Paralling a transformer directly connected to a generator

[bdn2004]

I work at a power plant that has one of it's main generators connected to an aging step up transformer. If the transformer failed it would take a long time to get a new one and if a spare is found it would take a long time to install it considering all the bus re-work, engineering time, and transporting logistics. I had an idea to parallel another future transformer. And another idea that to install the provisions for a spare to make the swap out as quick as possible.

Has anyone ever seen this done or have another idea? Are there issues to look out for?

 

[magoo2]

What you're proposing is not a bad idea. It's a switchable spare. If I understand your proposal, you're not really paralleling them in that only one of the transformers would operate at a given time.

Most of our GSUs are banks of single phase transformers and we normally have an in-place spare to cover a potential failure and long down time.

 

[dpc]

It probably makes more sense to just replace the existing transformer and keep the old one as a spare.

 

[ScottyUK]

If the old one is sitting at peace with the world - no deteriorating DGA or furan test results - then I'd leave it in place and keep the new one as a spare. Why take the chance and incur the cost moving the old just to use the new one?

What do you gain by having a parallel transformer if you have no means of isolating a faulted transformer? The whole scheme seems flawed to me - you need to put in breakers or disconnectors to be able to remove one transformer from the pair. Bus damage local to the transformer isn't unlikely if the transformer has a Very Bad Day so you need to have a means of removing the damaged section from service.


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

 

[slavag]

Hi.
I agree with Scotty. Paralleling w/o additional breakers and disconnectors is not so good solution. Used new one as spare. I'm not say about additional protection paralleling, isn't small additional cost.
Regards.
Slava

 

[bdn2004]

Keeping the new one as spare is an original good idea - thanks.

I didn't like the idea of simply replacing the transformer because:

1) new transformers are known to fail too - which leaves us right back where we are.

2) Installing a new transformer is at least a 45 - 60 day ordeal. And during that outage literally millions of dollars will be lost. That was the problem I was trying to fix.

I'm new to this industry...maybe it's just a problem you just live with.

The single line attached is basic, of course there would be disconnects, circuit breakers, and protective relays.

 

[dpc]

Replacement with new transformer for the old during a scheduled generator maintenance outage will cost much less than trying to install the new one when the old one fails and causes an unscheduled outage.

Buying a new transformer to sit as an unused spare does not make a lot of sense to me, and has essentially an infinite payback. The new transformer should have a much longer life (moving forward) than the old one, and should be more efficient, particularly regarding core losses.

The longer it sits around, the more likely you are to have a problem with it.

 

[slavag]

Dave.
Maybe better provide good test and seriuos maintanance to "old" trafo? How "old"?
But replace to new one? I don't know, maybe if "old" one is too bad.
Regards.
Slava

 

[ScottyUK]

dpc,

It's an insurance policy, not an asset with a payback time. A GSU transformer of that size will be on a leadtime of more than a year because a quite a few companies who made 'big' transformers have closed in the last ten years and all the available facilities are working at capacity. Same problem for generators - wreck a stator core and expect a **long** wait for a replacement.

Transformers are fairly predictable through DGA to identify thermal problems and furan analysis to monitor the insulation condition. It doesn't rule out the sudden death type of failure but that can happen to new or old. The efficiency gain is a fair point although even old transformers are very efficient so there is little room to improve.

Whichever transformer you decide to leave idle, make sure it is properly prepared for long term storage. Moisture ingress is your greatest enemy. Breathing type conservators need to be sealed off and an inert dry gas introduced above the oil for example.


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

 

[dpc]

All I'm saying is that looking out into the future, a new transformer will be less likely to fail than an old one (if you don't believe this, why buy a new one).

Given the cost of an unscheduled outage, the safer bet is to install the new one during a scheduled outage.

I agree that normally you will get some early warning on transformer failure.

 

[slavag]

Dave, today, I'm not sure:
"a new transformer will be less likely to fail than an old one". Is depend on where you bay new one. And old " hourse"
continue work w/o any problems. I heard from time to time about problems with new trafos ( mnf at placed.. but very cheaper).BTW, same with generators, motors.
Regards.
Slava

 

[7anoter4]

Hi bdn2004
I think you may do a compromise. If it is enough space, you may accomplish the new transformer parallel installation, that means: the 138 Kv side [SF6 Busducts for indoor installation or overhead line outdoor, connected directly after the main High Voltage Breaker ] and the medium voltage 20 KV-Isolated Phase Busduct , including heaters as usual, all this connected directly parallel to the existing Main Transformer but with no breaker. The new transformer, standing on the rails ,should be then disconnected and deployed 7-8 ft. back and should be concealed in a light but firm envelope in order to avoid moisture and dust. A kind of heating is indicated in this purpose also. The remaining opening in IphB shall be covered as live parts.
If the old transformer will deteriorate , the transformer replacing time will be shorter.
Best Regards

 

[bdn2004]

7anoter4,

..."the new transformer, standing on the rails ,should be then disconnected and deployed 7-8 ft. back and should be concealed in a light but firm envelope in order to avoid moisture and dust. A kind of heating is indicated in this purpose also. The remaining opening in IphB shall be covered as live parts."

you lost me there...

I'll be meeting with an engineering firm next week that specializes in switchyard design and will let you know how feasible this plan really is.