Transformer Loss Evaluation

PDS Notes – Vol. 2, No. 1


Transformer Loss Evaluation


Transformers are not 100% efficient. In other words, for every kWh of energy that goes into a transformer, some of it is lost as heat; the balance is delivered to the load. Modern transformers typically operate at efficiencies of 98 or 99%. The assumption is that this is very good and that all transformers are the same. Actually, this is not the case. Today’s transformer efficiencies are better than transformers manufactured 20, 30, or more years ago. The losses a transformer generates are still a real cost to your utility. The costs resulting from the transformer’s losses are present from the day it is energized until it is retired from service. Therefore, it pays to compare or more importantly, evaluate the cost of losses for your utility’s operation. If you compare two transformers with similar costs and losses, you can find significant long-term savings by purchasing a more efficient design. The types of losses associated with transformers include:

  • No load loss or iron loss – these are present when the transformer is energized.
  • Load loss or winding loss, these vary with the load on the transformer secondary.
  • Auxiliary losses – these are the losses associated with running pumps, fans, and other accessories.

For the typical distribution transformer installation, the cost of losses is greater than half of a transformer’s total owning and operating cost over the life of the installation. Consider a simple example; for a 25kVA overhead transformer with 100 watts NL and 350 watts load. If this transformer is loaded at 70% of nameplate and with energy costs of 5¢/kWH and demand costs of $16.6/kW/month, it will cost a utility $173 per year to operate due to transformer losses. If you add this up over 30 or 40 years of service, it is a very significant cost – much greater than the cost of the transformer.

Some organizations stopped evaluating losses when the DOE increased the efficiency requirements on distribution transformers. However, loss evaluation is an evaluation factor each utility needs to look at based on their individual cost parameters. These include energy cost, demand cost, loading practices, years of expected service, etc. Utilities should not accept a DOE transformer as the best buy for their system. Transformers that meet the DOE efficiency criteria provide the minimum efficiency values available for transformers. There are more efficient options available. For your utility, you need to look at your operating costs, and then determine the most cost effective solution in terms of transformer cost and efficiency for your operation. You need to make sure you are making the best decision on transformers purchased for your utility by evaluating transformer losses.

PDS, Inc. has the experience and expertise to assist you in updating or developing a distribution transformer loss evaluation program. Please feel free to call with any questions you might have.


PDS, Inc.