Distibution Transformer Explosions

I'm curious about the mechanics of power transformer expolsions. Keep in mind I don't have a real good understanding of transformers in general.

The other morning somthing (only guessing a transformer) made a loud bang, and then a very loud arcing sound, then we lost power. I was on my way to work, so I didn't have time to go look for the problem.

Assuming it was a transformer (thouse round can like shaped ones we have here in the US) What is likly to cause this on a cooler early morning (low demand) with no wind or storms? What exactly happens inside to cause the boom? What shorts against what? I would imagine they look black or burned after this happens? Do they spray oil all over? I have never managed to get a look at the aftermath of an explosion, and google didn'y yeild any photos.

Any current or former linemen hanging in the weeds here?

yep, that was most likely a transformer, although they usually arc before the explosion, not after. yes they spray boiling hot mineral\transformer oil everywhere, and sometimes the oil is on fire too. not something you want to be near. sometimes transformers just fail for no reason, apart from old age, or were faulty to begin with. Maybe there was an overload, or a power line fell down, it's pretty unormal for them to explode for no reason. they explode from the oil heating too much, expanding, then forcing the case to explode. but usually there is a short in the transformer, or it is being overloaded.

could of been an unbalanced load.

maybe?? although i don't know why the arcing sound came after the explosion, usually they arc, then explode?

I remember when Hurricane Juan hit here and I was just watching transformers exploding around the city...or at least flashes from some sort of electrical explosion...IE: Blue light produced.

Anyway there was at least 50 that I counted before our power went out.

Well, they are fairly HV... My guess would be over-temperature failure, or dielectric failure (inter-winding arcing). In any case, whether the arc caused a boom or the boom caused an arc is academic to the viewpoint of the repairman (but maybe not the system engineer).

well usually it arcs for a minute or so, then it explodes. the arcing or shorting heats up the oil, the oil then expands, increasing the pressure inside the transformers can, eventually the can bursts open, and flaming mineral oil sprays everywhere sounds fun

Is it just me?, or do other people think those horrible round American transformers look really old, cheap, and nasty?.

i think its because its easier to wind them like that? we don't have many of those round transformers here, only the big square ones that require 3 poles to hold up

things said:

i think its because its easier to wind them like that? we don't have many of those round transformers here, only the big square ones that require 3 poles to hold up

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We don't have many transformers on poles at all?, usually they are on the ground, inside a fenced area.

we have some in fences too, but we also have massive ones that hang on the power pole, but they require 2 more additional poles to hold them up

Here we have one on the ground for our house. It gives off a nice hum...

Yeah, in some ways, it makes alot more sense to have them on the ground. It is a damn heavy thing to have hanging that high up. I'm surprised I have never seen one fall with all the old poles around here.

The only advantage I can see to putting them up high is keeping the HV out of easy reach.

Do you have fewer transformers in the UK? Does the higher line voltage allow for more distance between them?

things - Got any pictures of the ones that take 3 poles? eek, that must be like having a bus 25 feet in the air!

Doesn't further away from ground mean fewer losses? Especially at high voltages? Or possibly the saftey issue of what happens if one explodes? Better up than down?

i don't have any pics at the moment, the one i saw with 3 poles is in townsville, but we have one near my school with two poles some reason the transformers don't hum??

Andy1845c said:

Do you have fewer transformers in the UK? Does the higher line voltage allow for more distance between them?

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I don't really know?, because they aren't stuck up in the air you don't really see them very often - but I think one transformer usually covers a fairly good sized area?.

Distribution Transformers

The USA uses more distribution tranformers than europe.

In the USA a transformer will serve a small number of customers (1 to 6) compared to europe where a transformer will serve a neighbourhood (1000s).

In the UK a common distribution transformer is 2MW supplied from 11kV. Customers taking 1MW may have their own transformer (this depends on the Power company or/and what the customer is prepared to pay).

Bigger transformers are more efficient which means that using less transformers may give lower overall losses in the distribution system. If a big transformer goes out of service a larger number of customers will be affected. A balance has to be struck.

The losses in large transformers give rise to high internal temperatures which degrade the insulation. Cooling can be with air passing over vanes sometimes assisted by fans. Circulating systems are used for cooling oil. The oil also acts as an insulator.

Failure can be due to oil leaking out. oil insulation failing or overload relay failure

Generally it is cheaper to install overhead plant especially with higher powers where the conductors can be cooled by the air. Underground Transmission lines require special cables and oil circulation cooling. These cables are preferred to be installed with flat gradients.

Burying cables gives better protection from the elements. Distribution lines in urban areas are more vulnerable to contractors shovels and pneumatic drills. Overhead plant can be more vulnerable to the attentions of vandals.

Overhead lines are easier to access for repair. Buried lines require more maintenance.




Some info about Buried v Underground

CheapSlider said:

In the USA a transformer will serve a small number of customers (1 to 6) compared to europe where a transformer will serve a neighbourhood (1000s).

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Thanks for that, that explains a lot!

Things and CheapSlider explain everything very well already.

I work a lot with distribution transformers and deal with cable faults.
As already said, previously overloading may have degraded the insulation to an extend that when temperatures changes occur cooling down or heating up the windings may make good contact and start arcing, this will create heat, expansion and bang. The mineral insulating and cooling oil will burn very well when heated.

That is the worst case scenario, normally a HT drop out fuse should operate before a transformer explodes, these go off with a bang like a shotgun and cause the sky to light up.
Although from my experience most TX's are fused with oversized fuses so protection doesn't always exist.

Sometimes deteriation of the oil may cause the problem, after years of service (30 Y + ) the oil tend to become acid and may cause low insulation resistance in the TX with increased leakage currents.

At substation TX's every half year oil samples are taken and are analysed in a lab for signs of carbon, ecythelene etc and so potential faults can be looked at well before something goes wrong. Ok these are 10 or 20 MVA and cost millions of dollars, for smaller dist TX's it is not worth doing.

We do load surveys and data readings in areas when complaints exist, These are usually pre warnings when overloading exist on the system or for pending cable faults or connection faillures.

Transformers are supposed to be designed to vent off the excess via a breather or explosion valve to a safe area.
This is not always possible on pole transformers.

I will take a few pics in the next week and provide the URL links for anyone interested.

Nigel, In New Zealand we mostly use the British system with 6.6 , 11 and 22 kV distribution voltages.
A lot of TX's are overhead 30 and 50 kVA rural, while in the city most are pad mounted TX's 100 to 300 kVA fed from OH or UG supplies and 500 kVA - 1 MVA near high load centres.
Some 325 kVA OH TX's are still in service, one only about 1 km away from my place which iw ill take a picci off.

Ok my $0.02 worth

great explanation ROLDALCO! so basically it gets heated in someway, the insulation starts to melt, arcing then happens, which heats the oil even more, the fuses (sometimes) fail to open, so it continues arcing, heating the oil till the can can't hold it anymore, the can explodes, and the arcs ignite the allready hot oil. The fuses are ment to stop that from happening, but they are sometimes rated for too much, and they let it happen, not very reliable