This paper presents a new online-ready mathematical model of a supercritical power boiler, which enables comprehensive analysis of the boiler dynamics. The model allows generating modified sliding curves for a power unit that is currently under construction in one of a Polish power plant. The modified sliding curves illustrate the transients of the steam at the turbine inlet, which varies depending on the power unit load. The developed model allows analysing the boiler operation during start-up from cold, warm, and hot states. Moreover, the model allows simulating the boiler operation when the power unit output increases by 7.5% and 5% in a specified period. To satisfy the requirement of increasing the power unit load by 5% in a set period, the live steam mass flow rate must be increased at the boiler outlet. For this purpose, the live steam pressure at the turbine inlet is reduced, and the fuel mass flow rate is increased simultaneously. This results in an increase in the live steam mass flow rate at the boiler outlet. The total increase in the live steam mass flow rate can be determined using the developed model. After the pressure decrease process is stopped, the required level in the steam mass flow rate at the boiler outlet should be achieved only by firing more fuel. The only approach to obtain the appropriate values of the target decrease in the pressure at a set load, in the range of the power unit loads from 40% to 80%, is to develop a mathematical model of the entire boiler and perform the simulations. All the simulations are performed using an in-house developed computer program. (C) 2019 Elsevier Ltd. All rights reserved.