Isochronous and Droop modes

[inamullah zafar khan]

We are operating 3 x 9 MW, 6 kV, 50 Hz gas turbines as part of Utilities power house of a large fertilizer complex which has a total load of 22 MW (mostly large induction motors) The power house is not connected with the public utility, i.e., we are operating our power house in an island mode. At the power house, we also have a load shedding system. In case any gas turbine trips, this load shedding system sheds the fertilizer plant load to bring the total load within the capacity of remaining gas turbine(s). The load shedding system actuates instantly on detection of either turbine trip signal or generator circuit breaker opening. All the gas turbines are being operated in droop mode (droop= 4 %).
On March 14, 2011, only two gas turbines were running normal (in droop mode) with a plant load of 15 MW. One of the gas turbines tripped on failure of turbine enclosure pressure (which occurred during ventilation fan changeover activity). Tripping of one gas turbine was followed by instant load shedding. However, the 2nd gas turbine also tripped (on generator over-frequency) resulting in total power failure. The plant management afterwards decided to always run all three gas turbines whenever the fertilizer complex total load exceeded 15 MW. The generator under frequency/over-frequency trip settings were: Under-Freq=49,0 Hz / 2.0 s, Over-Freq=51.0 Hz / 0.5 s.
On May 10, 2011, we were operating all three gas turbines (in droop mode) with a plant load of 21 MW. Tripping of one gas turbine (on actuation of a gas detector installed in the turbine enclosure) was followed by load shedding and the remaining two gas turbines remained stable, i.e., saved power failure.
In July 2011, the under-frequency/over-frequency trip settings of the generator were revised to have a wide band in frequency. The revised settings are: Under-Freq=47.5 Hz / 2.0 s, Over-Freq = 52.5 Hz / 3.0 s.
Now referring back to the incident of March 14, 2011, the investigation team has recommended to run at least one gas turbine in Isochronous mode to prevent total power failures. You are requested to comment on whether switching one gas turbine to Isochronous mode (while the other machines shall remain in droop mode) shall help in preventing blackouts in case any gas turbine trips.
Best regards,
INAM

 

[inamullah zafar khan]

Isochronous and Droop modes

In continuation of the above:
Suppose a situation when plant total load is 20 MW with following distribution: GT1 (Isochronous mode)=4 MW, GT2 (Droop mode)= 8 MW, GT3 (Droop mode)=8 MW and without any gas turbine tripping, a big block of load like 8 MW is shed (for example, due to a feeder tripping). What would be the response of Isochronous machine in this case? Will it go into reverse power in an attempt to maintain bus frequency? Please keep in mind that there is no communication link between the three turbines for load sharing and the power house is in island mode (i.e., not connected with public utility).
Regards,
INAM

 

[petersonra]

I am not sure this is the best forum for this type of question.

I participate in another forum where this kind of thing gets discussed at length on an almost daily basis. It does not have a lot of interest or relevancy to me so I find it a bit annoying at times, but it seems to interest a number of the posters there.

You might want to try control.com for this type of thing.

 

[don_resqcapt19]

Things like this are also often discussed at eng-tips.com.

 

[rcwilson]

When the load trips the frequency will start to increase immediately. How fast is determined by the inertia of the generators and the remaining loads. The governor on the isoch generator will try to maintain 50 Hz by backing off fuel until it hits the minimum fuel point. Meanwhile the two droop units will be backing off load until the system stabilizes. As the droop units back off, the isoch unit will pick up load.

How fast it occurs and whether it remains stable or trips requires some simulations with accurate data on the loads, generators and the controls.

 

[inamullah zafar khan]

Isochronous and Droop modes

But we fear that the Isochronous machine might go into reverse power (and consequently trip) in an attempt to bring the bus frequency to 50 Hz (when a big block of load suddenly sheds). How far is it true?
Regards,
INAM

 

[rcwilson]

But we fear that the Isochronous machine might go into reverse power (and consequently trip) in an attempt to bring the bus frequency to 50 Hz (when a big block of load suddenly sheds). How far is it true?

Without more information on the type of gas turbine, governor settings, reverse power settings, and a simulation of the interaction between the loads, the droop generators and the governor, it is very difficult to tell.

A gas turbine can take a lot of reverse power without damage. The turbine's compressor stages consume a lot of power; as much as 70% of the full load fuel flow just to keep the turbine at full speed no load. If the turbine generator is a single shaft machine with the generator directly connected to the power turbine it will pull reverse power to try and maintain frequency without reducing fuel to shutoff.

If it is a multi-shaft machine like an LM-2500, where the generator shaft is only connected to the output turbine, there is very little load or inertia on the generator shaft for reverse power and it will overspeed fast. (Think of it like a generator shaft connected to a "windmill" that is spun by the exhaust of the turbine. There is no mechanical connection between the inlet air compressor turbine and the generator. The generator is freewheeling).

Your system might be OK, as long as the governor doesn't throttle the fuel input back so far that the burners flames out.