4

- i CHAPTER 4. SCREENING ANALYSIS

TABLE OF CONTENTS

4.1 INTRODUCTION

................................ ................................ ............................... 4

- 1 4.2 DISCUSSION OF DESIGN OPTIONS

................................ .............................. 4

- 1 4.3 DESIGN OPTIONS NOT S CREENED OUT OF THE A NALYSIS

................. 4

- 3 4.3.1 Conductor Materials

................................ ................................ ............................. 4

- 3 4.3.2 Core Materials

................................ ................................ ................................ ...... 4

- 4 4.3.3 Core Deactivation Technology

................................ ................................ ............ 4

- 5 4.4 DESIGN OPTIONS SCREE NED OUT OF THE ANALY SIS

.......................... 4

- 6 4.4.1 Silver as a Conductor Material

................................ ................................ ............ 4

- 6 4.4.2 High

- Temperature Superconductors

................................ ................................ .... 4

- 7 4.4.3 Amorphous Core Material in Stacked Core Configuration

................................ . 4

- 7 4.4.4 Carbon Composite Materials for Heat Removal

................................ .................. 4

- 8 4.4.5 High

- Temperature Insulating Material

................................ ................................ 4

- 9 4.4.6 Solid

- State (Power Electronics) Technology

................................ ....................... 4

- 9 4.4.7 Nanotechnology Composites

................................ ................................ ............. 4

- 10 4.5 SUMMARY OF DESIGN OP TIONS SCREENED OUT

................................ 4

- 11

LIST OF TABLES

Table 4.2.1 General Loss

- Reduction Interventions for Distribution Transformers

........ 4

- 3 Table 4.3.1 Design Options Not Screened Out of the Analysis

................................ ...... 4

- 6 Table 4.5.1 Design Options Screened Out of the Analysis

................................ ........... 4

- 114

- 1 CHAPTER 4. SCREENING ANALYSIS

4.1 INTRODUCTION

The purpose of the screening analysis is to identify design options that improve distribution transformer efficiency and to determine which options the U.S. Department of Energy (DOE) will evaluate and which options will be screened out. DOE consults with industry, technical experts, and other interested parties in developing a list of design options for consideration. It then applies the following set of screening criteria to determine which design options are unsuitable for further consideration in the rulemaking ( Title of the Code of Federal Regulations, Part 430 ( 10 CFR Part 430 )

, s ubpart C, a ppendix A at 4(a)(4) and 5(b):

(1) Technological feasibility. Technologies incorporated in commercial products or in working prototypes will be considered technolog ically feasible;

(2) Practicability to manufacture, install, and service. If mass production of a technology in commercial products and reliable installation and servicing of the technology could be achieved on the scale necessary to serve the relevant market at the time of the effective date of the standard, then that technology will be considered practicable to manufacture, install, and service.

(3) Adverse impacts on product utility or product availability. If a technology is determined to have signific ant adverse impact on the utility of the product to significant subgroups or consumers, or result in the unavailability of any covered product type with performance characteristics (including reliability), features, sizes, capacities, and volumes that are substantially the same as products generally available in the U.S. at the time, it will not be considered further.

(4) Adverse impacts on health or safety. If it is determined that a technology will have significant adverse impacts on health or safety, it wi ll not be considered further.

This chapter discusses all the design options DOE considered for improving the energy efficiency of distribution transformers, and describes how DOE applied the screening criteria.

4.2 DISCUSSION OF DESIGN OPTIONS

There are sever al well

- established engineering practices and techniques for improving the efficiency of a distribution transformer. A transformer design can be made more energy

- efficient by improving the materials of construction (e.g., better quality core steel or wind ing material) and by modifying the geometric configuration of the core and winding assemblies.

Core and winding losses are not independent variables of transformer design; they are linked to each other by the heat they generate and by the physical spac e they occupy. Transformers are designed for a certain temperature rise, resulting from the heat generated by4

- 2

transformer losses during operation. The upper boundary on the temperature rise is a design constraint, based on industry practice and standards (Institute of Electrical and Electronics Engineers ( IEEE ) C57.12.00 and C57.12.01). If this temperature limitation is exceeded, it will accelerate the aging process of the insulation and reduce the operating life of the transformer.

