How long do electrical transformers last?

Mark R

Diamond Member
Oct 9, 1999
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While wandering idly down the street, i came across a power distribution transformer. A fairly standard final distribution one (11kV D>0.43kV y).

It looked a bit of a sorry state, but was making all the right noises. Anyway, it was labeled as having been manufactured in 1962 - so it's getting on a bit.

So I looked for a few more, and a couple that i could get close enough to - were also a good 30-40 years old. However, it seems as though keeping people away has become more of a priority in recent years - so anything that looked even marginally newer was way out of bounds.

So how long are power transformers supposed to last?

Presumably as long as you don't overheat them or short circuit them and don't let them rust, they should be pretty much good for a very, very long time. Even if you do look after them, what tends to kill them?
 

BrownTown

Diamond Member
Dec 1, 2005
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Well, the ones that I have seen that were "killed" were 400MVA oil filled transformer that ground faults and exploded. Aside from transformers that are completely destroyed I'd image that you could get wear and tear on the oil circulation pumps and fans that would require at least some changing of parts. Aside from cooling equipment I would say that you probably are talking about having things like rust slowly degrade a transformer. Also remember that the metal is expanding and contracting 60 times a second for years, thats gotta slowly cause embrittlement or other mechanical weakening over time. Also the insulation is put under stress by the vibration and temperature changes, over time I would also expect it to be weakened to the point of failure. Having said that, many transformers don't have a single moving part, so they should be relatively sturdy which is why they can often last decades.

As for keeping people away, I know when I have been to 500kV switchyards the transformers are surrounded on 3 sides by 20ft tall concrete walls to direct the force of an explosion away from critical equipment, and the people at the substations were not very keen on going anywhere near the transformers without good reason seeing as they are more or less at ticking time bomb (thousands of gallons of oil and sitting directly between hundreds of thousands of volts).
 

heyheybooboo

Diamond Member
Jun 29, 2007
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A non-refurbished transformer of that age is probably filled with Polychlorinated Biphenyls (PCBs) which in good condition is not so bad - unless a lineman has to spend the better part of the day with his arms immersed in the oil. I will venture a guess that the transformer has been refurbished if it is in as good of condition as you have reported

Most utilities have a good record of replacing PCB oil and refurbing older transformers. A refurbished transformer can continue to have a long service life.

Most utilities are aware of their *aging* infrastructure and replace or refurb things like transformers because, in the case of PCBs, the cost of an accident far outweighs that of routine maintenance.

 

PowerEngineer

Diamond Member
Oct 22, 2001
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As has already been suggested, power transformers can last a very long time. Even though they have no moving parts, they do generate heat through resistive losses in the windings (which increase with the square of the loading) and induced current losses in their cores (whenever energized).

These losses act to heat up the insides of the transformer, and the oil is meant to circulate in order to draw that heat out from the windings/core and dissapate it through the casing to the outside air. The really big power transformers (like the 400 MVA unit mentioned by BrownTown) often use radiators (with fans) and oil circulation pumps to improve the cooling of the transformer.

Over time, however, high core temperatures (caused by peaking loadings) and the effects of temperature cycling will make the insulation on the windings brittle and lead to cracks that short turns or ground the winding through the core. This causes much higher currents and greater heating that can cause explosion and fire if the transformer isn't quickly deenergized.

For the big transformers, utilities try to track the aging of the transformer through sampling both the oil and gases inside the transformer, looking for contaminants that indicate insulation breakdown. It's not unusual for these big transformers to be taken apart and rewound (with new insulated cable) when approach failure.
 

Modelworks

Lifer
Feb 22, 2007
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I had one on my house that was covered in rust and was probably there since the house was built in the late 50's. It was only replaced when they came out to replace the pole that was made with creosote.

Power transformers are what got me started in electronics when I was about 10.
A friend, a local ham operator showed me how to wind a power transformer from parts around an iron core using enameled wire and masking tape between the layers.

It peaked my interest because it seemed like magic how the power moved from one winding to the other. From then on we did projects like the crystal radio made from a coffee can and only 1 diode, again it interested me becuase the radio worked without a battery.

Power transformers are defenitely recycled in the US.
They contain a ton of valuable copper and other metals.

They also seem to be valuable in iraq.
They had trouble keeping power going because looters were stealing the wires and transformers to sell as scrap.
Had a lot of deaths from it as well.
 

