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Electrical tests of short models of polyethylene tape dielectric for a superconducting ac cable
Electrical tests on short models of the dielectric for a lapped polyethylene insulated superconducting cable are described. Impulse breakdown strengths and 50 Hz discharge inception fields are reported. The impulse strengths were encouraging but the discharge inception fields showed that further development is required,
Electrical tests of short models of polyethylene tape dielectric for a superconducting ac cable B. G. W i l l i a m s
This paper reports the preliminary results of a series of experiments to determine the electrical behaviour of cylindrical models of a polyethylene tape dielectric of the type proposed by Baylis 1 for a superconducting ac cable. The inner conductor was represented by strips o f copper laid helically on a tubular mesh of polyethylene or polypropylene with an outer diameter o f 50 ram. The conductor was covered in turn by a bedding layer, a conductor screen, the dielectric, and an outer conductor screen. These layers were built up from tapes in the manner described by Gibbons et al 2 for a room temperature, sulphur hexafluoride impregnated cable; the number and thickness of tapes in each sample are described in Table 1. The main difference between Gibbons' structure and the one reported here is that the superconducting cable models used carbon-loaded high density polyethylene bedding layer tapes, which were found to be more conducting and less prone to cracking at low temperatures than the low density tape used in the S F 6 cable. On the first few samples low density tape was used as the high density material was not available. The outer conductor on the models was made by wrapping tin foil around the outer screen. The samples were 375 mm long, consisting of a 125 mm test section flanked by 125 mm long terminating sections, which had stress cones incorporating capacitive voltage grading built on them. The samples were tested in the apparatus described by Williams, 3 and the cryogenic conditions were brought as close to 5 K and 4 bar pressure of helium as could be achieved. Measurements were obtained of the 50 Hz discharge inception field, and also of the impulse puncture strength when a 1/50/as voltage impulse was applied. The pattern o f the discharges on the display tube of the detector enabled them to be placed into one of three categories: 1. Discharges in a butt gap (the space between the edges of adjacent turns of a dielectric tape) surrounded by dielectric. 2. Discharges in a butt gap adjacent to one o f the screens.
o f a different type or greater magnitude with a higher inception voltage. These are recorded separately in Table I. It is desirable that the dielectric in a cable is discharge free at fields up to at least 12 MV m q . The results obtained show that most of the samples did not satisfy this requirement; however, from the impulse results (see below) it seems that the stress cones may have been influencing the field at the ends of the test section. This hypothesis is supported by a calculation of the field in the helium in the butt gaps, expected to be the weakest part; the values obtained were considerably lower than the breakdown field of helium at the relevant conditions of temperature and pressure, which suggests that the discharges were not caused by gas breakdown in the butt gaps. The proposed impulse voltage rating requires that the sample should have withstood a field of 82.5 MV m 1 in the dielectric. Only three samples punctured at a field lower than this value, and these had either the mechanically unsatisfactory low density bedding tapes, or no bedding layer at all. The puncture sites were, with one exception, under or near the edge of, one of the stress cones, where the distortion of the field is highest; the true value of the impulse strength of the dielectric in the test section may be higher than the value reported here. Five of the samples did not puncture, but failed the impulse test by flashing over the stress cones; this was probably caused by flaws in the manufacture of the stress cones, which were operating near their voltage limit. Using packs of polyethylene sheets between parallel electrades Meats 4 obtained discharge inception fields in the range 20 to 40 MV m "1 and impulse puncture strengths of 180 to 260 MV m q . These are considerably higher than the values reported here and indicate that an improvement may be possible. However the results obtained with the cylindrical samples indicate that the proposed impulse rating of 82.5 MV m -1 can probably be met, but that the occurrence of discharges needs further study if a 50 Hz operating field of 12 MV m -1 is to be achieved. This work was performed at the Central Electricity Research Laboratories and is published by permission of the Central Electricity Generating Board.
3. Corona into a large volume of gas. Where no such identification was made, it has been shown in Table I. In many cases it was possible to raise the voltage above the first inception voltage found and find discharges The author is with the Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, UK. Received 5 December 1974.
