We have two gas turbine installed in our site 701 and 702 being run by MARK-II and MARK-V control system. 701 was shutdown for major overhauling and Speedtronic calibration. At the moment 701 is running on ISO mode while 702 is on DROOP mode.

After the startup of 701 we have noticed that the rate of actuation of 70L and 70R relays has increased. Now they are switching very fast between ON & OFF state. Why?

On the other hand speed of 702 is varying from 5088 and 5090 while frequency is between 50.01-50.03 Hz.

Is these events are interlinked? what precautions we can take right now? any suggestions are most welcome.

 

It's likely the two are one. If your post is understood correctly, 701 is controlled by the Speedtronic Mark II, which was recently calibrated, and is being operated in Isoch speed control mode.

In the situation you describe with one unit in Isoch in parallel with a unit in Droop, the Isoch unit is the unit which controls the speed, and therefore the frequency, of the "system" (the load and the generators and their prime movers).

And, since the Mark II was recently "calibrated", this is likely the potential source of the speed difference.

Having said the above, a speed variation of 2 RPM out of 5090 RPM represents a differential of 0.03%, which is virtually negligible. We have no idea of the period of the oscillations, or the nature of the load on the units.

The tuning of the Isoch controller of the Mark II Speedtronic is likely just extremely tight.

It's been said many times before on control.com with regards to calibrations performed using or on Speedtronic turbine control systems: Before calibrating, the as-found condition should be ascertained and documented, and then, *and only then* should calibration be performed if the accuracy of the existing calibration is found to be out of spec.

More problems can be created where none existed by technicians performing unnecessary calibrations.

How is the Droop unit being operated? Is it being operated in straight Part Load Droop speed control or in Pre-Selected Load Control? Is it being operated differently from the way it was being operated before the major overhaul and Speedtronic calibration of 701?

Is the load being carried by 702 greater than the load being carried by 701? Is the load being carried by 701 at or near its Base Load setting? Is the load being carried by 702 at or near its Base Load rating?

Is there some other difference in the way the two units are being operated versus the way they were operated before the 701 major overhaul and Speedtronic calibration?

 

When you operate two generators in parallel, with one in droop and one in isochronous mode, the droop machine will load up to maximum load and the isochronous machine will take all the load swings. That explains why the 70R and 70L relays are operating continuously on the isochronous machine.

I am assuming you are isolated from a power grid, or these are the only units on a small power grid. Otherwise, I would expect you should have both units in droop.

 

CSA said:
> More problems can be created where none existed by technicians performing unnecessary calibrations. <

This is true outside of the power generation field as well. If the calibration is good enough, it's generally better to just leave it alone. In many cases you have a better chance of making the calibration worse than you do of improving it.

I like to distinguish between doing a "calibration check" and "adjusting the calibration". These are two different things and a lot of problems are caused when they are assumed to be the same.