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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 SCREENED OUT OF THE ANALYSIS . . . . . . . . . . . . . . 4-3

4.3.1 Conductor Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3

4.3.2 Core Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

4.4 DESIGN OPTIONS SCREENED OUT OF THE ANALYSIS . . . . . . . . . . . . . . . . . . . 4-5

4.4.1 Silver as a Conductor Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6

4.4.2 High-Temperature Superconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6

4.4.3 Amorphous Core Material in Stacked Core Configuration . . . . . . . . . . . . . . . . 4-7

4.4.4 Carbon Composite Materials for Heat Removal . . . . . . . . . . . . . . . . . . . . . . . . 4-7

4.4.5 High-Temperature Insulating Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

4.4.6 Solid-State (Power Electronics) Technology . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9

4.5 SUMMARY OF DESIGN OPTIONS SCREENED OUT . . . . . . . . . . . . . . . . . . . . . . . 4-9

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 . . . . . . . . . . . . . . . . . . . . . . 4-5

Table 4.5.1 Design Options Screened Out of the Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9
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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 Department will evaluate

and which options will be screened out. The Department 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 (10 CFR Part 430, Subpart C, Appendix A

at 4(a)(4) and 5(b):

 (1) Technological feasibility. Technologies incorporated in commercial products or in

working prototypes will be considered technologically 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 significant 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 will not be considered further.

This chapter discusses all the design options the Department considered for improving the

energy efficiency of distribution transformers, and describes how the Department applied the

screening criteria.

4.2 DISCUSSION OF DESIGN OPTIONS

There are several 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 winding material) and

by modifying the geometric configuration of the core and winding assemblies. 
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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 space they occupy. 

Transformers are designed for a certain temperature rise, resulting from the heat generated by

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 core and winding assemblies, a transformer has other nonelectromagnetic 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 more 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 point and associated

efficiency level, there can be several viable designs that achieve that efficiency level. The

Department found that a wide range of designs and efficiencies are technologically feasible using

common materials, engineering practices, and construction 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 ORNL report number 6847 published July 1996.)1

 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 screening

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 or different core-stacking techniques. Construction 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 controlled 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). 
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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

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.

4.3 DESIGN OPTIONS NOT SCREENED OUT OF THE ANALYSIS

The Department 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 copper 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 resistive

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 transformer 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.
a

 Registered trademark of Metglas, Inc., a wholly-owned subsidiary of Hitachi Metals, Ltd., Tokyo, Japan.

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 Considering the four screening criteria for this technology, the Department did not screen

out aluminum and copper as conductor materials. These materials are in commercial use today,

and DOE therefore found them to be technologically feasible. 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 impacts 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 past 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 grainoriented or domain-refined (i.e., laser or mechanically scribed steels).

Amorphous metal (or METGLASa

) material allows the construction of a low-loss core.

Amorphous metal is extremely 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 some

drawbacks: It 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 density 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 steels, both non-oriented hot-rolled and grainoriented cold-rolled, domain-refined grain-oriented electrical steel, and amorphous material

(wound core designs). The Department considered all of these core materials to be

technologically feasible, as they are used commercially 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 laser-scribed steels are available for use in both stacked- and woundcore configurations. However, at present the application of amorphous material is only a viable

design option in a wound core. Its manufacturers have not been successful in producing an
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amorphous product that can be used in a stacked-core configuration (discussed in section 4.4.3 of

this Chapter).

These core steels, high-silicon electrical steels, both non-oriented hot-rolled and grainoriented 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.

Table 4.3.1 summarizes the design options not screened out of the analysis.

Table 4.3.1 Design Options Not Screened Out of the Analysis

Design Issue Material

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

4.4 DESIGN OPTIONS SCREENED OUT OF THE ANALYSIS

The Department 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
4-6

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. The Department found that the use of silver 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. The Department 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. The Department is

also not aware of any adverse health or safety impacts associated with the use of this conductor

material.

However, the Department 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 during manufacturing process). In addition, due to limited availability,

silver is not feasible to use for mass production on the scale necessary to serve the U.S.

distribution transformer manufacturing industry. 

Thus, the Department screened silver out from further consideration 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 the 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 hightemperature, 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. For 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,

and complex cryogenic support components. Research to overcome these barriers is being

conducted, some of which is funded by the Department.

HTS materials were screened out of further consideration in this analysis because they fail

on two of the four screening criteria. First, the Department 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, the Department
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does not consider HTS technology a viable loss-reduction technology for distribution

transformers now or in the foreseeable future. Second, the Department 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

active cooling or they cease to function. With regard to the third screening criterion, the

Department is not aware of any adverse impacts on customer utility associated with these

materials. Similarly, the Department is not aware of any adverse impacts on health and safety

originating 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 Stacked 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 has not been demonstrated in a working design

by a manufacturer.

The Department screened out stacked core amorphous core material from further

consideration in the analysis. First, the Department is not aware of any working prototypes that

use amorphous core material in a stacked core configuration. Thus, the technological feasibility

of this material has not been demonstrated. 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, the Department is not aware of any adverse impacts on utility or availability to

consumers associated with this material. Similarly, for the fourth criterion, the Department 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 (see U.S. Patent 6,259,347 B1). While these results are impressive, a larger-
4-8

scale prototype distribution transformer has not been demonstrated, and if it were technologically

feasible, it would still be several years away from commercialization.

The Department assessed carbon 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

prototypes that incorporate this technology. The Department was not able to assess whether the

material meets or fails any of the other three screening criteria. Specifically, the Department

cannot determine whether transformers would be practicable to manufacture, install, and 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 impacts 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).

4.4.5 High-Temperature Insulating Material

The transformer industry conducts 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. 

The Department assessed high-temperature insulating materials, and found that the

material failed on the first screening criterion. The Department is not aware of any practical

high-temperature insulating or composite heat removal material, either in prototype form or in

commercial products. The Department was not able 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, the Department 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, the Department 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 (criterion 1).
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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. A small test transformer was built at Purdue University as a modeling

exercise, but no distribution transformer prototype has ever been manufactured using this

technology. 

The Department assessed the feasibility of solid-state (power electronics) technology, and

found that this technology failed on the first screening criterion. The Department is not aware of

any solid-state distribution transformer, either in prototype form or in a commercial product. The

Department was not able to assess whether solid-state transformer technology meets or fails any

of the remaining screening criteria. Due to the lack of a working prototype, the Department is

uncertain whether this technology may have certain issues that make them impracticable to

manufacture, install or service. Similarly, the Department is unable to assess whether there

would be any adverse impacts on consumer utility or availability associated with this technology. 

Finally, the Department is unable to assess whether there would be any adverse impacts on health

and safety aspects of a distribution transformer.

Thus, DOE screened solid-state power electronics transformer technology out of the

analysis due to technological infeasibility (criterion 1).

4.5 SUMMARY OF DESIGN OPTIONS SCREENED OUT

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

in Table 4.5.1. The design options that 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 Analysis

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 feasibility

Solid-State (Power Electronics) Technology Technological feasibility
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REFERENCES

1. 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