In addition to the c ore and winding assemblies, a transformer has other non

electromagnetic elements that may constrain the design of a transformer: the electrical insulation, insulating media (oil for liquid

- immersed transformers and air for dry

- type transformers), and the enclosure (the tank or case). Once the insulation requirements are set, a transformer design can vary both materials and geometry to reduce the losses.

Making a transformer more efficient (i.e., reducing electrical losses) is a design tradeoff between mo re expensive, lower

- loss materials, and the value a customer attaches to those losses. For a given efficiency level, the core and winding losses are generally inversely related

— reducing one usually increases the other. Additionally, at a given loading p oint and associated efficiency level, there can be several viable designs that achieve that efficiency level. DOE found that a wide range of designs and efficiencies are technologically feasible using common materials, engineering practices, and construct ion techniques (see Chapter 5).

Table 4.2

. 1 presents a general summary of the loss

- reduction approaches from which transformer design engineers may choose to build more energy

- efficient transformers. (This table was adapted from Table 2.2 in Oak Ridge National Laboratory ( ORNL ) report number 6847 published July 1996.) i For most of these approaches, there are clear tradeoffs between no

load (core) losses, load (winding) losses, and price.

Some of the approaches presented in Table 4.2

. 1 refer to specific technologies (e.g., lower

- loss core materials, lower

- loss conductor materials), while other approaches refer to transformer geometry modifications (e.g., core or conductor cross

- sectional area). This screenin g analysis considers the materials and technologies that may be used in transformer construction, but does not consider geometry or construction modifications such as a larger

cross

- sectional area

, different core

- stacking techniques , or symmetric cores . C onstruction methods and geometric modifications are inherent to the design and manufacturing process, and therefore are not a technology or design option considered in the screening analysis. These construction methods and geometric modifications are cont rolled by the transformer engineer and/or software design tool to improve the efficiency of resultant designs. Thus, they are applied to the designs prepared in the engineering analysis (see Chapter 5).4

- 3

Table 4.2

. 1 General Loss

- Reduction Interventions for Distribution Transformers

Loss

- Reduction Interventions No

- Load Losses Load Losses Effect on Price

Decrease Core Losses Use lower

- loss core materials Lower No Change * Higher

Decrease flux density by increasing core

cross

- sectional area Lower Higher Higher

Decrease flux density by decreasing volts/turn Lower Higher Higher

Decrease flux path length by decreasing conductor cross

- sectional area Lower Higher Lower

Use 120 ̊ symmetry in three

- phase cores** Lower No C hange TBD

Decrease Coil Losses Use lower

- loss conductor materials No Change Lower Higher

Decrease current density by increasing conductor cross

- sectional area Higher Lower Higher

Decrease current path length by decreasing

core cross

- sectional area Higher Lower Lower

Decrease current path length by increasing volts/turn Higher Lower Lower

* Amorphous

- core materials would result in higher load losses because flux density drops, requiring a larger core volume. ** Sometimes referred to as a ―hexa

- transformer‖ design.

4.3 DESIGN OPTIONS NOT S CREENED OUT OF THE A NALYSIS

DOE considers all distribution transformer design options currently in use by distribution transformer manufacturers to be viable. Viable design options include different conductor materials for coils and core materials.

4.3.1 Conductor Materials

Aluminum and cop per are used in current distribution transformer designs and are available for use in standard wire sizes and foils. When the two materials are applied in exactly the same manner, copper has a higher electrical conductivity and about 40 percent lower resi stive losses than aluminum. Compared to copper, aluminum is easier to form and work mechanically, and can be less expensive. By utilizing aluminum conductor material at a lower current density (i.e., larger conductor cross

- sectional area), aluminum trans former windings can be built with essentially the same load losses as copper. However, aluminum conductors increase core losses due to their larger core frames, necessitated by the larger winding space (―core window‖) through which the windings must pass. It is common for an efficient design option to have copper in the high

- voltage (HV) windings and aluminum at a lower current density in the low

voltage (LV) windings. In these LV windings, aluminum can be used in the form of flat, rolled foils to reduce eddy current losses.

Considering the four screening criteria for this technology, DOE did not screen out aluminum and copper as conductor materials. These materials are in commercial use today, and4

- 4

DOE therefore found them to be technologically feasi ble. They are obviously practicable to manufacture, install, and service because they have been used in mass production for many years and are expected to continue to be the primary winding materials for the foreseeable future. There are no adverse impac ts on consumer utility or reliability associated with the use of these conductor materials. Finally, there are no additional adverse impacts on health or safety associated with the use of these winding materials.

