Distribution Transformer Loading 1

[KVX10]

I have run across Anthony Pansini's text and it gives recommended loading for distribution transformers. I believe that some of the recommended kVA ratings seem high. (Just my opinion) For example loading a 167kVA padmount transformer for up to 370kVA load under Table 6-13. I understand that they are using Table 6-12 electric space heating.

What are your opinions?

See attachment for reference

 

[dpc]

Opinion on what? It depends how long it will be overloaded and how long you want the transformer to last. The tradeoffs are pretty well documented.

You also have to define your terms. Are you talking about connected load, peak demand load, average load or what? Winter versus summer makes a difference as well.

There is no question that utilities routinely undersize their distribution transformers if you look at the estimated peak demand loads. It is just more economical to replace the occasional transformer that fails due to overload than to size all transformers to carry the peak load without overloading.

 

[jghrist]

Peak load capacity depends on the duration of the peak load and the level of the off-peak load. If in doubt, compare with the IEEE Guide for Loading Mineral-Oil-Immersed Transformers, Std C57.91

Some utilities load transformers high enough to produce some loss-of-life on an economic basis, considering transformer losses and replacement costs.

370 kVA seems a bit high for a 167 kVA transformer, particularly if any load growth is anticipated, but I don't know what the criteria is used by Pansini.

 

[opmgr1]

I agree with dpc. You have to determine if the reduction in life expectancy is worth the revenue generated, bearing in mind that in addition to reduce life expectancy there is also significant increase in the load-loss of the transformer (loss revenue due to increase losses). IEEE C57.91-1995 gives very good information in this regard along with the Electric Power Distribution Handbook.

 

[opmgr1]

Distribution transformers can withstand being loaded more heavily when it is cold. according to ANSI for temp. <30 deg. C, the loading capability can be increased by 1% for each deg. C below 30 deg. C. Not sure how the document you attached arrived at the loading indicated under the winter column.

 

[waross]

The tables that the OP based his assumption on were estimated heating loads for a number of houses. This would include a demand factor.
As the temperature drops, the demand increases but the transformer is better able to withstand the overload in the cold ambient temperature.
The tables can not be construed to indicate that a 370KVA load may be fed from a 167KVA transformer under all conditions.
Peak loading as suggested by the tables would only be experienced for a short time during the year and not at all during mild winters. A few days per year of overload during extremely cold weather will not raise the transformer temperature very much. Transformer life is shortened by sustained, elevated temperatures but in this example the reduction in the life of the transformer will be marginal.
Most utilities will have their own loading tables which will take into account local conditions such as local building methods and local weather patterns. These tables may then be modified by a formula that takes into account the cost of a larger transformer vs the cost of the shortened life of the transformer. This adjustment may be calculated or it may be based on past experience.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[opmgr1]

This is an interesting situation because if we are to be guided by the C57.91 it means that for the transformer winding to remain at ambient temp. (30ºC) the surrounding temp. would have to fall to -91ºC

 

[lansford]

As most of the posters have noted utility practices allow overloading of the distribution tranformers based on a number of criteria.Based on my review of a number of utility standards, using this criteria, my experieince is that there are few failures due to this.

JIM

 

[jghrist]

This is an interesting situation because if we are to be guided by the C57.91 it means that for the transformer winding to remain at ambient temp. (30ºC) the surrounding temp. would have to fall to -91ºC

For normal life expectancy, the hot spot temperature is limited to 120°C, not 30°C.

 

[magoo2]

Isn't it 110C? 65C rise + 30C ambient + 15C allowance for hot spot.

 

[opmgr1]

Jghrist,
You misunderstood the point I was trying to make. At an ambient temperature of 30°C, the 80°C hottest-spot rise a 65°C rise unit (bear in mind there are 55°C rise units) gives a hottest spot winding temp. of 110°C, and at this temp the transformer still has a normal life expectancy. However the transformer kVA rating is based on an ambient temp. of 30°C.

 

[KVX10]

I was thinking in the same manner as WAROSS, but appreciate the other information you all presented.

DPC, sometimes I assume too much when asking for opinions, thanks for your input.

I am usually conservative when sizing transformers as I believe many are oversized, therefore inefficient, but when reading through this text, I noticed the higher than expected values which led me to post.