September 16, 1996

Ms. Ingrid M. Watson

Program Manager

Office of Codes and Standards

U. S. Department of Energy

1000 Independence Avenue, S.W.

Washington, DC 20585

Dear Ms. Watson:

NEMA is pleased to provide you with NEMA Standards Publication TP-1, "The

Guide for Determining Energy Efficiency for Distribution Transformers." This standard

was developed by the Transformer section and is available for use by the electrical

industry to promote the use of high efficiency transformers and to assist users in the

selection of such transformers.

As with other NEMA standards, this publication is periodically reviewed by the

Transformer section ofNEMA for revisions necessary to keep it up to date with

advancing technology.

In the Department of Energy's Interpretative Rule, it is indicated that additional

consideration will be given to market-driven and voluntary programs. The transformer

industry respectfully requests that DOE consider this standard as evidence of the

industry's pro-active efforts toward energy efficiency in lieu of a mandatory standards

program.

If you have any questions regarding this document, please feel free to contact me

(703) 841-3245.

National Electrical

Manufacturers Association

1300 North 17th Street, Suite 1847

Rosslyn, VA 22209

(703) 841-3200

FAX (703) 841-3300

EE-DET-97-550 COMMENT 00E
EE-DET-97-550 COMMENT 00E
NE A STANDARDS PUBLICATION TP 1-1996

Guide for Determining Energy Efficiency for Distribution Transformers

Published by

National Electrical Manufacturers Association

1300 North 17th Street, Suite 1847

Rosslyn, Virginia 22209

©1996 by National Electrical Manufacturers Association

EE-DET-97-550 COMMENT 00E
TP 1-1996

Page i

TABLE OF CONTENTS

Foreword .............................................................................................................................. i

Section 1 General ...................................................... ..........................................................................1

Section 2 Efficiency Evaluation for Electric Utilities .............................................................................5

Section 3 Evaluation Parameters for Commercial and Industrial Transformers .................................7

Section 4 Table of Energy Efficiency ...................................................................................................9

Appendix A Temperature Correction Factor for Dry Type Transformers .............................................. 11

Appendix B Bibliography .......................................................................................................................13

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Page ii

Foreword

This standard has been developed by the Transformer section and has been approved for publication by

the National Electrical Manufacturers Association. It is available for use by the electrical industry to

promote the use of high efficiency transformers and to assist users in the selection of such transformers.

This standard consists of two parts, sections 2 and 3, which offer a simplified methodology for the user to

apply to determine the equivalent first cost of core and load losses. This information will permit the

manufacturer to tailor the transformer design to the unique situation of each user. Section 4 defines the

minimum efficiency, based on kVA and voltage considerations, that is recommended if the user elects not

to follow the suggested methodology. It must be clearly understood that certain design criteria or

conditions of use may preclude the application of this standard. These exceptions are detailed in the body

of this document.

User needs and safety have been considered throughout the development of this document. This

publication is periodically reviewed by the Transformer section of NEMA for any revisions necessary to

keep it up to date with advancing technology. Proposed or recommended revisions should be submitted

to:

Vice President, Engineering Department

National Electrical Manufacturers Association

1300 North 17th Street, Suite 1847

Rosslyn, Virginia 22209

This Standards Publication was developed by the Transformer Products Section of the National Electrical

Manufacturers Association. At the time it was approved, the Transformer Products Section had the

following members.

-ABB Power T & D Company-Jefferson City, MO

Acme Electric Corporation-Lumberton, NC

-Cooper Power Systems-Waukesha, WI

Eaton Corp., Cutler-Hammer-Pittsburgh, PA

'General Electric-Hickory, NC

Hammond Mfg. Co.-Guelph, Ontario

Kentucky Assn. Elec. Cooperatives, Inc.-Louisville, KY

Magnetran Inc.-Voorhees, NJ

Niagara Transformer Corporation-Buffalo, NY

North American Transformer-Milipitas, CA

Olsun Electrics Corporation-Richmond, IL

Pauwels Transformers-Washington, MO

PDI-Sandston, VA

R.E. Uptegraff Mfg. Company-Scottsdale, PA

Siemans Energy & Automation-Jackson, MS

SMIT Transformers, Inc.-Ladson, SC

Southern Transformer Company-East Point, GA

Square D Company-Lexington, KY

Vantran Electric Corporation-Waco, TX

Virginia Transformer Corporation-Roanoke, VA

Waukesha Electric Company-Goldsboro, NC

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Page 1

1.1 SCOPE

Section 1

GENERAL

This standard in intended for use as a basis for determining the energy efficiency performance of the