CRYOGENICS. MARCH 1975
References
1 Baylis,J. A. Trans Roy Soc London A 275 205 (1973) 2 Gibbons, J. A. M., Howard, P. R., Skipper, D. J. Proc lEE 122 (1963) 89 3 Williams, B. G. Cryogenics 13 (1973) 613 4 Meats, R. J. Proc Third Int Conf Gas Discharges, IEE Conference Publication, 118 (1974) 419
135
. . t
(J1
"1"
E > :O C~
09
I
;0 -< O C) m Z
co o3
5x90
5x90
5x90
5x90
5x90
none
5x220
5 x 120
5x220
5x220
5x220
5
6
7
8
9
10
11
12
13
14
15
not identified not recorded CA: conductor adjacent DiV: dischargein butt gap
ni: nr:
5x220
5x90
4
17
5x90
3
5x220
5x90
2
16
5x90
2x120
2x120
2 x 120
2 x 120
2 x 120
2 x 120
2x100
2 x 100
2x100
2x100
2x100
2x100
2x100
2x100
2x100
2x100
2x100
No x thickness, /Jm
No x thickness, /Jm
1
Test
Inner screen tapes
Bedding layer tapes
14 x 125
14x75
14x75
14x75
14x75
12 x 75
7x125
7 x 125
7x125
7x125
7x125
7x125
7x125
7x125
5x125
14x125
14x125
No x thickness, /Jm
Dielectric tapes
50/50
33/67
33/67
33/67
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
Registration
Table 1. C o n s t r u c t i o n a n d electrical performance of models
5.6 5.9
2 x 120 2 x 120
6.5
9.8
2 x 120
10.7
4.8
2 x 120
2 x 120
7.8
2 x 100
2 x 120
4
12.2
2 x 100
13.1 15.9
DiV CA
6
23.8 23.0
DiV DiV
4 1.5
17
ni
14.9
13.3
ni ni
22.2
ni
corona
130
240
0.35
25
28
1.8
9
DiV
DiV
92.3
Not impulsed
Puncture at base of cone
Puncture at base of cone 92.3
DiV DiV
Puncture at base of cone
Puncture at base of cone
98
Puncture at base of cone
77 147
Puncture at base of cone
Puncture of test section
83.4
135.3
Puncture at base of cone
77.8
Stress cone flashover
CA
DiV
ni
Puncture at base of cone Stress cone flashover
DiV
100
19.5
129
Not impulsed CA
200
Stress cone flashover
Puncture at base of cone
Stress cone flashover 48
m
9.3
ni
CA
Stress cone flashover m
MV m "1
Impulse breakdown field Failure mode
nr
20
pC
Higher ac Higher ac discharge discharge magnitude type
11.7
15.3
MV m "1 rms
Higher ac discharge inception field
3
0.7
0.3
7
14.5
ni
130
2 x 100
CA
CA
CA
ni
nr
3.3
6.8
8.9
4O
nr
2 x 100
none
11.9
7.9
2 x 100 2 x 100
7.8
2 x 100
ni
9.6
2 x 100
ni
13.6
2 x 100
ni
7
75
14.1
2 x 100 48
pC
MV m "1 rms
No x thickness, /Jm
Lower ac Lower discharge discharge magnitude type
Lower ac discharge inception field
Outer screen tapes
Electrical tests of short models of polyethylene tape dielectric for a superconducting ac cable B. G. W i l l i a m s
This paper reports the preliminary results of a series of experiments to determine the electrical behaviour of cylindrical models of a polyethylene tape dielectric of the type proposed by Baylis 1 for a superconducting ac cable. The inner conductor was represented by strips o f copper laid helically on a tubular mesh of polyethylene or polypropylene with an outer diameter o f 50 ram. The conductor was covered in turn by a bedding layer, a conductor screen, the dielectric, and an outer conductor screen. These layers were built up from tapes in the manner described by Gibbons et al 2 for a room temperature, sulphur hexafluoride impregnated cable; the number and thickness of tapes in each sample are described in Table 1. The main difference between Gibbons' structure and the one reported here is that the superconducting cable models used carbon-loaded high density polyethylene bedding layer tapes, which were found to be more conducting and less prone to cracking at low temperatures than the low density tape used in the S F 6 cable. On the first few samples low density tape was used as the high density material was not available. The outer conductor on the models was made by wrapping tin foil around the outer screen. The samples were 375 mm long, consisting of a 125 mm test section flanked by 125 mm long terminating sections, which had stress cones incorporating capacitive voltage grading built on them. The samples were tested in the apparatus described by Williams, 3 and the cryogenic conditions were brought as close to 5 K and 4 bar pressure of helium as could be achieved. Measurements were obtained of the 50 Hz discharge inception field, and also of the impulse puncture strength when a 1/50/as voltage impulse was applied. The pattern o f the discharges on the display tube of the detector enabled them to be placed into one of three categories: 1. Discharges in a butt gap (the space between the edges of adjacent turns of a dielectric tape) surrounded by dielectric. 2. Discharges in a butt gap adjacent to one o f the screens.
o f a different type or greater magnitude with a higher inception voltage. These are recorded separately in Table I. It is desirable that the dielectric in a cable is discharge free at fields up to at least 12 MV m q . The results obtained show that most of the samples did not satisfy this requirement; however, from the impulse results (see below) it seems that the stress cones may have been influencing the field at the ends of the test section. This hypothesis is supported by a calculation of the field in the helium in the butt gaps, expected to be the weakest part; the values obtained were considerably lower than the breakdown field of helium at the relevant conditions of temperature and pressure, which suggests that the discharges were not caused by gas breakdown in the butt gaps. The proposed impulse voltage rating requires that the sample should have withstood a field of 82.5 MV m 1 in the dielectric. Only three samples punctured at a field lower than this value, and these had either the mechanically unsatisfactory low density bedding tapes, or no bedding layer at all. The puncture sites were, with one exception, under or near the edge of, one of the stress cones, where the distortion of the field is highest; the true value of the impulse strength of the dielectric in the test section may be higher than the value reported here. Five of the samples did not puncture, but failed the impulse test by flashing over the stress cones; this was probably caused by flaws in the manufacture of the stress cones, which were operating near their voltage limit. Using packs of polyethylene sheets between parallel electrades Meats 4 obtained discharge inception fields in the range 20 to 40 MV m "1 and impulse puncture strengths of 180 to 260 MV m q . These are considerably higher than the values reported here and indicate that an improvement may be possible. However the results obtained with the cylindrical samples indicate that the proposed impulse rating of 82.5 MV m -1 can probably be met, but that the occurrence of discharges needs further study if a 50 Hz operating field of 12 MV m -1 is to be achieved. This work was performed at the Central Electricity Research Laboratories and is published by permission of the Central Electricity Generating Board.
3. Corona into a large volume of gas. Where no such identification was made, it has been shown in Table I. In many cases it was possible to raise the voltage above the first inception voltage found and find discharges The author is with the Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, UK. Received 5 December 1974.
CRYOGENICS. MARCH 1975
References
1 Baylis,J. A. Trans Roy Soc London A 275 205 (1973) 2 Gibbons, J. A. M., Howard, P. R., Skipper, D. J. Proc lEE 122 (1963) 89 3 Williams, B. G. Cryogenics 13 (1973) 613 4 Meats, R. J. Proc Third Int Conf Gas Discharges, IEE Conference Publication, 118 (1974) 419
135
. . t
(J1
"1"
E > :O C~
09
I
;0 -< O C) m Z
co o3
5x90
5x90
5x90
5x90
5x90
none
5x220
5 x 120
5x220
5x220
5x220
5
6
7
8
9
10
11
12
13
14
15
not identified not recorded CA: conductor adjacent DiV: dischargein butt gap
ni: nr:
5x220
5x90
4
17
5x90
3
5x220
5x90
2
16
5x90
2x120
2x120
2 x 120
2 x 120
2 x 120
2 x 120
2x100
2 x 100
2x100
2x100
2x100
2x100
2x100
2x100
2x100
2x100
2x100
No x thickness, /Jm
No x thickness, /Jm
1
Test
Inner screen tapes
Bedding layer tapes
14 x 125
14x75
14x75
14x75
14x75
12 x 75
7x125
7 x 125
7x125
7x125
7x125
7x125
7x125
7x125
5x125
14x125
14x125
No x thickness, /Jm
Dielectric tapes
50/50
33/67
33/67
33/67
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
Registration
Table 1. C o n s t r u c t i o n a n d electrical performance of models
5.6 5.9
2 x 120 2 x 120
6.5
9.8
2 x 120
10.7
4.8
2 x 120
2 x 120
7.8
2 x 100
2 x 120
4
12.2
2 x 100
13.1 15.9
DiV CA
6
23.8 23.0
DiV DiV
4 1.5
17
ni
14.9
13.3
ni ni
22.2
ni
corona
130
240
0.35
25
28
1.8
9
DiV
DiV
92.3
Not impulsed
Puncture at base of cone
Puncture at base of cone 92.3
DiV DiV
Puncture at base of cone
Puncture at base of cone
98
Puncture at base of cone
77 147
Puncture at base of cone
Puncture of test section
83.4
135.3
Puncture at base of cone
77.8
Stress cone flashover
CA
DiV
ni
Puncture at base of cone Stress cone flashover
DiV
100
19.5
129
Not impulsed CA
200
Stress cone flashover
Puncture at base of cone
Stress cone flashover 48
m
9.3
ni
CA
Stress cone flashover m
MV m "1
Impulse breakdown field Failure mode
nr
20
pC
Higher ac Higher ac discharge discharge magnitude type
11.7
15.3
MV m "1 rms
Higher ac discharge inception field
3
0.7
0.3
7
14.5
ni
130
2 x 100
CA
CA
CA
ni
nr
3.3
6.8
8.9
4O
nr
2 x 100
none
11.9
7.9
2 x 100 2 x 100
7.8
2 x 100
ni
9.6
2 x 100
ni
13.6
2 x 100
ni
7
75
14.1
2 x 100 48
pC
MV m "1 rms
No x thickness, /Jm
Lower ac Lower discharge discharge magnitude type
Lower ac discharge inception field
Outer screen tapes