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Liquid nitrogen impregnated insulation for cryogenic power cables
The measured dissipation factor and breakdown strength of liquid nitrogen impregnated polyethylene paper insulation for cryogenic power cable are presented. The effect of screening, and impulse and ac deteriorations are discussed. The tests were carried out using model cable type specimens.
Liquid nitrogen impregnated insulation for cryogenic power cables M. Fukasawa and H. Nagano
Cryogenic power cables are a hopeful prospect for a large power transmission system, but much still has to be resolved before they can be put to practical use. One of the most important problems is the electrical insulation system, because even with cryogenic power cables high voltage and low loss insulation is needed for transmission of large powers.
16ram~ coppertube electrode
Leadtapeelectrode
At cryogenic temperatures, the electrical characteristics of popular dielectrics are claimed to be better than at room temperature, but there have been no satisfactory test results yet. In selecting insulation systems for cryogenic power cables, not only the electrical properties of materials but also the manufacture and installation of the cable have to be taken into account. In practice, there are obvious advantages in making these cables flexible. In this paper the electrical properties of liquid nitrogen impregnated polyethylene paper wrapped insulation are discussed.
••
~ j~iOOm~estmaterial
Stress
conej
Fig.1 Cross-sectioof n amodelcablespecimen
Test method Tests with liquid nitrogen impregnated dielectric materials were performed using a model cable specimen, whose crosssection is shown in Fig.1. In this model cable specimen, the central electrode is a 16 mm dia copper tube, around which the dielectric material in tape form is wrapped to give a total radial build up of about 1 ram. To prevent breakdown at the end of the specimen, stress cones are built and lead tapes wound for the outer electrode. This specimen construction was chosen as it can be relatively easily applied to practical cables. A simplified cross-section of the test apparatus is shown in Fig.2. The outer vessel is a conventional stainless steel dewar with a liquid nitrogen jacket. The pressure vessel, containing the liquid nitrogen for electrical insulation and the test specimen under investigation, is placed in the dewar which contains the liquid nitrogen coolant. When the pressure vessel is pressurized, the liquid in the pressure vessel is subcooled by the bulk of liquid in the dewar which is boiling at atmospheric pressure.
The authors are with Hitachi Cable Ltd, Research Laboratory, 5--1 Hitaka-cho, Hitachi-shi, Ibaraki-ken, Japan. Received 29 July 1974.
CRYOGENICS. NOVEMBER 1974
Preliminary tests Generally it is believed that the dissipation factor and the breakdown strength of liquid nitrogen impregnated insulation are affected by many factors such as: nature of dielectric material, moisture content of dielectric tape, impurity content of liquid nitrogen, liquid impregnating characteristic, and pressure. In the preliminary test, the following were studied. 1. The dissipation factor and the charge inception voltage of the specimen at atmospheric pressure or only pressurized liquid nitrogen showed great variation sometimes. It seems that the variation of these factors are due to bubbles which are generated by a small amount of heat. Subcooled liquid nitrogen can stop the generation of bubbles. The following tests were all carried out under 5 atm with 77 K subcooled liquid nitrogen. 2. For dielectric tape for the model cable specimen, many materials such as polyethylene film, polyester film, nylon film, polypropylene film, kraft paper, polyethylene paper, etc were studied. As the result, polyethylene paper tape was selected for a cryogenic power cable insulation. Liquid nitrogen impreg-
607
normal room (not controlled) have the same characteristics as the vacuum dried specimens which were placed in 0.005 torr, 60°C for 24 hours. This property is one of the most valuable merits of using a cryogenic power cable for storage and installation. Lead
4. Liquid nitrogen absorbs moisture, oxygen, etc when it contacts air. From the above mentioned result it seems a little moisture does not affect the electrical properties of this insulation system. At the same time, oxygen in liquid nitrogen does not show any effect if less than 1%. Maximum percentage of moisture and oxygen content is not known.
Pressure control valve Vapour outlet
On the basis of this preliminary test result, the following experiments were all carried out with subcooled liquid nitrogen, and without any special treatment for the model cable specimen and liquid nitrogen.