equipment covered and to assist in the proper selection of such equipment.

This standard covers single-phase and three-phase dry-type and liquid-filled distribution transformers as

defined in the following table:

Voltage

Class

Liquid

Rating

Dry

Rating

Single Phase

Three Phase

Single Phase

Three Phase

34.5 kV and Below

600 Volts and Below

10-833 kVA

15-2500 kVA

15-833 kVA

15-2500 kVA

Note: Includes all products at 1.2kV and below

Products excluded from this standard include:

a.

b.

C.

d.

e.

f.

9-

h.

i.

J•

k.

1.

M.

n.

0.

p•

Liquid-filled transformers below 10 kVA

Dry-type transformers below 15 kVA

Drives transformers, both AC and DC

All rectifier transformers and transformers designed for high harmonics

Autotransformers

Non-distribution transformers, such as UPS transformers

Special impedance, regulation and harmonic transformers

Regulating transformers

Sealed and non-ventilated, transformers

Retrofit transformers

Machine tool transformers

Welding transformers

Transformers with tap ranges greater than 15%

Transformers with frequency other than 60 Hz

Grounding transformers

Testing transformers

1.2 REFERENCE STANDARDS

Transformers shall meet the requirements for liquid-filled and dry-type transformers as contained in the

latest revision of the following standards:

American National Standards Institute, Inc.

11 West 42nd Street

New York, NY 10036

Primary Voltage

Secondary Voltage

C57.12.00-1993 IEEE Standard General Requirements for Liquid-Immersed

Distribution, Power and Regulating Transformers

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Page 2

C57.12.01-1989 IEEE Standard General Requirements for Dry-Type Distribution

and Power Transformers including those with Solid Cast and/or

Resin-Encapsulated Windings

C57.12.90-1993 IEEE Standard Test Code for Liquid-Immersed Distribution,

Power and Regulating Transformers and IEEE Guide for ShortCircuit Testing of Distribution and Power Transformers

C57.12.91-1979 IEEE Standard for Test Code for Dry-Type Distribution and Power

Transformers

National Electrical Manufacturers Association

1300 North 17th Street

Arlington, VA 22209

TR 1-1993 Transformers, Regulators and Reactors

ST 20-1992 Dry-Type Transformers for General Applications

1.3 DEFINITIONS

1.3.1 EC = Avoided cost of energy ($/kWhr-yr): The levelized avoided (incremental) cost for the next

kWh produced by the utility's generating units.

1.3.2 Efficiency tolerance : The nameplate efficiency is the value calculated based on the specified

losses of the transformer. A transformer is considered to be within an acceptable tolerance of its

nameplate efficiency if its no load and total losses meet the single unit tolerances defined in ANSI

standards C57.12.00 or C57.12.01.

1.3.3 FCR(%) = Fixed charge rate : The cost of carrying a capital investment.

1.3.4 HPY (hours): Hours per year (HPY) used in the A- and B-factor equations is generally taken as

8760. However, there may be exceptions, such as transformers connected to irrigation loads. In this

case, the transformer may be de-energized a portion of the year and hence, a value less than 8760 would

be used for HPY.

On the other hand, this transformer may remain energized all year but loaded only part of the year. In this

case, a reduced equivalent loss factor could be used to account for the unloaded portion of the year while

retaining the use of 8760 hours per year in both the A- and B-factor formulas.

1.3.5 LsF (%) - Loss factor: A measure of the annual average load losses to the peak value of load

losses on the distribution transformer.

