Transformers: ensuring efficient electrical system design

Electrical engineers can specify high-efficiency, 3-phase, low-voltage, dry-type transformers into facilities, taking into consideration the many parameters of transformer design to create a safe, effective electrical distribution system.


This article is peer-reviewed.

Learning objectives

  • Understand that the transformer is a primary component of power delivery and distribution.
  • Examine the use of 3-phase, dry-type transformers in facility design.
  • Learn the energy efficiency standards that define transformer design and specification.

A key component of power delivery and distribution is the transformer, which is an apparatus for reducing or increasing the voltage of an ac system. It is easier to transmit power at a higher voltage and a lower current. However, the higher voltage is unusable in daily operations. Therefore, it is necessary to transform it to a more usable voltage level for safe operation. It is difficult to get power across long distances at low voltage resulting in a need for reasonably low voltages at points of use.

Transformer on the third floor north of the NetApp Global Dynamic Lab 1 building. Courtesy: CRB/James West PhotographyDepending on where you live relative to a power plant, the number of transformers in the path of power delivery could be quite considerable. There could be a series of step-up, step-down, isolation, or voltage-correction transformers. You might see these in substations as you drive down the highway or see them up on poles as you pass through your neighborhood. After the power reaches your neighborhood, it travels through a transformer located outside, most likely a green box. Your neighbor may or may not be connected to the same one. Most people do not think about who or what is connected to the same transformer until a power outage. They are merely a topic of conversation for one to start thinking about the power distribution system’s construction, distribution, and a vast array of transformation requirements.

There are many types of transformers today, but the most common type is dry-type. Most 3-phase transformers used in commercial and industrial applications are dry-type transformers. This article examines the use of 3-phase, dry-type transformers in facility design and some important things to consider when specifying high-efficiency transformers.

Transformer efficiency standards

Many associations and agencies have been involved in 3-phase transformer efficiency, which has allowed this development in the U.S. to advance considerably since the 1990s. Transformers were, and are still, considered necessary components of an electrical distribution system. However, the “it is what it is” philosophy and no minimum-efficiency standards have been replaced with requirements for efficiency.

The first energy efficiency standard to affect 3-phase transformer design was written by the National Electrical Manufacturers Association (NEMA) and published in 1996 as the TP-1 standard, but was considered voluntary. The U.S. Environmental Protection Agency (EPA) got involved in transformer efficiency in 1999 by creating the Energy Star transformer and enforcing the requirements of the NEMA TP-1 standard. In 2000, the U.S. Department of Energy (DOE) started working on an energy-conservation standard that was finished in 2007. During the same period, the U.S. Congress passed the Energy Policy Act (EPACT) in 2005, providing energy-conservation standards for 3-phase, low-voltage, dry-type distribution transformers. The first DOE standard for 3-phase transformer efficiency was made mandatory in 2007, and the EPA Energy Star transformers ceased to exist because they met the same requirements set forth in EPACT. In 2010, NEMA introduced 3-phase, low-voltage, dry-type, premium transformer standards as the new benchmark for manufacturing requirements. This standard was replaced by a new DOE standard completed in 2013 and put into effect Jan. 1, 2016.

NetApp transformers in the Global Dynamic Lab 1 building. Courtesy: CRB/James West Photography

Low-voltage, 3-phase, dry-type transformer efficiency has always been driven at the manufacturing level. It is up to the manufacturer’s designers to use materials that will meet the requirements. The 1996 NEMA TP-1 standard included requirements for 97% efficiency for smaller transformers (up to 15 kVA) and up to 98.9% efficiency for larger transformers (1,000 kVA or more). The current 2016 DOE requirements for transformer efficiency mandate 97.79% efficiency for smaller transformers and up to 99.28% efficiency for the larger type. These efficiency requirements have forced manufacturers to look at the overall construction and electrical characteristics of all components included in their transformer designs.

Mechanically, the transformer is an apparatus with at least two sets of windings; the materials at the core are iron or complex metal alloys while the windings themselves usually are copper or aluminum wire. Most 3-phase, low-voltage, dry-type transformer designs take into consideration the magnetic properties of the components and even the coil winding geometry to achieve the highest efficiency possible from the transformer. Through these components and their configuration within the transformer, the internal losses have been reduced to meet the DOE requirements set forth today.

For all energy consumed in the U.S., the industrial and commercial sectors combined use the most, according to the Energy Information Administration. These sectors are where design engineers most often get involved in facility construction and distribution, to meet the needs of the operation using multiple voltages.

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