Development of REBCO superconducting power transformers in Japan

Physica C 469 (2009) 1726–1732

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Development of REBCO superconducting power transformers in Japan M. Iwakuma a,*, H. Hayashi b, H. Okamoto b, A. Tomioka c, M. Konno d, T. Saito e, Y. Iijima e, Y. Suzuki f, S. Yoshida f, Y. Yamada g, T. Izumi g, Y. Shiohara g a

Research Institute of Superconductor Science and Systems, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Kyushu Electric Power Co., Inc., 2-1-47 Shiobaru, Minami-ku, Fukuoka 815-8520, Japan c Fuji Electric Advanced Technology Co., Ltd., 1 Fujimachi, Hino-city 191-8502, Japan d Fuji Electric Systems Co., Ltd., 1-11-2 Ohsaki, Shinagawa-ku, Tokyo 141-0032, Japan e Fujikura Ltd., 1-5-1 Kiba, Koto-ku, Tokyo 135-8512, Japan f Taiyo Nippon Sanso Co. Ltd., 1-3-26 Koyama, Shinagawa-ku, Tokyo 142-8558, Japan g Superconductivity Research Laboratory, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan b

a r t i c l e

i n f o

Article history: Available online 2 June 2009 PACS: 74.25.Ha 84.71.b 84.71.Mn Keywords: YBCO Superconductor ac loss Transformer

a b s t r a c t In Japan we started a national project to develop a 66/6.9 kV–20 MVA transformer with REBCO superconducting tapes in 2006. This paper gives an overview of progress of the development of superconducting transformers in Japan and also describes the fundamental technologies studied before now to realize a 66/6.9 kV–20 MVA transformer as follows. To reduce the ac loss in REBCO superconducting thin tapes, authors proposed a new method different from the conventional technique of reducing the ac loss in superconducting multifilamentary wires. It consists of scribing process into a multifilamentary structure by laser or chemical etching, and a special winding process. Making a multilayered solenoidal coil with laser-scribed REBCO tapes, we verified the ac loss reduction in proportion to a filament width even in coil configuration. In addition, to realize a current capacity more than the rated secondary current of 2.4 kA, we first investigated the workability of REBCO tapes in the actual winding process with forming a transposed parallel conductor, where REBCO tapes were bent edgewise at transposing points. Making a test coil of a 24-strand parallel conductor, we verified no degradation of the critical current and nearly uniform current distribution among the tapes. The result suggests the applicability of the method of enhancing the current capacity by forming a parallel conductor with REBCO tapes. Further, to realize the dielectric strength regulated for the Japanese standards, i.e. lightning impulse withstand level of 350 kV and excess ac voltage of 140 kV, we made test coils and carried out dielectric breakdown tests. As a result, we got hold of the required insulation distance at the important points from the viewpoint of insulation design. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction In Japan we started a national project to develop superconducting cables, transformers and SMES by using REBCO tapes in 2006. As for transformers, the development target is a 66/6.9 kV–20 MVA superconducting transformer for power distribution substations. As compared with conventional transformers with copper windings, superconducting transformers have the merits of lightweight, compactness and high efficiency. They result from the low loss property and high current density of superconductors. To make more use of such remarkable properties, we have to reduce a one-turn voltage, which is an important parameter for the design of transformers, as compared with that of conventional ones. It corresponds to increasing an electric loading and decreasing a mag* Corresponding author. Tel.: +81 92 802 3831; fax: +81 92 802 3829. E-mail address: [email protected] (M. Iwakuma). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.05.246

netic loading. It means increasing number of turns and reducing the diameter of an iron core. However, it is difficult to remove an iron core even for superconducting transformers. The excitation current of a transformer is generally no more than 100th of a load current though the phase deviates by p/2. It is the benefit by use of an iron core. In case of no iron core, the excitation current needs to be enhanced so that the primary winding generates magnetic field as high as the saturation magnetic flux density of an iron core, i.e. around 1.7 T while the magnetic field for load current is usually no more than 0.3 T. On the other hand, even if a one-turn voltage is reduced and number of turns increases, the dimensions of the windings should become smaller than conventional one if we can make full use of the low loss property and high current density of superconducting windings. In advance of the national project, authors reviewed the development of superconducting transformers so far, whence we defined the problems. It was shown that the key issue for

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realization of superconducting transformers with REBCO tapes was the magnitude of the induced ac loss in superconducting windings. In this paper we first give an overview of progress of the development of superconducting transformers in Japan. Next we report the fundamental technologies studied before now on the following subjects; (1) ac loss reduction in REBCO superconducting thin tapes, (2) enhancement of current capacity up to kA-class without increment of ac loss density, (3) insulation distance to secure the dielectric strength regulated in the Japanese standards. The development schedule of the project is also shown.

