Folks,

I would be interested to hear your opinions of the pro's and con's of keeping a spare single phase transformer energized, but not on load, next to the three single phase units it 'supports'. The idea of energizing it is to keep it 'ready'. If you have experience, things to consider or comments I'd appreciate hearing from you. Thoughts about increased risk from through faults, switching surges and the like are welcome; as are thoughts on what terminations to use for other terminals - surge arresters etc - and what to do with protection schemes.

Appreciate your comments,

Tony McGrail

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Tony McGrail
Chair, New England Network

It's quite an open question and a lot would depend on the details of the particular installation, for me the key question is what benefit does keeping it energised provide?

Presumably it gives some assurance as to the condition, that it can withstand rated voltage but then I would think that assurance could be provided by other means. Testing on manufacture, perhaps followed up by post delivery testing to confirm no damage during transportation and an in-storage maintenance regime (including oil testing etc) perhaps even periodic electrical testing should provide assurance that the transformer remains fit for service.

Another question - can the unit be put into service from its storage position, or would it need dismantling and shifting? If it can stay in position then there's time savings associated with installation, cabling, auxiliaries ready to go etc. but if it needs dismantling then that benefit goes, having it assembled may even delay the installation programme.

What are the no-load losses and the cost? For something small it may be trivial but for a single phase tank on a large generator transformer you might be talking £30k/year in no-load losses alone not to mention the associated environmental impact just from keeping it energised.

My gut feel would be that simply keeping it in a dry but "off" environment would be better. This would minimise corrosion etc and oil degredation/water absorbtion, but minimise the longterm "wear" due to partial discharge or exposure to faults etc. You could energise it once every so often to satisfy yourself it's "ready" (maybe even monitor for any partial discharge during test periods), but I guess this might even have its own risks of damage due to inrush?, so you might not want to do it too often. Not my field really, just thoughts.

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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe

Hi,

All of our spare transformers as stored disconnected from the distribution system. I think we have a single bottle drykeep unit attached to each one and circulate the oil through this to keep the moisture level under control. We then carry out oil analysis at regular intervals as you would do for an energised transformer.

Prior to putting into service the transformer would be commissioned and tested in accordance with the manufacturers' recommendations. To be honest the size of our transformers doesn't make the swap out of them quick so to add in an extra couple of days for electrical testing doesn't create any issues for us.

I am however coming across this issue when putting plants into preservation and the decision of keeping transformers energised or not and i am leaning towards the de-energised option mostly due to the comment of ADJONES as to the losses you are paying for by keeping it energised.

Another issue I am looking at is that an energised but off load transformer will have cold oil and cold oil cannot retain as much moisture as hot oil, so there is an increased risk of moisture saturation witihin the oil leading to free water within the transformer. The consequences for this for an energised transformer are much worse than for one stored de-energised.

I would also want to think about how the transformer is going to be energised. If it is going to have its own breaker then that is an increased installation and maintenance cost that will need considering. The only protection system available would probably be some form of earth fault, you won't be able to implement differential protection and overcurrent won't be quick enough to protect the transformer, you could decrease overcurrent settings but you will still need a high enough setup to cater for the inrush during energisation. Putting protection on the secondary winding may also be problematic unless there are CTs within the bushings.

I also don't know if your transformer has a built in tapchanger. Our three phase units certainly do and if the transformer remained energised this would have to be exercised on a routine basis to prevent carbon build up on the contacts.

One last thing, you comment that the transformer will be sited next to the in-service units. Have you risk assessed this in terms of damage to the spare unit from catastophic failure of an energised unit? The last thing you want is to cause damage to the spare so that it becomes unuseable.

Kind regards

Donald Lane

Thanks to everyone who responded. Some great comments and some great things to think about.

Must admit that I'm against the idea of keeping the spare single phase unit energized for both technical and asset management reasons:
- technically, the energization is to test fitness for purpose, but failing the test could be catastrophic
- for asset management there are the no-load losses and there is the increased risk of either the unit itself failing or collateral damage causing it to fail, and the premature aging from being exposed to through faults.

But that's just me!

many thanks,

T.

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Tony McGrail
Chair, New England Network