NPS 2

[JJayG]

Negative Phase Sequence anybody? To include Negative Phase Sequence Trip Imminent alarm and philosophy. And if we get any ops engineers interested, preferable reactions to the above.
Usual rules apply, preferably in words of one sylable or less.

Have fun

 

[dpc]

This is in reference to what, exactly? Generator, motor, transformer, transmission line.....

 

[rbulsara]

No, Thanks! ...oh too many syllables?

Rafiq Bulsara

http://www.srengineersct.com

 

[JJayG]

That would be in reference to Generating plant and /or transformers attached to generating plant. More specifically Turbo Alternators both Steam Driven and also Gas turbines(Not that any of this makes any difference).

 

[slavag]

Hi JJayG.
NPS is standard and very importnat function for generators and motors. ( for other objects can be used too).
Usually for generators used two stages, 1 for alarm and second for trip. Often used IDMT curve for trip.
Setting depend on requerements of generator vendor.
NPS is overheating of rotor, vibration.
It's very shortly.

Best Regards.
Slava

 

[ScottyUK]

Negative sequence currents result in currents at double the line frequency to be induced in the rotor. The rotor of a turbo machine is a solid forging because of the mechanical stength required for high speed operation and therefore there are no laminations to limit the induced currents as you find in, say, a transformer or stator winding. The currents can be very large and have a significant heating effect on the rotor. The rotor is not an easy place to obtain temperature readings so a negative phase sequence relay is used to measure the NPS current at the stator from which the effect on the rotor can be implied and settings adjusted to ensure the rotor is protected. Settings are highly dependent on the rotor design and are usually determined by the generator manufacturer through testing.

An NPS relay normally has an IDMT curve as noted by Slava, and it should result in a unit trip, not an alarm. Rotor overheating is a very serious problem and it should not be left to an operator's opinion whether to trip the unit. An alarm which activates above a threshold is a worthwhile addition to give warning of an impending trip and allow the unit operator to inform the transmission system operator of a problem on the system. There is usually very little the operator of a generating unit can do to influence a negative sequence problem because it is almost always caused by a load imbalance out on the system.


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If we learn from our mistakes I'm getting a great education!

 

[JJayG]

Cheers Scotty, it's not that I would suggest leaving the decision to the operator,as you know, we operators reactions vary from slow(relatively speaking of course) to non existant, whereas engineers will make the decision only after the event has been thoroughly analysed to death and we are then told after weeks of deliberation what we did wrong and what we should have done.......... hindsight is a wonderful thing.
Ooops sorry, started going off on one there, where were we? Oh yes............
I was actualy just recalling an alarm on an old AEI Machine that declared "NPS Trip Imminent". Fortunately I was never subjected to it in anger.
Was also wondering what sort of fault or incident causes Negative sequence in the first place.

 

[waross]

I understood that Scotty was advocating pre-planning automatic trips. In these days of wild pricing swings there may be extreme pressure on the operators from management to stay online at all cost when the export price is high. A damaged rotor may be too large a price. Automatic trips also avoid the scenario of the poor operator who tripped off and saved the plant but lost his job.

Bill
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"Why not the best?"
Jimmy Carter