Coupling generator to transformer consequences 2

[switchdip]

Are there any consequences to coupling a generator to a large transformer? I have seen past postings that indicate this is a great way to eliminate inrush, but are there any factors that may be detrimental to the generator.
I am going to be supplying temporary power with (4) 2 MW – 480 Volt generators feeding a 13800 Volt bus. Each generator will have its own 2500 kVA transformer and a normally closed 15 kV switch on the high side. The main issue is that the 13800 side will be connected to a 30 MVA transformer. That is why we will start the first generator directly coupled to its transformer. When that generator is on line, we will sync the other 3 generators online.
I just want to make sure that there are not any issues on the generator alternator that I need to be aware of by energizing the transformers this way. I will slow down the soft starting of the voltage regulator to try to cut down on voltage overshoot.
Have also enclosed a simple one-line.

Any suggestions or comments are greatly appreciated.

 

[dpc]

So you never intend to parallel the generators with the Utility sources?

You can bring up the transformer with a generator, but you can't have any loads connected to the transformer while it is coming up, obviously.

The possible synchronizing scenarios will have to be carefully thought out and appropriate interlocking used to avoid closing in out of synch.

 

[switchdip]

Correct, we will not be paralleled to the Utility.

There will NOT be any loads on the system. We will bring the first generator on with its breaker closed and powering the transformer on startup. Once the generator is stable, the other generators will be synced and paralleled. When all four are on line, the load will be manually added.

We have all the synchronizing and load sharing controls already in place. Have done numerous jobs paralleling jobs. Just haven't started with a direct couple to a transformer this large.

Want to make sure it won't damage the alternator.

 

[dpc]

It shouldn't be a problem as long as the field is brought up gradually. It's done all the time for large generators connected to unit transformers.

There are others around here who have more experience with this than I do, so maybe they will be able to add something.

 

[ScottyUK]

A lot of plants built in recent years don't have an LV side circuit breaker at all, only at the HV side of the GSU transformer. In our case 'LV' is 16kV, 'HV' is 275kV and the transformer is 400MVA. I know of no problems likely to be brought about by this, other than the transformer core is usually nearer to saturation than the generator stator so V/Hz and overfluxing relays need to be set to protect the transformer rather than the generator. Not always the case though!


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[switchdip]

Yes, I can bring up the voltage slowly. Soft start at 120 Sec.

Thanks for all the responses.

 

[itsmoked]

So you do have another switch/breaker on the load side of the 30MVA correct?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Do your synchronizing at 13.8kV, then you won't even need 480V breakers on the generators. Just think of the generator and transformer as a unit that produces 2MW at 13.8kV. You will need to make sure your 13.8kV breakers are suitable for synchronizing duty. That way you'll never be subjected to inrush on the 2.5MVA transformers. That 30MVA transformer suggest that some times you are connect to the utility and some times to your generators; is that correct? Any chance you could arrange your controls to allow bringing that 30MVA up with the first generator, then parallel in the other generators? Inrush on 30MVA when supplied with 10MVA may be unpleasant.

 

[switchdip]

Yes there is a switch on the load side of the 30MVA transformer. That way the load can be brought on after all 4 generators are on line.

This is a temporary power solution only. Not permanent. Our generators are already equipped with the paralleling gear and breakers on them. This will only be used as a standby system. If the main utility fails, the bus will be dead. We will manually start the first generator with its breaker closed and power up the transformer. Then the other generators will be synced and paralleled to the bus. Then the load will be added by a switch on the load side of the 30MVA.

I guess my main question is if this will be detrimental to the generator / alternator by energizing the transformer this way.



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