Abnormal Load Shed behaviour of GTs (6FA Mark VI speedtronic control)


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An abnormal load shed behaviour of GT-1/2 was observed on Dated 04-Aug-2010 at 18:00 hrs. Complex was running on 195 MW load and system frequency was Low. 48.8 Hz. Suddenly Complex load dropped down to 183 MW. It was observed that GTG-1 which was previously running on 63.03 MW load came on 49.9 MW load (Total load shed was 13 MW). GTG-2 remained stable that time. After few seconds GTG-1 load started increasing but than GTG-2 shed load from 64.37 MW to 58 MW (Total load shed was 6.5 MW) which started increasing same time.

GTG-1 repeated its load shed behaviour at about 19:30 hrs and 20:00 hrs same day, But this time GTG-2 didn't shed load also this time frequency was not that low i.e. 48.8 Hz.

Next day at 07:09 hrs GTG-2 shed 4 MW but the frequency was normal.

GT-1 trend shows that when M/c shed its load, following main parameters were found disturbed,
IGVs angle
IBH valve
FG GCV positions / Exhaust Temps

I have also the trends of that sCenario if someone need that than i am ready to provide that trends but i need a solution this is a matter of serious concern
If these machines have DLN combustion systems, and they are being operated in off-frequency conditions, then all manner of odd things can be happening. 48.8 Hz is 97.6% of rated frequency, which means the unit was at 97.6% of rated speed. Which means the exhaust temperature was likely at or near the exhaust temperature limit of the machine, and depending on the combustion mode all manner of things can be happening at that time.

If the unit has DLN combustors, what were the combustion modes of the units at the time of the events? Were the combustion modes changing?

We don't know how these units are being operated (Part Load Control; External Load Control; some kind of load-sharing control; some kind of isochronous load-sharing control).

There are just too many variables to be able to say for sure precisely what happened.

Load is basically a function of fuel flow. And exhaust temperature control is a function of compressor discharge pressure. When the compressor speed is less than rated, then exhaust temperature limit will take effect sooner. And when IGV exhaust temperature control is added into the mix, and if the units have DLN combustors (which is suggested by the presence of IBH) then off-frequency operation, especially if it's not very stable (which off-frequency operation usually is not very stable) can play havoc with DLN machines.

Again, there are just too many variables here to be able to offer any reasonable analysis.

The machines have DLN combustion system. Combustion mode changes.

I understand everything but does not understand completely the off frequency condition. Does every machine run on off frequency condition or only that machine which have DLN combustion mode? Could you provide me with technical data regarding that operation. my email id is:
hashim.bukhari [at] ge.com.

Give me valuable suggestion that how we can cope with that problem as we have DLN combustion mode so that we will not face that problem in future.


The speed of a prime mover which is directly coupled (even through a reduction gear) to a synchronous generator (alternator) and being operated in parallel with other alternators and their prime movers on a grid is, for all intents and purposes, under the control of the frequency of the grid. The prime mover cannot rotate any faster or any slower than the speed which is directly proportional to the frequency of the grid to which the generator is connected. (The exception to this is when the prime mover is very powerful in relation to other prime movers operating on the grid.)

The formula which relates speed and frequency is:

F = (P * N) / 120

where F = Frequency (in Hz)
P = Number of poles of generator rotor
N = Speed of generator rotor (RPM)

Synchronous means "same as", in synchronism with (others), and synchronous generators operating in parallel are all LOCKED into the same frequency, and hence speed.

In most parts of the world, the frequency is pretty well controlled, except for relatively small excursions of plus/minus hundredths of a Hz. In other parts of the world, the frequency is regularly 1 Hz or more off the nominal rating.

When any combustion turbine which has an axial compressor coupled to the turbine operates a lower speed than rated then the axial compressor discharge pressure and flow is less than rated, which means that less fuel can be burned which means that less power can be produced.

Now, when a combustion turbine has DLN combustors with staged combustion modes and the frequency is unstable and/or low, then behaviour is completely unpredictable.

It's the "nature" of DLN combustion systems that they work best when the grid frequency is stable and near rated. It's also the "nature" of DLN combustion systems that they don't behave well when the grid frequency is unstable, and the maximum power output of any gas turbine is reduced when the grid frequency is lower than normal.

I see by your email address that you might be a GE employee. You should be working with GE to understand and resolve any issues which could be able to contribute to more predictable behaviour. About the only thing I could suggest, if the unit had DLN-I combustors, would be to switch it to Lean-Lean (or Extended Lean-Lean) combustion mode on frequency decrease to try to reduce the problems caused by combustion mode changes which can be very problematic with unstable frequency. But, we don't know what DLN combustion system is in use on the units at your site, so we can't make any specific recommendations--which should come from GE anyway.

I also suspect, and at least wonder, if there isn't some kind of load-sharing scheme in use at your site, and even some kind of Isochronous speed control which might be in use at your site. Both of which would have an affect on the behaviour of the units during a frequency (or load) excursion.

Please work with GE to resolve the issues at the site. As the units have DLN combustors, and we don't know all of the specifics of how the units are operated at your site, it's best to work through GE to understand all of the dynamics and the the possible contributors to instability at your site.