Residual magnetic field profiles and their current density profiles of coated conductors for fast and slow cut-off current operations

pISSN 1229-3008 eISSN 2287-6251

Progress in Superconductivity and Cryogenics

Vol.17, No.1, (2015), pp.17~20 http://dx.doi.org/10.9714/psac.2015.17.1.017

[```

1. INTRODUCTION

Coated conductor is an important candidate for power cable applications due to its low cost and high current density, which meats increasing demand in electric power applications [1, 2]. In order to design a high temperature superconducting (HTS) cable, we must study the transport properties of HTS tapes even for DC power transmission applications, because we should interrupt its current immediately if an accident happens. HTS tapes have characteristics of hysteresis and are sensitive to the magnetic field induced by the applied transport currents, which affects their performance in the cable.

A DC power cable system was conducted since 2009 when the Ag-sheathed BSCCO tape was relative cheaper and had higher critical current than the coated conductor [3]. As for a power cable system, high current is required for more than 10 kA [4]. Therefore, the magnetic field effects become significant which might affect the performance of HTS tapes themselves inside the cable [5].

Recently, the manufacture sufficiently makes strong and high critical current coated conductor, which is called as second generation (2G) HTS tape aiming to 1 kA of I

.

Residual magnetic field in HTS tapes was studied mostly by magnetization through the magnet [5, 6]. However, after assembling HTS tapes in the cable, the excited transport current produces the magnetic field throughout the cable [7, 8]. As for increasing electric energy consumption around the world, the high current-carrying capacity is needed for the power cable applications and finally straightens the magnetization of HTS tape in the cable. Thus we try to

study de-excitation effects of transport current for different current cut off operations through the residual magnetic field measurements.

In order to evaluate relation of the magnetic field with applied current we developed a scanning magnetic field measurements system by employing a Hall probe. This work presents the measurements of the vertical component of the magnetic fields above an YBCO coated conductor tape by varying applied current pattern. The coated conductor is placed in the liquid nitrogen bath. In the work, a transport current of 100 A, less than the critical current, is applied to the YBCO coated conductor. We measured the residual magnetic field distributions for slow and fast cut off the transport current. The results show differences of the magnetic field profiles and the corresponding current profiles by an inverse solution from the magnetic field measurement between fast and slow cut off current operations because of the hysteresis of coated conductor excited by the transport current.

2. SAMPLES AND EXPERIMENTAL METHOD 2.1. Samples

The YBCO coated conductors are 2G Amperium

Copper Laminated Wire in 12 mm (Type 8502) from American superconductor (AMSC) [2]. Table I presents the specifications of the YBCO coated conductor used in the experiment.

2.2. Scanning Hall Probe System

A scanning Hall probe system was developed to measure

Residual magnetic field profiles and their current density profiles of coated conductors for fast and slow cut-off current operations

J. Sun

, M. Tallouli

, O. Shyshkin

, M. Hamabe

, H. Watanabe

, N. Chikumoto

, and S. Yamaguchi

a

Chubu University, Kasugai, Aichi, Japan

b

V. N. Karazin Kharkiv National University, Ukraine

(Received 20 January 2015; revised or reviewed 16 March 2015; accepted 17 March 2015)

Abstract

Coated conductor is an important candidate for power cable applications due to its high current density. Even for DC power cable transmission, we must study the transport properties of HTS tapes after slow and fast discharge. In order to evaluate relation of the magnetic field with applied current we developed a scanning magnetic field measurements system by employing a Hall probe. This work presents the measurements of the magnetic fields above a coated conductor by varying applied current pattern. In the work, a transport current of 100 A, less than the critical current, is applied to YBCO coated conductor. We measured the residual magnetic field distributions after cut off the transport current with slow and fast operations. The results show differences of the magnetic field profiles and the corresponding current profiles by an inverse solution from the magnetic field measurement between these two operations because of the hysteresis of coated conductor excited by the transport current.

Keywords: Coated conductor, Current density, Hall probe, Magnetic field, Power transmission, Residual magnetic field

* Corresponding author: [email protected]

Residual magnetic field profiles and their current density profiles of coated conductors for fast and slow cut-off current operations

the magnetic field around the HTS tape as in Fig.1 [9].

Table II shows the specifications of the Hall probe. The samples and the Hall probe are immersed into liquid nitrogen filled in an open cryostat. The Hall probe is lifted above the conductor’s surface at a height h of 0.3 mm as in Fig.2. To scan the magnetic field, a DC brushless motor with a step of 0.05 mm controls the Hall probe across the tape surface at a constant altitude and the accuracy is better than 0.0025 mm. The voltage signals and commands are embedded into a computer. We developed a software to collect the data and control the scan system by a graphical LabVIEW language [10].

2.3. Current operations

To simulate a current off accident for an YBCO coated conductor, a circuit loop is built up. The current flows

TABLE I

S

S

O

CC

.

TABLE II

S

P

Fig. 1. Photos of the scanning Hall probe system. The inset is a zoom of Hall probe setup in liquid nitrogen.

Fig. 2. The experimental setup for the magnetic field measurement by a Hall probe. A Hall probe is placed above the tape surface at an altitude of h.

