In a test campaign to lower the minimum part-load of a combined cycle plant, a series of turndown tests on two GE Frame 9E gas turbines with DLN1 combustor technology were carried out under premix operation by Stadtwerke München (SWM). It has been found that the load can be reduced significantly compared to the conventional turndown ratio, before either CO emissions or combustion dynamics form the limiting factor of the turndown test. To exploit this potential safely and operate the gas turbines close to these physical limits, emissions and combustion dynamics must be monitored online. The azimuthal thermoacoustic mode that is observed in the can-annular machines is monitored with the IfTA PreCursor, based on the online determination of the modal decay rate. For this method, the acoustic pressure is measured at the cans around the gas turbine circumference to observe the azimuthal acoustic propagation that is enabled by the cross-firing tubes between the cans. Using this strategy to monitor CO emissions and thermoacoustic stability in real-time, a reduction of the minimal part-load limit by approximately 20% is achieved for the considered gas turbines. In must-run situations without demand for electricity generation, the operating costs can be directly reduced by the fuel savings. As an additional benefit, SWM can offer a broader power reserve for grid stabilization on the energy market. This monitoring strategy has been fully implemented in the control system and first experiences of the extended part-load limit are currently being gathered.

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