2. Progress of the development of superconducting transformers in Japan Twenty years ago Iwakuma et al. started the study of superconducting transformers using NbTi fine multifilamentary wires for ac use [1–3]. From the beginning of studies we experienced various difficult problems such as instability due to high resistivity matrix and uneven current sharing among the strands in multistrand cables and so on [4,5]. In addition, by making a numerical analysis on the response of superconducting windings against various kinds of surge from a power grid, we found out the most serious problems. When a lightning surge penetrates into superconducting windings, the temperature of superconducting windings easily raises beyond Tc due to the ac loss induced by oscillating current accompanied with voltage oscillation [6,7]. In addition, when a switching surge with a long duration of wave tail penetrates, the monotonously increasing current is superimposed and the magnitude of current increases up to several times as much as the rated current. It also causes the quench of superconducting windings [8]. It originates from too small specific heat of constituent materials around 4.2 K. We could not find a solution. Therefore Iwakuma et al. shifted the object of research and development to transformers with oxide superconducting wires, which are possible to operate even in liquid nitrogen at 64–77 K. Quench of the superconducting windings are not easily caused by the disturbances from a power grid since the specific heat is about one thousand times larger than that at liquid helium temperature. In addition, as long as we use subcooled liquid nitrogen at 64–77 K with an atmospheric pressure, cooling condition is much better and it is possible to secure as high dielectric strength as conventional insulation oil due to no bubble of subcooled liquid nitrogen. Before now Iwakuma et al. built four single-phase superconducting transformers on an experimental basis. Three were for a power grid; 6.6/3.3 kV–500 kVA, 22/6.9 kV–1 MVA and 66/6.9 kV–2 MVA [9–11]. The other was an onboard one for Shinkansen rolling stock; 25/1.2/0.4 kV–4 MVA [12]. In every transformer, Bi2223 silversheathed multifilamentary wires were wound. To enhance current capacity of flat superconducting wires, Iwakuma et al. introduced the configuration of parallel conductors. Parallel conductors were formed during the winding process differently from conventional method. Strands were insulated except the both terminals and transposed only at several points so as to be inductively equivalent with each other. So the transposition pitch was no less than several tens meter. Iwakuma et al. also investigated the ac loss properties and current sharing behavior among the constituent strands in the actual superconducting windings [13]. In addition Iwakuma et al. studied fundamental ac loss properties of typical superconducting parallel conductors theoretically and experimentally [14,15]. As a result, it was clarified that almost even current sharing and no additional ac loss are realized even if the transposition points deviate more or less from the optimum points. It means that the ac loss density in the windings composed of superconducting parallel conductors