4.3.2 Core Materials

Transformer cores in the p ast had relatively high losses, since they were fabricated from thick laminates of non

- oriented, low

- silicon, magnetic steels. Modern cores are made with steels that incorporate silicon (approximately 2

– 3 percent) and trace amounts of other elements, are cold

- rolled to thinner laminations, have improved laminar insulation, and may also be grain

oriented or domain

- refined (i.e., laser or mechanically scribed steels).

Amorphous metal material allows the construction of a low

- loss core. Amorphous metal is ex tremely thin, has high electrical resistivity, and has little or no magnetic domain definition. Cores made from this material can exhibit 60

– 70 percent lower core losses than one made of conventional steels. However, amorphous metal material does have so me drawbacks: i t saturates at a lower flux level of 1.57 Tesla versus 2.08 Tesla for conventional materials, and it has higher excitation requirements. Amorphous metal material is also fragile and requires special handling during the construction process . Additionally, these designs cannot be ―packed‖ as effectively into the winding window, causing the designs to have a space factor of 85 percent versus 95

– 98 percent for steel core materials, which increases losses. The net effect of the lower flux dens ity and higher space factor is a larger core with greater winding (conductor) losses and higher production costs.

The core steels considered in this screening analysis are all those found in commercial use today. These include high

- silicon electrical s teels, both non

- oriented hot

- rolled and grain

oriented cold

- rolled, domain

- refined grain

- oriented electrical steel, and amorphous material (wound

- core designs). DOE considered all of these core materials to be technologically feasible, as they are used co mmercially today (or in the past) by distribution transformer manufacturers at varying flux levels and lamination thicknesses. These commercially available high

- silicon, cold

rolled transformer steels, nominally designated M2

- M6, and domain

- refined or las er

- scribed steels are available for use in both stacked

- and wound

- core configurations. However, at present the application of amorphous material is only a viable design option in a wound core. No manufacturers currently produce an amorphous product that can be used in a stacked

- core configuration (discussed in section 4.4.3 of this c hapter).

These core steels, high

- silicon electrical ste els, both non

- oriented hot

- rolled and grain

oriented cold

- rolled, domain

- refined grain

- oriented, and amorphous material (wound core designs), are considered practicable to manufacture, install, and service, since they are core materials that are being used or that have been used by the distribution transformer industry. There are no known adverse impacts on consumer utility or reliability, and no known adverse impacts on health or safety associated with these core materials.4

- 5

4.3.3 Core Deactivation Technology

C ore deactivation technology employs a system of smaller transformers to replace a single, larger transformer. For example, three transformers sized at 25 k ilovolt

- ampere ( kVA ) and operated in parallel could replace a single 75 kVA transformer. The smalle r transformers that compose the system can then be activated and deactivated using core deactivation technology based on the loading demand.

Winding losses are proportionally smaller at lower load factors, but for any given current, a smaller transforme r will experience greater winding losses than a larger transformer. As a result, those losses may be more than offset by the smaller transformer’s reduced core losses. As loading increases, winding losses become proportionally larger and eventually outweig h the power saved by using the smaller core. At that point, the control unit (which consumes little power itself) switches on an additional transformer, reducing winding losses at the cost of additional core losses. The control unit knows how efficient eac h combination of transformers is for any given loading, and is constantly monitoring the unit’s power output so that it will use the optimal number of cores. In theory, there is no limit to the number of transformers that may be paralleled in this sort of system, but cost considerations would imply an optimal number.

DOE had the opportunity to speak with a company that is developing a core de activation technology. Noting that many dry

- type transformers are operated at very low loadings a large percentage o f the time (e.g., a building at night), the company seeks to reduce core losses by replacing a single, traditional transformer with two or more smaller units that could be activated and deactivated in response to load demands. A special unit controls whic h transformers are activated and deactivated in real

- time in response to the load demands.

While core deactivation could save energy over a real world loading cycle, those savings might not be represented in the current DOE test procedure. Presently, the test procedure specifies a single loading point of 50 percent for liquid

- immersed and medium

- voltage dry

- type transformers, and 35 percent for low

- voltage dry

- type. The real gain in efficiency for this technology is at loading points below the root mean square (RMS) loading specified in the test procedure, where some transformers in the system could be deactivated. At loadings where all transformers are activated, which may be the case at the test procedure loading, the combined core and coil losses of t he system of transformers could exceed those of a single, larger transformer. This would result in a lower efficiency for the system of transformers compared to the single, larger transformer. Therefore, DOE proposes allowing a weighted

- average loss calc ulation using various loadings to capture the energy savings potential of core deactivation technology. Further detail can be found in chapter 2 and chapter 5 of the technical support document.

