Transformers, K-rating, and Efficiency

[RDL914]

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

 

[prc]

First of all 60 kW load is not clear. Transformer load is always considered in kVA terms. Harmonic content of load current is also not clear. A K-13 transformer can handle 13 times more stray losses ( due to harmonics) than a K-1 transformer( which can handle only sinusoidal load), with out exceeding temperature limits. A K-4 unit can handle,in addition to 100 % fundamental, 16 % third harmonic current, 10 % of 5 th harmonic, 7 % of 7th and 5.5 % of 9th harmonic currents.K-13 can handle 200 % harmonic loading compared to K-4 unit. More efficient is a relative term. Today also there are less efficient and more efficient transformers.

 

[jmbelectrical]

Once you reduce the transformer size to 112.5KVA or 75KVA, significant modifications to the electrical distribution system, specifically, the primary-side and secondary-side overcurrent protective device ratings, may be necessary.

Have you considered the use of harmonic-mitigating transformers in lieu of K-rated transformers? Rather than simply being constructed to better withstand a certain degree of harmonic content, these transformers, if I remember correctly, use phase shifting and other techniques to actually improve the harmonic content on the primary-side of the distribution system.

 

[jghrist]

You need to know or estimate the loss factor (average loss over peak loss) to determine energy losses. You need to know what the load and no-load losses are for the existing transformer and for the proposed replacement. You need to know what your electricity rate structure is. With that information, you can calculate the losses with the existing and proposed transformers to compare the cost. You also need to know what the harmonic content of the load current is to determine what K rating you need.

 

[electricuwe]

I'd recommend to start at the point where the energy is used. Maybe you have still fluorescent ballasts with passive ballast or with ballasts without PFC. Exchanging this to state-of-the art fluorescent lighting or even LED lighting will reduce consumption as well as harmonics. Then, in a final step you may consider to replace the old transformer by transformers with lower rated power. Then you can expect savings only due to lower core (no-load) losses. A transformer larger then needed will be always better in regard to copper (load) losses.

 

[Desrod2]

When working for a utility we started a transformers replacement program and although there were some benefits identified replacement cost outweighed the benefit resulting from replacing the transformers with more efficient one. We decided to keep the existing installations as they were but changed the selection guides for the new installations.

You might want to do your study and wait for when you will replace you transformers.
Regards