4-i

CHAPTER 4. SCREENING ANALYSIS

TABLE OF CONTENTS

4.1 INTRODUCTION ............................................................................................... 4-1

4.2 DISCUSSION OF TECHNOLOGY OPTIONS .................................................. 4-1

4.3 TECHNOLOGY 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 TECHNOLOGY OPTIONS SCREENED OUT OF THE ANALYSIS .............. 4-5

4.4.1 Silver as a Conductor Material ............................................................................ 4-5

4.4.2 High-Temperature Superconductors .................................................................... 4-5

4.4.3 Amorphous Core Material in Stacked Core Configuration ................................. 4-6

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

4.4.5 High-Temperature Insulating Material ................................................................ 4-7

4.4.6 Solid-State (Power Electronics) Technology ....................................................... 4-8

4.4.7 Nanotechnology Composites ............................................................................... 4-9

4.5 SUMMARY OF TECHNOLOGY 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
4-1

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. As discussed in the 

technology assessment portion of chapter 3, DOE consults with industry, technical experts, and 

other interested parties to develop a list of technology options for further consideration. It then 

applies the following set of screening criteria to determine which technology options are 

unsuitable for further consideration in the rulemaking (Title of the Code of Federal Regulations, 

Part 430 (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 how DOE applied the four screening criteria to all of the technology 

options DOE considered in chapter 3. In the end, those technology options that are not screened 

out of the analysis become design options that DOE may consider for improving the energy 

efficiency of distribution transformers in the engineering analysis.. 

4.2 DISCUSSION OF TECHNOLOGY 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. 

 
4-2

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. DOE 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 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 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, different core-stacking techniques, or symmetric cores. Construction methods and 

geometric modifications are inherent to the design and manufacturing process, and therefore are 

not a technology 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).
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 Change 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 TECHNOLOGY OPTIONS NOT SCREENED OUT OF THE ANALYSIS

DOE considers all distribution transformer technology 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 lowvoltage (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, 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 
4-4

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 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). DOE 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, coldrolled transformer steels, nominally designated M2-M6, and domain-refined or laser-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 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. 
4-5

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

Core Deactivation Technology (Not applicable)

4.4 TECHNOLOGY OPTIONS SCREENED OUT OF THE ANALYSIS

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 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. 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 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, DOE 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 
4-6

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 DOE. 

 

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

fail on two of the four screening criteria. 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 technology 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 active 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 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 is not currently used by any manufacturers. 

DOE screened 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 process 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 
4-7

adverse impacts on utility or availability to consumers associated with this material. Similarly, 

for the fourth criterion, 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 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. 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, 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. 

DOE assessed high-temperature insulating 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 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, DOE is unable to assess whether 
4-8

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 (criterion 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 small test transformers have been built for research to 

assess the technology, 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 typically operating at 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 source 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, even before the inefficiencies of the high frequency magnetic components are 

considered, the power electronic devices consume more power than the total losses of 

conventional transformers. High-frequency magnetic losses are not much lower than lowfrequency magnetic losses. This makes the total loss higher than what can be achieved with 

conventional, low-frequency magnetics. 

Additionally, it would not be practicable to manufacture transformers using this 

technology on the scale necessary to serve the distribution 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 electronic circuit cards for the signal electronics, 

wave soldering, aluminum heat sinks, power electronic semiconductors, sintered cores, and 

unique winding equipment. Ferrite magnetic core materials are also required instead of silicon 

iron sheeted cores. 

DOE assessed the feasibility of solid-state (power electronics) technology, and found that 

this technology failed on the first and second screening criteria. DOE is not aware of any solidstate distribution transformers that can achieve improvements in efficiency, either in prototype 

form or in a commercial product. DOE 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 

screening 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 aspects of a distribution 

transformer.
4-9

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 transformers 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 nanotechnology 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 

uncertain 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 technology. 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).

4.5 SUMMARY OF TECHNOLOGY 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;

Practicability to manufacture, install, and service

Nanotechnology Composites Technological feasibility
4-10

REFERENCES

 

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: 

http://cta.ornl.gov/cta/Publications/Reports/ORNL-6847.pdf