Our gas turbine is GE 6FA Mark V Control system. During our last startup once gas turbine reached FSNL, we given stop command without synchronize the breaker. Turbine starts Coasting down. but once reached 90% of speed turbine stays at 90% of speed for 8 Minutes. After 8 minutes turbine starts coasting down. Why 8 minutes holding required at 90% of speed while gt in coasting down? IGV position 48 to 33 dga.
There is a problem with the Shutdown FSR Control Constants, and/or the Minimum FSR Control Constant, that prevented the fuel from being reduced enough to allow the turbine speed to decelerate while flame is being maintained (called a fired shutdown). There is a timer of approximately 8 minutes that is initiated when a STOP is initiated that will just shut off the fuel if the turbine doesn't reach flame-out speed (usually less than approximately 20% TNH) in the specified time.
During a fired shutdown the FSR is decreased in steps as the turbine speed decreases until it reaches approximately 20% TNH (on gas fuel; approximately 50-60% TNH on liquid fuel) at which point the Mark* will shut off the fuel and the unit will then coast down to cooldown operation. If the heating values of the fuel have increased then the step changes of FSR might not be sufficient to allow the turbine to deceleration to the next step change. If the turbine speed doesn't decrease during a fired shutdown the Mark* will shut off the fuel after approximately 8 minutes to protect the unit. And then the unit will "coast down" (without flame). Amazingly enough, the Shutdown FSR is NOT based on a deceleration rate but is just a function of a series of step changes (decreases) as the speed accelerates. If the fuel is "too strong," or the fuel control valve LVDT calibrations are incorrect, or the Shutdown or Minimum FSR Control Constants are incorrect, or there is a problem with the fuel supply pressure (being too high--particularly on liquid fuel) then the unit will not decelerate while on a fired shutdown and will behave as you described.
This problem usually occurs after a"calibration" of the fuel control valve LVDTs (that was not done properly), OR a change in the heating values of the fuel, OR the incorrect substitution of a fuel control valve, OR a problem with the fuel supply pressure. Occasionally, the Minimum- or Shutdown FSR Control Constants are simply marginal and need to be adjusted.
But, the behaviour you described is not normal fired shutdown behaviour but is the Mark* responding to a problem by simply shutting off the fuel after approximately 8 minutes if the turbine speed fails to decelerate in, what should be, a reasonable time to flame-out speed (the normal speed below which fuel will normally be shut off during a proper fired shutdown (STOP)).
Hope this helps!