Running Unsynchronized Generator for Long Period

[Afifex]

I Work in Ammonia/Urea Plant in Turbomachinery Maintenance. One of the steam turbines in the plant drives electrical generator to supply the plant with electricity. Sometimes in case of steam shortage in the plant, generator turbine is switched off, and power is supplied from the grid. This can take place two or three times per month.
From the electrical perspective, is there any danger for the Generator if the turbine-generator train is kept running at operating speed without synchronization during steam shortage time instead of starting-up and shutting down the turbine?

What could be measured, must be measured exactly.

 

[ScottyUK]

The generator won't mind being at sync idle provided its field is maintained, but most turbines don't like long periods at zero load or in a reverse power condition. Is this a condensing machine? They bring more problems.

 

[Afifex]

Please clarify what is meant by "...provided its field is maintained"

What could be measured, must be measured exactly.

 

[ScottyUK]

Well, if you drop excitation off the generator field then it won't run as a synchronous machine anymore. It might continue to run as an induction machine, but the rotor forging will get very hot and will probably sustain damage.

 

[edison123]

Scotty - I think OP means the turbine running with a low steam.

Afifex - Yes, you can keep the turbogenerator running without excitation as long as it is disconnected from the grid. But, it might not any good to your turbine with such a low steam. Check with your turbine OEM.

Muthu

http://www.edison.co.in

 

[waross]

The issues wll be with the turbine more than the generator.
If the generator is disconnected from the grid (as in running unsynchronized)there should be no issues with the generator.
If the generator is running less than about 97% of sync. speed it is well to disconnect the field.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Bill, I think - perhaps wrongly - that he's intending to use the generator as a motor to keep the turbine spinning. I'm not sure what that will achieve operationally, but that wasn't the question asked. Re-start will be interesting because the machine will be cold and at sync speed when steam is admitted.

edit: I'm wrong, see later.

 

[waross]

You may e correct Scotty. Until the OP clarifies, flip a coin.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

No, my mistake - you guys are right. The OP said "without synchronisation".

So he wants to hold the machine at sync idle. The problems will all be turbine problems as far as I can see, but the type of problems will depend on the turbine type.

 

[Afifex]

Thank you guys for your responses
What I meant is to run the turbine at its operating speed but with very low steam, just to rotate it, but there would be no excitation for the generator to produce power.

What could be measured, must be measured exactly.

 

[ScottyUK]

Hi Afifex,

You haven't told us much about the turbine. Is this a condensing turbine, or a letdown turbine? A letdown machine can possibly handle this type of operating mode, but a condensing type will likely have problems. The problems will be more numerous on a large machine where the physical size of the LP blades can introduce issues that aren't seen on smaller types.

 

[krisys]

Hi Afifex,

Ensure that you are operating the turbine-generator set with rated speed only.
Just because the set is not synchronized, don't allow the speed to drop or raise above the rated speed. (Hope the over/under frequency protections will be in service).

If the speed goes beyond the envelop, there could be a mechanical resonance and resulting excessive vibration and even breakage of the shaft.

Please ask your mechanical rotating guy to refer the torsional analysis report and ensure the upper and lower critical speeds.

 

[ScottyUK]

Even at sync idle there are loads of potential problems with long-term operation at no-load. Most condensing turbines have a minimum stable load which needs to be achieved pretty soon after synchronisation.

Some of the potential problems, depending on unit size, include: poor steam admission characteristics, differential expansion, condenser vacuum, gland sealing, LP blade cooling, water drop formation at the back end of the LP section.

 

[Afifex]

It is 20MW condensing turbine type.

What could be measured, must be measured exactly.

 

[crshears]

Not a good idea to motor a turbine from the grid unless sufficient "ventilating" steam is provided; as previously stated, talk to your turbine manufacturer.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

crshears,

You're making the same mistaken interpretation as me - he's wanting to hold the set at synchronous idle so it's effectively ready to synchronise, but the breaker isn't actually closed. There will be some steam, but not enough.

 

[crshears]

Hey Scotty,

Ah. I see. Mistake of mine was to see "supplied from grid" in the first part of the post as meaning the turbine, not the plant.

Good catch.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]