Increasing transformer efficiency

Transformers are fundamental components in electrical distribution systems for commercial buildings.


This article has been peer-reviewed.Learning objectives

  • Learn the history of transformer efficiency regulation.
  • Understand how transformer efficiency is optimized.
  • Know how to select a transformer type for specific applications.

The most efficient way to transport power from point A to point B is to use the highest voltage available.Higher voltage equates to lower currents for a constant amount of power. Losses are a result of current and wire resistance (P = I2 * R). Therefore, decreasing current has a much greater return in reducing power losses than increasing wire size to decrease resistance.

A transformer is required at each point along the distribution path where a change in voltage is necessary. Power produced at a utility company power plant will change voltage numerous times via transformers before reaching the end user.

Figure 1: Pad-mounted transformers such as those shown in the photo are used with underground electrical power distribution to convert higher voltages to lower voltages. Courtesy: CFE MediaUtility companies typically use medium voltages (5 kV or 15 kV) to distribute power to customers.Commonly used local distribution voltages are 4,160 V, 12.47 kV, and 13.8 kV, while transmission lines that traverse greater distances use higher voltages such as 69 kV and higher. The voltage may be boosted at the production source to a higher voltage for transmission and then reduced to a medium voltage at a neighborhood utility substation. Voltage is reduced again (typically 480 V) at a utility pad-mounted transformer to provide service to an individual building, then reduced to 208 Y, 120 V via a dry-type transformer in a building electrical room (see Figure 1).

The concept of using the highest voltage available also applies to power distribution within buildings. In large commercial buildings, 480-V, 3-phase, 4-wire power is commonly used to serve large mechanical equipment. Lighting is typically served at 277 V while 120 V power is needed for receptacle loads.

Today, as design teams and building owners strive for high-performance, net zero buildings, maximizing transformer efficiencies is essential. After a transformer is placed in a building and energized, it begins consuming energy and is never turned off. Even when a building is unoccupied and all loads on the transformer are turned off, the transformer itself continues to consume energy 24 hr/day. The losses of a transformer contribute to heat in the building. More efficient transformers require less cooling to maintain a desired room temperature, which in turn saves more energy.

Standards and regulations history

Over time, both industry and government regulatory agencies-National Electrical Manufacturers Association (NEMA) and U.S. Dept. of Energy (DOE), respectively-have worked in tandem to advance the efficiency of transformers. In 1992, the DOE commissioned a study of transformer efficiency. The result of the study revealed that on average, low-voltage dry-type transformers are loaded to only 35% of the nameplate rating. Because equipment is sized for maximum peak demand using the conservative diversification factors allowed by the National Electrical Code (NEC) and loads can fluctuate based on time of day or time of year, the results of the DOE study were not surprising to design engineers. In the 1990s, many transformers were designed for maximum efficiency at much higher loading, when in reality most transformers will never experience 100% load. The 35% load identified by the DOE study provides a clear baseline for improving performance.

The next advance occurred in 2002. NEMA published its TP-1: Guide for Determining Energy Efficiency for Distribution Transformers. This standard defined minimum efficiency levels required for NEMA Class 1 efficiency for low-voltage dry-type transformers, medium-voltage dry-type transformers, and liquid-filled distribution transformers.

For common 3-phase dry-type distribution transformers, the minimum required efficiencies ranged from 97.0% for a 15 kVA transformer to 98.9% for a 1,000 kVA transformer. NEMA also published its TP 2: Standard Test Method for Measuring the Energy Consumption of Distribution Transformers, and the TP 3:Standard for the Labeling of Distribution Transformer Efficiency. These standards specified testing and labeling requirements related to NEMA TP-1 transformers.

Following the publication of the NEMA TP 1 standards, a variety of compliant products became available on the market. Design professionals embraced these standards and included the requirement for NEMA TP 1 compliant transformers, tested to the NEMA TP 2 standard in specifications, shortly after the standards were published.

Then in 2005, the DOE passed the Energy Policy Act of 2005. This legislation required all dry-type transformers rated 600 V or less and manufactured and installed in the U.S. to meet the NEMA TP 1 requirements as of Jan. 1, 2007. Prior to this regulation taking effect, the DOE did not regulate the energy efficiency levels of transformers. The DOE soon followed with regulations for liquid-immersed and medium-voltage dry-type distribution transformers. In 2010, the DOE published a formal Code of Federal Regulation for energy efficiency levels for liquid-immersed and medium-voltage dry-type distribution transformers. The regulation was coded as 10 CFR 431. The next advance occurred in 2009 when NEMA launched its NEMA Premium Efficiency Transformer Program. This required transformers dubbed NEMA Premium to have 30% fewer losses than the DOE 10 CFR 431 regulation, which was still being formalized at that time. Most major transformer manufacturers bought into the program and agreed to make transformers meeting this new NEMA Premium standard available to the marketplace.

At this time, the efficiency levels in the current regulations were based on the 2002 TP-1 standard created by NEMA. With an industry agreed-upon standard in place and manufacturers committed to creating product, it seemed inevitable that the DOE would follow the NEMA Premium standard as it had done with NEMA TP-1.

Table produced by DLR Group Data was obtained from US Government Electronic Code of Federal Regulations and NEMA.However, this was not to be. On April 18, 2013, the DOE published a final ruling (78 FR 23335) amending its 10 CFR 431 energy efficiency standards for low-voltage dry-type, liquid-immersed, and medium-voltage dry-type distribution transformers. The new standards will be placed into effect Jan. 1, 2016. The new efficiency levels are similar to the NEMA Premium levels, but are not a consistent 30% decrease in losses across the board (see "Candidate and trail standard levels" on page xx). For low-voltage dry-type transformers, the percentage decrease in losses varies from 29% to 36% depending on size (see Table1).

It is anticipated that manufacturers will evolve their current NEMA Premium lines to meet the DOE's new 10 CFR 431 requirements while striving to maintain compliance with the NEMA Premium standard. The DOE predicts that the amended energy efficiency standards will save consumers up to $12.9 billion for equipment sold from 2016 to 2045.

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