applied superconductivity_05

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5 Transformers

SummaryR.F. Giese

Argonne National LaboratoryPotential Application of HTSCs to Power Transformers

B.W. McConnellOak Ridge National Laboratory


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50 Applied Superconductivity

Summary

One-sixth of the annual losses associated with transmitting electricity over the nationalgrid occur in power transformers. Losses in power transformers are equal in magnitudeto the output of five large-scale, base-load power plants. Installation of superconductingpower transformers could reduce these losses.

Section 5 considers a design for a l,OOO-MVA generation step-up transformerwith superconducting windings. incorporating Nb3Sn developed by Westinghouse ElectricCorp. under contract to DOE in 1981. This design, together with the cost assumptions,formed the basis of B.W. McConnells following evaluation of the potential impact of thenew high-temperature superconductors (HTSCs) on power transformers. Since almostnothing is known concerning the AC properties of the new HTSCs, Nb3Sn properties wereassumed (except for the high critical temperature). The results of this analysis indicatethat use of the new HTSCs will result in total life-cycle costs that are 35% lower thanfor Nb3Sn and 60% lower than for conventional power transformers of this size.

To date, no full-scale superconducting power transformer has been built ortested. This is probably due in large part to the high value electric utilities assign toreliability; failure of the power transformer could result in a shutdown of the entiregenerating plant. Furthermore, although the cost savings associated with thesuperconducting power transformer appear to be substantial, the power transformeritself represents only a small part of the entire generating plant.


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Transformers 51

Potential Application of HTSCsto Power Transformers

5.1 INTRODUCTIONThe recent discoveries of materials that are superconducting at temperatures above theboiling point (77 K) of liquid nitrogen (LN2) may allow the development of powerapparatus with significantly higher operating efficiencies and, hence, greatly reducedoperating costs. These materials also might have the advantage of remaining in thesuperconducting state at significantly higher magnetic fields than previously seen inType I and II superconductors. (However, the high field region has not yet been studied indetail.) At present, these high-temperature superconductors (HTSCs) appear to beextremely brittle and have a low current density (nominally 100 A/cm). However,reports of wires and ribbons fabricated from the materials offer hope that potentialfabrication problems can be solved. In addition, IBMs announced increase of the currentdensity in thin films by a factor of 100 is encouraging.

The use of LN2 as a coolant implies immediate economic advantages over thepreviously required liquid helium (LHe). LN2 is considerably less expensive, because thebasic raw material is free and the production process is considerably more efficient. Infact, the process is so inexpensive that the operation of HTSC apparatus at LN2temperatures may well be considered for other technical reasons, even if higher-temperature superconductors are found.

This section presents a first evaluation of power transformers ss onetechnological application of the new HTSCs. This evaluation is based on the followinggeneral assumptions:

1. Extension of previous designs using LHe superconductors to theHTSC operating region is possible.

2. These materials will prove no more difficult to fabricate intoworking configurations than existing applications using Nb3Sn.

3. Adequate bulk current carrying capability can be obtained.4. The AC properties of the materials will be favorable or can be

made favorable.

In addition, the best technological estimates of realistic improvements inoperating efficiencies consistent with other engineering constraints are applied wherepossible. No credit is taken for the higher heat capacities or the greater thermaloperating range present at LN2 temperatures. These latter credits may well further


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improve the HTSC economic advantage and may provide for technical solutions to someperplexing problems seen in LHe designs. Also, no credit is taken for the elimination ofany iron or the subsequent reduction in losses that may be possible with these materials.

Transformer technology is evaluated using the set of baseline economicassumptions presented in App. A. The total l ife-cycle costs (TLCC) are compared forconventional and HTSC applications, and a time to break-even is estimated. Potentialproblems and research areas for the technology are summarized.

5.2 APPLICATION OF SUPERCONDUCTORS TO POWER TRANSFORMERS

5.2.1 Method of AnalysisThe application of high-magnetic-field, high-current-density Type I I super-conductors has presented a challenge to power engineers for the last 30 years. However,

the design of a power transformer using Type I I superconductors has proven to be anextremely difficult engineering problem. First attempts at designing a superconductingtransformer began in 1961 and continued through 1981. Over a ZO-yr period, a trulyviable design was not found. However,DOE/Westinghouse (DOE/WH) project

near the end of this period, a jointdid succeed in achieving a transformer design

that showed favorable economic results and appeared capable of prolonged steady-stateoperation. Prior to this 1981 design, designs were unsuccessful due to a lack ofknowledge of AC losses in Type I I superconductors, the excessive volumes of theconfigurations, and high AC losses due to large AC magnetic fields or largesuperconductor volumes.

The 1981 DOE/WH study produced a design for a l,OOO-MVA generation step-uptransformer, which had superconducting windings and operated at LHe temperatures.The study included an economic comparison of the new design with a conventional designof the same rating. The superconducting design was seen to have an economic advantageas a result of (1) a careful design of the conductors and windings, which substantiallyreduced AC losses, and (2) the inclusion of all costs associated with ownership over thetransformers useful lifetimes (i.e., TLCC).

