A method for diagnosing which parts of a Combined Cycle Gas Turbine (CCGT) power plant are responsible for performance deviations is presented. When the overall power output and efficiency deviate from their baseline value, application of the method allows the determination of the component(s) of the plant, responsible for this deviation. The level of depth of this assessment depends on the number of quantities measured. It is demonstrated that a minimal number of measurements can be used to allocate differences between the gas turbine and the steam part of the plant. Additional data can then be used to identify deviating components in more detail. The influence of measurement uncertainty and the exploitation of different measurements in order to check consistency and improve reliability of the results are discussed.

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