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Single-shaft power trains have the GT and the ST driving a single generator. In multi-shaft systems each GT and ST drives its own generator, typically in a 2x1 configuration — two GTs with their own HRSGs supplying steam to one ST.

With single-shaft combined cycle plants, the GT and ST are both connected to a single generator. The GT is ready to start producing power long before its exhaust can generate enough steam to run the ST. This leaves plant owners two options: they can install and fire up an auxiliary boiler to spin the ST until the HRSG is on line, or they can let the GT alone drive the generator until there is enough HRSG steam to allow the ST to take over part of the load.


The second arrangement is cheaper in terms of both capital and operating costs. To make this work, a self-synchronizing clutch is installed between the ST and the generator so the GT can start generating power and then the ST can join in once it is up to synchronous speed.