Specimen
Dielectric strength
Liquid nitrogen for insulating
Generally, in the oil impregnated kraft paper wrapped insulation, the dielectric strength of thin paper insulation is higher and is not more scattered than that of the thick paper insulation when the total thickness is constant, because of the surface effect and the masking effect of the weak points. But the thick paper has good mechanical properties.
Liquid nitrogen for cooling
Fig.2
It is thought that the flexible cryogenic cable also needs a few thicknesses of insulating tapes to endure the tape wrapping stress, the cable bending stress, and the electrical stress.
Cross-section of test apparatus
The ac impulse and dc breakdown tests were carried out using model cable specimens with thin and thick, polyethylene paper and kraft paper liquid nitrogen impregnated insulation. Breakdown test results are shown in Fig.3 and physical properties of insulating tapes are shown in Table 1.
nated polyethylene paper insulation has excellent dielectric properties and liquid impregnating characteristics. 3. In OF cable, kraft papers for insulation are impregnated after vacuum drying. But in liquid nitrogen impregnated polyethylene paper insulation, vacuum drying is not needed. Virgin specimens which were constructed and stored in a
In Fig.3, the liquid nitrogen impregnated kraft paper insulation shows the same characteristics as the oil impregnated kraft paper insulation, that is, the dielectric strength of the
Breakdown strength, kV mm "1 60
50
Polyethylene paper
aC
80
90
100
,
I
110
120
Thick paper Thin paper Thick paper
Kraft paper Thin paper Polyethylene paper
70
OqtO 0
I°t
°1 o o
Thick paper Thin paper
Impulse Thick paper
O
Kraft paper Thin paper Polyethylene paper de
Thick paper Thin paper Thick paper
Kraft paper
°
t
°
°
I
°
°
I
°
Thin paper Fig.3
608
Breakdown strength of specimens
CRYOGENICS. NOVEMBER 1974
Table 3. Dissipation factor and breakdown data of model cable specimen without screening
Table 1. Physical properties of insulating material Material
Thickness, #m
Polyethylene paper Thick paper 126 +- 17 Thin paper 64 +- 16 Kraft paper Thick paper 150 Thin paper 80
Density, Permeability, gcm 3 s (100 cm3) -[ 0.68 0.62 0.65 1.10
Dissipation factor,. %
1 021 +_481 392 *- 207 960 3 000
Insulating material Polyethylene paper
A p p l i e d voltage, k V
Polyester film
5
0,001
0.122
0.066
10
0,001
0.123
0.066
15
0,001
0.126
0.065
20
0,001
thin paper insulation is higher than that of the thick paper insulation. As the thin paper insulation has more insulation layers and surface layers than the thick paper insulation for the same thickness, then in the thin paper insulation: 1. Electron awflanches are hard to grow being obstructed by surface barriers.
AC b r e a k d o w n strength, k V m m -1
2. The increase of insulating layers supply weak points in insulators.
Kraft paper
53.0
Impulse b r e a k d o w n strength, k V m m "1
102
-
0.067
50.0
40.0
95.0
-
3. Butt-gaps between wound tapes are more shallow. For this reason, it seems that the breakdown of liquid nitrogen impregnated wrapped insulation is caused by electron avalanche. In polyethylene paper insulation, the breakdown strength of the thin paper insulation is not always higher than that of the thick paper insulation. It is because of this the permeability of thin polyethylene paper is very low. Electrical breakdown occurs at the weakest point in the insulator and low density and low permeability is the weakest point. Electrical properties of liquid nitrogen impregnated polyethylene paper insulation are as follows. ac breakdown strength Impulse breakdown strength dc breakdown strength Permitivity Dissipation factor
53 kV mm "l 102 kV mm q 100 kV mm q 1.7 less than 0.001%
These dielectric strengths are not too low and varied, and these results satisfy to a certain extent our requirements.