Fig. 3. Measured waveforms of the transport current for slow (a) and fast (b) cut-off operations.

through a shunt resistor of 0.25 mΩ. The transport current can be down quickly by a breaker and slowly by controlling the output of the power supply. Firstly a constant current of 100 A is applied to a coated conductor. The current is cut off by two methods: one is slow down the current by a controller in a speed of 1 A/s as in Fig. 3a and the other one is by a none-fuse break. Fig. 3b shows a measured waveform of cut-off 100 A current by an oscilloscope.

Therefore the cut-off time are about 100 s and 0.1 ms for slow and fast operations, respectively. After cut-off the current of 100 A, we measured the vertical component of the residual magnetic field above the tape surface by a Hall probe.

3. RESULTS AND CALCULATIONS 3.1. Residual magnetic field measurement

Fig. 4 presents the vertical component of the residual magnetic field profiles after slow and fast cut-off the transport current. The profiles are obtained at three different positions along the conductor length y-direction and they show similar curves between each other. The residual magnetic field profiles are almost uniformly distributed for different positions at y-direction, which is different from that when the current is applied to the conductor. Non-uniform exists for magnetic field distributions along the conductor width x-direction.

Magnetic field profiles are almost symmetrical distributed along x direction as in Fig. 4.

18

J. Sun, M. Tallouli, O. Shyshkin, M. Hamabe, H. Watanabe, N. Chikumoto, and S. Yamaguchi

Fig. 4. Measured residual magnetic field profiles above the YBCO coated conductor after cut off a current of 100 A by slow (a) and fast (b) operations.

As for fast and slow current cut-off operations, the residual magnetic profiles are similar and have both peak and valley near the conductor’s edge. However, their intensity of the observed peak/valley is different between slow and fast operations. The peak of magnetic field is about 1 mT for fast operation and about four times that (0.25 mT) of slow operation as in Fig. 4.

3.2. Current density profiles by an inverse solution The corresponding current density profiles are obtained by solving an inverse problem from the measured magnetic field [11, 12]. We solve the inverse problem on the assumption that the current density is uniformly distributed along the tape thickness because of high aspect ratio of width to thickness, 12 mm / 0.001 mm = 12000 and also along the tape length as shown observed magnetic field profiles at three different x-direction positions. The calculation method is described in detail in [11]. Briefly, the measured magnetic field at a constant height above the conductor’s surface can be obtained as Equation (1) by Biot-Savart law. Fig. 5 shows a scheme of calculated model used in the inverse solution. As in Fig. 5, the coated conductor is divided into n sub-elements and has a uniform sheet current density, J

(x

,0) across its cross section.

The sheet current density of ith sub-element J

(x

,0) is obtained by inversely solving the equation, similar to Carrera’s groups [12],

, ( 1 )

Fig. 5. A schematic cross-sectional diagram of the calculated model used in the inverse solution.

Fig. 6. Calculated sheet current density for different current cut-off by slow (a) and fast (b) operations in the YBCO coated conductor. A solid bar indicates a coated conductors width.

where, the vacuum permeability, µ

_{= 4π × 10}

_{H m}

_and the distances between the Hall probe at (x

,h) and ith sub-element at (x

,0), (i+1)th sub-element at (x

,0) are given by respectively.

, ( 2 )

and

, ( 3 )

A Mathwork code was developed to get an inverse solution to J

(x

,0) from a series of linear equations of (1) by Cramer’s rule. Therefore, the sheet current density is calculated as above for each magnetic field profile at three y-direction positions for slow and fast cut off operations as shown in Fig. 6.

The accuracy of the calculation could be controlled in two ways simultaneously as follows. The first check that could be done is to calculate the magnetic field profiles by means of the direct application of Biot-Savart law for the current density profiles shown in Fig. 6, and then to compare these calculated magnetic field profiles with the measured profiles shown on Fig 4. Fig. 7 shows the calculated profiles of the magnetic field from the calculated sheet current density in comparison with the measured ones, which shows a good agreement with each other.

Concerning the present calculation the difference between measured and calculated values of the magnetic field is less than 5%.

An additional check could be done from the point of view that the total residual current in the HTS tape should be zero. For the calculation presented above the value of residual current varies between 0.005 A and 0.01 A.

Similar to magnetic field profiles as in Fig. 4, the calculated current density profiles are different between slow and fast operations because of hysteresis of the superconductor excited by the transport current due to different operations in corresponding to the residual

19

Residual magnetic field profiles and their current density profiles of coated conductors for fast and slow cut-off current operations

Fig. 7. Calculated profiles of the magnetic field from the calculated sheet current density in comparison with the measured ones.

magnetic field. In the center of the tape, the sheet current densities are similar with each other and almost zero. The sheet current density for fast operation becomes stronger than that for slow operation at the tape edges. Big difference of sheet current density at the edges of the conductor leads to the difference of residual magnetic field at the edges between fast and slow operations. From the calculated sheet current density for each sub-element, the induced total current I

can be estimated by

(4)

The induced total current is about 12.5 A for fast operation on contrary to 1.5 A for slow operation. Through the measured residual magnetic field and the analysis of the current density, we speculates that the quality in the tapes from the current cut off operations is needed to be considered in the cable applications.

