Table 1. A dry-type transformer with an assumed constant load of 37.5 kVA.

Transformers are electrical equipment designed to convert one alternating current voltage to another. They are essential in electrical distribution systems and are widely used to raise voltages for transmission over long distances and then reduce the voltage of the power line (4-35 kilovolts) to a voltage suitable and safe for facility equipment (120 to 480 volts).

Dry-type distribution transformers are usually found inside larger commercial/industrial facilities and are generally owned by the facility. There are more than 40 million utility-owned, liquid-filled units currently in service-usually serving smaller facilities. These transformers are usually low-cost, lightweight units, located outside the building.

Efficiency

Table 2. When the load is only 26 kVA, the high-efficiency unit still shows a 1-year payback.

Distribution transformers are very efficient, with losses of less than 0.25% in large units. According to the U.S. Environmental Protection Agency's (EPA) Energy Star program, most large facility distribution transformers convert at least 95% of input power into usable output power. Smaller units have efficiencies of 98% or above. However, when the overall losses of the many transformation steps in a distribution system are considered, these losses can add up. The EPA has collected information about the efficiency of transformers since April 1996 when it introduced specifications for energy-efficient transformers as part of its Energy Star labeling program.

In addition, when facilities close for the day or the weekend and most electrical equipment is turned off, the load on the transformers decreases. The no-load losses in these lightly loaded transformers increase (as a percentage), causing a loss of efficiency.

Transformer losses in power distribution networks can exceed 3% of the total electrical power generated and are estimated to total 140 billion kilowatt-hours (kWh) per year in the U.S. The Energy Star program estimates that converting these transformers to higher efficiency units would reduce wasted electricity by about 61 billion kWh each year.

Reducing losses can increase transformer efficiency. There are two components that make up transformer losses. The first is "core" loss (also called no-load loss), which is the result of the magnetizing and de-magnetizing of the core during normal operation. Core loss occurs whenever the transformer is energized; core loss does not vary with load. Amorphous iron, a new type of core material that reduces core loss, has recently become available. Developed in Europe, amorphous iron is made of rapidly cooled molten metal alloy. Amorphous iron is expensive but reduces core loss to less than 30% those of conventional steel cores. An alternative, less expensive core material is silicone steel, which has losses higher than amorphous iron, but less than standard carbon steel.

The second component of loss is called coil or load loss, because the efficiency losses occur in the primary and secondary coils of the transformer. Coil loss is a function of the resistance of the winding materials and varies with the load on the transformer.

The choice of winding material-copper or aluminum-affects efficiency. Copper is a better electrical conductor than any other metal except silver. Electricity flowing through copper wires meets far less resistance than it does in aluminum or steel wires of the same diameter. Copper wires result in lower electrical losses, which appear as unwanted heat. Engineers call these resistive, or I2R, losses. Use of copper windings minimizes transformer full-load losses and may permit smaller cores, minimizing core (no-load) losses. Another way to reduce transformer losses is to use larger diameter wires that allow current to flow more easily.

Sizing distribution transformers to meet their expected loading also greatly influences transformer efficiency. Greatly oversized transformers can contribute to inefficiency, but when transformers are appropriately matched to their loads, efficiency increases. Core losses continue whenever the transformer is energized; therefore, when the expected loading is variable or unknown, the size selection is a compromise between core loss and coil loss.

Tables 1 and 2 show the effect of proper transformer sizing on losses and the payback associated with more heavily loading a more efficient transformer. The higher efficiency is associated with lower temperature rise (80?C vs. 150?C above ambient), but the temperature rise alone does not always guarantee a more efficient unit.

The total cost of owning and operating a transformer must be evaluated, since the unit will be in service for decades. The only proper method to evaluate alternatives is to request the manufacturer or bidder to supply the load and no-load losses, in watts. Then, simple multiplication can reveal anticipated losses at planned loading levels. Frequently, a small increase in purchase price will secure a unit with lower operating costs. For many applications, very short paybacks are possible.

Until recently electric utilities have accepted the losses from their less efficient distribution transformers. Making the case for utilities to purchase energy-efficient transformers is difficult. The reasons include:

• High first cost

• Costs passed on to the end user

• Other financial considerations

Increased attention is now being focused on the savings in environmental emissions that could become more important than dollar savings. In addition, the U.S. Department of Energy is considering whether legislation should be applied to transformer standards.

Energy Star's web site includes two downloadable files that allow utilities to evaluate transformer options. Go to: yosemite1.epa.gov/ estar/consumers.nsf/content/utility_distribution_transformers.htm, and click on "Utility Distribution."

Promoting Efficiency

Table 3. Criteria are based on a 35% nameplate load and a temperature of 75 degrees C.

The Copper Development Association (CDA) for several years has been promoting a wide-ranging program developed to reduce electric bills and reduce emissions from power plants through electrical-energy efficiency. This program includes promoting energy-efficient transformers.

