It is nowadays a common understanding that, due to the uneven availability of renewable energy sources, the operation of traditional power generation plants and especially of combined cycles has to be more flexible and subject to more frequent and, most probably, colder starts than in the past. This phenomenon translates into a negative impact on maintenance intervals and operating costs as resulting by a higher increase rate of equivalent operating hours. In addition, the optimization of start-up time, largely driven by initial components temperature, has become a key performance indicator for the profitability of such plants.

The case study presented in this paper deals with a steam turbine in a combined cycle power plant, commissioned on the early 2000s. The turbine is currently operating daily for half of the year and occasionally for the other half, collecting about 150 warm and 30 cold starts per year.

The application to the steam turbine of a Thermal Warming System (TWS) is analyzed in detail by assessing casing and rotor temperature distribution, in transient operation. The control algorithms, that allows maximizing the system effectiveness while safeguarding against possible issues due to uneven temperature distributions, are also discussed. The resulting increase in average starting temperature, as evidenced by online rotor temperature calculation, is then considered with respect to its benefits for maintenance optimization and plant profitability.

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