Dynamic Modelling, Validation and Analysis of Coal- fired Supercritical Once-through Boiler-Turbine-Generator Systems Under Stringent UK Grid Requirement

This study presents a dynamic model and analysis of supercritical coal-fired power plant (SCPP). The model includes the main component of the once-through boiler-turbine-electric generator system of a SCPP. The first principle model was developed under the assumptions that the temperature, pressure and flowrate changes in the axial direction in the once-through boiler and assumed well mixed radially. The model parameters were estimated from design and operational data from a reference 600 MWe SCPP. The model was validated with plant operational data for steady state conditions at full load, and at transient load ramp. The model shows relative error of less than 3 % in predicting the reference plant measurements. The model was further used to simulate the flexibility of the SCPP for rapid load changes and variations in system frequency under stringent UK grid system. The UK grid code stipulates that depending on the operating load as a percentage of the registered capacity (MCR), up to 10 % of registered capacity has to be supplied within 10-30 s of a 0.5 Hz frequency drop occurring over a 10-s period. The simulation results show that using turbine throttling approach (about 4 % MCR), extraction stop (about 5 % MCR) or condensate stop (about 2 % MCR) individually was not sufficient to meet the grid requirement. On the other hand, the study shows that a combination of turbine throttling, extraction stop and/or condensate stop can achieve a 10 % increase in generating capacity (MCR) of a SCPP within 10 s to 30 s of primary frequency change as required by the UK grid.