Off-design steady performance and operating characteristics of single and two shaft gas turbines for electric power generation have been investigated comparatively. A set of balance equations has been derived based on validated component models. A simultaneous calculation scheme has been employed, which is flexible to various engine configurations. Part-load performance analyses of two commercial gas turbines have been carried out to compare operating characteristics between single and two shaft engines. The predicted performance characteristics of both engines coincide soundly with the manufacturer’s data and also correspond with the inherent characteristics of each configuration. The adoption of the VIGV modulation has been addressed in order to examine the possibility of leveling up the heat recovery capacity by maintaining a high turbine exhaust temperature (TET) when those gas turbines are used for combined cycle plants. Maintaining TET at its design value as far as the VIGV modulation allows has been simulated and it has been determined that the TET control is possible at up to 40% and 50% load in the single and two shaft engine, respectively. Combined cycle performances have also been investigated for two engine configurations in different operating modes. While the VIGV modulation produces a favorable influence over the combined cycle performance of the single shaft configuration, the two shaft engine does not appear to be effectively improved by the VIGV modulation since the degradation of gas turbine performance counteracts the advantage of the higher performance of the bottoming (steam turbine) cycle.

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