6B Gas Turbine

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phd

We have 6B gas turbine running at part load with grid import of 10MW. Suddenly Grid disconnected due to earth fault both GT's reached to the island mode. Both GT's on the DROOP mode. We were trying to keep one m/c on ISOCH mode & other on the Droop mode, but whenever ISOCH mode selection done on one m/c it is taking the full load of other DROOP mode GT. Before tripping of grid Gt-1 running 10 mW & GT-2 @ 20 MW part Load.

After tripping load increased for GT-1 to 15 MW & GT 2 on 25MW. What could be the problem for this? It is a 6B Gas turbine with liquid fuel operation.
 
It is not completely clear which machine is DROOP and which is ISOCH. Normal effect of running 2 machines in parallel and islanded is the DROOP machine will load up fully and the ISOCH machine will take the load swings. In that scenario, I would expect that GT-1 is ISOCH and GT-2 is DROOP, from the loads you have stated (GT-1 = 15 MW and GT-2 = 25 MW). In fact, I would have expected the DROOP machine to be more like 35 MW and the ISOCH machine to be 5 MW, unless you have done something to force the ISOCH machine to a higher load.

Is the DROOP machine is some sort of load setpoint control (pre-selected load, maybe)?
 
Thank you sir for explanation.

GT-1 & GT-2 before tripping of grid were on the Droop mode with 10 & 21 MW respectively with GT-1 part Load & GT-2 Preselect Load. After tripping just to share the load we have taken out GT-2 immediately to Part load but mean time GT-1 has taken load of 5 MW grid import. Both GT's running in islanding mode with GT-1 15 MW & GT-2 22 MW respectively in droop mode in part load. We tried to keep GT-1 in ISOCH but as soon as ISOCH selected it is taking load from the GT-2 which is on Part Load Droop mode 22 MW. after sometime we thought will keep GT-2 in ISOCH & GT-1 in dorop mode that also did not happen it gave the same result as earlier taking the load of other GT.

So WHAT could have been done to Run Both GT's on ISOCH & DROOP Mode with no change in load after selecting mode. In both GT's no load control active during that period.

Thanks for your reply
 
Dear phd,

One main parameter to be adjusted before selecting the Isochronous is the system frequency at 50 Hz as the machine isochronous is already set in the Speed Tronic at 50 Hz, therefore MUST be adjusted first, otherwise will occur the following:

1- If you select isochronous in case of system frequency is higher than 50 Hz: Speed Tronic will try to adjust the frequency by reducing power to the minimum and may reach reverse power (this will happen very fast may be within few seconds)

2- If you select isochronous in case of system frequency is lower than 50 Hz: Speed Tronic will try to adjust the frequency by increasing power very fast hence the GT may trip by Exhaust temperature high.

PLEASE TRY TO ADJUST THE FREQUENCY NETWORK AT 50 EXACTLY THEN SELECT ISOCHRONOUS, YOU WILL NOTICE MACHINE WILL OPERATE VERY SMOOTHLY
 
P

Process Value

The curious case of isochronous mode

Well, i have seen this happen 4-5 times. From what you say this is what happened.

1. as you were under import (10MW) form the grid, when the machines got islanded the GT's would have come to droop mode with a lesser frequency. i do not know the load or the droop setting but lets assume that it is around 49.7 for 50Hz machine.

2. Now the two machines are in droop with 49.7 Hz and you suddenly turn on the iso mode in one machine. In GE the standard iso setpoint is the nominal frequency with a allowed deviation of (0.1Hz). Thus the frequency setpoint value of the machine (the one put on iso) raised from 49.7 to 50. This is the reason for the sudden loading of the isochronous generator. As your home load remains a constant the other generator load reduced and tripped in low forward power or reverser power?? i guess.

what should you do to make this work ?

well bring both machines to 50Hz by adjusting the raise lower command in droop mode. Once the machines have settled in 50Hz turn on the iso mode. There will still be as load swing but it will be small. Once the machines stabilise it runs quite well.

As i have said before i have seen operators and engineers (including myself. In a handful of tripping caused by me, this one happens to be the second one, if my memory serves right. but hey that's how you learn.) do this mistake. It’s up to you to go for this operation and explain to the people the standard operating procedure for putting droop in the GE machines. In my parent refinery after 4-5 similar trippings over a space of two years, they stopped this and were happy with the machine just running on droop, which is a very stable one for island operation.
 
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