This article is excerpted from "Premium-Efficiency Motors and Transformers", a CD-ROM available fromCDA by calling 888/480-MOTR, or through the Publications List.

Transformer losses are produced by the electrical current flowing in the coils and the magnetic fieldalternating in the core. The losses associated with the coils are called the load losses, while the lossesproduced in the core are called no-load losses.

What Are Load Losses?

Load losses vary according to the loading on the transformer. They include heat losses and eddy currents inthe primary and secondary conductors of the transformer.

Heat losses, or I2R losses, in the winding materials contribute the largest part of the load losses. They arecreated by resistance of the conductor to the flow of current or electrons. The electron motion causes theconductor molecules to move and produce friction and heat. The energy generated by this motion can becalculated using the formula:

Watts = (volts)(amperes) or VI.

According to Ohm’s law, V=RI, or the voltage drop across a resistor equals the amount of resistance in theresistor, R, multiplied by the current, I, flowing in the resistor. Hence, heat losses equal (I)(RI) or I2R.

Transformer designers cannot change I, or the current portion of the I2R losses, which are determined by theload requirements. They can only change the resistance or R part of the I2R by using a material that has alow resistance per cross-sectional area without adding significantly to the cost of the transformer. Mosttransformer designers have found copper the best conductor considering the weight, size, cost andresistance of the conductor. Designers can also reduce the resistance of the conductor by increasing thecross-sectional area of the conductor.

What Are No-load Losses?

No-load losses are caused by the magnetizing current needed to energize the core of the transformer, anddo not vary according to the loading on the transformer. They are constant and occur 24 hours a day, 365days a year, regardless of the load, hence the term no-load losses. They can be categorized into fivecomponents: hysteresis losses in the core laminations, eddy current losses in the core laminations, I2Rlosses due to no-load current, stray eddy current losses in core clamps, bolts and other core components,and dielectric losses. Hysteresis losses and eddy current losses contribute over 99% of the no-load losses,while stray eddy current, dielectric losses, and I2R losses due to no-load current are small and consequentlyoften neglected. Thinner lamination of the core steel reduces eddy current losses.

The biggest contributor to no-load losses is hysteresis losses. Hysteresis losses come from the molecules inthe core laminations resisting being magnetized and demagnetized by the alternating magnetic field. Thisresistance by the molecules causes friction that results in heat. The Greek word, hysteresis, means "to lag"and refers to the fact that the magnetic flux lags behind the magnetic force. Choice of size and type of corematerial reduces hysteresis losses.

Values of Transformer Losses (A and B Values)

The values of transformer losses are important to the purchaser of a transformer who wants to select themost cost-effective transformer for their application. The use of A and B factors is a method followed bymost electric utilities and many large industrial customers to capitalize the future value of no-load losses(which relate to the cost to supply system capacity) and load losses (which relate to the cost of incrementalenergy). Put another way, A values provide an estimate of the equivalent present cost of future no-loadlosses, while B values provide an estimate of the equivalent present cost of future load losses. Most utilitiesregularly update their avoided cost of capacity and energy (typically on an annual basis), and use “A” and“B” values when specifying a transformer. Most smaller end users typically use life-cycle -cost evaluationmethods, discussed in another article on this web site.

When evaluating various transformer designs, the assumed value of transformer losses (A and B values) willcontribute to determining the efficiency of transformer to be purchased. Assuming a high value fortransformer losses will generally result in purchase of a more efficient unit; assuming a lower value of losseswill result in purchase of a less efficient unit. What value of losses should be assumed?

The total owning cost (TOC) method provides an effective way to evaluate various transformer initialpurchase prices and cost of losses. The goal is to choose a transformer that meets specifications andsimultaneously has the lowest TOC. The A and B values include the cost of no-load and load losses in theTOC formula:

TOC = NLL x A + LL x B + C

Where,

TOC = capitalized total owning cost,NLL = no-load loss in watts,A = capitalized cost per rated watt of NLL (A value),LL = load loss in watts at the transformer's rated load,B = capitalized cost per rated watt of LL (B value),C = the initial cost of the transformer including transportation, sales tax, and other costs to prepare it for service.

What Is the A Value?

