Transformers, K-rating, and Efficiency

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RDL914

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Hello All,

This is my first post so bear with me. I've been asked to do some very preliminary research into transformer efficiency and determine if utility cost savings can be obtained by swapping out old transformers with new, "energy efficient" transformers. I am not designing a system by any means.

The facility is a 15 floor office building. The existing transformer on a typical floor is 480-208Y/120V, 150kVA, K-13 rated, and the peak demand is estimated at 60kW. The peak demand is not expected to increase.

My understanding of K rated transformers is that they are used when non-linear loads such as florescent lighting and electronics such as computers can damage a standard transformer. However, since the peak demand appears to be 50% or less of the 150kVA, I would think K-13 is overkill, and am wondering if these can be replaced with K-4 or non-K rated but energy efficient transformers.

In short, my question is how should the following variables be considered:
  1. 50% peak demand on a K-13 transformer - stay with K-13 but reduce transformer to 112.5 or 75kVA?
  2. Stay with 150kVA but reduce to K-4 or Type EE model?

For the sake of this post, I am disregarding the demolition and replacement cost to do this, and am strictly looking at the cost savings over time given a more "efficient" transformer for this application.

Thanks for the help,

RDL
 
captainwireman said:
Take into account many "efficient" transformers will have a different aic rating and may not be compatible with the existing installation.
Click to expand...
Transformers do not have AIC ratings. Only over current protective devices can have AIC ratings.

Transformers have "per cent impedance" (%Z) which determines the amount of available short circuit amps (SCA) they provide. Device AIC ratings must exceed the available SCA.
 
jim dungar said:
Transformers do not have AIC ratings. Only over current protective devices can have AIC ratings.

Transformers have "per cent impedance" (%Z) which determines the amount of available short circuit amps (SCA) they provide. Device AIC ratings must exceed the available SCA.
Click to expand...

Yes, poorly written. Transformers with a higher efficiency rating can impose a higher available fault current on overcurrent protection and may exceed the SCCR rating for the equipment. Do the calculations as there is a liability issue.
 
RDL914 said:
This is my first post so bear with me. I've been asked to do some very preliminary research into transformer efficiency and determine if utility cost savings can be obtained by swapping out old transformers with new, "energy efficient" transformers. I am not designing a system by any means.

The facility is a 15 floor office building. The existing transformer on a typical floor is 480-208Y/120V, 150kVA, K-13 rated, and the peak demand is estimated at 60kW. The peak demand is not expected to increase.

My understanding of K rated transformers is that they are used when non-linear loads such as florescent lighting and electronics such as computers can damage a standard transformer. However, since the peak demand appears to be 50% or less of the 150kVA, I would think K-13 is overkill, and am wondering if these can be replaced with K-4 or non-K rated but energy efficient transformers.

In short, my question is how should the following variables be considered:
  1. 50% peak demand on a K-13 transformer - stay with K-13 but reduce transformer to 112.5 or 75kVA?
  2. Stay with 150kVA but reduce to K-4 or Type EE model?
Click to expand...

The most important thing you need to do is to determine the hour-by-hour loading profile of your transformers.
The predominant losses in a transformer come from the resistance of the windings (I?R losses), therefore lowering the load effectively improves the efficiency.

Very few transformers are appreciably loaded, this is why the 'new EE transformers' (built under the TP1) were required to assume a loading of only 35% as the target point for efficiency.

In my experience, probably less than .5% of general use transformers need to be anything more than K-1.
 
RDL914 said:
Hello All,

This is my first post so bear with me. I've been asked to do some very preliminary research into transformer efficiency and determine if utility cost savings can be obtained by swapping out old transformers with new, "energy efficient" transformers. I am not designing a system by any means.

The facility is a 15 floor office building. The existing transformer on a typical floor is 480-208Y/120V, 150kVA, K-13 rated, and the peak demand is estimated at 60kW. The peak demand is not expected to increase.

My understanding of K rated transformers is that they are used when non-linear loads such as florescent lighting and electronics such as computers can damage a standard transformer. However, since the peak demand appears to be 50% or less of the 150kVA, I would think K-13 is overkill, and am wondering if these can be replaced with K-4 or non-K rated but energy efficient transformers.

In short, my question is how should the following variables be considered:
  1. 50% peak demand on a K-13 transformer - stay with K-13 but reduce transformer to 112.5 or 75kVA?
  2. Stay with 150kVA but reduce to K-4 or Type EE model?

For the sake of this post, I am disregarding the demolition and replacement cost to do this, and am strictly looking at the cost savings over time given a more "efficient" transformer for this application.

Thanks for the help,

RDL
Click to expand...
Check the total harmonic distortion of your load. The K rating of a transformer is related to it. Higher is the distortion, higher is the K-rating. A K-rated transformer rating can be equal to the total harmonic load. If a derated ordinary transfomer is used to handle harmonic loads, its losses are still higher than a corresponding smaller K-rated transformer.
 
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