1.3.6 LL: Load Loss watts at rated temperature rise plus 20°C, i.e., 85° C for 65° C rise and 170° C for

150°C rise.

1.3.7 LM - Loss multiplier (%): A measure of system losses from generation through the

transmission and distribution system.

1.3.8 Nameplate efficiency : Rounded efficiency to 1/10 of 1 percent. This calculated number which

is derived from the parameters of output kVA divided by out put kVA plus losses multiplied by 100% at a

given percent load and at specified reference temperature.

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Page 3

Mathematically, the relationship is as follows:

%E= 100x(PxkVAx1000)

PxkVAxl000+NL+LLxP2 xT

Where:

P = Per unit load

0.35 for low voltage (600 volt class) dry type transformers

= 0.50 for medium voltage liquid and dry transformers

kVA = nameplate kVA rating

NL = No load (core) loss at 20`C in watts

LL = Load loss at its full load reference temperature, consistent with

C57.12.00 (liquid) and C57.12.01 (dry) in watts.

T = Load loss temperature correction factor to correct specified

temperatures, i.e., 75°C for dry- and 85°C for liquid-transformers.

For liquid-filled transformers, the factor T is 1.0, since efficiency is stated at 85°C. Dry type transformers

are significantly affected because 75°C is well below the full load rise. For dry-type transformers, the

factor T assumes 10% winding eddy loss, as described in the appendix; however, the actual winding eddy

loss may be different than 10%.

1.3.9 P: Per unit load.

1.3.10 PL-Equivalent annual peak load (on the transformer): The average annual peak load seen

by the transformer that is equivalent to an initial peak load with an estimated load growth rate and a

maximum allowable load before change-out is required.

1.3.11 RF (%)-Peak loss responsibility factor: A measure of the diversity of the load on the

transformer.

1.3.12 SC = Avoided cost of system capacity ($/kW-yr): The average avoided (incremental) cost of

generation, transmission and primary distribution capacity required to supply the next kW of load to the

distribution transformer coincident with the peak load.

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Section 2

EFFICIENCY EVALUATION FOR ELECTRIC UTILITIES

Electric utilities purchase distribution transformers based on a life-cycle cost methodology which accounts

for the economic value of electrical losses over the useful life of the transformer. The following is a brief

summary of the equivalent first cost (EFC), total owning cost (TOC) method which is most commonly

utilized:

TOCEFC = Transformer first cost + Cost of core IossEFC + Cost of load IossEFC

Transformer first cost = Cost of acquisition

Cost of core IossEFC = A($/Watt) x core loss watts

Cost of load IossEFC = B($/Watt) x load loss watts

Where:

A = Equivalent first cost of no load losses ($/Watt)

B = Equivalent first cost of load losses ($/Watt)

The A and B factors are functions of the energy costs and the capacity costs associated with the

transformer no-load losses and load losses (including financial costs).

Avoided costs are generally used in the calculation of total owning cost (TOC) evaluations for distribution

transformers. As previously described, components of TOC are the costs to the utility to supply

distribution transformer core (no load) and load losses, commonly referred to as the A- and B-factors.

These distribution transformer losses represent additional (incremental) load that the utility must serve

over the life of the transformer. If the utility's avoided costs reflect its long-range incremental costs taken

over the transformer's economic life, they can be used to determine the system capacity and energy cost

of the transformer core and load losses.

Most utilities regularly determine their avoided costs of capacity and energy. Assuming that those values

reflect the continuing economic impact over the transformer's life, they should be used directly in the loss

evaluation process.

Avoided costs of energy and generating capacity that are used in evaluating distribution transformers

should be determined on a long-range incremental cost basis, using generation production costing and

expansion planning. The long-range incremental cost factors include the utility load characteristics,

projected peak loads (demand), existing and future generation capability and availability, generation

maintenance, generation reliability, costs associated with construction of new generation plants, current

and projected fuel prices, short-range purchase/sale contracts and long-range purchase/sale contracts.

The generation expansion includes planning reserve margins sufficient to maintain system reliability at a

high level based on objective criteria.