Table 4.3

. 1 summarizes the design options not screened out of the analysis.4

- 6

Table 4.3

. 1 Design Options Not Screened Out of the Analysis

Design Issue Materi al

Conductor Materials for Coils Aluminum (wire and sheet) Copper (wire and sheet)

Core Materials Cold

- Rolled High Silicon (CRHiSi) Steel
CRHiSi Domain

- Refined Steels Amorphous Materials in Wound Core Core Deactivation Technology ( N ot applicable)

4.4 DESIGN OPTIONS SCREE NED OUT OF THE ANALY SIS

DOE screened out the following design options from further consideration because they do not meet the screening criteria:

1. Silver as a conductor material

2. High

- temperature superconductors

3. Amorphous core material in stacked core configuration

4. Carbon composite materials for heat removal

5. High

- temperature insulating material

6. Solid

- state (power electronics) technology

7. Nanotechnology Composites

4.4.1 Silver as a Conductor Material

The electrical conductivity of silver exceeds that of copper, aluminum, and other normal metals at room temperature (25 ̊ Celsius). However, silver has a lower melting point, a lower tensile strength, and limited availability. DOE found that the use of si lver as a conductor is technologically feasible, since distribution transformers with silver windings were built during World War II because of a war

- time shortage of copper. DOE believes the use of silver as a conductor would not have any adverse impacts on consumer utility or reliability, as it can readily replace copper or aluminum in this application. DOE is also not aware of any adverse health or safety impacts associated with the use of this conductor material.

However, DOE screened out silver as a conductor material because it is impracticable to manufacture, install, and service. Silver conductor designs are constrained by lower operating temperatures (adding to manufacturing complexity) and lower tensile strength (material can easily break durin g manufacturing process). In addition, due to limited availability, silver is not4

- 7

feasible to use for mass production on the scale necessary to serve the U.S. distribution transformer manufacturing industry.

Thus, DOE screened silver out from further c onsideration as a conductor material in the analysis due to its impracticability to manufacture, install, and service (criterion 2).

4.4.2 High

- Temperature Superconductors

A new class of high

- temperature superconducting (HTS) materials was discovered in 1987. These new materials become superconducting at temperatures close to that of liquid nitrogen, a readily available coolant that is considerably less expensive than liquid helium, the coolant for the previous generation of superconducting materials. After th e discovery of these materials, research programs were launched worldwide to explore the use of superconducting material in power transformers. However, the use of superconductors, both low

- and high

temperature, in transformer manufacturing has proven to be an elusive goal. Low

- temperature superconductors (liquid helium

- cooled) are physically possible but not feasible for commercial use, since these units are often unable to return to the superconducting state following a high fault current condition. F or HTS (liquid nitrogen

- cooled), a few demonstration power transformers have been built, but a prototype distribution transformer has not been constructed. Design constraints include unique conductors, unacceptable alternating current variation losses,

an d complex cryogenic support components. Research to overcome these barriers is being conducted, some of which is funded by DOE

. HTS materials were screened out of further consideration in this analysis because they fail on two of the four screening crit eria. First, DOE does not consider HTS materials to be technologically feasible because a HTS distribution transformer has never been built. Additionally, due to technical issues associated with HTS power transformers, DOE does not consider HTS technolog y a viable loss

- reduction technology for distribution transformers now or in the foreseeable future. Second, DOE does not consider HTS materials to be practicable to manufacture because they are extremely brittle (built of ceramic composites), are orders of magnitude more expensive than conventional conductor material, and are not mass

- produced in a manner that would meet the demands of today’s distribution transformer market. Furthermore, they are not reliable in service because they require continuous a ctive cooling or they cease to function. With regard to the third screening criterion, DOE is not aware of any adverse impacts on customer utility associated with these materials. Similarly, DOE is not aware of any adverse impacts on health and safety or iginating from the use of HTS materials.

Thus, DOE screened HTS materials out of the analysis because of technological infeasibility (criterion 1) and impracticability to manufacture, install, and service (criterion 2).