is equal to that in a single strand under the same condition. On the other hand, the ac losses in the Bi2223 superconducting windings of the above-mentioned four transformers were not as small as we could get the merits of superconducting transformers. For example, the ac loss in the 25/1.2/0.4 kV–4 MVA onboard transformer was 7 kW at the rated operation. The weight of the superconducting transformer itself was 2.1 ton and it was 70% of conventional one. However, the required cryocooler should be larger and heavier than the transformer. It originated from the too large ac loss in a Bi2223 strand. Many efforts to reduce the ac loss in Bi2223 silver-sheathed multifilamentary wires were made until now. However, it has not been realized. What we have to do to realize high-performance superconducting transformers is reducing the ac loss in a superconducting strand itself. Under such a situation, authors studied out a solution to a set of abovementioned problems. It is a transformer with REBCO superconducting thin tapes. We proposed a new idea to reduce the ac loss in a superconducting thin tape even in a coil configuration instead of the conventional concept of ac loss reduction for a superconducting multifilamentary wire. Then we started a new national project to develop superconducting cables, transformers and SMES with REBCO superconducting thin tapes. Hereafter we report the fundamental technologies that we studied before now to realize a 66/6.9 kV–20 MVA superconducting transformer and also the development plan and schedule. 3. ac loss reduction of REBCO superconducting tapes The ac loss induced in the windings due to the perpendicular component of applied magnetic field is the most part of the total heat load of superconducting transformers [13], which determines the required cooling capacity of the cryocooler. In case that ac loss is so large, the size and weight of the cryocooler itself may be larger than a conventional transformer. To make use of superconducting transformers, ac loss induced by the perpendicular component of applied magnetic field should to be reduced at any rate. We studied out a new method to reduce the ac loss of superconducting thin tapes. It is possible to easily reduce the ac loss in a short sample by scribing a superconducting layer including a stabilizing layer in parallel to tape edges into multifilamentary structure by using laser or chemical etching. However, in coil configuration, all of the filaments are connected at the terminals. So shielding current is induced and it generates much coupling current loss. It may result in no effect of scribing. Hence we introduced a new special winding process so as to suppress the shielding current. To verify the validity of the method, we made test coils using YBCO tapes fabricated IBAD-PLD method. The parameters of the tapes are listed in Table 1. The width was 10 mm. The substrate was Hastelloy 100 lm thick. On the surface of a YBCO layer, stabilizing silver was evaporated into a thickness of 10 lm. We scribed the tapes by laser into multifilamentary structure so that the filaments were arrayed in parallel from end to end of a tape as illustrated in Fig. 1. All filaments in a tape are insulated with each

Table 1 Parameters of YBCO superconducting tapes fabricated by IBAD-PLD method. Substrate

Hastelloy

Thickness of substrate Buffer layer Thickness of buffer layer Thickness of YBCO Thickness of Ag stabilizing silver layer Width of tape Self field Ic at 77 K

100 lm Gd2Zr2O7, CeO2 1 lm 2.1–2.3 mm 10 lm 10 mm 230–260 A

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other on the identical substrate since the buffer layer of Gd2Zr2O7 and CeO2 with a thickness of about 1 lm in total is an insulator electrically. We fabricated 3-, 5-, 10- and 20-filament tapes. The photograph of the YBCO tapes scribed into 10- and 20-filament structure is shown in Fig. 2. We also confirmed that the actual resistances between the filaments were larger than 10 kX/cm. First we investigated the ac loss properties of the short scribed tapes by a pickup coil method. The straight sample tapes 60 mm long were stacked into three layers and they were inserted into the center of a saddle-shaped pickup coil [16]. We measured the ac loss by applying magnetic field in perpendicular to the wide surface of the tapes at 77 K. Fig. 3 shows the observed magnetization curves of non-scribed tapes, 5-filament and 10-filament tapes. The magnetization be-

Laser beam

Silver YBCO Buffer layer Hastelloy Fig. 1. Illustration of the process into multifilamentary structure by laser scribing.

comes smaller with larger number of filaments, i.e. in proportion to the width of a filament. Fig. 4 shows the observed amplitude dependences of the ac loss in non-scribed, 3-, 5-, 10- and 20-filament tapes. The ac loss per a unit volume of superconductor for the larger amplitude than the penetration field, which corresponds to the breaking point of an ac loss curve, is generally approximated as the following expression:

W¼

a cw B c mn

ð1Þ

where a and c are pin parameters, Bm is the magnetic field amplitude, w is the width of a tape, n is the number of filaments. Here, as a pin model, Irie–Yamafuji model which is expressed as

