4160 V single phase from a 3 Phase 2 MW Genset ? 4

[vrparts]

We are going to loop test/season the core of several large generators. The largest is about 1200 MW. The OEM says they usually just hook up to one phase of the 4160 breaker for a large motor. Plant engineering says NO to single phase loads on the 4160 V bus. Last time this was tried they simply used one phase of a 2.5 MW Genset. The voltage went to 5600V and the current which was calculated to be 297 went above 350. The generator lasted about 15 minutes before the "magic smoke" escaped from the field.
I know small 3 phase generators can be connected Zig-Zag for single phase loads but I can find no reference for 4160 generators.

 

[ScottyUK]

Yes, it can be connected zig-zag if all the leads are brought out. Just watch your AVR sensing VT connections. The other thing to watch is a minimum excitation limiter which might also be responsible for the over-voltage. You'll either need to set it right back or disable it. Bear in mind that a single phase load will generate massive negative phase sequence heating in the rotor of the generator, so don't run it like this for long under load: leave it running at idle if possible to get cooling air through the machine. Oversize the genny too. I've done this a few times with LV sets for primary injection on transformers and it works ok. Don't try to explain to the rental company what you are doing, it will only lead to confusion!


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

 

[edison123]

Hi Scotty

How do you connect in zig-zag in a 3 phase alternator winding ? Could you possibly post any diagrams here ?

Kumar

 

[ScottyUK]

Hi Kumar,

See one of the bottom diagrams. Those are 12 lead machines, I expect this will be a six lead type, and I'm assuming they are all brought out. I expect they probably will be because at this size the cost of a stator differential relay is justified.

http://www.smokstak.com/forum/attachment.php?attachmentid=3756

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

 

[rbulsara]

Interesting. Thanks Scotty.

 

[ScottyUK]

vrparts,


You got a spare 4160V transformer? I can understand your plant guys not wanting to create a voltage unbalance on the 4160V network with a huge single phase load because it would be detrimental to the motors on the bus, but if you have spare transformer you can dedicate to the test you might have some options: the next voltage level up (13kV?) will likely be big enough to carry the unbalanced load without major voltage imbalance. You should be able to calculate the imbalance to a reasonable degree of accuracy and see whether it would unbalance the 13kV bus voltage enough to worry about.


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

 

[edison123]

Scotty

For some reason, that site isn't opening. May be due to the under-seas cable fault ? I will try to access it later. Thanks.

 

[ScottyUK]

Still seems ok from the UK. Another of the great mysteries of the internet!


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

 

[itsmoked]

edison123; It is just a picture. Are you saying you can't see the picture?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wolf39]

Hi vrparts:

This is a strange posting.

Loop tests for seasoning usually are done on hydro generator stator cores stacked in the field. However, the 1200 MW stator core you mentioned definitely belongs to a turbogenerator. These are always stacked in the factory, not in the field.

If the OEM executes the factory loop test as mentioned, you possibly can do the same in the field. Let us know the loop test data (amperage, voltage, kVA rating). If you require a single phase load of, lets say, 500 kVA, you can expect "smoke" coming from the damper (amortisseur) winding of a 2.5 MW generator, not from the field winding. If your generator has a nameplate rating of, lets say, 20 MW or higher, there should be no problem from the generator side. The damper winding then is capable to cope with the inverse current in question. For the transformer you have to check somewhere else.

For "seasoning" a stator core you may have to magnetize for a duration of up to 24 hours.

Regards

Wolf

http://www.hydropower-consult.com

 

[edison123]

Yes, Keith. I am not getting the picture .

 

[itsmoked]

Can you see it now?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

LOL. Thanks Keith.

I got it now, literally and "figuratively".

 

[vrparts]

Wolf et all;

I type slowly so I’ll try to keep it short. Here is more of the story. These are big four pole, Hydrogen Cooled machines. Most units like these have an end iron heating problem, if operated under excited. We never needed the capability so it wasn’t a big deal. When we increase MW out, new interconnect standards require us to recover the capability or stay off the grid. Therefore we get new rotors and about 2 foot of new stator end iron with better ventilation.
There will be HP, LP and MSR work as part of the project. Space will be at a premium. The Generators are at 3 different sites. I envision a single trailer with a generator and breaker, which we back up to the Turbine deck and connect to the loop. One of our rental equipment suppliers has a design using multiple trailers with Generators, step up transformers, load banks for the other phases etc.
Using house power or bringing power in from the sub station 6 times is a paperwork nightmare.
I think it would sure be worth the effort to reconnect a generator, but the work is about a year away and so far I am having no luck stirring up interest.

Al

 

[waross]

Either the double delta connection or the zig-zag connection will result in approximately equal current in all of the generator windings.
The current will be in phase with one phase, leading in the second phase and lagging in the third phase.
If the voltage and current in a previous connection went too high it was probably caused by a problem with the sense voltage connections to the AVR. You should have good control of the voltage. I see a lot of smaller sets connected single phase with either zig-zag or double delta and both connections work well. By small set, I mean up to around 300 KVA.
Scotty and I usually agree but we do see differences of scale. Rotor heating is not serious enough to notice on the small machines and the manufacturers allow full current when connected for single phase. As the size of the machine increases, rotor heating becomes more serious.
When Scotty suggests running at idle for cooldown, I believe that he means electrical idle, that is full speed for maximum cooling air movement but no electrical load.

Bill
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"Why not the best?"
Jimmy Carter