Screening The dissipation factor of liquid nitrogen impregnated polyethylene paper insulation is less than 0.001% when screening is not used. But high voltage cryogenic power cable also needs screening. Especially in cryogenic power cables, it must be ensured that the screening reduces the dissipation factor, as the cooling efficiency is low. Some screening materials such as carbon paper, aluminized polyester fihn, and aluminized kraft paper were used and tested. The measured dissipation factors of model cable specimens with and without screening are shown in Tables 2 and 3. The screenings were placed around the outside of the inner copper electrode and the inside of the outer lead electrode. In Table 3, the dissipation factors of liquid nitrogen impregnated kraft paper and polyester film specimens are 0.12% and 0.07% respectively. On the other hand, in Table 2, the dissipation factor of polyethylene paper specimen using paperlike screening such as carbon paper and aluminized kraft paper is about 0.02% and the dissipation factor using fihn-
Table 2. Dissipation factor and breakdown data of model cable specimen with screening Dissipation factor, % Screening material
A p p l i e d voltage, k V
CB paper
Aluminized polyester
AI sputtered polyester
Aluminized kraft paper
Coated CB paper*
5
0.020
0.001
0.001
0.020
0.001
10
0.021
0.001
0.001
0.023
0.001
15
0.022
0.001
0.001
0.024
0.001
20
0.023
0.001
0.001
0.026
0.001
AC b r e a k d o w n strength kV mm 1 t
52.0
Impulse b r e a k d o w n strength, kV mm q 102
53.9
55.6
-
-
55.0 110
51.3 --
*Silicon grease coated CB paper tlnsulation thickness of specimen is 1 mm
C R Y O G E N I C S . N O V E M B E R 1974
609
like screening such as aluminized or aluminium sputtered polyester fdm is less than 0.001%. Taking into account that the permeability of kraft paper and carbon paper is about 1 000 s/lO0 cm 3 and the permeability of the film is extremely large, it seems that the dissipation factor of the polyethylene paper specimen, which uses the paper-like specimen, is due to some particles in the screening tape or adhesive. Also it seems that the effect of polyester film itself when it is used as screening for the polyethylene paper specimen is so small that the reduction of the dissipation factor is not detectable, as dielectric characteristic is a total phenomenon of material, compared with breakdown characteristic which is a partial phenomenon in the weakest part of the material. This phenomenon reappeared when a silicon grease coated carbon paper was used. The dissipation factors of liquid nitrogen impregnated silicon grease coated polyethylene paper specimen without a screening and liquid nitrogen impregnated polyethylene paper specimen with a silicon grease coated CB paper are also less than 0.001%, that is, silicon grease does not reduce the dissipation factor of polyethylene paper specimen and coated silicon grease on the surface of carbon paper prevents the reduction of the dissipation factor. The breakdown strengths of model cable specimens do not depend on whether they are with or without screening, shown in Tables 2, 3.
Impulse deterioration t e s t If corona charge causes harmful damage to the dielectric material impulse voltage, which causes a partial discharge, results in dielectric deterioration even if the applied voltage is less than the breakdown voltage. Impulse deterioration was studied by applying 1 000 shots of the same impulse voltage to the same specimen. Four
kinds of impulse voltage 80 kV, - 8 0 kV, 90 kV, and - 9 0 kV (1 x 40 standard wave) respectively were applied. If impulse voltage causes damage to grow in the model cable specimen, the specimen will be broken down by repeating the same impulse voltage. But all specimens endured 1 000 shots at the same impulse voltage except one specimen which was broken down at first, shown in Fig.4. It is thought that this breakdown voltage 90 kV is within a permissible deviation of the mean impulse breakdown voltage 102.0 + 6.2 kV from Fig.3. Next, the ac breakdown voltage of the specimens, for which 1 000 shots were applied at the same impulse voltage, will be lower than that of a virgin specimen, if impulse voltage deteriorates the delectric. But the ac breakdown voltage of the specimen which was already applied was 57.0 -+ 1.7 kV, and the breakdown voltage of the virgin specimen was 53.0 + 2.4 kV, that is, it appeared that the ac breakdown voltage of the model cable specimen is not decreased by prior application of impulse voltage. A C deterioration test AC deterioration tests have been carried out by applying a constant voltage until breakdown of the specimen. Test apparatus was improved for this to prevent extreme contamination of liquid nitrogen. The test results are shown in Fig.5. The ac breakdown voltage (V, kV) decreases with application time (t, h) exponentially, which is the same as for many other dielectric materials. The relation between V and t is described as follows. V = 47.7 x t'°.°46 Although 30 kV was applied, all the specimen did not breakdown after 1 000 hours and we found there was no corona charge above 1 pC up to 30 kV. AC breakdown strength after applying 1 000 times of impulse voltage, kV mm "1
No of applications 0
250
500
750
1 000 OK
+80 kV
0
30
40
50
60 X
OK .... O K
X
OK OK
- 8 0 kV Impulse voltage applied
~, X
OK
X
OK
+90 kV
OK .... O K
X X ~(
| [ (Breakdown occurs first time) - 9 0 kV
OK
X
OK Fig,4
610
Impulse deterioration test result
CRYOGENICS. NOVEMBER 1974
1. Liquid nitrogen impregnated polyethylene paper wrapped insulation has certain suitable characteristics for cryogenic power cable applications, that is, dielectric properties and liquid impregnating characteristics. Its dissipation factor is less than 0.001% and mean breakdown strengths are 53,102, and 100 kV mm "1 to ac impulse, and dc voltage respectively.
>
y =47.4t -°'°46
60 Initial breakdown voltage
0
+' 5 0 o
40
Breakdown~
< 30
0
Nobreakdown
0.I
I
J
I
I
I0
I00
ICXDO
Applicationtime, h Fig.5
AC deterioration test result
2. The film-like screening such as coated carbon paper and metallized polyester does not reduce the dissipation factor of this insulation system. 3. Liquid nitrogen impregnated polyethylene paper insulation does not deteriorate when 1 000 shots of the same impulse voltage are applied. 4. AC deterioration tests were carried out. It seems that the ac deterioration is small, but is permissible for practical use.
In ac deterioration tests, the dissipation factor was measured about every 100 hours. The dissipation factor was less than 0.001% every time and no reduction was detected.
Considering the experimental data for liquid nitrogen impregnated wrapped insulation, only tentative hypotheses are justified until more data and operating experience have been accumulated.
It seems that the ac deterioration is small, but is permissible for practical use. In addition it seems more data and operating experience should be accumulated.
References Jefferies, M. J., Mathes, K. N. IEEE Trans Power Apparatus and Systems PAS-89 No 8 (1970)
Conclusion
Belanger, B. C. 'Lightning impulse and switching sulge breakdown of liquid nitrogen impregnated wrapped dielectrics for cryogenic cables' Submitted to the Winter Power Conference, February 1971
A liquid nitrogen impregnated insulation system was discussed and following results were derived.
PROCEEDINGS OF THE
International
CryogenicEngineering Conference As for previous conferences in the series, the ICEC5 Proceedings are to be published by IPC Science and Technology Press Ltd.
Reserve your copy now f r o m : IPC Science and Technology Press Ltd, IPC House 32 High Street, Guildford, Surrey, England GU1 3 EW.
The Proceedings, which are planned for publication in November 1974, will contain all the papers presented at Kyoto, as well as a summary of the discussion, in one A4-sized hardbacked book. The ICEC5 Proceedings will be an invaluable source of reference on the latest developments and applications in cryogenic engineering.
C R Y O G E N I C S . N O V E M B E R 1974
611
Liquid nitrogen impregnated insulation for cryogenic power cables M. Fukasawa and H. Nagano
Cryogenic power cables are a hopeful prospect for a large power transmission system, but much still has to be resolved before they can be put to practical use. One of the most important problems is the electrical insulation system, because even with cryogenic power cables high voltage and low loss insulation is needed for transmission of large powers.
16ram~ coppertube electrode
Leadtapeelectrode
At cryogenic temperatures, the electrical characteristics of popular dielectrics are claimed to be better than at room temperature, but there have been no satisfactory test results yet. In selecting insulation systems for cryogenic power cables, not only the electrical properties of materials but also the manufacture and installation of the cable have to be taken into account. In practice, there are obvious advantages in making these cables flexible. In this paper the electrical properties of liquid nitrogen impregnated polyethylene paper wrapped insulation are discussed.
••
~ j~iOOm~estmaterial
Stress
conej
Fig.1 Cross-sectioof n amodelcablespecimen
Test method Tests with liquid nitrogen impregnated dielectric materials were performed using a model cable specimen, whose crosssection is shown in Fig.1. In this model cable specimen, the central electrode is a 16 mm dia copper tube, around which the dielectric material in tape form is wrapped to give a total radial build up of about 1 ram. To prevent breakdown at the end of the specimen, stress cones are built and lead tapes wound for the outer electrode. This specimen construction was chosen as it can be relatively easily applied to practical cables. A simplified cross-section of the test apparatus is shown in Fig.2. The outer vessel is a conventional stainless steel dewar with a liquid nitrogen jacket. The pressure vessel, containing the liquid nitrogen for electrical insulation and the test specimen under investigation, is placed in the dewar which contains the liquid nitrogen coolant. When the pressure vessel is pressurized, the liquid in the pressure vessel is subcooled by the bulk of liquid in the dewar which is boiling at atmospheric pressure.
The authors are with Hitachi Cable Ltd, Research Laboratory, 5--1 Hitaka-cho, Hitachi-shi, Ibaraki-ken, Japan. Received 29 July 1974.
CRYOGENICS. NOVEMBER 1974
Preliminary tests Generally it is believed that the dissipation factor and the breakdown strength of liquid nitrogen impregnated insulation are affected by many factors such as: nature of dielectric material, moisture content of dielectric tape, impurity content of liquid nitrogen, liquid impregnating characteristic, and pressure. In the preliminary test, the following were studied. 1. The dissipation factor and the charge inception voltage of the specimen at atmospheric pressure or only pressurized liquid nitrogen showed great variation sometimes. It seems that the variation of these factors are due to bubbles which are generated by a small amount of heat. Subcooled liquid nitrogen can stop the generation of bubbles. The following tests were all carried out under 5 atm with 77 K subcooled liquid nitrogen. 2. For dielectric tape for the model cable specimen, many materials such as polyethylene film, polyester film, nylon film, polypropylene film, kraft paper, polyethylene paper, etc were studied. As the result, polyethylene paper tape was selected for a cryogenic power cable insulation. Liquid nitrogen impreg-
607
normal room (not controlled) have the same characteristics as the vacuum dried specimens which were placed in 0.005 torr, 60°C for 24 hours. This property is one of the most valuable merits of using a cryogenic power cable for storage and installation. Lead
4. Liquid nitrogen absorbs moisture, oxygen, etc when it contacts air. From the above mentioned result it seems a little moisture does not affect the electrical properties of this insulation system. At the same time, oxygen in liquid nitrogen does not show any effect if less than 1%. Maximum percentage of moisture and oxygen content is not known.
Pressure control valve Vapour outlet
On the basis of this preliminary test result, the following experiments were all carried out with subcooled liquid nitrogen, and without any special treatment for the model cable specimen and liquid nitrogen.
Specimen
Dielectric strength
Liquid nitrogen for insulating
Generally, in the oil impregnated kraft paper wrapped insulation, the dielectric strength of thin paper insulation is higher and is not more scattered than that of the thick paper insulation when the total thickness is constant, because of the surface effect and the masking effect of the weak points. But the thick paper has good mechanical properties.
Liquid nitrogen for cooling
Fig.2
It is thought that the flexible cryogenic cable also needs a few thicknesses of insulating tapes to endure the tape wrapping stress, the cable bending stress, and the electrical stress.
Cross-section of test apparatus
The ac impulse and dc breakdown tests were carried out using model cable specimens with thin and thick, polyethylene paper and kraft paper liquid nitrogen impregnated insulation. Breakdown test results are shown in Fig.3 and physical properties of insulating tapes are shown in Table 1.
nated polyethylene paper insulation has excellent dielectric properties and liquid impregnating characteristics. 3. In OF cable, kraft papers for insulation are impregnated after vacuum drying. But in liquid nitrogen impregnated polyethylene paper insulation, vacuum drying is not needed. Virgin specimens which were constructed and stored in a
In Fig.3, the liquid nitrogen impregnated kraft paper insulation shows the same characteristics as the oil impregnated kraft paper insulation, that is, the dielectric strength of the
Breakdown strength, kV mm "1 60
50
Polyethylene paper
aC
80
90
100
,
I
110
120
Thick paper Thin paper Thick paper
Kraft paper Thin paper Polyethylene paper
70
OqtO 0
I°t
°1 o o
Thick paper Thin paper
Impulse Thick paper
O
Kraft paper Thin paper Polyethylene paper de
Thick paper Thin paper Thick paper
Kraft paper
°
t
°
°
I
°
°
I
°
Thin paper Fig.3
608
Breakdown strength of specimens
CRYOGENICS. NOVEMBER 1974
Table 3. Dissipation factor and breakdown data of model cable specimen without screening
Table 1. Physical properties of insulating material Material
Thickness, #m
Polyethylene paper Thick paper 126 +- 17 Thin paper 64 +- 16 Kraft paper Thick paper 150 Thin paper 80
Density, Permeability, gcm 3 s (100 cm3) -[ 0.68 0.62 0.65 1.10
Dissipation factor,. %
1 021 +_481 392 *- 207 960 3 000
Insulating material Polyethylene paper
A p p l i e d voltage, k V
Polyester film
5
0,001
0.122
0.066
10
0,001
0.123
0.066
15
0,001
0.126
0.065
20
0,001
thin paper insulation is higher than that of the thick paper insulation. As the thin paper insulation has more insulation layers and surface layers than the thick paper insulation for the same thickness, then in the thin paper insulation: 1. Electron awflanches are hard to grow being obstructed by surface barriers.
AC b r e a k d o w n strength, k V m m -1
2. The increase of insulating layers supply weak points in insulators.
Kraft paper
53.0
Impulse b r e a k d o w n strength, k V m m "1
102
-
0.067
50.0
40.0
95.0
-
3. Butt-gaps between wound tapes are more shallow. For this reason, it seems that the breakdown of liquid nitrogen impregnated wrapped insulation is caused by electron avalanche. In polyethylene paper insulation, the breakdown strength of the thin paper insulation is not always higher than that of the thick paper insulation. It is because of this the permeability of thin polyethylene paper is very low. Electrical breakdown occurs at the weakest point in the insulator and low density and low permeability is the weakest point. Electrical properties of liquid nitrogen impregnated polyethylene paper insulation are as follows. ac breakdown strength Impulse breakdown strength dc breakdown strength Permitivity Dissipation factor
53 kV mm "l 102 kV mm q 100 kV mm q 1.7 less than 0.001%
These dielectric strengths are not too low and varied, and these results satisfy to a certain extent our requirements.
Screening The dissipation factor of liquid nitrogen impregnated polyethylene paper insulation is less than 0.001% when screening is not used. But high voltage cryogenic power cable also needs screening. Especially in cryogenic power cables, it must be ensured that the screening reduces the dissipation factor, as the cooling efficiency is low. Some screening materials such as carbon paper, aluminized polyester fihn, and aluminized kraft paper were used and tested. The measured dissipation factors of model cable specimens with and without screening are shown in Tables 2 and 3. The screenings were placed around the outside of the inner copper electrode and the inside of the outer lead electrode. In Table 3, the dissipation factors of liquid nitrogen impregnated kraft paper and polyester film specimens are 0.12% and 0.07% respectively. On the other hand, in Table 2, the dissipation factor of polyethylene paper specimen using paperlike screening such as carbon paper and aluminized kraft paper is about 0.02% and the dissipation factor using fihn-
Table 2. Dissipation factor and breakdown data of model cable specimen with screening Dissipation factor, % Screening material