The “A” value is an estimate of the present value of future capital cost (nonload- dependent) items at a givenpoint in time. It can vary over time as utilities re-evaluate their costs on a periodic basis. (In other words, the“A” value is the answer to the question, “what is a watt of no-load loss over the life of the transformer worthto me today?”) Even if there is no load, there is capital that is devoted to fixed capacity to generate, transmitand distribute electricity, which contribute to the “A” value. The loading that may change daily on thetransformer does not affect the no-load loss value. It is calculated using the following formula:

A = [SC + (EC x 8760)] x 0.001 / [FC] = Cost of No-Load Loss in $/watt

Where,

SC = Annual Cost of System Capacity in $/kW-year (SC is the levelized annual cost of generation, transmissionand primary distribution capacity required to supply one watt of load to the distribution transformer coincident withthe peak load).

EC = Energy Cost (EC is the levelized annual cost per kWh of fuel, including inflation, escalation, and any otherfuel related components of operation or maintenance costs that are proportional to the energy output of thegenerating units).

8,760 = hours per year

FC = Fixed Charge on capital per year (FC is the levelized annual revenue required to carry and repay thetransformer investment obligation and pay related taxes, all expressed as a per-unit quantity of the original).

0.001 = conversion from kilowatts to watts.

What Is the B Value?

Similar to the way the “A” value is determined, the “B” value is an estimate of the present value of futurevariable, or load-dependent, cost items at a given point in time. (In other words, the “B” value is the answerto the question, “what is a watt of load loss over the life of the transformer worth to me today?”) The “B”value can also change over time as utilities revaluate their costs on a periodic basis, but once determined, itis a constant value for a given transformer purchase. The cost of load losses, or “B” value, is calculatedusing the following formula:

B = [(SC x RF) + (EC x 8,760 x LF)] (PL)2 (0.001) / (FC) = Cost of Load Loss Cost $/watt

Where,

RF = Peak Loss Responsibility Factor (RF is the composite responsibility factor that reduces the system capacityrequirements for load losses since the peak transformer losses do not necessarily occur at peak time).

LF = Annual Loss Factor (LF is the ratio of the annual average load loss to the peak value of the load loss in thetransformer).

PL = Uniform Equivalent Annual Peak Load (PL is the levelized peak load per year over the life of the transformer.Transformer life cycle is defined as the useful life of the asset and is usually assumed to be 30-35 years).

Specifying A and B Values

For custom-designed transformers, manufacturers optimize the design of the unit to the specified A and Bvalues resulting in a transformer designed to the lowest total owning cost, rather than one designed forcheapest first cost.

In situations where A and B values have not been determined (or the enduser does not utilize or specifythem), such as occur in commercial or small industrial applications, the suggested technique to maximizetransformer efficiency is to obtain the no-load and full-load loss values of a specific transformer, in watts.This method is discussed in the article “Transformer Life-Cycle Cost”, elsewhere on this web site.

Proper Transformer Sizing and Copper Windings Mean Lowest TotalOwning Cost, Fastest Payback

Transformers are used in virtually every commercial and industrial building, from the service transformerreducing the distribution voltage to a more usable voltage for the building, to step-down transformers servingindividual floors, to small transformers for individual apparatus or functions. Typically a transformer is a long-lived device that can be in service for decades.

Over such a long life span, the operating cost of a transformer can greatly exceed the initial price, soselection of the right transformer for economic performance involves looking at proper size (capacity) andefficiency.

And efficiency means looking at both the core steel and the winding material.

Transformer Losses

In the simplest terms, there are two components to transformer losses: core losses (also called no-loadlosses); and coil losses (called load losses).

The core losses originate in the steel core of the transformer, caused by the magnetizing current needed toenergize the core. They are constant, irrespective of the load on the transformer (thus the name no-load).They continue to waste energy as long as the transformer is energized. No-load losses do, however, varywith the size (kVA) of the transformer, and the core steel selected; hence the emphasis on proper sizing.

The coil losses (load losses) originate in the primary and secondary coils of the transformer, and are a resultof the resistance of the winding material. That's where selection of copper windings can make a difference inlosses.

Proper Sizing

Transformers are sometimes placed into a speculative setting in advance of occupancy, so the engineerdoes not necessarily know the load that will be placed on the unit. As the installer is often not the partypaying the electric bill, there can be a tendency to oversize the transformer capacity relative to the load it willactually see. Since the no-load loss is a function of the kVA capacity of the transformer, careful selection ofthe transformer capacity closer to the intended task will ensure lowest core loss.

