Over Frequency Tripping of a 12 MW Steam Turbine Generator

The combination of over frequency and reverse power seems odd. Do you have any indication of how the load varied just before the trips?

A rapid drop in load would of course cause all generating sources on the grid to speed up, but then they should restabilize to nominal frequency. However, the various governors could have different response rates. What is odd is that if your generator is over frequency, it should be feeding excess power into the grid, rather than absorbing reverse power from the grid.

One of the best CR4 guys on issues like this is RAMConsult, from whom we can hope to hear.

Well put. I find it also very strange too.

I tend to think that possibly some part of the equipment has a fault, as an alternator connected to the grid, cannot overspeed, it just takes more "load" from the grid, maybe too much load!!

But the frequency remains "held" by the grid....

I feel that possibly the over frequency is just an indication that actually happens AFTER the alternator has dropped off and the speed control, cannot reduce the speed/frequency to the no load situation fast enough.

So I feel that the problem lies a short time before the incident.

This could even be something like a very large local motor being suddenly being shut off, combined with a rise in voltage from the grid.

I tend to assume, maybe wrongly here, that the grid is inherently very stable, voltage changes are relatively slow and frequency is held to being exact over 24 hours average......

Naturally, the alternator will "follow" the frequency of the grid if it changes, so theoretically if the grid frequency rises, it may even rise about the level at which will cause the alternator to simply drop off, though I personally have never ever heard of such a situation.....the grid is usually totally stable...

Maybe a proper mains monitor needs to be attached (permanently!), it might give a better indication of where the problem lies......

Grid frequency is almost always changing by usually small amounts. If some big load is started somewhere, all of the rotating generators on the grid slow down by a little bit, until their governors bring them back up to speed. Conversely, if some big load drops off, everything speeds up until the governors bring the speed back down to nominal frequency.

It is possible to use this effect to shed loads when grid frequency drops, and then restore them when grid frequency recovers. One of my former workplaces has done a lot of work in this area. (Pacific Northwest National Laboratory = Battelle.)

These deviations are periodically adjusted over time to keep the average grid frequency (nearly) constant, which keeps your clocks on time.

I don't believe in prayer, but damn, that was uncanny!

Dear Mr.jimmy23,

Over Speeding indicates, the load n the Generator is cutoff instantly or the governor has developed some problem or too sensitive.

Pl.check the Turbine Governor Sensitivity and Over-Speed Setting. The governor Speed Droop Character should be checked and adjusted as referred in the Instuction/Operation Manual.

Pl. report back - how the problem was sorted out - as we are eager to know it.

DHAYANANDHAN.S

Over-frequency is load rejection, external to your equipment. Loss of generator excitation would normally be covered by other protective equipment, voltage sensing equipment, and protective devices directly on the field supply.

Your reverse power 32 indicates that you were motoring your generator for a short time (assuming no powerhouse loads on the 13.8kV side). This may have happened when your turbine governor reacted to the over speed caused by the load rejection, but before the 13.8kV breaker had a chance to open.

Your settings may have overlapped a little, but erred on the side of caution.

12MW is very small for pretty much any utility, so it is unlikely you could survive a large sectionalization of the external load, and perhaps the automatic controls to switch to isochronous mode for the governor are not enabled.

How is your powerhouse fed, electrically?