This evaluation of HTSC application to transformers is based on (1) a carefulextension of the results of the 1981 study using the ANL guidelines for TLCC analysis inthe economic evaluation, (2) the inclusion of common costs that were not previouslyconsidered, and (3) a conservative replacement of the HTSC design during the design lifeof the system. This last change in the economic evaluation is based upon the presenttrend of replacing or overhauling large power transformers at the midpoint of the 30-yrbook life. In this evaluation, the conventional transformer is not replaced during itslifetime; however, the HTSC transformer is replaced at the 10th and 20th years.

The design parameters of the generator step-up transformer under study are:Power 1,000 MVAVoltage 22-500 kVBasic impulse level 1,300 kV


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Transformers 53

Impedance 12%Construction Three-phase, core-form

The original economic study considered only the components of the two designsthat would differ (i.e., core, windings, refrigeration, and losses). Other items, such asthe tank, manufacturing, instrumentation, and bushings, were not included because theircosts were judged to be the same for both designs. Relative costs were computed, with100 being the total cost of the items considered for a conventional unit. The presentstudy includes these latter costs to obtain a more realistic economic evaluation. Theeconomic parameters used in the present study are those provided in App. A. The lossesinclude (1) for the conventional design, conductor IR losses, iron (hysteresis) and stray(or unknown) losses, and dielectric losses and (2) for the superconducting design,conductor AC losses, iron and stray losses, dielectric losses, heat leakage through theleads and dewar, and input power to the refrigerator.

The procedure for adapting the results of the previous study at LHe temperatureto a design at LN2 temperature was to identify the most significant items that would bechanged and to estimate the impact of these changes on the cost. The items that wereidentified are:

1. Refrigeration plant,2. Power requirements for refrigerator to remove low-temperature

losses,3. Superconducting windings, and4. Thermal insulation around superconducting windings.The refrigeration plant is required to remove about 2,000 W from the low-

temperature area. From Fig. 10 of Ref. 1, the efficiency of such a refrigerator is about18% of the Carnot efficiency. Combined with a Carnot efficiency of 77/(300 - 77) andexpressed as a reciprocal of efficiency, the coefficient of performance for an LN2refrigerator is calculated to be 16.1. For this study, a more conservative value of 20 isassumed. Using the two coefficients-of-performance (COP) values, the cost of an LN2refrigerator was determined (from Fig. 11 of Ref. 1) to be about one-eighth that of anLHe refrigerator.

The second item to be altered was the cost of powering the refrigeration. Thiswas accounted for by multiplying the portion of the cost of losses attributable to therefrigeration by the ratio (20/400) of the COP of the two systems. In both cases, therefrigeration plant itself and the cost of refrigeration power, the resultant value issufficiently low that it no longer represents a significant portion of the total cost.Therefore, the result is not sensitive to the exact value of the applied correction factors.

Finally, the superconducting windings and the thermal insulation were assumed tobe equal to LHe values. These represent a small portion of the total costs. The HTSCmaterials are undefined at this time, although there are indications that their brittlenessand difficult handling characteristics will be quite similar to those of Nb3Sn. Since


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fabrication costs will be a large part of the total cost of this material and fabricationprocesses may be quite similar, it is reasonable to assume that the cost of the newmaterial will be close to that of the old. Perturbations on the costs of these materialswere evaluated, and an extreme case, which demonstrates the effect on the final result,is included in Fig. 5.1.

5.2.2 ReaultaThe results of the comparison are shown in Table 5.1, with the base data for the

conventional and LHe-cooled units taken from Table 6 of Ref. 2. The assumption is madethat the previously ignored costs of the tank, manufacturing, instrumentation, andbushings account for about 94% of the capital cost of a conventional transformer. Thisvalue is added to the cost of all three designs, and the other component costs areadjusted so that the TLCC of the conventional transformer continues to be expressed as100, as in the original study.

The result of this adjustment is to show a present-value, life-cycle savings of60% for the complete transformer.

On the basis of the data from Table 5.1, the effect of significant changes in thecost of the superconducting materials was explored. Factors of up to 10 times were

Conventional /go-

Superconductor(Materials costs =10 x costs of LHe

HTSCL (Materials costs =costs of LHe

10 superconductor)000 4 8 12Time i;r)

20 24 28

FIGURE 5.1 Relative Costs of 1,006-MVA Power Transformers(costs are normalized to the cumulative costs of theeomrentional system in year 30)


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Transformers 55

TABLE 5.1 Relative Costs of 1,000~MVA Transformers (costs are nor-malized to the cumulative costs of the conventional system in yeer 30)

Conventional SuperconductingCost I tem Case 1 Case 2 LHe HTSC

Conventional materials 0.47 0.63Superconducting materials 0 0Refrigeration plant 0 0Miscellaneous costs 7.28 9.83EfficiencyCost of lossesb 0.997 0.99792.25 92.25Total life-cycle costsC 100.00 102.71Percent savingsd -3

