My GTG, Frame 5 Mark 6e controlled, PG 5371PA was running normal at 17 MW, in ISOCHRONOUS MODE. Due to some electrical fault the load came down to 2 MW, and RPM hike from 5100 to 5217. Next we observed that ISO mode change over to Droop of itself.
My question is that what should be the probable cause of change over from ISO to Droop mode?
Without being able to see the application code running in the Mark VIe at your site it is impossible to say with any degree of certainty what happened and why.
In general there is usually a contact from a tie breaker (a breaker that connects ("ties") the generator to a grid) that is connected to the Mark* that prevents operating the unit in Isochronous mode when the tie breaker is closed AND often it is used to automatically switch to Isochronous mode when the tie breaker opens, separating the unit from the grid.
There may also be some logic in the application code that prevents operation in Isochronous mode below some load. When it comes to Isochronous mode there are MANY beliefs and operating procedures in use at different plants, almost as many different beliefs and practices as there are plants. It's a very controversial topic and many people have very firmly held ideas and beliefs; some of them are well-founded and others are not--but they get incorporated into logic and application code because, well, because they do. And, over years can cause much discussion and many problems and lots of wonder.
But, again, without being able to examine the application code running in the Mark VIe, the configuration of the plant tie breaker and generator breaker, and a better definition of the fault which occurred it's impossible to say anything more than: You need to review the application code and the I/O configuration and whatever data you may have from the event (Trip Log data, etc.) and try to determine what happened and why. The programming and configuration of sites which can operate in both Isochronous and Droop speed control can be very different and is NOT standard.
Hope this helps!