How much load can a single phase 75KVA OH TX really handle? 3

[brussell250]

I am trying to size the single phase TX's i need. I will sometimes need to overload them based on the nameplate. I have personally witnessed many TX's pushed way past their rating. These will be new ABB or Power Partners TX's. I'm sure there is a safety factor I want to say that my utility uses 167% or something like that to size them.....I know heat is a factor also so there is more than one answer....

Thanks,

Bill

 

[wayne440]

Another factor - How long do you want them to last?

 

[davidbeach]

How hard are they to replace when they fail? How adverse is the publicity when they fail?

From your Industrial tag, I'd assume you're on the customer side rather than the utility side. If so you probably don't actually have any options beyond that allowed by the relevant electrical code, the NEC if in the USA. Utilities are bound by different rules.

 

[brussell250]

I would like it to last for at least a year. Changing out a NG TX is not a big problem; the reason I am asking this question is so I don't have to go down that rd and up them to a 100's which are a foot taller and weigh 250+ lbs. more...I don't care about utilities or code, I want to know what a 75KVA OH TX can handle on a regular basis i.e. I have seen plenty of 15's and 25's pushed way past their rating and can hold the load no prob......on the other hand I have seen plenty of 10's, 15's, 25's and 37.5's blow up. What I want to know is what is the safety factor on a OH TX or what is its emergency 25 degree C rating? Again ABB or Power Partners....

Thanks for your help,

Bill

 

[jghrist]

There is no safety factor as such on transformer loading. The rating is based on normal insulation life when loaded at 100% of rating when the ambient temperature is 40°C. You can load it higher than that for short periods of time if the loading during the rest of the time is less than rated. You can allow higher loading if you are willing to have decreased insulation life. There is an IEEE guide to show how to calculate how much you can load the transformer for different load cycles and different levels of loss of life. It is complicated. I worked for a utility that worked out the economics with their typical residaential load cycles and determined that it was economical to install transformers with peak loads of 125% of rating and not to replace them until the load grew to 300% of rated.

 

[wayne440]

You might get more meaningful answers if you will better define the frequency, duration and extent of your planned overload.

 

[MAGTiger]

These transformer commonly see 250% of rated current for relatively short periods say 1.5 hours, mainly in the morning when everyone wakes up. Then in the afternoon again a high overload maybe 150-200% but lasting a few hours, then 10-20% for about 8 hours,... The oil provides a significant thermal mass and allows it to heat up more slowly. Were it a dry typ then the thermal time constant would be half or less and you would not want to overload more than 30%. But the pole top units are oil filled. Figure an absolute max current of 250% and an average load factor of 70% in a 24 hour period and you should be fine. I worked at the Power Partners plant in Athens in the 70' WOW that seems like a different lifetime. Back then it was Westinghouse. Power Partners should be able to provdie you with a permissible load profile based on expected thermal loss of life.

Neil