0.38 0.380.50 0.501.76 0.2212.67 12.670.9985 0.999246.14 25.5362.20 39.6138 60

aIncludes tank, manufacturing, instrumentation, and bushings.bPl-esent value based on 11.55% discount rate, 30-yr book life,and 4% inflation; the capacity factor is 80%.The conventional Case 1 unit is assumed to have a full operatinglife of 30 yt. The conventional Case 2 unit is replaced at15 yr, and the superconducting units are replaced in the 10thand 20th years. The present values of the capital costs areadjusted to reflect these assumptions.dCompared with the conventional Case 1 unit.

applied to this cost, and the effect on present-value cost of the LN3 superconductingtransformer was computed. The results of this comparison indicate that significantvariations in the value of these materials do not greatly change the final result. Thefabricated materials cost used in the LHe study was $150/lb.

Figure 5.1 shows the relative costs of the three designs as a function of time andincludes a case where the cost of the LN3 superconducting materials exceeds the cost ofLHe superconducting materials ten-fold. The payback time for the LN2 design, aboutthree years, is still less than five years if the superconducting material is ten times ascostly. Also, the conventional transformer has several distinct capital advantages in thisanalysis. I f the superconducting transformers are assumed to have an effective life of30 yr, the break-even time is about six months for the HTSC base case. The incrementalcapital costs were calculated to be $4,835/MVA, which means that an HTSC transformerin the l,OOO-MVA size range can have about 250% greater equivalent capital costs than aconventional transformer.


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5.3 TRANSFORMER DESIGN FEATURESSeveral design features should be considered in an evaluation of the results of

this study:l The LHe transformer design is based on using Nb$n as the

superconductor, with a current density of about lo5 A/cm2.Present superconducting materials at higher temperatures may notbe able to sustain currents of this level within the near future. Ifthe current density cannot be increased to at least this level, theHTSC transformer size would become excessive.

l The LHe transformer design had an unresolved technical problemconcerning short-circuit conditions. If a quench occurred as a resultof overcurrent, the LHe refrigeration could not provide adequatecooling to return the windings to the superconducting state.Because of the substantially lower cost of LN2 refrigeration,sufficient cooling capacity could feasibly be included to overcomethis problem.

l Reliability becomes a primary concern when a complicatedapparatus, such as a cryogenic refrigerator, is installed in asystem. However, replacing an LHe refrigerator with an LN2 unitgreatly simplifies the system, and the cost of the LN2 unit issufficiently low that redundancy can be built into the system withlittle economic penalty.

l A central concept in the transformer design used in this study is aconfiguration of four windings, with a main and an auxiliary windingat each voltage level. At a selected overcurrent level, the mainwindings switch to the normal conduction state in response to themagnetic leakage field strength, with the auxiliary windings thencarrying the current and limiting it to a small multiple of the full-load current while remaining superconducting.

l The LHe or LNg superconducting designs will be physically the samevolume as a conventional unit, but they should have a moderateweight advantage. Hence, transportation costs will be comparable.

5.4 CONCLUSIONSThe technology of power transformers, which represents a potential application

for the new HTSCs, has been evaluated. This evaluation was predominantly an economicscoping study developed from previous work on a similar device using earlier, LHe-basedtechnology. Power transformers show a strong potential for significant cost reductionsusing HTSCs when evaluated on a life-cycle basis. Break-even occurs at between sixmonths and three years, and the analysis is considered to be conservative (i.e., favorableto conventional technologies).


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Transformers 57

These evaluations assume that the new HTSCs can be made to perform at leastas well as LHe superconducting materials in their magnetic, current density, andmaterial properties. Specifically, the AC properties of the HTSCs have not yet beendetermined, but they are expected to be similar to the earlier Type I I superconductors.I f this is indeed the case, AC power applications may not be so easily achievable.However, the knowledge gained in applying LHe materials to both AC and DC powerdevices should reduce the amount of time required to achieve useful applications. Forexample, the 20-yr period required to produce a reasonable power transformer may becut in half for the HTSC application.

Several key areas of research appear to have been uncovered by this evaluation.The obvious need for higher current densities and bulk current capability has beenpreviously stated by many researchers. A better understanding of HTSC physics andmaterial properties is also needed. In particular, experimental and theoretical researchon HTSC properties under time-varying magnetic fields must be conducted as soon aspossible.

I f the HTSCs reported to exist above 150 K are consistently reproducible, somesevere thermal difficulties encountered in earlier designs for transformers may beessentially solved by operating these HTSC materials at LN2 temperatures. A moredetailed study of the application of HTSCs to transformers could also identify certainneeded properties that may be producible by materials researchers.

5.5 REFERENCES1. Westinghouse Electric Corp., Appl icati on of Low Temperature Technology to Power

Transformers, U.S. Dept. of Energy Report DOE-ET-29324-l (Feb. 1982).2. Riemersma, H., et al., App li cati on of Superconduct ing Technology t o Pow er Trans-

formers, IEEE Trans. on Power Apparatus and Systems, PAS-100(7):3398-3407 (J uly1991).

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