J c ðBÞ ¼ aBc1

ð2Þ

is supposed. The ac loss in a short YBCO tape was reduced by scribing in reverse proportion to the number of filaments as theoretically predicted. Next we wound an eight-turn single layer solenoidal coil using a three-filament YBCO tape with a length of 3 m. We investigated the current sharing behavior among the filaments in liquid nitrogen at 77 K applying ac transport current up to 100 A in amplitude. Fig. 5 shows the current waveforms in the respective filaments together with the total current in case of 240 Hz. We can see that uniform current distribution among the filaments is realized. Paying attention to the fact that so-called shielding current is loop current, we can see that uniform current distribution shows that no shielding current is induced among the filaments. Further we fabricated a 16-layer solenoidal coil with a three-filament YBCO tape. The number of turn per layer was six. The photograph of the coil is shown in Fig. 6 and the parameters are listed in Table 2. The total length of a tape was about 24 m. We also fabricated one more coil with the identical dimensions and structure using a non-scribed tape for comparison. The calculated maximum radial and axial components of magnetic field inside the winding were 0.07 and 0.14 T, respectively, at an operating current of 100 A. Here we designed the coils, by referring to the observed ac loss curves in short sample tapes shown in Fig. 4, so that the

Fig. 2. Photograph of 10 mm wide YBCO tapes scribed into 10- and 20-filament structure.

108

40

107

Ac Loss [J/m3cycle]

-µ0M ( T ) per unit volume of YBCO

30 20 10 0

106

105

-10 10 4

-20 -30 -40 -2

reference 5-filament 10-filament

-1

0

1

2

B(T) Fig. 3. Observed magnetization curves of non-scribed, 5- and 10-filament YBCO tapes.

103 10-3

Ic=260A-2.3um-10mm@77K Ic=230A-2.1um-10mm@77K Ic=230A-2.1um-10/3mm@77K Ic=260A-2.3um-10/5mm@77K Ic=260A-2.3um-10/10mm@77K Ic=260A-2.3um-10/20mm@77K

10-2

10-1

100

Field Amplitude [T] Fig. 4. Observed magnetic field amplitude dependences of ac loss in short nonscribed, 3-, 5-, 10- and 20-filament YBCO tapes.

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Total current Filament 1 Filament 2 Filament 3

Filament 1 Filament 2 Filament 3

@240Hz

100

1

0.5

0

0

-50

V (V)

I (A)

50

-0.5

-100 0

0.002

0.004

0.006

0.008

-1 0.01

time (sec) Fig. 5. Observed waveforms of the branch current in each filament and the total transport current. The voltage waveforms that were observed from the potential taps soldered to the individual filaments are also shown by broken lines. Frequency was 240 Hz.

ac loss due to the perpendicular magnetic field against the wide surface of YBCO tapes was the major part of the total ac loss in either coil. Applying ac transport current at 0.01–400 Hz, we measured the ac losses of the coils in liquid nitrogen at 77 K by electrical and thermal methods. The observed ac losses are shown in Fig. 7. We can see that the ac loss of the coil wound with a three-filament tape is reduced to 1/3 as compared with a non-scribed one. Fig. 8 shows the frequency dependences of the observed ac loss in the coil wound with a three-filament tape with a current amplitude as parameter. No frequency dependence was observed. It proves that the ac loss is only the sum of hysteresis loss in each filament. 4. Insulation distance required for 66 kV superconducting transformers According to the domestic standards on transformers in Japan, LIWL and withstand ac voltage for 66/6.9 kV power transformers are regulated as 350 kV and 140 kV, respectively. We aim at getting hold of the required insulation distance in the peculiar system which is mainly composed of REBCO superconducting thin tapes and subcooled liquid nitrogen so as to realize the regulated dielectric strength. To make a dielectric breakdown test, we made model coils so as to simulate the actual situation (1) between primary and secondary windings, henceforth called as between a and (2) between turns. The photographs of the respective model coils and schematic illustrations are shown in Fig. 9. The bobbins were made of GFRP. YBCO superconducting tapes and copper tapes were used as the electrodes. They had the identical dimensions of 5 mm in width and around 0.11 mm in thickness. In the model coils between a, the electrodes were arranged facing each other with a GFRP bobbin between as shown in Fig. 9a. On the other hand, in the model coil between turns, the both electrodes were co-wound side by side with a constant gap as shown in Fig. 9b. The insulation distances were varied as a parameter. Since the dielectric breakdown tests in the identical condition should be carried out 3–5 times at least, many pieces of model coils with the identical structure and dimensions were prepared for every test condition. The dielectric tests were carried out in subcooled liquid nitrogen at 66 K. In the impulse voltage dielectric test, the wave height was 350 kV, the duration of wave front was 1.2 ls and the duration

Fig. 6. Photograph of a six-turn and 16-layer solenoidal coil with a three-filament YBCO tape.

