Enhancing the operational flexibility and safety of an ultra-supercritical power unit coupled with external water storage during the state conversion process

The increasing penetration of renewable energy necessitates deep peak shaving in coal-fired power plants (CFPPs), exacerbating the trade-off between operational flexibility and safety, particularly during state conversion at ultra-low load. To address this, a flexibility-oriented thermal system configuration coupled with an external water storage tank is proposed first. The high-temperature saturated water from the separator will be stored during the dry-to-wet state conversion process and used to heat the feedwater during the wet-to-dry process, thereby enhancing the separator's steam generation capability. Subsequently, a real-time temperature monitoring model of the overtemperature-prone regions is developed by applying the Proper Orthogonal Decomposition (POD) algorithm to indicate the safety status. On this basis, the revised control strategies combining the dimensionality-order-reduction method are proposed to rapidly obtain the safety margins for parameter regulation, which aims to enhance operational flexibility while ensuring safety. Comparative evaluations are conducted with a Ve of 1.0% - 2.5% Pe/min. During the wet-to-dry process, the conversion duration can be shortened by up to 13.8%, and the flexibility indicator can be increased from 3.66 to 4.26. The overtemperature is completely avoided, and the warning time proportion is reduced by up to 57.1%. Similarly, dry-to-wet conversion shows improved stability in power control and a maximum reduction in wall temperature of 7.6°C. Overall, the proposed strategies strike an optimal balance between flexibility and safety at ultra-low load and provide a forward-looking solution for CFPPs under the new operating paradigm.