moisture in dry type indoor transformer windings

[Andy32821]

Hi,

I would like to turn off a group of dry type 480-208/120 transformers a few times a month to save on the power bill.

A concern that keeps getting raised is moisture in the windings.

These transformers are in an air conditioned control room so the humidity can be indirectly controlled and directly controlled by a de-humidifier if needed.

My question is: What is the highest electric room humidity I can allow and still insure that the transformer insulation is not going to get impregnated with moisture?

A lot of you guys have already helped me a great deal on a related thread about in-rush current, breaker trips, and such and I don’t wish to annoy you by re-hashing those topics.

But any information on preventing dry type indoor transformer winding moisture problems would be greatly appreciated.

The transformers in question are 300kva three phase GE k-factor cat # 9T23B3479G13.

Thanks,

Andy

 

[DanDel]

First off, the kW power loss of idling transformers is very low, and all of it is dissipated as heat.

You will probably have low enough humidity if there is any kind of AC and/or dehumidifier, anyway, I would imagine the level would be below 20-30%?

Please consider that you may do more damage to the transformers by de- and re-energizing them a few times a month. This would impose more financial issues than by any humidity or kW loss.

 

[dpc]

http://www.geindustrial.com/publibrary/checkout/Installation and Instruction|475A667AAP001|PDF

You have to stay above the dew point as a minimum requirement. If there is any visible condensation, you have a problem.

Also, cleaning the windings is more important since the gunk that accumulates on the windings over time will tend to absorb moisture.

You should probably contact a manufacturer and get their recommendations for long-term storage.

 

[Electic]

I don't believe moisture ingress is generally a problem for dry type transformers with encapsulated windings.

"Drying out" windings is a term mostly applied to large oil filled power transformers, where the oil forms the insulation, and water can be absorbed and contaminate the insulation qualities. Even smaller oil filled distribution transformers are not generally "dryed out" prior to putting into service, probably because the insulation isn't critically stressed (?).

Having said that, I would make sure your humidity does not allow mold growth or corrosion problems. The core should be sealed.

 

[dpc]

Also, maybe take a look at ANSI publication C57.94 that covers installation and maintenance of dry-type transformers. I'm too lazy to go look at it right now, so I don't know if it will be helpful at all. I'd get a manufacturer's opinion. They probably have a lot of these stored all the time somewhere.

 

[rovineye]

Why not just throw in some anti-condesation heaters? I have VPI insulated transformers (the largest being 7500 kVA), and we shut them down frequently, sometimes for long durations. Only rudamentary environmental controls in the space, if any. We have anti-condensation heaters in the bottom of the transformer enclosure (6 each at 150 watts).

 

[dpc]

He's trying to save energy, so the heaters would probably be a last resort. As I recall he is talking about a large number of 300 kVA transformers.

 

[Zogzog]

I did a feasbillity study for one of the big 3 automotive companies on this a few years ago, same idea, try to save some money. Long story short, you wont save any money and you increase the risk of the transformer failing or at least reducing life expetancy. There is no answer to your humidty question, the higher the humidity the more moisture a dry type transformer will absorb.

If you insist, there is a procedure to put a dry type transformer in lay-up, I think it is in the Red Book, but basically you clean it, test it, install a small heater, wrap it. All you need to do is keep the inside temp a few degress warmer than the outside. Of course you really should test it again before bringing it back on line, all these things cost money and you save little.

I recommend not doing this but if you do the best think to do is contact the manufacturer for a layup procedure.

 

[itsmoked]

Folks we're talking $800/day of

the kW power loss of idling transformers is very low

See thread: thread238-125977

We are also talking about an steady A/C'd space. By definition it will have lower humidity than the same unconditioned space.

We are talking relatively small temperature excursions, ever.


How at $800/day can you NOT save energy?

Andy32821; With a herd of transformers like that I would call the maker and ask. Tell them how many you have,(that will get their attention), and the temperature excursions seen in the space, and the typical humidity. Ask their opinion.

Tell us what they say!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Andy32821]

Hi Guys,

GE was non committal, but said if there is no moisture problem and the transformers are switched unloaded they don’t expect a significant decrease in transformer life. And they would only say that verbally.

The Florida Solar Energy Center has reviewed this project as part of a larger energy project and said the actual dollar savings will be close to double my estimates.

Probably about $175,000 per year per building.

Management has already bought in a two year trial period.

So it’s now up to me to try to minimize the impact of the switching.

Our biggest challenge is to not impact production (show activity).

The cost of replacing a couple of transformers and breakers a year should not impact the savings much.

Any ideas you guys have on protecting the transformers is welcome. Heaters are not practical but we can beef up our pm’s. Other than keeping the transformers vacuumed out once a year and keeping the electric room clean and dry I don’t know much else to do.


Thanks,
Andy

 

[dpc]

Andy,

I think just vacuuming the windings will help a lot.

 

[Zogzog]

Small in reference to the KVA rating of the transformer, "no load losses" are approximately 1% or less for small and medium size transformers. So your 300kVA transformer would draw about 3kVA, it is inductive so lets say .85PF, so thats 2.55kW times 24 hrs = 61.2 kWH/day, lets assume (I am guessing you are an industrial customer) $.05/khw. Thats $3.06/day for 1 300kVA transformer x 365 days $1160.50/transformer each year.

Somehow you are going to save $175,000 per year per building?? So you are going to idle about 150 of these transformers per building? What kind of place do you have there?

Now 1 failed transformer will cost you what $6k-$7k? Plus installation, labor, shipping, parts, lets say $10k to replace?

 

[dpc]

Zogzog,

See the link to the previous thread in itsmoked's comment. We've been through all this before.

 

[Zogzog]

I did, he had no basis for his numbers. Also they had dozens and said there were no shows, i am sure the convention center still had some loads on. I will take my math over that post anyday.

EEJamies math in the other post even put the OP's claim into question.

 

[Andy32821]

Hi Zogzog,

I understand your doubts. Most people have the same doubts when they see the numbers. But please remember this is only a tiny fraction of the million dollar a month utility bill for this center.

These transformers feed only the thousands of floor boxes you see in a convention center. (Close to two million square feet of floor box area).

The wattage usage is taken from pq meters on the switch boards.

There are sixteen 4000 amp 480 switchboards just to feed the floor boxes and another forty switchboards for the other loads.

As I said the outside reviewers said my numbers were low, but they assumed we could shut off the transformers even between back to back shows. That may not be practical. They also included the air conditioning loads, demand reduction, and such.

I will be glad to let you review the numbers for me. I have made some embarrassing mistakes in the past and I love peer review, but I think you will find these numbers have been vetted pretty well.