High-Efficiency Copper-Wound Transformers Save Energy and Dollars

Dry-type transformers, for a variety of reasons, have largely replaced oil-filled units within industrial, commercial and institutional buildings in the USA. However, unlike oil-filled units in the utility grid, little attention is often paid to the energy efficiency of dry-type units. Unlike motors, they have no moving parts to wear out, and so are expected to last 20 years or more.

Efficiency is seldom specified when buying dry-type transformers. Values of 95% or higher are typical, and differences between high- and low-efficiency units are only 1% to 2%, with a significant first-cost premium for the more efficient units.

But first cost is not the last cost of any transformer. Complete life-cycle costs must be carefully examined along with the economics of high-efficiency dry-type transformers.

Life-Cycle Costing

When a lower cost, 98% efficient unit and a higher cost unit at 99% are compared, that means that the losses are actually cut in half.

Conductor losses, also known as coil losses or load losses, should not be overlooked – for one reason because such losses vary by the square of the load. That means a fully loaded transformer has four times the load losses compared with one running at 50% of its design load. In many applications, transformers are heavily loaded. But even at lower load factors, if the units are not efficient (and many are not), losses mount rapidly.

Thus, the continuing costs of transformer losses should be balanced with the savings to be gained from efficient units – savings which go on year after year – quickly paying back the extra first cost.

Other benefits pile up as well. Efficient transformers run cooler, and thus more reliably, because of decreased stress on insulation materials. As a result, they will have a higher overload capacity, an important issue in dry-type transformers. The ultimate result can be units with a smaller kVA rating actually doing the same job, with attendant first-cost savings.

Efficiency vs. Temperature Rise

Efficiency and temperature rise are related, but separate issues. Inefficiency is purely and simply the generation of waste heat- I²R losses in the transformer windings. To the extent heat is generated, the unit is inefficient. The temperature rise, on the other hand, results not only from how much heat is generated, but also from how much is removed.

An inefficient dry-type transformer can, in fact, run cool by incorporating into its design larger air passages or fan-cooling. High-efficiency transformers eliminate the need for these extra cooling costs.

The Economics of Efficiency

The examples below are of high-efficiency, copper-wound transformers designed to achieve an 80°C temperature rise and high efficiency. These are compared to standard, run-of-the-mill units using aluminum, the low-cost winding material, and designed to a 150°C rise.

The 1,500 kVA unit represents a typical, large dry-type transformer used for industrial and some larger commercial applications. In these applications, duty cycles can be quite high. Both 65% and 85% are used in the examples. Paybacks in high-electricity-cost areas can be as fast as one year for the design shown.

For light commercial applications, such as lighting circuits in office buildings, smaller units, such as the 75 kVA example shown, are used. Here, duty cycles would typically be lower. The sample calculations are for 50% and 75% duty cycles. In these applications, however, the customer is likely to pay a higher electricity rate than is the case in industrial applications. But here, again, paybacks can be short – as little as 1.1 years.

Once the unit's first cost premium is paid back, those energy savings continue to accumulate for the decades the transformer will be in service.