we have two Frame -5 GTG with Mark-IV controller. One runs at a time and other one is kept on standby condition. We have no power supply form Grid. So we run gtg in Ischo mode. Normally our plant load is 17.5 MW.

On 27th Aug-08 near about 2:30 pm we started one compressor in our plant which consume 1.6 MW power. When we started the compressor, GTG output frequency drop down and it was automatically changed to Droop mode. At that time IGV was full open condition.

Instantly we stopped the Compressor.
After thar Operator Changed the mode from Droop to ISCHO manually. We checked the maximum power output but it was in limit i.e. 18.5MW.(GTG Max. o/p =25MW Rated). So what is the reason of this auto change over and frequency droop.

 

A couple of things could have caused your frequency to fall:

1. Do you have a temperature derating curve for your GTG? As far as I know the frame 5 GTG is rated at 25MW at ISO temperature of 15 deg C. At higher temperatures its rating is lower. What was the inlet air temperature at the time of your incident? Conversly, what was the GTG exhaust temperature before you started the compressor? Was it close to its base load rating? If yes, then your GTG was practically at full load, or close to, and when you started the compressor, there was not enough reserve on the GTG.

2. I believe the starting of the said compressor is a routine job at your works. Apart from (1) above, what was done differently on the day of the incident?

3. You have to be careful, as a compressor rated at 1.6MW will consume much more power during acceleration, when the speed of the motor approaches the peak power point, before it reaches the normal operating speed. Do you know the maximum power demand of the motor during start-up, or do you have the power curve of the motor from standstill to synchronous speed?

Most probably you had a combination of the above which sent your GTG in overload and thus the drop in frequency.

 

We checked the Exhust Temp. and inlet temp. both were in normal range. Also there was sufficent margine of Max. Load. The compressor maximum power consumption is 1.8MW. My question is why the GTG change its mode from Ischo to Droop mode?
How to check the logic behind it?

 

Going through the logic of a MkIV controller fitted on a Frame 6 GT (which I would believe would be practically the same as a Frame 5 GT, only the controlling parameters would be different), it seems that during the compressor startup, one of the controllers (R,S or T) went temporarily offline, most probably due to the sudden change of parameters caused by the sudden load presented to the GT by the compressor. The logic behind it is this:

GT to operate in ISOCH mode, logic point L83SCI has to be on. This is held on by L83REC (which must be OFF). It seems that in your case L83REC came on as L3QDATA temporarily went off and then came back on. Now L3QDATA will go temporarily off if one of the controllers went offline. Obviously this is one theory about your problem, you have to check all the logic that disengages ISOCH control mode, but at least it should lead you where to look for. This theory may hold especially due to the fact that after the system settled you managed to switch back to ISOCH mode (i.e. the controller had 'realigned' and went back on line). Check the controllers' (R,S and T) analogue data at the time of the change-over, and see whether one of the controller's data was different from the others'.

 

Next day we checked the History data but we could not find any abnormal data. Is there any other option to check the data of that particular date? Where can I get the set points of the values when it changed from Isoch to Droop mode?

 

You should obtain the services of someone who's knowledgeable in Mark IV Speedtronic turbine controls. jojo has provided the typical names of logic signals active when a turbine is operating in Isoch mode. If you can't read through your logic and determine what might have caused the event (because in a typical application there's nothing which would cause this to occur, therefore your application is either non-typical or there are other factors in operation here) then you should avail yourself of someone who can read through the Speedtronic elementary and provide the information you require. Without being able to see your Speedtronic elementary, we can only comment on typical sequencing.