IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011 3261

Innovative Calorimetric AC Loss Measurement ofHTSC for Power Applications

K. W. See, C. D. Cook, and S. X. Dou

Abstract—The applications of high-temperature supercon-ductors (HTS) in electric power components have been widelyreported and various studies have been made to define theiralternating current (AC) losses-a key design parameter for manypractical high power electrical engineering applications. However,very few studies over the range 25 to 45 K have been conductedeven though this is one of the favored temperature ranges forcost-effective applications of HTS. Methods and techniques usedto characterize and measure these losses have been so far groupedinto ‘electrical’ and ‘calorimetric’ approaches with externalconditions set to resemble the application conditions. In thispaper, we present an approach using the calorimetric method toaccurately determine losses in the superimposed AC and DC fieldslikely to be experienced in practical devices such as Fault CurrentLimiters. This technique provides great simplification comparedto pick-up coil and lock-in amplifier methods and is applied toa lower temperature range. The preliminary loss data at 40 Kwill be presented in applied AC magnetic fields with DC fields upto 1T. The data of losses obtained on this sample will allow theestimation and minimization of losses in practical high power HTScoils and will be used in the verification of numerical coil models.

Index Terms—AC losses, calorimetric method, high-temperaturesuperconductors, superimposed AC and DC.

I. INTRODUCTION

R ECENT significant progress in high-temperature super-conductors (HTS) practical application includes the de-

velopment of the fault current limiter, a critical device in anelectric power network that is used to suppress high short circuitcurrent. In the past few years, the superconducting fault currentlimiter (SFCL) has been extensively used in power grids. Gen-erally, SFCL’s can be divided into two primary classes based ontheir working principle, the resistive limiter and the inductivelimiter. Both of these, however, have to cope with the quenchingof the superconductor and thus can suffer from troublesome longrecovery times. The solution was first suggested by B. P. Rajuet al. in early 1980’s [1] by introducing a superconducting dcbias. Later developments included a discussion in 1998 by T.Verhaege and Y. Laumond [2] on a saturated iron core conceptfor a fault current limiter (SICFCL). SICFCL does not need thequench of superconductivity to create sufficient impedance forfault current limiting and the only superconducting portion is a

Manuscript received August 02, 2010; accepted November 09, 2010. Dateof publication December 23, 2010; date of current version May 27, 2011. Thiswork was financially supported by Australian Research Council Linkage ProjectLP0989352.

The authors are with the Faculty of Engineering, University of Wollongong,NSW 2522, Australia (e-mail: kws216@uow.edu.au).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TASC.2010.2092741

dc coil which is used to supply permanent ampere turns to mag-netize the iron cores. The novel concept of this design has beenextended to recent simulation work for Medium Voltage distri-bution systems by S. B. Abbott et al. [3]. Despite the growinginterest in this application, AC loss studies on superconductorssubject to similar condition have yet to be done, particularly inthe temperature range of interest. This is unfortunate as accu-rate knowledge of AC loss from such a device is necessary foroptimizing the dimensions and the cryogenic costs of the su-perconducting part. The superconducting coil in a SICFCL willbe simultaneously exposed to both DC self field and AC mag-netic field from the AC winding. Hence, the work presented inthis paper will investigate the conditions in a SICFCL in orderto allow an accurate estimation of the AC loss. It will includeboth the experimental data and will be supported by theoreticalstudies which are demonstrated to provide a close approxima-tion of the actual losses. This will begin with the worst case con-dition that the sample will be exposed perpendicularly to bothDC and AC external magnetic fields at the intermediate tem-perature 40 K, which is within the typical range for practicalapplications of superconducting materials.

The vigorous development of the(Bi-2223) superconductor by various research groups has re-sulted in significant progress in fabricating an engineering con-ductor with high critical current and a long length by use ofthe silver sheath method. Hence, the commercially producedbi2223 tape provided by Sumitomo Electric [4] is used in thisinvestigation. The ac losses are commonly measured by mag-netization or electrical technique or by using the calorimetricmethod [5], [6]. In the magnetic method, the ac loss is mea-sured by the change of magnetization of the tape whereas inelectrical technique, ac current is driven through the sampleand the voltage across the sample is measured. The calorimetricmethods involve the measurement rate of cryogen boil-off or thetemperature increase of the sample. For most practical purposes,the total energy loss regardless of its transport or hysteresis com-ponents is of interest. Therefore, a calorimetric method is usedhere that measures all the losses in the sample; in this case, theloss results from the applied AC and DC background fields.