4.4.3 Amorphous Core Material in Stack ed Core Configuration

As discussed in section 4.3.2 , amorphous material is considered a viable core material in a wound

- core configuration. However, stacked amorphous core material is not presently a viable design option for distribution transformers, and is not currently used by any manufacturers

.4

- 8

DOE s creened out stacked core amorphous core material from further consideration in the analysis. First, DOE is not aware of any working prototypes that use amorphous core material in a purely stacked core configuration. Thus, the technological feasibility of this material has not been demonstrated. DOE is aware of at least one manufacturer that utilized a variation of an amorphous core in a stacked core configuration. This patented design process involved joining multiple amorphous strips together. The pro cess was not economically feasible in the United States, and is not currently used by any U.S. manufacturers.

Second, the material has not demonstrated its practicability with respect to manufacturing, and therefore cannot be assessed as to its ability to meet the demand of mass production nor demonstrate its reliability in service. Considering the third criterion, DOE is not aware of any adverse impacts on utility or availability to consumers associated with this material. Similarly, for the fourth crit erion, DOE is not aware of any adverse impacts on health and safety from the use of amorphous core material in stacked core configuration.

Thus, DOE screened amorphous core materials in stacked core configuration out of the analysis due to technological infeasibility (criterion 1) and impracticability to manufacture, install, and service (criterion 2).

4.4.4 Carbon Composite Materials for Heat Removal

A new technology that may prove effective in future transformer designs is the use of carbon fiber composites for heat removal. These materials offer good heat conduction and electrical insulation performance. The U.S. Naval Research Laboratory built small (less than 1 kVA), high

- frequency transformers with this technology and demonstrated a 35 percent size and core loss reduction. While these results are impressive, a larger

- scale prototype distribution transformer has not been demonstrated, and if it were technologically feasible, it would still be several years away from commercialization.

DOE assessed carbo n composite materials for heat removal from distribution transformers, and found the material failed the first screening criterion. These materials for heat removal failed the first screening criterion because there are no commercial products or working
p rototypes that incorporate this technology. DOE was not able to assess whether the material meets or fails any of the other three screening criteria. Specifically, DOE cannot determine whether transformers would be practicable to manufacture, install, an d service with this new material, since the application of the technology in a distribution transformer design has not been determined. Similarly, any potential adverse impacts on consumer utility or availability cannot be assessed, and any adverse impact s on health and safety cannot be determined at this time.

Thus, DOE screened carbon composite materials for heat removal out of the analysis due to technological infeasibility (criterion 1).4

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4.4.5 High

- Temperature Insulating Material

The transformer industry c onducts research and development on insulating materials. While potentially improving dielectric performance, industry studies this technology to create an electrical insulation that can withstand higher operating temperatures, and to create an electrical insulation that conducts heat more effectively out of the core

- coil assembly. Increasing electrical insulation performance would result in smaller effective core and coil volumes, and therefore reduce operating losses.

DOE assessed high

- temperature in sulating materials, and found that the material failed on the first screening criterion. DOE is not aware of any practical high

- temperature insulating or composite heat removal material, either in prototype form or in commercial products. DOE was not abl e to assess whether the material meets or fails any of the other three screening criteria. Transformers are built today with standard grades of insulation (up to 220 ̊ Celsius); however, it is uncertain whether higher temperature materials may have certain issues that make them impracticable to manufacture, install, or service. Similarly, DOE is unable to assess whether there would be any adverse impacts on consumer utility or availability due to the lack of a working prototype. Finally, DOE is unable to assess whether there would be any adverse impacts on health and safety aspects of a distribution transformer because of this material.

Thus, DOE screened high

- temperature insulating materials out of the analysis due to technological infeasibility (criteri on 1).

4.4.6 Solid

- State (Power Electronics) Technology

The application of solid

- state (power electronics) technology to transformers is in the early stages of research. DOE is aware that s mall test transformers have been built for research to assess the techn ology

, but no commercial distribution transformer product offering has ever been manufactured using this technology. This is largely due to the fact that the designs have been prohibitively expensive and less

- efficient than a standard transformer design.