DrPizza

Administrator Elite Member Goat Whisperer
Mar 5, 2001
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www.slatebrookfarm.com
Originally posted by: heyheybooboo
A non-refurbished transformer of that age is probably filled with Polychlorinated Biphenyls (PCBs) which in good condition is not so bad - unless a lineman has to spend the better part of the day with his arms immersed in the oil. I will venture a guess that the transformer has been refurbished if it is in as good of condition as you have reported

Most utilities have a good record of replacing PCB oil and refurbing older transformers. A refurbished transformer can continue to have a long service life.

Most utilities are aware of their *aging* infrastructure and replace or refurb things like transformers because, in the case of PCBs, the cost of an accident far outweighs that of routine maintenance.

Years ago, when I had a transformer in front of my house dripping oil fairly steadily (a few drops an hour in warmer temperatures), Niagara Mohawk assured me that none of their transformers in service had PCB's in them.
 

F1shF4t

Golden Member
Oct 18, 2005
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Originally posted by: PowerEngineer

As has already been suggested, power transformers can last a very long time. Even though they have no moving parts, they do generate heat through resistive losses in the windings (which increase with the square of the loading) and induced current losses in their cores (whenever energized).

These losses act to heat up the insides of the transformer, and the oil is meant to circulate in order to draw that heat out from the windings/core and dissapate it through the casing to the outside air. The really big power transformers (like the 400 MVA unit mentioned by BrownTown) often use radiators (with fans) and oil circulation pumps to improve the cooling of the transformer.

Over time, however, high core temperatures (caused by peaking loadings) and the effects of temperature cycling will make the insulation on the windings brittle and lead to cracks that short turns or ground the winding through the core. This causes much higher currents and greater heating that can cause explosion and fire if the transformer isn't quickly deenergized.

For the big transformers, utilities try to track the aging of the transformer through sampling both the oil and gases inside the transformer, looking for contaminants that indicate insulation breakdown. It's not unusual for these big transformers to be taken apart and rewound (with new insulated cable) when approach failure.


Just another point which can be added is that how long transformers will last depends on how they are loaded. For example where i work transformers are generally considered to have 3 ratings.

First one is normal load where one day of operation is equal to one day of its life. Then you have the long term emergency cyclic rating where the transformer uses one month of its usable life for one day of continuous operation and the short term rating where it uses a month of its life for one hour of continuous operation.
 

HVAC

Member
May 27, 2001
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Heat: outright melting of parts or accelerated degradation or corrosion of parts.
Mechanical failure: if not properly secured by design or manufacture, transformer coil windings will vibrate as power cycles through them. If allowed to vibrate unchecked, they will bang into each other and/or eventually break.
Material compatibility: dissimilar material issues with high current. Oil additives breaking down over time.
Accident: getting run over, etc....
Maintenance: introducing impurities or failure to properly seal after service.
 

jagec

Lifer
Apr 30, 2004
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A lot more of them get hit by lightning and explode than die of old age, it seems.
 

Analog

Lifer
Jan 7, 2002
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Heat is the biggest culprit of failure. Heat can come from environment and loading.
 

futuristicmonkey

Golden Member
Feb 29, 2004
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Originally posted by: PowerEngineer

As has already been suggested, power transformers can last a very long time. Even though they have no moving parts, they do generate heat through resistive losses in the windings (which increase with the square of the loading) and induced current losses in their cores (whenever energized).

These losses act to heat up the insides of the transformer, and the oil is meant to circulate in order to draw that heat out from the windings/core and dissapate it through the casing to the outside air. The really big power transformers (like the 400 MVA unit mentioned by BrownTown) often use radiators (with fans) and oil circulation pumps to improve the cooling of the transformer.

Over time, however, high core temperatures (caused by peaking loadings) and the effects of temperature cycling will make the insulation on the windings brittle and lead to cracks that short turns or ground the winding through the core. This causes much higher currents and greater heating that can cause explosion and fire if the transformer isn't quickly deenergized.

For the big transformers, utilities try to track the aging of the transformer through sampling both the oil and gases inside the transformer, looking for contaminants that indicate insulation breakdown. It's not unusual for these big transformers to be taken apart and rewound (with new insulated cable) when approach failure.

Rule of thumb is that even a 10 degree celsius rise over the transformer's rating will halve the insulation's life.

 

JohnCU

Banned
Dec 9, 2000
16,530
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orr overloading them if the face plate happens to contain the wrong ratings information... :eek: