Development and tests of extruded ethylenepropylene-rubber-insulated superconducting cable

Crygenics 35 ( 1995) 805-807 0 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 001 I-2275/95/$10.00

Development and tests of extruded ethylenepropylene-rubber-insulated superconducting cable M. Kosaki, M. Nagao, A. Minoda, M. Nagata+* and S. Tanaka”

Y. Mizuno*,

N. Hirata+,

Toyohashi University of Technology, Tempaku, Toyohashi *Nagoya Institute of Technology, Nagoya, Japan +Chubu Electrical Power Inc., Co., Japan *Fujikura, Ltd, Japan ‘The Furukawa Electrical Co. Ltd, Japan

441, Japan

The simultaneous application of the design voltage (20 kVrms) and current (2 kArms) to ethylenepropylene-rubber (EPRI-insulated superconducting cable, cooled by liquid helium, was successfully carried out. The superconductor was a niobium layer clad on a copper pipe. The EPR insulation was extruded simultaneously with semiconducting electrostatic shielding layers. A specific advantage of this cable design is the complete exclusion of the cryogenic helium from the electrical insulation structure. Keywords: ethylenepropylene

rubber; superconducting

cable; insulation

The extensive use of an extruded crosslinked polyethylene (XLPE) insulation for power transmission cables and the excellent insulation capabilities of polymeric materials at cryogenic temperatures suggest the challenge of applying them to the electrical insulation of a superconducting cable. After a long and systematic survey and research on material properties, we concluded that ethylenepropylene rubber (EPR) would be best suited for this purpose. Thus, we undertook the development and testing of extruded EPRinsulated superconducting cables.

The superconductor is a niobium layer clad on a copper pipe specially manufactured for this cable development. The critical current is 8 kA at 4.2 K. The a.c. loss of this superconductor is somewhat smaller than that of the earlier version with helically wound superconductors’.

Design of the superconducting

Electrical insulation

cable

The structure of the present superconducting cable is illustrated in Figure 1; component diameters in millimetres are given in parentheses. Cable specifications are as follows: a.c. voltage to ground, 20 kVrms; a.c. transmitting current, Cor~g.ed

Figure cable.

pol~erhykne

1 Structure of extruded Dimensions are in mm

pipe(65

0)

Corrugated

EPR-insulated

aluminium c+4OD

152)

superconducting

2 kArms; and cable length, minal bushings.

15 m, one piece with two ter-

Conductor

The specific features of this cable are the separation of liquid or gaseous helium coolant from the electrical insulation. The common practice of electrically insulating a superconducting cable is to lap the conductor with plastic tape and to circulate cryogenic helium through the insulation. This design is similar to that of an oil-filled cable and has the distinct advantage of absorbing the thermal contraction. However, there is the drawback of exposing the electrically vulnerable helium to the high electric field region. If a partial discharge starts in the butt gap, the longterm reliability of the electrical insulation is severely impaired. On the other hand, the extruded-polymer insulation design is customary for conventional cables, such as CV cables. If the helium coolant is confined in the centre core pipe, as shown in Figure 1, it is possible to isolate the

Cryogenics

1995 Volume

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Extruded

ethylenepropylene-rubber-insulated

superconducting

helium from the electrical insulation. This design is ideal for insulation reliability because plastic material usually has better insulation capability at cryogenic temperatures: low dielectric loss, improved electrical strength, high electrical tree resistivity, water tree free, and elimination of void of partial discharge by thermal contraction of the bulk. The drawback is the fragility of plastic materials at cryogenic temperatures’. We employed extruded low-density polyethylene (LDPE) and XLPE in the insulation design of our superconducting cable with partial success because they cracked at 170 and 40 K, respectively. EPR proved to have better cryogenic performance than polyethylene, as shown in T&P I, chiefly because the inorganic fillers incorporated for the mechanical strength at room temperature help to decrease the thermal contraction’. Therefore, EPR was selected for the main insulation of the present cable: it was simultaneously extruded with the semiconducting electrostatic shielding layers.

Thermal

Figure

2

University

cable: M. Kosaki et al.

Installation of superconducting of Technology

cable at Toyohashi

insulation

The thermal insulation consisted of multilayered superinsulation and vacuum. An intermediate thermal shield with liquid nitrogen was not incorporated, for simplicity of the cable structure.

Terminal

bushings

The electrical insulation of the terminal bushings was EPR, whose cold end can withstand the repeated thermal cycles from room to liquid-helium temperature. The central pipes of the bushing containing the braided copper wires enabled evaporated helium gas to flow through and to exchange the heat generated internally by Joule loss effectively.

TIME[hl Figure

3

previous

Temperature

changes during the cooling tests

study’. The complete

testing facility

is shown in

Figure 2.

Test results

Testing facility

Cooling test The cooling system consisted of three sections: a cable test set-up to be cooled, a controlled cooling apparatus using evaporated cold nitrogen vapour, and a liquid-helium container. The inlet of one bushing was tied to the controlled precooling apparatus, and the inlet of another to the liquidhelium container. Temperature at the points of concern and the thermal contraction of the insulation surfaces in both the circumferential and longitudinal directions were monitored and acquired in a data acquisition system. The current and voltage test circuit was the same as that used in the

1 Thermal temperature

Table

contraction

of polymers

Thermal contraction (%)

Low-density polyethylene (LDPE) Semiconducting LDPE Crosslinked polyethylene (XLPE) Semiconducting XLPE Ethylenepropylene rubber (EPR) Semiconducting EPR

Cryogenics

at liquid-nitrogen

z

Material

806

Temperature profiles during the cooling test are shown in Figure 3. Although there were local delays in the cooling, by using precooling, the temperatures eventually (after 50 h) came down to about 100 K. The coolant was then switched to liquid helium; the innermost copper pipe was tilled with it 20 h later. At the centre of the cable. the tenperature on the surface of the EPR insulation was 20 K. Contraction curves are shown in Figure 4. The rate of contraction increases with lowering of the temperature. The

1995 Volume

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Figure 4 Contraction of the EPR surface of the cable during the cooling test

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Extruded

ethylenepropylene-rubber-insulated

circumferential contractions were larger than the longitudinal, which suggests that lengthwise contraction was restricted. Note that there were no discontinuities in the curves, which clearly indicates the absence of crack formation during cooling. This was confirmed by the voltage test immediately afterward. Voltage

superconducting 100

The design voltage of 20 kVrms was applied after the copper pipe was filled with liquid helium. There was no problem in the cable withstanding the design voltage. As expected, the PD detector combined with a logical noise discriminator detected no partial discharge along the full length of the cable up to 20 kVrms. So far, we have been able to cool similarly extruded EPR cables with liquid helium more than 10 times without any physical damage’. The same thing can be reported for the two EPR bushings. These results indicate that EPR has dependable mechanical properties in the cryogenic temperature region.

Cable

A

30 n

10 :

_I

ACLOSS --B-

a

3

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_-_ H’8 f

0.03 ; 0. 01 300

loss

a

0.1 7

test

cable: M. Kosaki et al.

Joule loss of bushing (estimateri) __Q__ Joule loss of Cu pipe

Hf

(estimated) n

L/ 1000

500

2000 301O(I

Current [Al Figure

5

Loss

in the cable during the current test

simultaneously for 5 min. Partial discharge was absent during the test, and there was no abnormal change in helium gas evaporation. It can be said that this test was a success.

Conclusions Current

test

Current tests were performed after the copper pipe was filled with liquid helium. Currents of 500, 1000, 1500 and 2000 Arms were applied to the cable for 300 s, and currents of 3000 and 4000 Arms were applied for 15 and 10 s, respectively, without quenching. The cable loss was estimated from the amount of evaporated helium gas; it is shown in Figure 5. The estimated cable loss was consistent with the calculated cable loss including the loss at the bushing with normal conductors, which was much larger than the a.c. loss of niobium superconductors. Simultaneous

voltage

and current

test

After the copper pipe had been filled with liquid helium, a voltage of 20 kVrms and a current of 2 kArms were applied

The development and tests of extruded EPR-insulated niobium superconducting cable were carried out. The cable, with a rating of 20 kVrms and 2kArms, successfully passed the liquid-helium cooling test, the voltage test, the current test and the simultaneous voltage and current test. This cable could be a significant breakthrough in superconducting cable design.

References Kosaki, M., Nagao, M., Mizuno, Y., Shimizu, N. and Horii, K. Development of extruded polymer insulated superconducting cable Cryqenics (1992) 32 885-894 Mizuno, Y., Mitsuyama, Y., Nagao, M. and Kosaki, M. Evaluation of ethylene-propylene rubber as electrical insulating material for superconducting cable IEEE Tran.c E/K Insul ( 1992) EL27 110% III7

Cryogenics

1995 Volume

35, Number

11

807