Aero Gas Turbine trapped in cooldown during stop command


I am using TM2500 aero gas turbine with Woodward control.
During running conditions, the turbine is controlled by NPT/NSD speed control. No problem at running condition.
When the operator initiate stop command, the generator breaker is open and the turbine initiate cool down sequence.
As per logic the turbine should reduce speed to N25 references speed @6300RPM and the control move to high speed rotor control. The high speed shaft should rotate @ 6300 rpm for 5 minutes. Then Fuel chopper signal should chop the fuel.

unfortunately the 5 minutes timer is resetting after 120 sec or some time 240 sec I.e. different time. This making the cooldown cycle in trap as the timer is never completed 5 minutes.
The time can be reset in only two condition. 1. If the turbine is not in minimum speed reference .

2. If the control is not NGG control. High speed shaft control.

We found that some time the Turbine control is moving from NGG control.

we are facing this for last 6 months.

Any one can advice why this is happening or how we can get rid of the problem.
Thanks in advance
Without being able to ”see” what's in GAP it's impossible to say what's happening. Sounds like you need to closely monitor why the unit is not staying in the mode it needs to be to complete the cooldown timer. Maybe the fuel control valve is allowing excessive fuel flow because it needs refurbishment or replacement.

There's just too much we don't know and can't observe to be more concise. We also don't know what kind of monitoring capability the unit has.

Please write back to let us know what you find to be the cause, and how you resolve the problem!