The literature reveals the existence of a variety of calorimetrictechniques [7], [8] to determine the ac losses. Most of these arecarried out in a liquid nitrogen bath at 77 K with either trans-port current or applied field only, or with both simultaneously.One may extend S. P. Ashworth et al.’s [9] efforts in devel-oping a local calorimetric method by using a differential ther-mocouple to measure transport current loss and N. Chakrabortyet al.’s [10] in developing a bolometric measurement by usingthe sheath material (eg. Ag) as one of the junctions in the ther-mocouple that is used to measure the temperature rise. In the

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3262 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011

Fig. 1. Overview of the measuring system. The setup is axial symmetric withsuperconducting magnet coil inside the helium reservoir and AC field coil insidethe sample space. The sample is insulated from the coils with G10 phenolicbobbin and exposed to a vertical homogenous magnetic field.

technique used here a new calorimetric apparatus for short sam-ples ( 2.5 cm) with adequate accuracy of AC loss measurementhas been developed.

II. EXPERIMENTAL PROCEDURE

The measuring system consists of two parts, the loss measure-ment and the calibration system. The measurement techniqueused for the losses is by detection of the temperature incrementon the sample by using commercially available Cernox’s RTDsensor. This sensor measured the change in resistance that cor-responds to a change in temperature. There is no need to cali-brate the sensor because only the changes of the resistance withthe calibration system need to be acquired and compared withthe loss measurement. Fig. 1 shows the superconducting magnetcryostat system that is used in this measurement system.

The sample is placed inside the sample holder with transversefield direction from both DC and AC superconducting magneticcoils. The cryostat has an insulating vacuum space between thesample space and the helium reservoir and also between the he-lium and the nitrogen reservoir to reduce heat load and to main-tain temperature stability. To achieve a variable temperature, thebottom and top heater have to be varied accordingly and at thesame time the throttle needle valve on the top flange needs tobe adjusted. This valve serves as a channel between the heliumreservoir and the sample space. The sensor on the respectiveheater shows the temperature in the chamber and hence suggeststhe temperature stability inside the sample space. The currentleads to both DC and AC coils are made of high temperaturesuperconductor to reduce the joule heating that could affect thesensitivity of the measurement. The sample holder is made en-tirely from G10 phenolic material to avoid any induced currentfrom the applied AC field. Fig. 2 shows the AC solenoid (6.0

Fig. 2. The sample is placed symmetrically inside the AC superconducting coilwith G10 phenolic sample holder. The coil is separated from the sample spaceby G10 insulating bobbin.

cm in height and an inner diameter of 3.0 cm) with the sampleplaced inside the insulating bobbin. Apart from supporting thesolenoid, the bobbin is also used to enclose the sample to restrictany heat transfer.

Included in the package that is not shown in Fig. 2 is a smallheater coil made of high resistance wire and temperature sen-sors. The uniqueness of this measurement system is that the cal-ibration method does not need the sample to be in contact withthe current lead or any conducting elements. This is to elimi-nate the end effects associated with short length samples andalso the uncertainties of the thermal properties of the materialsinvolved (if any) which are usually poorly known at low temper-atures. The calibration process is done in exactly the same con-figuration as the ac loss measurement except for the changing ofcurrent lead between the ac and dc power supply. This changewill not affect the result as it only takes place outside the in-sulated sample space. Fig. 3 shows the schematic arrangementand placement of the sensors, heater and sample. The heater iswound around a thin layer of nylon and is placed symmetricallyaround the sample. Heat loss from the heater to the ambient isrestricted as it is enclosed between the G10 bobbin and the nylonlayer. As the sample is closest to the heater, the total heat or en-ergy produced will be instantaneously measured by the sensor.This is because the heating wire has extremely low heat capacityat low temperature.

Two sensors are used to measure the changes of temperaturefrom the sample and the insulated region. The sample is againthermally insulated in the center region with a block of G10 ma-terial, thereby producing a measureable and sensitive tempera-ture change under the applied field condition. As the calibrationand the loss measurements are made at the same temperature,the sensors need not to be calibrated because the sensitivity ofthe sensors varies according to the change of temper-ature. Before taking the measurement, the sample is replacedwith a dummy insulating material to investigate the response ofthe sensors under the applied field condition and to measure anyadditional heat flow in or out of the system.

SEE et al.: INNOVATIVE CALORIMETRIC AC LOSS MEASUREMENT OF HTSC FOR POWER APPLICATIONS 3263

Fig. 3. Schematic sample holder, heater and sensors circuit. The heater iswound axial symmetrically around the sample and properly sealed to ensurethe total heat produced is measure by the sensor.

III. RESULTS AND DISCUSSION

The silver sheathed bi2223 tape was cut into 2.5 cm lengthand loaded into the sample holder as described above. Thesample was first cooled down to liquid helium temperature( 4.2 K) and then heated to 40 K by using the bottom and topheater. The power supply to the heaters is manually controlledby the temperature controller and it determines the level ofstability at the desired temperature. A good stability will have atemperature variation of 0.5 K or less at the set point. With goodthermal insulation, the sample temperature will vary steadilyand slowly until it reaches equilibrium with the heat transferwithin the heater and the helium input from the reservoir. Sincethe heaters and the amount of helium input can be varied, thesystem is capable of achieving any desired temperature from10 K to 100 K with reasonable stability.

