Modeling and experimental validation of subcritical thermal power units for dynamic response characteristics analysis under ultra-low load in power systems

With the increasing penetration of renewable energy, coal-fired thermal power units are required to provide reliable primary frequency regulation (PFR) under ultra-low load conditions (below 40% turbine heat acceptance (THA)), where conventional grid-oriented models using a fixed boiler heat storage coefficient may lead to large simulation errors. To address this problem, this study develops a dynamic PFR model for subcritical drum-boiler units that explicitly describes the boiler's transient heat transfer, phase change, and pressure/flow coupling within the PFR time scale. To make the model applicable under variable operating conditions, a steady-state distributed control system (DCS)-based initialization and parameter-identification method is further proposed, so that key boiler parameters can be updated without disturbance tests. A 600 MW subcritical thermal power unit is used for validation under 30% and 40% turbine heat acceptance. The results show that the proposed model improves the average accuracy of power output simulation from 77.91% to 91.70% and the average accuracy of main steam pressure simulation from 90.58% to 96.93%, compared with the standard model. The model also reproduces the rapid steam-flow increase, subsequent pressure drop, and corresponding power response during PFR more realistically. Furthermore, the mechanism analysis shows that the unit exhibits stronger PFR capability at 40% THA than at 30% THA, although the difference gradually narrows as the control valve opening increases. The proposed model provides a physically interpretable and computationally efficient tool for assessing the realistic PFR capability of subcritical thermal power units under ultra-low load conditions.