Ac Loss (J/cycle)

100

10-1

10-2

10-3

10mm-3filament No scribing 10mm-3filament (boil-off gas) No scribing (boil-off gas)

Table 2 Parameters of test coils for the verification of low ac loss. Outer diameter of winding Inner diameter of winding Height of winding Number of turns per a layer Number of layers Length of a YBCO tape

98 mm 60 mm 72 mm 6 16 23.6 m

101

I (A)

102

Fig. 7. Current amplitude dependences of ac losses in the coils wound with nonscribed and three-filament tapes. The losses are measured by an electric and a boiloff technique.

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100

400

Breakdown voltage (kV)

AC Loss (J/cycle)

350

-1

10

300 No breakdown

250 200 150 100

AC breakdown voltage (kVrms) Impulse breakdown voltage (kVpeak)

50

10-2

0 85A 70A 50A 30A

10-3 10-2

0

10

20

30

40

50

60

Distance between alfa (mm) Fig. 10. Dependences of the dielectric breakdown voltage on insulation distance in the model coils between the primary winding and secondary one.

10-1

100

Frequency (Hz)

101

102

Fig. 8. Frequency dependence of the ac loss in the coil wound with a three-filament tape.

of wave tail was 50 ls. In the ac voltage dielectric test, ac voltage of 140 kV was applied for 1 min. Fig. 10 shows the observed dependences of the breakdown voltage on insulation distance in the case of the model coils between a. The breakdown voltage for excess ac voltage increased monotonically with insulation distance. We can see from that the insulation distance between a is required to be more than 45 mm to satisfy the regulated ac withstand voltage of 140 kV. On the other hand, with respect to 350 kV dielectric impulse voltage test, no break-

down was observed in case that insulation distance was more than 30 mm. Fig. 11 shows the observed dependences of the breakdown voltage on insulation distance in the case of the model coils between turns. Breakdown voltage increased monotonically with insulation distance, i.e. distance between turns, for both of ac and impulse voltage. The straight line of 17.5 kV represents the target dielectric strength against an impulse voltage. It was estimated by a numerical analysis of the voltage oscillation due to a lightning surge. In addition the straight line of 66 V represents the target dielectric strength against ac excess voltage. We can see that 1 mm is enough for intervals between turns from the viewpoint of dielectric strength.

Fig. 9. Photographs and the schematic illustrations of the model coils for the dielectric breakdown test. (a) Model coils between the primary and secondary windings. (b) Model coils between turns.

M. Iwakuma et al. / Physica C 469 (2009) 1726–1732

1731

Breakdown voltage (kV)

120 AC breakdown voltage (kVrms) Impulse breakdown voltage (kVpeak)

100 80 60 40 20 0

0

1

2

3

4

5

6

Distance between turns (mm) Fig. 11. Dependences of the dielectric breakdown voltage on insulation distance in the model coils between turns.

We have obtained the basic knowledge on the required insulation distance to design a 66 kV transformer using REBCO superconducting tapes and subcooled liquid nitrogen. 5. Large current capacity The secondary current of a 66/6.9 kV–20 MVA transformer is 1.67 kArms. That is current amplitude is 2.37 kA. Therefore the critical current of the secondary winding should be more than 3 kA even in case that a safety coefficient is 1.3. We have to realize it without increment of ac loss density. We decided to adopt the configuration of parallel conductors by which we successfully enlarged current capacity without additional ac loss due to forming those with Bi2223 wires [13]. As a first step of development, we aimed at verifying the applicability of the method of enhancing current capacity by forming a parallel conductor with REBCO superconducting tapes. The constituent stands in parallel conductors should be bent more or less edgewise at the transposition point though REBCO tapes are much more rigid than Bi2223 wires. In the beginning we investigated the workability of REBCO tapes in the actual winding process. We fabricated a test coil forming two pieces of 12-strand parallel conductors, i.e. a 24-strand parallel conductor with REBCO tapes 5 mm in width during the winding process. The 12-strand parallel conductors were cowound side by side into a one-layer solenoide. So the constituent strands, i.e. REBCO tapes were transposed 11 times in each parallel conductor so as to be inductively equivalent with each other. The parameters of the test coil are listed in Table 3. The photograph of the test coil is shown in Fig. 12. To verify the workability, we investigated dc transport property of the test coil at 64 and 77 K. Fig. 13 shows the observed V–I characteristics. The critical current, Ic, at 77 K, which was defined at the criterion of 1 lV/cm, was 1.4 kA. The maximum axial and radial components of magnetic field at that time were 0.119 T and 0.095 T respectively. No degradation of Ic was observed as can be seen by referring to the Ic–B curve of a single tape. Ic at 64 K was more than 2 kA and it was impossible to measure because of limitations of a power supply. Table 3 Parameters of a test coil wound with a 24-strand parallel conductor. Cross section of a superconductor Inner diameter Height of winding Turn number Transposition Total length of REBCO tapes

