We have G.E gas turbines frame 9...and frame 6. my question is during fuel change over why it is not recommended to use preselect load command....? Also a times due to any sort of disturbance if the turbine starts hunting...should the load select command be removed from preselect....

 

The below presumes that a GE-design heavy duty gas turbine is being operated at <b>part load</b> (i.e., NOT at Base Load). Many sites put in a Preselected Load Control setpoint greater than rated load (i.e., greater than Base Load) to drive the unit to temperature control (Base Load). (They do this in the mistaken belief that the unit will produce more power than if they select Base Load; yet another myth and wives' tale about GE-design heavy duty gas turbine operation.)

The descriptions below do not apply to operations with Preselected Load Control enabled with a setpoint greater than Base Load resulting in the unit operating with exhaust temperature control enabled (at Base Load).

GE's implementation of Preselected Load Control actually over-rides Droop Speed Control, so any frequency/speed disturbance on the grid is responded to exactly opposite as it should be.

For example, in the event of a frequency decrease (below nominal), Droop Speed Control would try to increase the power output. BUT Preselected Load Control, if enabled, will try to drive the power back to the Preselected Load Control Setpoint, which would be less than Droop Speed Control would be trying achieve. This will actually result in some "hunting" of the Speedtronic as Droop Speed Control will be trying to increase the power but Preselected Load Control will be trying to decrease the power.

The opposite is true for a frequency/speed increase when Preselected Load Control is enabled.

I would be very interested to know if you loaded a turbine to some load using Preselected Load Control, then disabled Preselected Load Control (see below), how much would load change during the course of several hours or more.

My experience says that sometimes load output is more stable and doesn't change by very much even if ambient conditions change by 10 or 15 deg C. In fact, many implementations of Preselected Load Control have not been tuned very well, such that there are pretty constant load oscillations of +/- 1 to 2 MW during operation with Preselected Load Control Enabled, and so disabling it will actually result in more stable power output because of reduced hunting of the control system.

But, most power plant operators and their supervisors, being <b>intensely resistant to change</b> and <b>staunchly against trying anything new</b>, will <b>resist</b> any attempt to operate without Preselected Load Control enabled, even for a few minutes.

As for using Preselected Load Control when performing fuel transfers (fuel "changeovers"), it's a similar situation. If there are any load swings due to fuel-related issues then both the Droop Speed Control and Preselected Load Control functions will be driving FSR to try to maintain load, and that can make the load swings worse. It's very easy to just "bump" the RAISE- or LOWER SPD/LOAD buttons/targets while operating in Preselected Load Control to disable Preselected Load Control and then enable the fuel transfer. If fuel matching has been done correctly *and* there are no fuel-related issues (air in the liquid fuel line; gas fuel pressure/flow problems) then the fuel splitter algorithm will execute a smooth fuel transfer which should result in a relatively "bump-less" power output. (Again, presuming no fuel-related issues, and a reasonably well-performed fuel matching process was executed.)

Many sites believe that by having Preselected Load Control enabled when performing fuel transfers that it will smooth out any load swings. When it was possible to get operators and/or their supervisors to try the fuel transfer with Preselected Load Control disabled, often it was found that it was slightly smoother than with it enabled. (Again, presuming no fuel-related issues, and a reasonably well-performed fuel matching process was executed.)

Hope this helps! If you have different experiences, we'd like to know about them.