Method for predicting and using the exhaust gas temperatures for control

Abstract

A method and apparatus are disclosed for protecting gas turbines (two- and three-shaft) from damage or destruction during dangerous transients. In this situation, fuel flow must be reduced, as needed, to keep the exhaust gas temperature below a maximum limit, thereby avoiding damage to blading in the turbine's first-stage. The dynamic action of the temperature downstream of the combustion chamber may be estimated and then used to correct the temperature measured by thermocouples. For this approach, the turbine is instrumented with an inlet pressure transmitter and an outlet temperature transmitter. The method dynamically estimates temperatures downstream of the combustion chamber and reduces the fuel flow rate if the estimated temperature approaches the temperature limit. Existing thermocouple measurements (EGT) are used as well as the pressure of air exiting the compressor (CDP) upstream of the combustion chamber. By employing a first-order lag and a steady-state gain, the measured CDP is used to predict a 'correction' EGT signal which is then added to the measured EGT, resulting in a corrected EGT. Once the corrected EGT has been calculated, its first time-derivative is used to adjust the safety margin between a maximum allowable limit temperature and a lower predetermined temperature set point.