The Consortium for Energy Efficiency (CEE) is a national, non-profit public benefits corporation that promotes the manufacture and purchase of energy-efficient products and services (www.ceeformt.org /ind/trnsfm/trnsfm-main.php3).

CEE is promoting high-efficiency performance guidelines for low voltage distribution transformers by partnering with Energy Star to adopt the NEMA standard TP-1.

Energy Star also promotes the use of energy-efficient transformers by adopting the NEMA standard TP-1. NEMA (National Electrical Manufacturers Association) has been developing standards for the electrical manufacturing industry for more than 70 years and is one of the leading standards development organizations in the world. In so doing NEMA contributes to an orderly marketplace and helps ensure the public safety by developing technical standards that are in the best interests of the industry and the users of its products.

TP-1

NEMA published TP-1 in 1996. TP-1, a "Guide for Determining Energy Efficiency for Distribution Transformers," is intended for use as a basis for determining the energy efficiency of certain single-phase and three-phase dry-type and liquid-filled distribution transformers and to assist in the proper selection of such equipment. Copies of TP-1 can be ordered from the NEMA website (www.nema.org). The guide lists minimum efficiencies based on transformer size, but has limitations. There are no other consensus standards for transformer efficiency.

The most serious limitation of TP-1 is that it assumes a low load factor (35-50%), which critics say is not representative for industrial environments and is not recommended by manufacturers because of the waste of load capacity. The larger core results in higher losses. The TP-1 minimum standard may be properly applied to utilities and applications where the transformer loading is unknown or highly variable. For many industrial or large commercial applications, where loading is better defined, operating hours are long, or electricity costs are higher than the national average, selecting a transformer of higher efficiency and lower coil loss may be a better economic choice than the minimum standard.

Many object that the TP-1 guideline is too easy to meet and does not push the state of the art in transformer efficiency. Since the standard is for linear loads only, others point to the lack of guidelines for non-linear loads.

Potential Savings

Energy Star estimates that each Energy Star labeled transformer can save $100 to $300 each year (at $0.075/kWh), resulting in an average payback period of 2 to 5 years, depending on transformer size. After this initial payback, this investment will continue to pay back over the rest of its 25-30 year life. Larger commercial facilities with as many as ten low-voltage transformers can save up to $3000 each year.

According to the Oak Ridge National Laboratory's Report 6925, the cumulative energy saving potential nationally is enormous: over 2.5 quadrillion British thermal units (BTU) of primary energy could be saved if every added transformer were at least as efficient as the Energy Star minimum, and over 10 quadrillion BTUs if the highest available efficiency levels were adopted. (1 quadrillion Btu = 2.93 x 1011 kWh)

Energy Star's web site includes a transformer efficiency calculator that allows engineers and building personnel to quickly evaluate options by comparing efficiencies and operating costs of Energy Star transformers with other models (yosemite1.epa.gov/ESTAR/consumers.nsf/content/comm_indust_transformers.htm). Click on "Calculate my Savings."

Product Criteria

Over 20 manufacturers produce Energy Star listed transformers. Key product criteria for Energy Star labeled single-phase and three-phase commercial and industrial transformers is shown in table 3.

Massachusetts and Minnesota have passed legislation that adopts the NEMA TP-1 standard as the minimum efficiency guideline for the installation of new equipment. Massachusetts adopted TP-1 in January 2000. Minnesota's building code that mandates TP-1 went into effect in July 1999. The State of Wisconsin Master Specifications adopted TP-1 in March 2000.

California AB 970 directs the California Energy Commission to amend the current Appliance Energy Efficiency Standards (including transformers) at the earliest feasible date. The principle reason is to respond to growth trends in peak demand for electricity that has strained the adequacy and reliability of the California electrical system.

The Energy Policy Act of 1992 directed the Department of Energy (DOE) to consider minimum efficiency standards when they were technically feasible, economically justified, and saved energy. DOE made that determination in 1997 but has not adopted a standard yet. NEMA and CEE are urging the adoption of their TP-1 standard, while CDA is promoting higher loading levels (around 75%) and other changes to increase efficiency levels.

There are several barriers preventing the use of higher efficiency transformers. Many end users are not familiar with energy-efficient transformers and the economics of their use. Electrical contractors purchase most dry-type transformers for end users and they do not pay the facility electric bills, nor do they usually plan ahead to order high-efficiency units that are usually not stocked at electrical distributors. In addition, there is usually a high mark-up for the high-efficiency units.

Energy users should consider energy-efficient transformers for their facilities and implement either a minimum standard for the purchase of their distribution transformers by specifying TP-1 or adopt the CDA recommendations for higher efficiency. A cost-benefit analysis will show what is the best investment. eun

About the author: John Fetters, CEM, CLEP, is Energy User News Fundamentals of Energy Series editor. He company, Effective Lighting Solutions, addresses the lighting energy needs of commercial, industrial, and institutional end users.

This article was developed with assistance from the Copper Development Association (www. copper.org) and the U.S. EPA's Energy Star Transformers Program (yosemite1.epa.gov/estar/consumers.nsf/content/transformers_gateway_page.htm