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]

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

[Bruiser]

Inam,
Firstly, what a well written post! And a fascinating question, I'm enjoying thinking about it.

As a general comment, I think that load sharing should be easy to implement, and I would be surprised if the equipment you need for this (CTs, field control, rack control) are not already in place.

Also, the frequency bands should reflect the droop settings, noting that gas sets are a bit 'sloppy' in their response. I often annoy purists when I ask for solid reasons as to why 50Hz is so important - turning the gain down is a great starting point.

In response to the scenario, I think that the 8MW load shed would cause reverse power. The two droop sets will move along their curve a bit, but there will still be close to 4MW of 'motoring' going on - droop sets driving the isoc set. Even if you revise the 4MW isoc setting to something a bit higher, there will probably be another scenario that produces the same result.

This leads to a comparison of which is the more elegant solution - implement load sharing or try to find a sensible response to reverse power. There are other scenarios to consider, too. What happens if the isoc set goes off line for any of the historical reasons you've mentioned?

[jbevett]

INAM

When we run our plants one unit will always be Isochronous or Zero Droop and one at 30 or 3% droop. This will ensure one (Zero Droop Unit) generator is the lead meaning what it says goes. Isochronous just means it will maintain the Freq you set it at and droop will lose some of the speed when taking on load changes. Your droop and gain settings are very important in this mode. Droop meaning how much of the load will be picked up in 3% it will pick up that 3% and gain will determine how fast you pick up that load and return to rated Freq. In most cases when you have multiple generators at droop they will fight over the load and more often than not the mechanically stronger unit will win and pull that entire load from the weaker unit. I deal in diesel units but governor systems just like engines work on the same principles with small differences here and there.

[hazim]

pls give me

[hazim]

plse give your advise
we have gas turbinegenerator type ge frame 5 with exitation prismic A10 , our problem is when we operate generator in isochronus mode and our load reach 15 mw , 10 mvar the excitation increased the voltage value to 11.8 kv and generator protection trriped on v/f protection , pls help me to solve the problem

[hazim]

plse give your advise
we have gas turbinegenerator type ge frame 5 with exitation prismic A10 , our problem is when we operate generator in isochronus mode and our load reach 15 mw , 10 mvar the excitation increased the voltage value to 11.8 kv and generator protection trriped on v/f protection , pls help me to solve the problem

[hazim]

plse give your advise
we have gas turbinegenerator type ge frame 5 with exitation prismic A10 , our problem is when we operate generator in isochronus mode and our load reach 15 mw , 10 mvar the excitation increased the voltage value to 11.8 kv and generator protection trriped on v/f protection , pls help me to solve the problem