Once the sample stabilizes at 40 K, pulses of ac currentwith variable frequency, amplitude and duration can be appliedthrough the magnetizing coil to generate the magnetic field. Inthis paper, results of ac loss for applied sinusoidal field at 50 Hzwith and without DC background field are presented. It is notedthat the field is applied in the perpendicular direction to the tapeflat face. The response of the temperature sensors in the appliedfield without the presence of the sample returns no observabletemperature changes and hence gives confidence that no erroris introduced during the ac loss measurement. Fig. 4 shows theloss measurement conducted on the sample as a function ofthe magnetic field at 0T, 0.5T and 1.0T DC background field.The resistance value corresponds to the temperature rise on thesample and it is directly related to the ac losses of the HTS.

In exactly the same way and arrangement as the loss mea-surement, a dc current is passed through the heating coil for 60seconds and the changes of the sensor reading are recorded forcalibration purposes. The total power supply to the heating coilis measured by placing an external resistive shunt connected tothe dc circuit and the voltage measured across the heating coilterminal inside the chamber. During the calibration experiment,the sample is in the superconducting state and not exposed toany external applied magnetic field. In doing so, all the changesfrom the sensor will be from the calibration coil alone. Fig. 5

Fig. 4. Resistance readings from the sensor associated with the ac losses in thesample tape exposed to an ac applied field perpendicular to the tape face at 40K with and without dc background field. The resistances are plotted against therms applied magnetic field at 50 Hz frequency.

Fig. 5. Calibration curve relating the coil dissipation to measured sensorreading in resistance. The line is a linear fit to the data.

contains a typical set of calibration data at 40 K with the dissi-pation shown in J/m. The calibration constant is obtained fromthe changes in temperature measured by the changes in resis-tance. The calibration constant as calculated from the slope is 6(mJ/m)/ohm.

The results shown in Fig. 6 are the ac losses obtained fromthe calibration constant in J/m/cycle. The figure also shown theresult extracted from Sumitomo [4] at 77 K without applied DCfield for comparison. At higher fields, the losses are lower forthe case with applied DC background field as opposed to thelower applied field. When applied to a DC background field, thecritical current density, of the superconductor is reduced andso the loss is reduced. On the other hand, this does not occur atthe lower applied field region and it can be seen that the lossesare more significant for higher DC background fields. This be-havior has actually been reported by Y. Fukumoto [11] in thepast but with a parallel applied field. Apart from having muchhigher ac losses with perpendicular alignment, the critical statemodel would still be applicable as the hysteresis loss is inversely

3264 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011

Fig. 6. AC losses as a function of the applied transverse magnetic field at 40K and 50 Hz with different DC background magnetic field. The result at 77 Kfrom Sumitomo without background field is also shown for comparison.

proportional to for induced magnetic fields lower than fullpenetration field and vice versa for higher induced fields [12].This is explained in the same manner for the case at 77 K and40 K.

IV. CONCLUSIONS

The apparatus presented for ac loss measurement has provedto be capable of giving results for superconducting wire exposedto conditions that have rarely been investigated previously. Thistechnique in principle provides reasonable accuracy over wideranges of temperatures, magnetic fields and frequencies. Thetechnique is appropriate for short samples at approximately 3cm in length or less, and provides a good measure of the perfor-mance of newly developed or prototype conductors. The smallvolume of the chamber offers a great stability in temperature andhomogeneity in applied magnetic field. The preliminary data inFig. 6 is consistent with the anticipated losses as described in

the critical state model. Additional work is currently being car-ried out to extend the range and accuracies of measurements.

ACKNOWLEDGMENT

The authors thank Dr. Joseph Horvat for valuable commentson the work performed here and Dr. Frank Darmann, from Zen-ergy Power, for providing supporting materials in this work.

REFERENCES

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[2] T. Verhaege and Y. Laumond, “Fault current limiters,” in Handbook ofApplied Superconductivity. Bristol, U.K.: IOP Publishing, 1998, pp.1691–1702.

[3] S. B. Abbott, D. A. Robinson, S. Perera, F. A. Darmann, C. J. Hawley,and T. P. Beales, “Simulation of HTS saturable core-type FCLs forMV distribution systems,” IEEE Trans. Power Delivery, vol. 21, pp.1013–1018, Apr. 2006.

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[10] N. Chakraborty, A. V. Volkozub, and A. D. Caplin, “Bolometric mea-surement of ac loss in HTS tapes: A novel approach of microwatt sen-sitivity,” Supercond. Sci. Technol., vol. 13, pp. 1062–1066, Mar. 2000.

[11] Y. Fukumoto, H. J. Wiesmann, M. Garber, and M. Suenaga, “Al-ternating current losses in mono- and multicored silver sheathed������� �� � � tapes at � �� � in direct currentmagnetic fields,” J. Appl. Phys., vol. 78, pp. 4584–4590, Oct. 1995.

[12] M. Wilson, Superconducting Magnet. Oxford: Oxford UniversityPress, 1983.