed power capacity—to comprehensively evaluate the marginal impacts of key factors on system performance. Results based on a modified IEEE 14-bus system indicate that under the fixed high scenario, PSH can achieve free arbitrage with the best economic performance, but does not account for capacity fluctuation risks. Under the fixed low scenario, persistent congestion leads to substantial wind and solar curtailment, significantly increased operating costs, and prolonged full loading of lines, posing the highest security risk. Under the time-varying scenario, PSH is forced to pump at full power during nighttime and generate continuously during daytime, effectively alleviating line flow pressure. Although the operating cost is slightly higher than that of the fixed high scenario, line overload is avoided, renewable energy accommodation is significantly improved, and substantial security gains are achieved at a moderate economic cost. This paper reveals the forcing mechanism of time-varying transmission capacity on PSH scheduling strategies, verifies the feasibility of the time-varying capacity strategy as an effective trade-off between economics and security, and provides a theoretical basis for optimal PSH operation and transmission capacity management in power systems with high shares of renewable energy.