10 mm  0.25 mm u350 mm 526.6 mm 36 turn = 3 turn  12 block 11 times 960 m = 40 m  12 tapes  2 lines

Fig. 12. Photograph and schematic illustration of the test coil wound with a 24strand parallel conductor.

In addition we investigated the ac transport property. Rogowski coils were attached to every tape near one of terminals. Applying 50 Hz ac current to the test coil at 66 K, we investigated the current sharing behavior among the tapes. The observed branch current waveforms are shown in Fig. 14a together with the total transport current. No phase shift among each branch current was observed. We verified the quite stable operation up to 1 kArms, which was limited due to the capacity of ac power line. The result suggests the applicability of the method of enhancing current capacity by forming a parallel conductor with REBCO tapes. Fig. 14b shows the relative branch current ratios against the total current in the case of 1 kArms. The branch current in each tape deviated by 20% to 30% from even sharing. It seems due to the fact that the tapes were evenly transposed in transposition pitch regardless of magnetic field distribution. It suggests the necessity of improving the design of transposing position. 6. Development schedule We will further develop the fundamental technology including the abovementioned subjects to realize a 66/6.9 kV–20 MVA super-

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6 Coil due to contact resistance Superconductor

5

V (mV)

4 3 2 1 0

0

500

1000

1500

I (A)

(a) 4

Coil due to contact resistance Superconducting winding

V (mV)

3

conducting power transformer. We plan to make a 2–6 MVA class transformer on an experimental basis after 4 years. After the test of the transformer, we will develop the full scale one. In the present project we will also develop the technique to add a current limiting function to transformers. The development of current limiters with YBCO superconducting tapes has been made worldwide. Though Ic values in the windings of current limiters are almost uniform spatially, that in the case of transformers distributes over the windings according to magnetic field distribution. It seems more difficult to realize it than current limiters and superconducting cables. However it will enhance the advantage of superconducting transformers. In addition it will reduce the cost of equipment in a power grid as compared with the case of juxtaposition of a transformer and a current limiter. We plan to develop a 6.9/1.2 kV–400 kVA superconducting transformer with over current limiting function for 4 years besides a 66 kV transformer.

7. Summary 2 1 0 0

500

1000

1500

2000

(b) Fig. 13. Observed V–I characteristics of the test coil wound with a 24-strand parallel conductor at (a) 77 K and (b) 64 K.

We have developed the fundamental technologies to build a 66/ 6.9 kV–20 MVA transformer with REBCO superconducting tapes. We proposed a new method to reduce the ac loss in superconducting thin tapes and verified it. We also got hold of the basically required insulation distance in the system that is composed of REBCO tapes and subcooled liquid nitrogen. Further we verified the applicability of the method of enhancing current capacity due to forming parallel conductors with REBCO tapes. We will further improve such technologies and also develop the technology to add current limiting function to superconducting transformers with aim at combining both technologies when we build a 66/ 6.9 kV–20 MVA transformer on a full scale.

References

Fig. 14. (a) Observed waveforms of the branch current in each REBCO strand of a 24-strand parallel conductor. (b) Ratios of the branch current in each strand to the total current.

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