Investigation on load cycling flexibility and safety considering dry-to-wet state conversion of an ultra-supercritical power unit under deep peak shaving conditions

Coal-fired power plants serve as the basic guarantee and system regulation power source in China's power grid. Integrating renewable energy requires coal-fired power plants to maintain load cycling operational flexibility even under deep peak-shaving conditions. From this perspective, the safety constraints limiting flexibility enhancement at ultra-low loads were analyzed. Based on that, the flexibility thresholds without and with state conversion of an ultra-supercritical power unit were explored, considering wall temperature and material thermo-mechanical characteristics. The maximum load ramping down rate, when keeping dry state across the entire load variation process, was determined to be 1.0% Pe/min. It is subject to the overtemperature constraints of the water wall. To address this, the revised control strategies were proposed, taking the wall temperature variation rates as the feedforward signal and the criterion for determining the state conversion. The results show that the maximum allowable load ramping down rate can be increased from 1.0% to 2.5% Pe/min by executing the state conversion process to the wet state. Higher rates will exacerbate excessive thermal stresses and unacceptable material degradation. The minimum state conversion loads under various limitation scenarios were discussed. The optimal state conversion loads can be obtained to maximize dry state duration within given safety margins, which are 25.2%, 25.8%, and 26.5% Pe corresponding to 1.5%, 2.0% and 2.5% Pe/min, respectively. The results of this study are demonstrably implementable and significant in actual power generation units.