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 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 Config uration . . . . . . . . . . . . . . . . 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 A nalysis . . . . . . . . . . . . . . . . . . . . . . 4-5 Table 4.5.1 Design Options Screened Out of the Analy sis . . . . . . . . . . . . . . . . . . . . . . . . . . 4-94-1 CHAPTER 4. SCREENING ANALYSIS

4.1 INTRODUCTION The purpose of the screening analysis is to identif y design options that improve distribution transformer efficiency and to determin e 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 developinga list of design options for consideration. It then applies the following set of screening criteri a to determine which design options are unsuitable for further consideration in the rulemak ing (10 CFR Part 430, Subpart C, Appendix A at 4(a)(4) and 5(b): (1) Technological feasibility. Technologies incorp orated in commercial products or in working prototypes will be considered technological ly feasible; (2) Practicability to manufacture, install, and ser vice. If mass production of a technology in commercial products and reliable installation and s ervicing 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 o f the product to significant subgroups or consumers, or result in the unavailability of any c overed product type with performance characteristics (including reliability), features, sizes, capacities, and volumes that are substantially the same as products generally availa ble 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 D epartment 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 prac tices and techniques for improving the efficiency of a distribution transformer. A transf ormer design can be made more energy-efficient by improving the materials of construction (e.g., b etter quality core steel or winding material) and by modifying the geometric configuration of the cor e and winding assemblies.4-2

Core and winding losses are not independent variabl es 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 standard s (Institute of Electrical and Electronics Engineers IEEE C57.12.00 and C57.12.01). If this t emperature 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 des ign of a transformer: the electrical insulation, insulating media (oil for liquid-immers ed 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 geom etry to reduce the losses. Making a transformer more efficient (i.e., reducingelectrical 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. Addition ally, at a given loading point and associated efficiency level, there can be several viable desig ns that achieve that efficiency level. The Department found that a wide range of designs and e fficiencies are technologically feasible using common materials, engineering practices, and constr uction 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 mo re 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 betw een no-load (core) losses, load (winding) losses, and price. Some of the approaches presented in Table 4.2.1 ref er to specific technologies (e.g., lower-loss core materials, lower-loss conductor mat erials), while other approaches refer to transformer geometry modifications (e.g., core or c onductor 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 modi fications such as a larger cross-sectional area or different core-stacking techniques. Constr uction methods and geometric modifications are inherent to the design and manufacturing proces s, 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 eng ineer and/or software design tool to improve the efficiency of resultant designs. Thus, they ar e 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

Decrease Coil Losses Use lower-loss conductor materials No Change Lower Hig her

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 loa d losses because flux density drops, requiring a la rger core volume.

4.3 DESIGN OPTIONS NOT SCREENED OUT OF THE ANALYSIS The Department considers all distribution transform er 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 distributio n 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 con ductivity and about 40 percent lower resistive losses than aluminum. Compared to copper, aluminumis easier to form and work mechanically, and can be less expensive. By utilizing aluminum c onductor material at a lower current density (i.e., larger conductor cross-sectional area), alum inum transformer windings can be built with essentially the same load losses as copper. Howeve r, aluminum conductors increase core losses due to their larger core frames, necessitated by th e 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 us ed in the form of flat, rolled foils to reduce eddy current losses.a

Registered trademark of Metglas, Inc., a wholly-o wned subsidiary of Hitachi Metals, Ltd., Tokyo, Jap an.

4-4 Considering the four screening criteria for this t echnology, the Department did not screen out aluminum and copper as conductor materials. Th ese 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 wind ing 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 additio nal 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 l osses, since they were fabricated from thick laminates of non-oriented, low-silicon, magne tic 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 l aminar insulation, and may also be grainoriented or domain-refined (i.e., laser or mechanic ally scribed steels). Amorphous metal (or METGLAS a ) material allows the construction of a low-loss co re. Amorphous metal is extremely thin, has high electri cal resistivity, and has little or no magnetic domain definition. Cores made from this material c an exhibit 60–70 percent lower core losses than one made of conventional steels. However, amo rphous 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 requirement s. Amorphous metal material is also fragile and requires special handling during the constructi on 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 mate rials, which increases losses. The net effect of the lower flux density and higher space factor i s a larger core with greater winding (conductor) losses and higher production costs. The core steels considered in this screening analys is are all those found in commercial use today. These include high-silicon electrical steel s, both non-oriented hot-rolled and grainoriented cold-rolled, domain-refined grain-oriented electrical steel, and amorphous material (wound core designs). The Department considered al l of these core materials to be technologically feasible, as they are used commerci ally today (or in the past) by distribution transformer manufacturers at varying flux levels an d lamination thicknesses. These commercially available high-silicon, cold-rolled tr ansformer 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 appli cation of amorphous material is only a viable design option in a wound core. Its manufacturers h ave not been successful in producing an4-5

amorphous product that can be used in a stacked-cor e 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 use d by the distribution transformer industry. There are no known adverse impacts on consumer util ity or reliability, and no known adverse impacts on health or safety associated with these c ore materials. Table 4.3.1 summarizes the design options not scree ned out of the analysis. Table 4.3.1 Design Options Not Screened Out of the A nalysis

