Isochronous and Droop modes

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    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,



    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).



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