Solid

- state technology has not achieved the same efficiency levels as standard transformer designs, and the designs come at a high cost. The electronic transformer functionally consists of a high frequency chopper operating at typically 20 kilohertz ( kHz ) , a high frequency step

- down transformer at the chopping frequency, and a power frequency modulator at the 60 Hz frequency with a large commutating capacitor. Fundamentally

, there must be a minimum of two sets of power electronic devices, one at the sour ce side (high voltage primary ) and one at the load side (low voltage secondary ). The forward voltage drop in each power switching device is a minimum of 1.0 volt. The significant currents passing through each device result in very high losses. Hence, ev en before the inefficiencies of the high frequency magnetic components are considered , the power electronic devices consume more p ower than the total losses of conventional transformers. High

- frequency magnetic losses are not much lower than low

frequency magnetic losses. This makes the total loss higher than what can be achieved with conventional

, low

- frequency magnetics.4

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Additionally, it would not be practicable to manufacture transformers using this technology on the scale necessary to serve the distr ibution transformer market. A manufacturer wishing to use the technology would need an entirely new manufacturing facility to handle this unique design. The manufacturer would need e lectronic circuit cards for the signal electronics, wave soldering, alum inum heat sinks, power electronic semiconductors, sintered cores, and unique winding equipment

. Ferrite magnetic core materials are also required instea d of silicon iron sheeted cores.

DOE assessed the feasibility of solid

- state (power electronics) te chnology, and found that this technology failed on the first and second screening criteria

. DOE is not aware of any solid

state distribution transformer s that can achieve improvements in efficiency , either in prototype form or in a commercial product. DO E also does not believe the technology would be practicable to manufacture on the scale necessary to serve the distribution transformer market. DOE was not able to assess whether solid

- state transformer technology meets or fails any of the remaining scree ning criteria. DOE is unable to assess whether there would be any adverse impacts on consumer utility or availability associated with this technology. Finally, DOE is unable to assess whether there would be any adverse impacts on health and safety aspect s of a distribution transformer.

Thus, DOE screened solid

- state power electronics transformer technology out of the analysis due to technological infeasibility and practicability to manufacture, install, and service ( criteria 1 and 2 ).

4.4.7 Nanotechnology Composites

DOE understands that the nanotechnology field is actively researching ways to produce bulk material with desirable properties on the molecular scale. Some of these materials may have high resistivity, high permeability, or other properties that make them attractive for use in electrical transformers. DOE knows of no current commercial efforts to employ these materials in distribution t ransformers and no prototype designs using this technology.

DOE assessed the feasibility of nanotechnology composites, and found that this technology failed on the first screening criterion. DOE is not aware of any distribution transformer using nanotech nology composites, either in prototype form or in a commercial product. DOE was not able to assess whether nanotechnology composite transformers meet or fail any of the remaining screening criteria. Due to the lack of a working prototype, DOE is uncertai n whether this technology may have certain issues that make it impracticable to manufacture, install or service. Similarly, DOE is unable to assess whether there would be any adverse impacts on consumer utility or availability associated with this technol ogy. Finally, DOE is unable to assess whether there would be any adverse impacts on health and safety aspects of a distribution transformer.

Thus, DOE screened nanotechnology composites out of the analysis due to technological infeasibility (criterion 1)

.4

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4.5 SUMMARY OF DESIGN OP TIONS SCREENED OUT

Those design options that DOE screened out from further consideration are listed below in Table 4.5

. 1 . The design options th at DOE did not screen out of the analysis are listed in Table 4.3

. 1

.
Table 4.5

. 1 Design Options Screened Out of the Ana lysis

Design Option Excluded Screening Criteria
Silver as a Conductor Material Practicability to manufacture, install, and service High

- Temperature Superconductors Technological feasibility; Practicability to manufacture, install, and service Amorphous Core Material in Stacked Core Configuration Technological feasibility; Practicability to manufacture, install, and service Carbon Composite Materials for Heat Removal Technological feasibility High

- Temperature Insulating Material Technological feasibil ity
Solid

- State (Power Electronics) Technology Technological feasibility ; Practicability to manufacture, install, and service Nanotechnology Composites Technological feasibility4

- 12 i ORNL, 1996. Oak Ridge National Laboratory, Determination Analysis of Energy Conservation Standards for Distribution Transformers, P.R. Barnes, J.W. Van Dyke, B.W. McConnell, S. Das, ORNL report number 6847, Oak Ridge National Laboratory, Oak Ridge, Tennessee. Published July 1996. Available online at:

www.eere .energy.gov/buildings/appliance_standards/ commercial/dist_transformers.html