What happens if a utility transformer exceeds its ratings temporarily?

[Cartoon1]

I'm trying to get basic knowledge of what typically happens when a pad mounted utility transformer exceeds its ratings load temporarily. What is usually their ratings threshold. So for example lets assume we have a 500KVA transformer and the load it is feeding temporarily spikes (5-10minutes to 550KVA). Does this cause the transformer to fail or is it cable of handling higher ratings for a period of time? I ask this because i have seen buildings loads peak demand temporarily exceeds the transformer ratings.

Thank you

 

[jim dungar]

Transformers are temperature sensitive devices. If additional cooling is added to a transformer, it is capable of supplying more than it nominal rating.

 

[winnie]

Transformers have 'thermal mass', meaning that it takes time for them to heat up (or cool down). Transformer damage is also a thermally sensitive aging process.

This means that a transformer can tolerate limited time overloads without any sort of damage at all. Additionally, if a transformer is overloaded to the point that the temperature becomes excessive for a period of time, the transformer will likely survive with a reduction in life, not an instant failure.

-Jon

 

[LarryFine]

So for example lets assume we have a 500KVA transformer and the load it is feeding temporarily spikes (5-10minutes to 550KVA).

Happens all the time. It gets hot, but doesn't suddenly fail.

 

[zbang]

Many power companies play the odds of overload vs failure- they may let a transformer run for 8 hours at 150% load knowing that it'll then get 16 hours at under 30% and cool off. They also generally know the ages of the transformers, expected lifetime, cost to replace, etc, so might let one run hotter than usual because they know, statistically, when it will fail and can expect to replace it then.

 

[sgtsparky]

I had to do life analysis on 3500 kva transformers in a log mill, the life of a transformer is not affected by small short duration overcurrents. At 120% continuous we did not have to add fans at 130 we would have need fans but could not run continuously beyond this without reducing the life of the transformer but could run for 4 hours at 150 and if 2 hours at 180% but then it had to cool down, I don’t remember where I found the info but there are curves for transformers and how loading affects their life, a small overload 10% it should be able to handle continuously with no problems.

 

[Joethemechanic]

500KVA transformer and the load it is feeding temporarily spikes (5-10minutes to 550KVA).

10% for 5 - 10 minutes, I would expect no noticeable effect on transformer life.

 

[Ken_S]

Many power companies play the odds of overload vs failure- they may let a transformer run for 8 hours at 150% load knowing that it'll then get 16 hours at under 30% and cool off. They also generally know the ages of the transformers, expected lifetime, cost to replace, etc, so might let one run hotter than usual because they know, statistically, when it will fail and can expect to replace it then.

I was surprised, but not too surprised when I heard this right from the mouth of a utility company engineer

 

[Joethemechanic]

Let's not forget that larger transformers cause low lagging power factor problems

 

[petersonra]

The same thing happens with transformers not owned by the utility. You can typically overload a transformer pretty substantially for short periods of time without a whole lot of problems.

 

[GoldDigger]

Often you will see a very noticeable red light on a POCO pole pig when the transformer temperature goes above the normal operating limit. Usually as the result of an overload, but also potentially because of an internal fault too small to blow primary fuses.
Not sure whether it resets on its own when the temperature drops.

 

[Julius Right]

Let's say it is about a dry-type transformer. In IEEE Std C57.96-1999 there are some tables for permissible transformer overload.
For instance: Table 5—Daily loads above rating to give normal life expectancy in 30 average ambient for transformers with a 30 min time constant.
For example, if the transformer overload is for not more than 30 minutes and after then the load will be no more than 90%, then 1.47 times the rating kVA it is permissible.

 

[AlbertL57]

Often you will see a very noticeable red light on a POCO pole pig when the transformer temperature goes above the normal operating limit. Usually as the result of an overload, but also potentially because of an internal fault too small to blow primary fuses.
Not sure whether it resets on its own when the temperature drops.