Energy Star (TP-1) Transformers May Not be Efficient Enough

The Energy Star label is applied to transformers that meet a certain minimal standard for efficiency knownas NEMA TP-1 (NEMA stands for the National Electrical Manufacturers Association.) The full name of thestandard is "Guide for Determining Energy Efficiency for Distribution Transformers"1 or "NEMA StandardsPublication TP-1-1996." It is intended to promote the manufacture and use of energy efficient transformersby establishing minimum efficiency standards, albeit with certain assumptions built-in. It contains a simplifiedmethod for evaluating the first cost of transformers along with the costs of core and load losses. It alsopresents tables of minimum transformer efficiencies based on kVA size, voltages, and liquid or dry-type.

Unfortunately, there is nothing especially efficient nor cutting-edge about transformers that meet TP-1. Yes,they are an improvement over so-called "standard" transformers, still made and sold widely, and they areconditionally required in certain states for new construction. However, many transformers are available fromvarious manufacturers that exceed the efficiency levels of TP-1, and may provide a fast payback of theirpurchase price.

The efficiency standards in NEMA TP-1 are based on certain assumptions that may result in the selection ofless-than-optimally efficient transformers. One key assumption is that low voltage (600-volt class) dry-type(typical commercial or industrial) transformers are loaded at 35 per cent of their nameplate rating. Formedium voltage and liquid-filled transformers the assumed loading is 50% of nameplate rating. Another

underlying part of the economic rational for the standard is an assumed electricity cost of 6 cents per kWh.

Both these assumptions may be too low for industrial and commercial users, who often can more accuratelypredict their load requirements, or who may be paying more than 6 cents per kWh, particularly at peaktimes. In fact, recommended loading for economic sizing of a transformer is typically around 75% ofnameplate (35% load, if constant, means the transformer is oversized and wasting core loss as well as wellas higher purchase price.)

The table below, provided by Olsun Electrics, compares a "standard efficiency" 75 kVA transformer to analuminum-wound TP-1 model, a copper-wound TP-1 model, and a "premium efficiency" copper-wound unit,at various loading levels. As the table shows, choosing a more efficient, copper-wound transformer thatexceeds the minimal efficiencies of TP-1 (and Energy Star) can pay back its price premium in as little as oneyear.

Noteworthy is the fact that the TP-1 (Energy Star) efficiency, copper-wound unit, loaded at 75% of itsnameplate capacity (column 7), saves over $88 per year compared to an aluminum-wound TP-1 model(column 6), but costs only $85 more initially. At only 50% loading, the copper TP-1 unit (column 11) savesabout $50 per year compared to the same aluminum unit (column 10). No-load loss (core) is reduced from350 to 320 watts because the greater conductivity of copper windings allows a smaller core to be used, soenergy continues to be saved even at light loading levels.

For even greater savings, the premium efficiency, copper-wound unit saves over $401 per year at 75%loading (column 8), compared to the aluminum TP-1 model (column 6), and costs only $1235 additional. Infact, over a 20-year life (neglecting the time value of money), the total owning cost of the premium efficiency,copper-wound model is $12,399.60 compared to $25,447.00 for the standard efficiency model. The 20-yeartotal ownership cost to buy and operate the premium efficiency transformer is less than one-half the cost ofthe standard model.

Payback time comparison for 75kVA Dry-Type transformers(1) (2) (3) (4) (5) (6) (7) (8)

Standard(Aluminum)

TP-1(Aluminum)

TP-1(Copper)

Premium(Copper)

Standard(Aluminum)

TP-1(Aluminum)

TP-1(Copper)

Premium(Copper)

100 100 100 100 75 75 75 75% of nameplate loadCore loss(w)Conductorloss (w)Total loss(w)Efficiency(%)

3752829320495.9

3501874222497.12

3201670199097.42

190993118398.45

3751591196696.62

3501054140497.56

320940126097.81

190559749

98.69

Transformercost ($) 1336 1979 2064 3214 1336 1979 2064 3214

Additionalcostcomparedwithstandardunit ($)

643 728 1878 643 728 1878

Energycost/year($)

1964.69 1363.76 1220.27 725.42 1205.55 860.93 772.63 459.29

Annualenergy costsavingcomparedwithstandardunit ($)

