Dynamic simulation of a triple-pressure combined-cycle plant: Hot start-up and shutdown

Abstract

The operation of combined-cycle power plants is increasingly determined by frequent start-ups and shutdowns for grid balancing. This study investigates the capability of a comprehensive process simulation model to predict the transient response of a triple-pressure heat recovery steam generator (HRSG) with reheater to the start-up and shutdown procedures of a heavy-duty gas turbine. The model is based on geometry data, system descriptions and heat transfer calculations established in the original HRSG design. The numerical solution approach and the practical development of a suitable model structure, including the required control circuits, are explained. Detailed simulation results are presented, using initial conditions that correspond to a previous overnight shutdown. Calculations are performed for a complete operating cycle of the plant, where the following main phases are distinguished: start-up procedure, load-following operation, design operation and shutdown procedure. The numerical model is validated with measurement data of the commercial power plant for each pressure stage, yielding good agreement. Deviation from the transient behaviour of the real plant is discussed with regard to modelling assumptions and incomplete information on components outside the HRSG system boundaries.