Phased construction of Combined Cycle power plants provide power producers the flexibility to install and operate their gas turbines while the steam portion of the plant is erected. This installation sequence presents a unique challenge, from a performance test perspective, of how to accurately determine the new and clean performance of the combined cycle plant had the entire facility been constructed and tested all at once. Specifically, at the time the entire combined cycle is ready to test, the gas turbines will have accrued operating time and have some associated degradation.
During the first phase of construction, the gas turbines are installed and commissioned to operate in simple cycle mode. This is followed by the installation and commissioning of the steam cycle in the second phase of construction. Between these two phases, the gas turbines operate commercially in simple cycle mode. The time lag between the first and second phase varies from a few months to a few years. As such, the thermal performance of the combined cycle plant cannot be measured by means of a single test, instead a series of tests need to be conducted at each phase of installation in order to properly account for the new and clean performance of the equipment when determining the combined cycle plant output and heat rate.
The current ASME PTC 46-1996 test code provides a method for conducting phased tests in section 5.5.3. With the evolution of gas turbines and combined cycle plant technologies, a better understanding of gas turbine degradation phenomenon and an increasing need for utilities to be able to monitor the overall plant performance on a more regular basis, it is worthwhile to re-visit the phased testing methodology outlined in the Code. For instance, the degradation accrued by the gas turbine during simple cycle operation impacts its output, heat rate, air flow and exhaust temperature. The Code accounts for all of these impacts, with exception to the impact of the change in exhaust temperature. Additionally, the calculation method outlined in the Code assumes that the firing levels of the gas turbine remain unchanged when switching operation between simple and combined cycle modes. This assumption may or may not hold true in all instances because, with advanced control systems in place, the gas turbine operation would have been optimized for combined cycle operation. Hence, additional testing and analysis may be required to accurately measure the thermal performance of the combined cycle plant. Lastly, the current Code test methodology requires a set of correction curves dedicated solely for the purposes of phased testing. Analytical studies, prompted by recent test experiences, have been completed with the aid of thermodynamic models to identify the extent to which the test accuracy will be improved by accounting for all the key parameters versus the traditional phased testing methodology outlined in PTC 46-1996.
This paper presents the results of these studies; discusses the key parameters that impact the phased testing philosophy; and outlines a simplified phased test calculation methodology to aid in the accurate determination of combined cycle thermal performance while leaving the plant owners with a set of correction curves that can be utilized not only for the phased test but also for any future performance monitoring activities or predictive analysis.