Droop to Isoch Generator Mode Causes Trip

How much MW is shared by each generator when one of the CGTG's is changed to isochronous mode?

When the grid is disconnected, 40 MW load is decreased from your system and due to decrease of the load, the generator's frequency is increased (this is the characteristics of droop mode generators). See the previous posting http://cr4.globalspec.com/thread/42805 for isoch and droop mode definition.

The moment the CGTC is changed to isoch mode, it attempts to change the frequency to 60 Hz from 61.5 Hz by decreasing the input fuel and hence the load shared by this generator is decreased to a value lower than 2% load and it trips.

You can try this:

After the grid is disconnected, manually increase the load of the CGTG (to be run in isoch mode) as much as possible (by transferring load from other generators), and then change it to isoch mode. For manually transferring load, you can slowly increase the frequency of the CGTC and decrease the frequency of the other generator. This is a little tricky operation but the generator operators are supposed to be able to do it.

Another alternative is to decrease the system frequency to 60 Hz from 61.5 Hz. (the system should have a common method to decease the frequency) and then change to isoch mode.

- MS

Thank you msamad for the explanation.

After changing one unit to Isoch, it's 16 MW load was shared by the CGTGs and STGs and the droop CGTGs jumped instantly from 16 MW to 22MW. After this change was achieved per the droop function, the speed kept increasing steadily over long period of about a minute with no load change. Why??

The reason why the frequency was changing as load was changing is very clear. The first solution mentioned is neither reliable nor secure and complicated for the operators but the second one has been adopted. I am looking for an automated solution.

Also, from readings and other replies in other forums low forward power is not needed for Gas Turbines and used for Steam Turbines.

The speed is increased because of the increase of frequency – the frequency is directly proportional to speed.

In one of the project, we used Woodward's EGCP-3 LS (Engine Generator Control Package Load Sharing Model) for load sharing of the engine generators. I am not sure it works for the CGTG and STG. You can consult with Woodward for the solution.

- MS

Thank you msamad,

The remaining generators speed decreased and MW increased as per the droop setting but after that the speed kept increasing (not decreased) for about one minute. This is another speed change other than the droop reaction. Every 6 to 9 seconds the frequesncy was getting up.