Pad Mount transformers

[hardworkingstiff]

I'm of the understanding that a PoCo pad mount transformer should be able to operate at 150% of it's rating (for an hour or so) without damaging the transformer. I've also been told that no noticeable voltage drop would occur.

Am I of the correct understanding?

How far above the transformer rating can it operate before damage?

Thanks

 

[chris kennedy]

How far above the transformer rating can it operate before damage?

I'm no EE Lou but it is my understanding that this is a product of the temperature rise of the equipment.

From Siemens Speed Fax;

-115? C rise, can tolerate 15% continuous overload.

-80? C rise, can tolerate 30% continuous overload.

 

[jim dungar]

I'm of the understanding that a PoCo pad mount transformer should be able to operate at 150% of it's rating (for an hour or so) without damaging the transformer. I've also been told that no noticeable voltage drop would occur.

Am I of the correct understanding?

How far above the transformer rating can it operate before damage?

Thanks

I would not be surprised to see a POCO transformer operate above 200% for many hours at a time.

Heat build up is what causes all transformers to fail (excluding short circuits). Theoretically, you could overload a transformer by 200% for 12 hours then turn it off for 12 hours, and it would last for its normal life expectancy. In reality it takes longer for a transformer to cool down then it does to heat up, so the period of loading would more likely be 200% for 8 hours followed by <50% load for 16 hours. There are formulas to figure out the amount of cooling time required based the ambient temperature and the actual loading profile.

 

[zog]

It is not the load, or even the tempature that does the damage, it is the tempature rise, which is the delta T from the core to the outer windings (or ambient). This is why transformers are rated in a max rise temp, the difference in tempature across the windings causes different expansion rates, the inside of the winding expands more than the outside, which will crack the insulation causing premature failure.

A common for life expectancy of transformers is as follows:

-Class ?A? insulation - 8C above max temp rating= ? life expectancy
-Class ?B? insulation - 12C above max temp rating= ? life expectancy

 

[hardworkingstiff]

Thank you all.

The reason I was asking is I've often installed 800-amp and 1,000-amp panels (single-phase) for marinas and PoCo usually will feed it with a 100-KVA transformer. I try to get them to allow me to install the feeders from the transformer to our service equipment (putting CTs in the transformer) and size it all per the NEC, that way we have less voltage drop in the system.

The loads at marinas are usually light except for holidays or nice weekends. In the evenings everyone is on their boat with the AC running and cooking and drinking and just having fun. That's when the service is pushed to it's limits, and since the PoCo transformer is sized to 1/2 or less than the service, well I was just wondering what the consequences might be.