Selecting and sizing transformers to achieve energy efficiency

Electrical engineers must know when and where to specify dry-type transformers. Energy efficiency guides play a vital role in this process.


This article is peer-reviewed.Learning objectives

  • Learn to select and specify dry-type power transformers.
  • Apply knowledge of cost and payback of transformers to their selection and specification.
  • Outline all codes, standards, and regulations for power transformers.

Transformers are integral components of any power distribution system. As a consulting engineer, selecting and specifying the proper transformer for the operation is an essential part of electrical system design. The focus of this article is on dry-type power distribution transformers and how to select the right transformer for the job.

Table 1: As the K-factor of a transformer increases, the better it is built to withstand nonlinear loading and related harmonics in the electrical system. This table provides an overview of K-factor ratings and their intended load type. Courtesy: RTM Engineering Consultants

There are many things to consider when specifying a dry-type power distribution transformer (see Figure 1). Weighing each of the options to select the right transformer for the job can be a balancing act. The efficiency of a transformer is a critical design component, and understanding the physics surrounding efficiency is important. Key factors to consider that are related to efficiency include transformer loading, the presence of system harmonics, K-factor, and temperature rise. Another major movement in the world of transformer efficiency centers on the U.S. Department of Energy (DOE) federal efficiency regulations. As regulations become stricter, and as manufacturers adopt and adapt to these new regulations, engineers should explore how transformers are evolving to keep up with these standards. Lastly, the payback and upfront cost of transformers are aspects a consulting engineer should review when selecting a more energy-efficient transformer.

Transformer build and efficiency

When considering a transformer selection, it’s important to understand exactly how transformers work and what contributes to their efficiency. This is the first component of a quality product specification. When you understand the physics behind how a transformer operates and what makes it efficient, this gives you the ability to constructively review the product specification and compare it against others. In this day and age, manufacturers are already up against rigid federal guidelines in terms of producing efficient transformers. Understanding the basic principles behind transformer build and operation is a fundamental building block for making the correct transformer selection.

A sign of inefficiency when it comes to transformer operation is the presence of excess heat. Any heat generated by a transformer is a direct result of inefficiency and losses within the transformer. Some of the key contributing factors to transformer inefficiency are conductor and core losses.

Conductor losses are attributed to lack of winding efficiency. The windings are typically made up of either copper or aluminum. The construction material is certainly a consideration, as it relates to conductor losses. While copper conductors have better current-carrying properties, aluminum conductors can closely match the current-carrying properties of copper when they are sized properly. The weight of copper as compared with aluminum can be more than three times greater, but the weight of the copper is offset by its current-carrying abilities. When comparing the two, strictly on a weight basis, aluminum has better current-carrying capacity than copper. This is an important consideration when it comes to transformer selection. Most manufacturers offer transformers with copper or aluminum windings, providing the same efficiency ratings, while the copper-wound transformers can be more expensive given the higher comparative cost of copper.

The losses attributed to winding resistance also are critical components to the efficiency of a transformer. While no conductor is ever 100% efficient, properly sized winding conductors play a huge role in transformer efficiency. Manufacturers can perform testing as well as properly size the conductors based on and around a transformer specification, but it is also the role of the consulting engineer to understand these specifications and how the transformer was constructed and designed to perform under certain conditions.

When it comes to core losses, the main factors are flux leakage, eddy currents, and excitation. Flux leakage is the magnetic flux created by the primary transformer winding that does not pass over to the secondary winding. Transformer manufacturers work to limit the amount of flux leakage in their transformer designs. The winding and core manufacturing processes can play a role in limiting flux leakage.

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