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 op tions 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) technology4-6

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 De partment found that the use of silver as a conductor is technologically feasible, since distri bution transformers with silver windings were built during World War II because of a war-time sho rtage 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 al uminum in this application. The Department is also not aware of any adverse health or safety impa cts associated with the use of this conductor material. However, the Department screened out silver as a co nductor material because it is impracticable to manufacture, install, and service. Silver conductor designs are constrained by lower operating temperatures (adding to manufacturi ng complexity) and lower tensile strength (material can easily break during manufacturing pro cess). In addition, due to limited availability, silver is not feasible to use for mass production o n the scale necessary to serve the U.S. distribution transformer manufacturing industry. Thus, the Department screened silver out from furth er 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 (HT S) materials was discovered in 1987. These new materials become superconducting at temperatures close to that of liquid nitrogen, a readily available coolant that is consi derably less expensive than liquid helium, the coolant for the previous generation of superconduct ing materials. After the discovery of these materials, research programs were launched worldwid e to explore the use of superconducting material in power transformers. However, the use o f superconductors, both low- and hightemperature, in transformer manufacturing has prove n to be an elusive goal. Low-temperature superconductors (liquid helium-cooled) are physical ly possible but not feasible for commercial use, since these units are often unable to return t o 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 distr ibution transformer has not been constructed. Design constraints include unique conductors, unacc eptable alternating current variation losses, and complex cryogenic support components. Researchto overcome these barriers is being conducted, some of which is funded by the Departmen t.

HTS materials were screened out of further consider ation 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 distribut ion transformer has never been built. Additionally, due to technical issues associated wi th HTS power transformers, the Department4-7

does not consider HTS technology a viable loss-redu ction technology for distribution transformers now or in the foreseeable future. Sec ond, 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 reli able in service because they require continuous active cooling or they cease to function. With reg ard to the third screening criterion, the Department is not aware of any adverse impacts on c ustomer 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 analysi s because of technological infeasibility (criterion 1) and impracticability to manufacture, install, and service (criterion 2). 4.4.3 Amorphous Core Material in Stacked Core Config uration As discussed in section 4.3.2, amorphous material i s considered a viable core material in a wound-core configuration. However, stacked amorp hous core material is not presently a viable design option for distribution transformers, and ha s 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 Departme nt is not aware of any working prototypes that use amorphous core material in a stacked core confi guration. 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 f rom the use of amorphous core material in stacked core configuration. Thus, DOE screened amorphous core materials in stac ked core configuration out of the analysis due to technological infeasibility (criter ion 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 technol ogy 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 be en demonstrated, and if it were technologically feasible, it would still be several years away fromcommercialization. The Department assessed carbon composite materials for heat removal from distribution transformers, and found the material failed the fir st screening criterion. These materials for heat removal failed the first screening criterion becaus e there are no commercial products or working prototypes that incorporate this technology. The D epartment was not able to assess whether the material meets or fails any of the other three scre ening criteria. Specifically, the Department cannot determine whether transformers would be prac ticable to manufacture, install, and service with this new material, since the application of th e technology in a distribution transformer design has not been determined. Similarly, any pot ential 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 h eat removal out of the analysis due to technological infeasibility (criterion 1). 4.4.5 High-Temperature Insulating Material The transformer industry conducts research and deve lopment on insulating materials. While potentially improving dielectric performance, industry studies this technology to create an electrical insulation that can withstand higher ope rating 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 effe ctive core and coil volumes, and therefore reduce operating losses. The Department assessed high-temperature insulatingmaterials, 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 remov al material, either in prototype form or in commercial products. The Department was not able t o assess whether the material meets or fails any of the other three screening criteria. Transfo rmers are built today with standard grades of insulation (up to 220° Celsius); however, it is unc ertain whether higher temperature materials may have certain issues that make them impracticabl e to manufacture, install, or service. Similarly, the Department is unable to assess wheth er 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 mate rials out of the analysis due to technological infeasibility (criterion 1).4-9

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-st ate (power electronics) technology, and found that this technology failed on the first scre ening criterion. The Department is not aware of any solid-state distribution transformer, either inprototype 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 la ck 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 De partment 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 tr ansformer technology out of the analysis due to technological infeasibility (criter ion 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 no t screen out of the analysis are listed in Table 4.3.1. Table 4.5.1 Design Options Screened Out of the Analy sis

Design Option Excluded Screening Criteria Silver as a Conductor Material Practicability to man ufacture, install, and service High-Temperature Superconductors Technological feasi bility; 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 Technolo gical feasibility High-Temperature Insulating Material Technological f easibility Solid-State (Power Electronics) Technology Technolog ical feasibility4-10 REFERENCES 1. ORNL, 1996. Oak Ridge National Laboratory, Dete rmination Analysis of Energy Conservation Standards for Distribution Transformer s, 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. Availa ble online at: www.eere.energy.gov/buildings/appliance_standards/ commercial/dist_transformers.html