4. SUMMARY AND CONCLUSIONS

A scanning Hall probe system is developed to study magnetic property of HTS tape conductor. We measured different residual magnetic field above YBCO tape coated conductor for slow and fast cut off of the transport current.

Different de-excitation process of the transport current affects the magnetic property of the coated conductor.

Through solving the inverse problem from the measured magnetic field, the calculated current density profiles are different between slow and fast cut-off the transport current due to hysteresis of YBCO coated conductor. The residual magnetic field measurements show the instability of transport current density and induced current in a coated conductor for different current cut off operations. This information needs to be considered into the cable applications for discharge processes in coated conductors due to hysteresis of superconductors.

ACKNOWLEDGMENT

The authors thank Dr. A. Iiyoshi, Chancellor of Chubu University for his continuous encouragement through this work and Dr. Hiroshi Fujiwara, a professor of Keio University and the president of Nano-Opt Energy Corporation, for his financial support on the construction of the CASER-II. Parts of this work were supported by a grant of Strategic Research Foundation Grant-aided Project for Private Universities Ministry of Education, Culture, Sports, Science and Technology (MEXT). We acknowledge Mr. K.

Yoshimura for his help on experimental setup and also thanks are given to Prof. H. Takano, Prof. A. Ninomiya and Dr. F. Tosin.

REFERENCES

[1] Y. Takahashi, Y. Aoki, T. Hasegawa, Y. Iijima, T. Saito, T.

Watanabe, I. Hirabayashi, Y. Yamada, T. Maeda, T. Honjo, and Y.

Shiohara, “Preparation of YBCO coated conductor by the TFA-MOD method,” IEEE Tran. Appl. Supercond., vol. 13, pp.

2508-2511, 2003.

[2] M. W. Rupich, X. Li, S. Sathyamurthy, C. Thieme, K. Demoranvill, J. Gannon, and S. Fleshler, “Second generation wire development at AMSC,” IEEE Trans. Appl. Supercond., vol. 23, p. 6601205, 2013.

[3] S. Yamaguchi, T. Kawahara, M. Hamabe, H. Watanabe, Y. Ivanov, J. Sun, and A. Iiyoshi, “Experiment of 200-meter sperconducting DC cable system in Chubu University,” Physica C, vol. 471, pp.

1300-1303, 2011.

[4] W.V. Hassenzahl, S.E.C. Eckroad, P.M. Grant, B. Gregory, and S.

Nilsson, “A high-power superconducting DC cable,” IEEE Trans.

Appl. Supercond., vol. 19, pp. 1756-1761, 2009.

[5] J. Sun, H. Watanambe, M. Hamabe, T. Kawahara, and S.

Yamaguchi, “Effects of HTS tape arrangements to increase critical current for the DC power cable,” IEEE Trans. Appl. Supercond., vol.

23, p. 5401104, 2013.

[6] S. Baba, R. Inada, T. Makihara, Y. Nakamura, A. Oota, S.

Sakamoto, S. Li, and P.X. Zhang, “Evaluation of remanant field distributions on Bi2223 tapes with oxide barriers by using scanning Hall-probe microscopy,” J. Phys.: Conf. Ser., vol. 234, p. 022004, 2010.

[7] C. Gu, T. Qu, and Z. Han, “Measurement and calculation of residual magnetic field in a Bi2223/Ag magnet,” IEEE Trans. Appl.

Supercond., vol. 17, pp.2394-2397, 2007.

[8] J. Sun, H. Watanabe, M. Hamabe, N. Yamamoto, T. Kawahara, and S. Yamaguchi, “Critical current behavior of a BSCCO tape in the stacked conductors under different current feeding mode,” Physica C: Superconductivity, vol. 494, pp. 297-301, 2013.

[9] R. Inada, S. Baba, R. Ohtsu, T. Makihara, S. Sakamoto, and A. Oota,

“Longitudinal uniformity of commercial Bi2223 characterized by scanning Hall-probe,” IEEE Trans. Appl. Supercond., vol. 21, pp.

2816-2819, 2011.

[10] J. Sun, M. Emoto, K. Yoshimura, Y. Ivanov, H. Watamabe, M.

Hamabe, T. Kawahara, and S. Yamaguchi, “Development and operation of the measurement system for a 200 m HTS cable,” Proc.

ICEC 23 – ICMC 2010, pp. 1035-1040, Wroclaw 2011.

[11] M. Tallouli, J. Sun, O. Shyshkin, A. Ninomiya, M. Hamabe, H.

Wataanbe, N. Chikumoto, S. Kaddour, S. Yamaguchi, “Residual magnetic field measurement of BSCCO and YBCO HTS tapes by a Hall probe,” IEEE Transl. Appl. Supercond., Accepted for publication.

[12] M. Carrera, X. Granados, J. Amoros, R. Maynou, T. Puig. And X.

Oborados, “Current distribution in HTSC tapes obtained by inververse problem calculation,” J. Phys.: Conf. Ser., vol. 234, p.

012009, 2010.

20