GE Mark VI Fast Unload on Shutdown

On site we have a GE gas turbine 9FA DLN2.0 with package 4 compressor enhancement and Mark VI controller. We have noticed some times (6 out of 100 shutdown) a fast unload of GT until the breaker opening. IN more details when the plant is shutting down, GT decreases load up to 55 MW. Then waits until the ST generator breaker opens and ST is tripped, at that point takes normal shutdown command and reduces load. Normally, in the vast majority of cases, the unload is done within 2, 5-3 minutes. In some cases the GT unloaded fast, in 45-50 seconds. On all these 6 cases we have noticed no strange thing, no speed increase, no frequency increase, no strange alarm. We have only seen that the PM4 gas supply was kept until 52G breaker opening and then stopped. Usually the PM4 stops at 20-30 MW depending on combustion reference temperature TTRF1.

GE engineering was requested to ask, they said that the acceleration kicked in and unloaded the unit and the proposed to change the control constant FSKMIND4_SD from 15.7 to 17 % (Shutdown Value Of FSKMIND4[0]). Can you please share any similar experience? My understanding is that the controller works on OPEN LOOP and goes to fast unload in order to avoid loss of flame and in the same time keep a continuously decreasing firing temperature.

<i>Based on the information provided </i> and without being able to see the application code running in the turbine control panel, the data you are reviewing, AND the alarm logs for the data you are reviewing. The likely probable cause is that some condition was active or had been active causing the PM4 to remain latched in and/or a condition was in effect that resulted in a faster than normal reduction of FSR and load.

There are occasions when certain conditions occur that were not anticipated by the OEM, or which were not properly acknowledged and resolved by the operators, that result in one rung unintentionally being latched (or unlatched, as the case may be) that cause the sequence to go awry.

CSA thanks for your reply. GE stated that acceleration kicks in and for that reason the unit unloads quickly, FSRACC constant, FSR value is the same as FSRACC during this time, the strange thing is that actually there is no speed acceleration of high grid frequency, speed is 2999-3001 rpm and grid frequency is 49.99-50.01 Hz.