Siemens SGT 800 Barring Block

We have two (2) SGT 800s (50MW each) on a recently commissioned CHP plant, we have had approximately 5 incidents where the machines went into 40hrs barring block as a result of varying issues. In a nutshell, the barring block is a protection for the GT if barring is not achieved on coast down, hence time required to have machine properly cooled prior to start. But staff experience to avoid this issue translates to a reliability issue. Barring block is initiated 10 minutes after loss of power to starting motor, 30 minutes if ventilation is lost.

My questions are:
Does any one here operates the SGT 800 and has experienced this and how did to avoid the recurrences?
Does any one here operates the SGT 800 and carried out OEM modifications on this such as DC barring or Barring back-up?
 
Dear MPS

The barring block is protection which acts when the turbine is ordered into Cooling down time after any stop from warm status. During the cooling down, the turbine is cranked at low speed by the Starter motor. if for any reason, the cranking is stopped beyond the specified time (Usually 10 minutes), ten the barring function is blocked (Usually for 10 hours). This is meant to prevent a restart wit warm shaft as it increases occurrences of bending, vibration and damage.
The barring is blocked mostly on loss of AC power, so the motor is stopped. If you are operating your turbine as a power generation unit you shall have a back up power supply in case of turbine trip. For this machine you will need a quite big diesel generator if you are operating in an island mode (Not connected to any other grid).
Another alternative is the one you mentioned, the DC barring. Like the oil lubrication pumps, when the AC power goes off, the VFD will take DC power from battery bank and run the pumps. The same principle can be applied to the Starter (barring motor).
Of course you will need a huge battery bank or banks to run the barring motor for several hours. One of my thought, though I have not seen in practice would be to have the DC barring but with a control program which will operate it 2 to 3 minutes every 9 minutes, hence you might save on battery capacity.

Regards
Mario
 
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