The application of the TOC methodology using A- and B-factors, as described above, is common place in

the electric utility industry.

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Section 3

EVALUATION PARAMETERS FOR

COMMERCIAL AND INDUSTRIAL TRANSFORMERS

3.1 INTRODUCTION

While many aspects of the TOC for industrial or commercial placed transformers parallels that used by

the utility, there are, nonetheless, differences that require the use of a different procedure . This procedure

would at least provide a differentiation among designs that reflect the users constraints.

The following is a review of the equivalent first cost TOC method:

TOCEFC = Price + cost of core loss + cost of load loss

Cost of core IOSSEFC = A($/watt) x core loss watts

Cost of load IossEFC = B ($/watts) x load loss watts

Where:

A = Equivalent First Cost of No Load Losses

= PW(IS) (EL) (HPY) / 1000

Where:

PW(IS) = Present worth of an inflation series

a = per unit inflation index

i = per unit interest rate

n = number of years

Where:

EL = purchasers cost of electricity ($/kWh)

B = Equivalent first cost of load losses

= AxP2xT

An alternative approach to specific no load and load losses of a transformer is to measure energy

efficiency at the specific load which is most typical of how it will be used . The general expression for

energy efficiency is:

%Efficiency = 100 (output load)/(output load + losses)

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Page 8

This expression can be tailored to the commercial/industrial segment as:

% E=

100 x (P x kVA x l000)

PxkVAxl000+NL+LLxP2 xT

Where:

P = Per unit load

= 0.35 for low voltage (600 volt class) dry type transformers

= 0.50 for medium voltage liquid and dry transformers

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Page 9

Section 4

TABLES OF ENERGY EFFICIENCY

The following tables of energy efficiencies would define the minimum efficiency to be categorized as

NEMA class 1 efficiency, per this standard. These efficiencies apply to both the utility and the

commercial/industrial segment, that do not use evaluation parameters as defined in either Section 2, for

utility transformers, or Section 3, for commercial/industrial transformers.

Table 4-1

NEMA CLASS 1 EFFICIENCY LEVELS FOR LIQUID-FILLED DISTRIBUTION TRANSFORMERS

Reference Condition Temperature % of Nameplate Load

Load Loss 85°C 50%

No Load Loss 20°C 50%

kVA

Single Phase

Efficiency kVA

Three Phase

Efficiency

10 98.3 15 98.0

15 98.5 30 98.3

25 98.7 45 98.5

37.5 98.8 75 98.7

50 98.9 112.5 98.8

75 99.0 150 98.9

100 99.0 225 99.0

167 99.1 300 99.0

250 99.2 500 99.1

333 99.2 750 99.2

500 99.3 1000 99.2

667 99.4 1500 99.3

833 99.4 2000 99.4

2500 99.4

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Table 4-2

NEMA CLASS I EFFICIENCY LEVELS FOR DRY-TYPE DISTRIBUTION TRANSFORMERS

Reference Condition Temperature % of Nameplate Load

Low Voltage 75°C 35%

Medium Voltage 75°C 50%

Single Phase

Efficiency

Three Phase

Efficiency

kVA Low Voltage Medium Voltage kVA Low Voltage Medium Voltage

15 97.7 97.6 15 97.0 96.8

25 98.0 97.9 30 97.5 97.3

37.5 98.2 98.1 45 97.7 97.6

50 98.3 98.2 75 98.0 97.9

75 98.5 98.4 112.5 98.2 98.1

100 98.6 98.5 150 98.3 98.2

167 98.7 98.7 225 98.5 98.4

250 98.8 98.8 300 98.6 98.5

333 98.9 98.9 500 98.7 98.7

500 - 99.0 750 98.8 98.8

667 - 99.0 1000 98.9 98.9

833 - 99.1 1500 - 99.0

2000 - 99.0

2500 - 99.1

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Appendix A

TEMPERATURE CORRECTION FACTOR FOR DRY TYPE TRANSFORMERS

A.1 ASSUMPTIONS

a. Stray and eddy losses-10% of load loss

b. Conductor material-copper and aluminum

c. Candidate full load temperature rises 80, 115, 150

d. 20°C Ambient

A.2 ALGORITHMS

a. Electrical resistance at 75°C vs. full load reference

R75 F+75°C

Rref F+Trise+Ambient

Where:

