Comparative Analysis of Off-Design Performance Characteristics of Single and Two-Shaft Industrial Gas Turbines

Abstract

Gas turbines for electric power generation usually operate at part-load conditions for a considerable amount of the time. Regardless of gas turbine configurations, performance generally degrades with a reduction in power. Accordingly, attention should be paid to the task of enhancing the part-load performance of the gas turbine itself and furthermore, the performance of applied systems such as the combined cycle and cogeneration power plants. Most gas turbines developed for electric power generation (heavy-duty) have a single-shaft configuration as shown in Fig. 1. The speed of the generator driven by the gas turbine should remain constant during the load variation. Therefore, the single shaft engine is advantageous in the event of the load being shed because the compressor acts as a very efficient brake. For this reason, regulation of output speed is easier to achieve than other configurations, 1. Aeroderivative gas turbines have long been representatives of multishaft power generation engines. Recently, a couple of heavy-duty engines with the two-shaft configuration shown in Fig. 2 have also been introduced. Meanwhile, during the part-load operation of modern single shaft heavy-duty gas turbines, the airflow rate is actively controlled by modulating the setting angles of the variable inlet guide vane (VIGV) and the variable stator vanes (VSVs) in order to maintain the highest possible exhaust gas temperature (design temperature or higher). Thus, heat recovery capacity is greater compared with the maximum airflow control (fuel only control). Practical mass reduction limit is 10% to 30% corresponding to 20% to 50% in load reduction, 23. In multishaft engines, compressor shaft speed and thus airflow rate decrease as power reduces. Their part-load exhaust temperatures are higher than those of single shaft engines without VIGV control.