600.94 744.42 1239.28 344.62 432.92 746.26

Payback 1.07 0.98 1.52 1.87 1.68 2.52

(9) (10) (11) (12) (13) (14) (15) (16)

Standard(Aluminum)

TP-1(Aluminum)

TP-1(Copper)

Premium(Copper)

Standard(Aluminum)

TP-1(Aluminum)

TP-1(Copper)

Premium(Copper)

50 50 50 50 35 35 35 35% of nameplate loadCore loss(w)Conductorloss (w)Total loss(w)Efficiency(%)

375707108297.19

350469819

97.86

320418738

98.07

190248438

98.84

3751591196696.62

350176526

98.04

320157477

98.04

190113303

98.86

Transformercost ($) 1336 1979 2064 3214 1336 1979 2064 3214

Additionalcostcomparedwithstandardunit ($)

643 728 1878 643 728 1878

Energycost/year($)

663.48 502.21 452.54 268.58 1205.55 322.54 292.50 185.80

Annualenergy costsavingcomparedwithstandardunit ($)

161.27 210.94 394.90 69.90 99.95 206.65

Paybackperiod (yrs) 3.99 3.45 4.76 9.20 7.28 9.09

Courtesy: Olsun Electrics, Richmond, IL.Notes:

1. Standard and Aluminum TP-1 units are 150ºC rise, copper TP-1 unit is 115°C rise, Premium unit is 80° C rise.2. Loss values at 100%, 75% and 50% nameplate load are at reference temperature.3. Loss values at 35% nameplate load are at 75°C in accordance with TP-1.4. Energy cost assumed to be $0.07/kWh.

Specifying to Minimize Owning Cost

Whenever possible, always compare competing transformer models by asking for the load and no-loadlosses, in watts, and look at the total cost of ownership. If possible, perform life cycle cost analysis(discussed elsewhere on this Web site). Remember that no-load losses are constant whenever thetransformer is energized. Specifying copper windings can minimize both the load loss and the no-load loss,by allowing for a smaller core. If the load is known or can be predicted, choose a transformer that will beloaded to about 75% of its nameplate rating. Oversizing the unit increases the no-load losses, as well as thepurchase price, unnecessarily.

If the actual losses in watts are not available, and you are seeking the transformer with the lowest losses,choose a transformer with 80° C rise, core of grade M 6 steel or better, and copper windings. Specifying alower temperature rise transformer results in a unit with higher overload capability. For example, an 80°Crise dry-type unit using 220ºC insulation, has 70°C reserve capacity. This allows the 80°C unit to operatewith an overload capability of 15-30% without affecting the transformer life expectancy. Also, a coolerrunning transformer means a more reliable unit, and more up-time.

Overcoming Transformer LossesBy Philip J.A. Ling, P.E., Powersmiths Corp.

Aug 29, 2003 12:00 PM

A more efficient transformer can pay for itself manytimes over during its 25-year lifespan

The transformer plays a key role in an electricalsystem s efficiency and power quality, yet 95% ofbuying decisions are based solely on first cost. Buyingbased on life cycle cost would save literally hundreds ofthousands of dollars in operating losses over theinstalled life for transformers in a typical facility. In fact,the United States Environmental Protection Agency (EPA) estimates that 60 to 80 billion kWhannually can be attributed to transformer losses. These losses cost end-users $3 to $4 billion, andcan tie up nine days of U.S.generating capacity annually.

When feeding the increasinglyelectronic nature of connectedequipment, distortion of the voltagewaveform can reduce the operatingreliability of both the electricalsystem and the connectedequipment. Users can lower energyrates and avoid high transformerlosses by investing in energy-efficient transformers. This articlecompares the efficiency of differenttransformer types including lowtemperature rise, Energy Star, K-rated, as well as new transformersthat are designed to minimize lifecycle cost.

Underestimating loss data.Electronic equipment and other nonlinear loads now make up most of the load on transformers inmany facilities. Even in the average office, many individuals plug in mostly computers, printers,scanners, and other electronics to 120V receptacles. The load profile of electronic equipment fromthe computer in the office to the variable speed drive in the factory drives both additional lossesand unwanted distortion, according to IEEE Standard 519-1992. Since transformer manufacturerstest only under ideal (linear) conditions, as called for in present construction standards, asubstantial gap exists between published loss data and actual losses incurred after installation (Fig.1). In fact, test results published in a 1996 IEEE Transaction paper documented an almost tripling oftransformer losses when feeding 60kW of computer load rather than linear load.