F = thermal coefficient of resistance

Faluminum = 225

Fcopper = 234.5

Ambient = 20°C

Full Load Rise Aluminum Copper

80 0.9231 0.9253

115 0.8333 0.8376

150 0.7595 0.7651

b. Temperature correction factor

TF = (LI) R75 + LE Rref

Rref R75

L, = PU load loss due to d - c resistance = 0.9

L, varies directly with temperature

LE = PU load loss due to strays and eddies = 0.1

LE varies inversely with temperature

TF = 0.9 R75 + 0.1 Rref

Rref R75

Full Load Rise Aluminum Copper

80 0.9391 0.9408

115 0.8700 0.8732

150 0.8152 0.8193

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Page 13

Appendix B

BIBLIOGRAPHY

C57.12.10-1988 American National Standard for Transformers - 230 KV and below, 833/958

through 8333/1041 7kVA, single phase, and 750/862 through

60000/80000/100000kVA, Three Phase without Load Tap Changing; and

3750/4687 through 60000/80000/100000 kVA with Load Tap Changing - Safety

Requirements.

C57.12.20-1988 American National Standard for Transformers - Overhead-Type Distribution

Transformers, 500 kVA and Smaller; High Voltage, 34500 and Below; Low

Voltage, 7970/13800Y and Below

C57.12.22-1989

C57.12.23-1993

C57.12.24-1988

American National Standard for Transformers - Pad-Mounted, CompartmentalType, Self-Cooled, Three-Phase Distribution Transformers with High Voltage

Bushings, 2500 kVA and Smaller; High Voltage, 34500GrdY/19920 Volts and

Below; Low Voltage, 480 Voltages and Below - Requirements

IEEE Standard for Transformers - Underground-type, Self-Cooled, Single-phase

Distribution Transformers with Separable, Insulated, High Voltage Connectors;

High Voltage (24940 GrdY/14400 V and Below) and Low Voltage (240/120 V, 176

kVA and Smaller)

American National Standard for Transformers - Underground-type, Three-Phase

Distribution Transformers, 2500kVA and Smaller; High Voltage 34500

GrdY/19920 Volts and Below; Low Voltage, 480 Volts and Below - Requirements

C57.12.25-1990 American National Standard for Transformers - Pad-Mounted, Compartmentaltype, Self-Cooled, Single-Phase Distribution Transformers with Separable

Insulated High Voltage 34500 GrdY/19920 Volts and Below; Low Voltage,

240/120 Volts; 167 kVA and Smaller - Requirements

C57.12.26-1992 IEEE Standard for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers for Use with Separable Insulated High-Voltage

Connectors (34500 GrdY/19920 V and Below; 2500 kVA and Smaller

C57.12.40-1992 American National Standard for Transformers - Subway and Vault Types (Liquid

Immersed) - Requirements

C57.12.50-1981

C57.12.51-1981

C57.12.52-1987

American National Standard Requirements for Ventilated Dry-Type Distribution

Transformers, 1 to 500 kVA, Single-Phase, and 15 to 500 kVA, Three-Phase, with

High Voltage 601 to 34500 Volts, Low-Voltage 120 to 600 Volts

American National Standard Requirements for Ventilated Dry-Type Power

Transformers, 501 kVA and Larger, Three-Phase, with High Voltage 601 to 34500

Volts, Low Voltage 208Y/120 to 4160 Volts

American National Standard Requirements for Sealed Dry-Type Power

Transformers, 501 kVA and Larger, Three-Phase, with High Voltage 601 to 34500

Volts, Low-Voltage 208Y/120 to 4160 Volts

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Page 14

C57.12.57-1987 American National Standard for Transformers - Ventilated Dry-Type Network

