500 KVA transformer max loading

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lilsparky

Member
Location
Madison, Al
Hello, it's me again. Here's my question today:

I have an existing 500 KVA (12.47/480) outdoor transformer. I am upsizing the loads on the secondary size. So far I have calculated 362 kva of load which includes my multiplier for future. What is the max. design that should be used for a 500 kva transformer? is it 20%? am I already close to the max?

Thanks as always!!
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Why should you think that you can't load a 500 KVA transformer to 500 KVA? :? If you design a new building, and calculate the load at 500 KVA, the utility company is likely to give you a 250 KVA transformer anyway. :happyyes:

 

lilsparky

Member
Location
Madison, Al
the 500 kva is an existing transformer and I am just adding loads to it...well readjusting..I am removing motor loads but the HVAC loads are to remain as is. I guess I thought a transformer wasn't suppose to be fully loaded? NO? :?
 

mike_kilroy

Senior Member
Location
United States
the 500 kva is an existing transformer and I am just adding loads to it...well readjusting..I am removing motor loads but the HVAC loads are to remain as is. I guess I thought a transformer wasn't suppose to be fully loaded? NO? :?

NO.

If I sell a 500kva xfmr it sure better be able to work with 500kva load all day long or I misrepresented my product. a transformer is designed like a motor to work to its continuous rating with a certain temperature rise above ambient. depending on the design, this typically 115, 130, or 150 degree C rise above 40 degree C ambient at full load.
 

MSU

Member
Location
Hattiesburg, MS
As a general rule, Utilities will size a distribution transformer at 1.2-2HP connected per KVA of transformer capacity, depending upon the anticipated load factor or load diversity of the load. More information is needed to accurately size a transformer, but a liquid filled transformer should be able to serve its rated KVA in load 24x7 assuming a specified ambient temperature and a specified wind. It only cares that its rated hot spot insulation temp is not exceeded.

I commonly see 20-40DegC ambient specs and 2-4MPH wind specs.

Losses are commonly broken into no load (core) losses, and load (conductor) losses. Since conductor losses vary as the square of the current, a reduction from 1PU current to .8PU will reduce load losses from 1^2 to .8^2, or 100% to 64%. Load losses usually account for 70-85% of total losses depending on the transformer design. Core losses are pretty constant 24/7.
 

templdl

Senior Member
Location
Wisconsin
As a general rule, Utilities will size a distribution transformer at 1.2-2HP connected per KVA of transformer capacity, depending upon the anticipated load factor or load diversity of the load. More information is needed to accurately size a transformer, but a liquid filled transformer should be able to serve its rated KVA in load 24x7 assuming a specified ambient temperature and a specified wind. It only cares that its rated hot spot insulation temp is not exceeded.

I commonly see 20-40DegC ambient specs and 2-4MPH wind specs.

Losses are commonly broken into no load (core) losses, and load (conductor) losses. Since conductor losses vary as the square of the current, a reduction from 1PU current to .8PU will reduce load losses from 1^2 to .8^2, or 100% to 64%. Load losses usually account for 70-85% of total losses depending on the transformer design. Core losses are pretty constant 24/7.
You are on the right track. Load lose only have to do with is the efficiency at those given load points. And, yes, both the insulation class and the hot spot as allowed are the target points. I included some of the following in one of my posts a few weeks ago or so using a dry type transformer as an example. No matter what the kva rating at a given temperature rating is, 80degC, 115degC, or 150degC one thing will always stay the same is the 220degC insulation class and the 30degC hot spot allowance.
What will vary is the ambient temperature up to 220degC insulation class less the hot spot allowance of 30degC which is 190degC. No matter what the 190degC should never be exceeded. Now take the 190deC-150degC rated transformer and you end of with 40degC which is the max ambient temperature. Reduce the ambient temperature and the transformer is capable of some overload without exceeding the 190degC. A transformer that is rated 115degC has the advantage of 150-115=35degC. As such its rating starts 35degC lower than the 150 so you have 35degC to work with which equated to the ability to be overloaded 15% w/o exceeding the 190degC target. Get get that ambient down some such as 25degC and you gain some more overload capacity.
The, is you start with transformer rated a 80degC....well you should understand how it works. Yes, you get a basic 30% overload capacity w/o playing with the ambient temperature.

It used to be that you could expect the transformer life expectancy to be some 30year and sometime more. Today, however, not as much overload capacity is specified and the transformers run warmer for longer periods of time. Transformers running a close to that 190degC mark are expected to only last maybe 3-5 years.
 

MSU

Member
Location
Hattiesburg, MS
templdl,

Good explanation. Agreed. Most of the time, assumptions are made because the loading is usually pretty dynamic and not clearly defined.

I like to see most equipment, especially motors and transformers loaded to 75-90% of nameplate. The longevity gains are generally worth the initial cost penalty.
 

templdl

Senior Member
Location
Wisconsin
Often it is not a "choice" in the field for overloading transformers as it happens little by little as time goes by. The transformer goes into overload and troubles start to happen and it's "DID I DO THAT?" Then it's time to revisit the loading. This stuff is more often than not out of sight out of mind until problems start to happen then sometimes turns into a panic mode.
 

BJ Conner

Senior Member
Location
97006
Transformer Loading

Transformer Loading

Transformer loading is more complex the bigger they get. There are ANSI standards that will let you overload them and determine the shortened life resulting from that.
Most utilities base their load on 24 hr periods. The loading is different in different climates and considers air conditioning and/or electtric heat.
The engineering is in a cook book usually conserative ( even if doesn't seem like it).
Ambient temperature has a big effect. Oil filed transformers can be overload 1.8 % for ever degree below 32 F. That about 100% at 30 below. I have used that for designing circuits for block heaters in parking lots and office trailer complexes ( where they had electric heat).

If the abmient temperature is high the loading has to be lowered. That is the main reason you see tranfomer fires in heat wave.
 
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