Transformers have two major components that drive losses: the core and the coils. The typical coreis an assembly of laminated steel. Core losses are mostly related to magnetizing or energizing thecore. These losses, also known as no-load losses, are present the entire time the transformer ispowered on, regardless of whether there s any load or not.

Core losses are roughly constant from no-load to full-load when feeding linear loads. They representa continuous cost, 24/7, for the 25- to 40-year life of the transformer. A common 75kVAcommercial transformer has about 400W in no-load losses. At $.10/kWh, this represents acontinuous cost of $350/yr or $14,000 over a 40-year life, eclipsing the purchase cost many timesover. And remember, this is just the cost for powering the unit. The cost of powering the load itselffar exceeds this cost.

The coil losses, commonly referredto as load losses, are associated withfeeding power to the connected load.For linear loads, these losses arepredominately I2R losses. In otherwords, load losses increase by thesquare of current from no-load tofull-load, driven by the resistance ofthe coil. Fig. 2 shows a graphicalrepresentation of how transformerlosses increase with loading.

Since a wide variety of transformersserve different purposes, actuallosses incurred in the field will varysubstantially from one installation toanother. Load level varies widely,with some installations running very heavily loaded and others more lightly loaded. This differencesubstantially affects actual losses incurred.

To calculate the cost of these losses, one must refer to the billing structure of the electric utility.This varies across the country and may involve kWh, kW peak demand, and kVA charges. Taking anexample from Fig. 2, at 60% load the transformer has about 1,500W of losses. If the user is beingbilled only on kilowatt-hour consumption at a rate of $.10/kWh, the operating cost would be 1.5kWx $.10/kWh, which equals $.15/hr or roughly $1,300/year the same order of magnitude as thepurchase price of the transformer. Although some utilities charge by kVA or kW, most charge acombination of a kWh rate and a peak demand charge. Additional distribution or environmentalcosts are also common surcharges included in electrical bills, so be sure to look beyond the cost perkWh.

Comparing transformer losses. Only a limited amount of field data is available on transformerlosses due to the high cost of gathering detailed data from a reasonable number of individualtransformers. Faced with this lack of comprehensive field data, the remaining graphs in this articlerepresent our years of field experience with a combination of published efficiency data under linearand nonlinear load conditions and independent testing, as well as before/after field measurementsto build a series of representative loss curves for different transformers as accurately as possiblewith the data available.

Standard transformer The standard transformer is built to deliver its nameplate kVA rating underlinear load only and is UL Listed on this basis. As it has the lowest purchase price on the market, itrepresents the majority of transformer purchases made across the country. When feeding electronicequipment, substantial derating is required on the order of 50% or more to prevent overheatingand premature failure, according to IEEE Standard 1100-1992. Along with its high operating cost,other factors include a substantial loss in capacity and distortion of the voltage to connectedequipment.

Low temperature rise transformer Transformers with a low operating temperature rise have oftenbeen purchased with energy savings in mind, as published full load losses are substantially lowerthan those of many other transformers. These transformers are traditionally available in either 80°Cor 115°C operating temperature rise, as opposed to the standard 150°C rise that represents themajority of low-voltage, 3-phase, dry-type transformer sales.

The low temperature rise transformer is designed to run cooler than a standard transformer whenfully loaded. To meet this objective, manufacturers typically use a larger core and winding set,resulting in higher no-load losses (more core), but lower load losses (more coil). Since total lossesare the sum of both core and coil losses, the low rise transformer will have higher losses than othertransformers at low load levels where core losses predominate, but lower losses when heavilyloaded, since coil losses predominateat high load levels.

From Fig. 3, it s evident that at lessthan 60% load, it actually costsmore to operate the 80°C risetransformer than the standard 150°Crise transformer. Depending on thesize and manufacturer, the break-even point can be as high as 80%.Since many transformers are loadedto less than 50% capacity, use of an80°C rise transformer is often acommitment to higher energycosts the exact opposite of whatwas intended. Another limitation withthe low temperature rise transformeris that it s UL Listing applies whenfeeding linear loads only.