Interesting - I remember seeing those lights occasionally when I was little, back in the '60s. I think they were introduced in the '50s, when a lot of utilities were seeing major load increases from the growing use of window air conditioners ("room coolers"), and wanted a way to finding transformers that needed to be upsized. As for resetting - I believe they were manually reset with a hook stick.

 

[AlbertL57]

Often you will see a very noticeable red light on a POCO pole pig when the transformer temperature goes above the normal operating limit. Usually as the result of an overload, but also potentially because of an internal fault too small to blow primary fuses.
Not sure whether it resets on its own when the temperature drops.

One transformer manufacturer offered an alternative - instead of an overload light, it used a buzzer. The idea was that nearby residents would hear the unusual noise, figure there was something wrong with the transformer, and call the utility. I remember a trade magazine ad that described a field test, showing how quickly people called to report the sound.

 

[augie47]

One transformer manufacturer offered an alternative - instead of an overload light, it used a buzzer. The idea was that nearby residents would hear the unusual noise, figure there was something wrong with the transformer, and call the utility. I remember a trade magazine ad that described a field test, showing how quickly people called to report the sound.

Won't work with old folks like me with tinnitus

 

[Fred B]

Often you will see a very noticeable red light on a POCO pole pig when the transformer temperature goes above the normal operating limit. Usually as the result of an overload, but also potentially because of an internal fault too small to blow primary fuses.
Not sure whether it resets on its own when the temperature drops.

Don't know, but there are some around me that have had the red light for years and now have burn marks and severe rusting on the transformer but still POCO doing nothing. Even called on one and no trucks showed up. Think they don't bother until they are not getting any money because power went out to the customer.

 

[Jraef]

Many power companies play the odds of overload vs failure- they may let a transformer run for 8 hours at 150% load knowing that it'll then get 16 hours at under 30% and cool off. They also generally know the ages of the transformers, expected lifetime, cost to replace, etc, so might let one run hotter than usual because they know, statistically, when it will fail and can expect to replace it then.

Once I called the PoCo for a project where they had a 750kVA padmount xfmr installed and we needed to upgrade to 1,000kVA for a new machine. They sent a crew out; one guy opened the transformer cabinet door, changed the fuses, then changed the stickers to say 1,000kVA, the other guys ran new conductors to the meter cabinet, changed the CTs, closed the doors and left...

 

[Tulsa Electrician]

Once I called the PoCo for a project where they had a 750kVA padmount xfmr installed and we needed to upgrade to 1,000kVA for a new machine. They sent a crew out; one guy opened the transformer cabinet door, changed the fuses, then changed the stickers to say 1,000kVA, the other guys ran new conductors to the meter cabinet, changed the CTs, closed the doors and left...

Now that's funny

 

[Cesna]

Some utility padmounts can be adjusted for extended draw exceeding anywhere from 150 to 180% for 2 to 6 hours. I've also seen power studies of aerial 167kva three pot banks with sustained draw over 850 amps at 480V without issue. Every major utility has different variance standards. With that said, they will fuse with whatever is in the guys truck at the time so be diligent.

 

[garbo]

I talked to our local utility company detailer about concerns of them always supplying undersized transformers. Told me the do this to save on energy while transformers are lightly loaded and saving money on smaller transformers.I did a 800 amp 240 volt service where there was an existing 200 & 600 amp services. They refused to replace the already overloaded 75 KVA transformer. Told me that its oil filled and being up on a pole is air cooled. The 800 amp service was for an injection molding shop that ran 24/6 and load was always over 600 amps. Original transformer burned out in maybe two months and replacement undersized 75 KVA 6 months. In other words they told me that a transformer rated for 180 amps is large enough to supply over 1,000 amps. Of course the first replacement transformers light grey painted enclosure had darkened burn marks after a few weeks. Owner of the business had his lawyer stating that because of the Ultility company penny pinching they would sue for lost production if the second utility transformer burnt out early. PECO/Exelon horrible for never wanting to replace frayed rubber covered ( RW maybe ) on underground services. They told me that thier 80 year old #6 copper wire was large enough to feed a 100 & 200 amp service in a 4 floor building.