Transformers 2500 kVA and Below, three-Phase, with High Voltage 34500 Volts

and Below, Low-Voltage 216Y/125 and 480/277 Volts - Requirements

C57.12.59-1989 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

C57.12.70-1978 American National Standard Terminal Marking and Connections for Distribution

and Power Transformers

C57.12.80-1978 IEEE Standard Terminology for Power and Distribution Transformers

C57.12.90-1993 IEEE Standard Test Code for Liquid-Immersed Distribution, Power and

Regulating Transformers and IEEE Guide for Short-Circuit Testing of Distribution

and Power Transformers

C57.12.91-1979 IEEE Standard Test Code for Dry-Type Distribution and Power Transformers

C57.91-1981 IEEE Guide for Loading Mineral-Oil-Immersed Overhead and Pad-Mounted

Distribution Transformers Rated 500 kVA and Less with 65°C or 55°C Average

Winding Rise

C57.92-1981 IEEE Guide for Loading Mineral-Oil-Immersed Power Transformers Up to and

including 100 MVA with 65°C or 55°C Average Winding Rise

C57.96-1989 IEEE Guide for Loading Dry-Type Distribution and Power Transformers

C57.109-1993 IEEE Guide for Liquid-Immersed Transformer Through-Fault Current Duration

C57.110-1986 IEEE Recommended Practice for Establishing Transformer Capability When

Supplying Nonsinusoidal Load Currents

EE-DET-97-550 COMMENT 00E
NEMA T.. ®TI .... i u ]'u

The purpose of NEMA standards, their classification , and status are set forth in certain clauses of the

NEMA Standardization Policies and Procedures manual and are referenced below:

Purpose c

National Electrical Manufacturers Association Standards are adopted in the public interest and are

designed to eliminate misunderstandings between the manufacturer and the purchaser and to assist the

purchaser in selecting and obtaining the proper product for their particular needs. Existence of a National

Electrical Manufacturers Association Standard does not in any respect preclude any member or

nonmember from manufacturing or selling products not conforming to the standard.

Definition of a Standard

A standard of the National Electrical Manufacturers Association defines a product, process, or

procedure with reference to one or more of the following; nomenclature, composition, construction,

dimensions, tolerances, safety, operating characteristics, performance, rating, testing, and the service for

which they are designed.

(Standardization Policies and Procedures, p. 1)

Di mensions

Where dimensions are given for interchangeability purposes, alternate dimensions satisfying the

other provisions of the Standards Publication may be capable of otherwise equivalent performance.

(Standardization Policies and Procedures, p. 6)

Ca- of Standards

National Electrical Manufacturers Association Standards are of three classes, which have received

the affirmative vote of at least two-thirds of the Subdivision votes cast in the affirmative or negative:

1. NEMA Standard, which relates to a product, process, or procedure commercially standardized and

subject to repetitive manufacture.

2. Suggested Standard for Future Design, which may not have been regularly applied to a commercial

product, but which suggests a sound engineering approach to future development.

3. Adoptive Standard, which is adopted in whole or in part from the standards of another organization,

either domestic, regional, or international.

(Standardization Policies and Procedures, p. 5)

A __.i_ °L- ed L_ - mi ng Information

Authorized Engineering Information consists of explanatory data or other engineering information of

an informative character not falling within the classification of NEMA Standard or Suggested Standard for

Future Design, which standard has received the affirmative vote of at least two-thirds of the Subdivision

votes cast in the affirmative or negative.

(Standardization Policies and Procedures, p. 5)

I dentification of Status

Standards in NEMA Standards Publications are identified as "NEMA Standard," "Suggested Standard

for Future Design," or "Adoptive Standard." These indicate the status of the standard. A statement

incorporating the auxiliary verb "shall" indicates that compliance with a requirement is mandatory for

compliance with the standard. These classes of standards are identified in the foreword or throughout the

text.

The material identified as "Authorized Engineering Information" is designated similarly. Statements

incorporating other auxiliary verbs such as "should," "may," etc. refer to the authorized engineering

information and not to procedures required for compliance with the standard.

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