Energy Star transformer In 1998, the EPA included a high-efficiency transformer program underthe Energy Star banner. For a reference document, the EPA settled on NEMA TP-1 Guide forDetermining Energy Efficiency for Distribution Transformers. The NEMA TP-1 standard establishesrequired efficiencies at 35% load for low-voltage, dry-type transformers, and at 50% load for liquid-filled and medium-voltage, dry-type transformers. In a bid to move the first-cost driven market tohigher efficiency transformers, several states including New York, California, Minnesota, andMassachusetts, adopted NEMA TP-1into law.

However, NEMA TP-1/ENERGY STARtransformer efficiencies referencetest data under linear loadconditions. This results in publishedefficiencies that are much higherthan experienced in the real worlddue to the additional lossesassociated with the widespread useof electronic equipment. Ironically,transformers feeding harmonic-richloads are exempt from meetingNEMA TP-1 benchmark efficiencies.Like the standard transformer, theEnergy Star transformer is built todeliver its nameplate kVA ratingunder linear load and is UL Listed onthis basis. And like standardtransformers, Energy Star transformers exhibit increased losses, loss of capacity, and increasedvoltage distortion when feeding electronic equipment. The Energy Star-compliant transformer ismore efficient than the standard transformer as shown in Fig. 4 (linear loading).

K-rated transformer Unlike standard transformers, which are designed to feed linear loads onlyand lose capacity when feeding nonlinear loads, K-rated transformers are designed to feed nonlinearloads with harmonic content up to their nameplate rating. The UL Listing is maintained as long asthe load profile has a K-factor lower than the K-rating of the transformer. Industry standard ratingsinclude K4, K13, and K20, with K4 and K13 being the most frequently specified. A higher K-ratingrepresents the capability to withstand higher harmonic content.

K-rating is a heat survival rating, nota treatment of associated powerquality issues like voltage distortion,and efficiency isn t typicallydiscussed. Surviving the extra heatmeans using more core and coilmaterial, and sometimes use ofdifferent construction techniques.Depending on the manufacturer sdesign, harmonic losses may bereduced to varying degrees.Ironically, even though thedesignated use of the K-ratedtransformer is to feed nonlinear load,manufacturers publish their loss dataunder linear load conditions.

The need for commissioning.When energy savings are driving part or all of the justification for selecting a particular transformer,it s important that these savings are indeed present once the transformer is installed. This meanscommissioning the transformer for energy performance after installation. In fact, some rebateprograms and other life cycle-oriented programs like Leadership in Efficiency and EnvironmentalDesign call for ongoing product commissioning.

As electronic equipment has become more integrated into our daily lives, transformer losses haveadded a substantial hidden energy cost to the overall operating costs of many buildings. If properlyapplied, energy-efficient transformers can help deliver substantial energy savings and power qualityimprovements.

Ling is vice president technology of Powersmiths Corp. in Irving, Texas.

Sidebar: Pay Me Now or Pay Me Later

Electric utilities have traditionally purchased their distribution transformers based on life cyclecosting, or total cost of ownership (TCO), where the cost of losses is factored into the buyingprocess as they understand the cost of ongoing operating losses. As a result, the trend in thismarket segment is the use of higher efficiency transformers.

The opposite trend is in place in the commercial/industrial world, where the standard low voltagestep-down transformer is widely considered a commodity. The only perceived differentiator isupfront cost, or purchase price, since the lowest first cost wins.

Commercial transformer specifications rarely set a minimum efficiency requirement. As you wouldexpect, building a less efficient transformer is cheaper than building a more efficient one, so atypical low-first-cost transformer will have a low upfront cost but substantially higher operatingcost. And the lifetime cost of the operating losses far exceeds the purchase cost.

The typical buying process makes the situation worse. Traditionally, the consulting engineerspecifies a generic transformer, and a contractor purchases it from a wholesaler. The contractortypically focuses on first cost since the winning bidder is based on providing the lowest bid. The enduser, who pays the electricity bill for the next 40 years, is neither involved in the selection processnor educated about the true operating cost of the unit or the potential savings from using a moreefficient unit. As a result, the fight is over first cost, which for the transformer is on the order of 4%of the life cycle cost. In the end, the end user is stuck with high operating cost, the other 96% ofthe life cycle cost.