A p p l i e d voltage, k V
CB paper
Aluminized polyester
AI sputtered polyester
Aluminized kraft paper
Coated CB paper*
5
0.020
0.001
0.001
0.020
0.001
10
0.021
0.001
0.001
0.023
0.001
15
0.022
0.001
0.001
0.024
0.001
20
0.023
0.001
0.001
0.026
0.001
AC b r e a k d o w n strength kV mm 1 t
52.0
Impulse b r e a k d o w n strength, kV mm q 102
53.9
55.6
-
-
55.0 110
51.3 --
*Silicon grease coated CB paper tlnsulation thickness of specimen is 1 mm
C R Y O G E N I C S . N O V E M B E R 1974
609
like screening such as aluminized or aluminium sputtered polyester fdm is less than 0.001%. Taking into account that the permeability of kraft paper and carbon paper is about 1 000 s/lO0 cm 3 and the permeability of the film is extremely large, it seems that the dissipation factor of the polyethylene paper specimen, which uses the paper-like specimen, is due to some particles in the screening tape or adhesive. Also it seems that the effect of polyester film itself when it is used as screening for the polyethylene paper specimen is so small that the reduction of the dissipation factor is not detectable, as dielectric characteristic is a total phenomenon of material, compared with breakdown characteristic which is a partial phenomenon in the weakest part of the material. This phenomenon reappeared when a silicon grease coated carbon paper was used. The dissipation factors of liquid nitrogen impregnated silicon grease coated polyethylene paper specimen without a screening and liquid nitrogen impregnated polyethylene paper specimen with a silicon grease coated CB paper are also less than 0.001%, that is, silicon grease does not reduce the dissipation factor of polyethylene paper specimen and coated silicon grease on the surface of carbon paper prevents the reduction of the dissipation factor. The breakdown strengths of model cable specimens do not depend on whether they are with or without screening, shown in Tables 2, 3.
Impulse deterioration t e s t If corona charge causes harmful damage to the dielectric material impulse voltage, which causes a partial discharge, results in dielectric deterioration even if the applied voltage is less than the breakdown voltage. Impulse deterioration was studied by applying 1 000 shots of the same impulse voltage to the same specimen. Four
kinds of impulse voltage 80 kV, - 8 0 kV, 90 kV, and - 9 0 kV (1 x 40 standard wave) respectively were applied. If impulse voltage causes damage to grow in the model cable specimen, the specimen will be broken down by repeating the same impulse voltage. But all specimens endured 1 000 shots at the same impulse voltage except one specimen which was broken down at first, shown in Fig.4. It is thought that this breakdown voltage 90 kV is within a permissible deviation of the mean impulse breakdown voltage 102.0 + 6.2 kV from Fig.3. Next, the ac breakdown voltage of the specimens, for which 1 000 shots were applied at the same impulse voltage, will be lower than that of a virgin specimen, if impulse voltage deteriorates the delectric. But the ac breakdown voltage of the specimen which was already applied was 57.0 -+ 1.7 kV, and the breakdown voltage of the virgin specimen was 53.0 + 2.4 kV, that is, it appeared that the ac breakdown voltage of the model cable specimen is not decreased by prior application of impulse voltage. A C deterioration test AC deterioration tests have been carried out by applying a constant voltage until breakdown of the specimen. Test apparatus was improved for this to prevent extreme contamination of liquid nitrogen. The test results are shown in Fig.5. The ac breakdown voltage (V, kV) decreases with application time (t, h) exponentially, which is the same as for many other dielectric materials. The relation between V and t is described as follows. V = 47.7 x t'°.°46 Although 30 kV was applied, all the specimen did not breakdown after 1 000 hours and we found there was no corona charge above 1 pC up to 30 kV. AC breakdown strength after applying 1 000 times of impulse voltage, kV mm "1
No of applications 0
250
500
750
1 000 OK
+80 kV
0
30
40
50
60 X
OK .... O K
X
OK OK
- 8 0 kV Impulse voltage applied
~, X
OK
X
OK
+90 kV
OK .... O K
X X ~(
| [ (Breakdown occurs first time) - 9 0 kV
OK
X
OK Fig,4
610
Impulse deterioration test result
CRYOGENICS. NOVEMBER 1974
1. Liquid nitrogen impregnated polyethylene paper wrapped insulation has certain suitable characteristics for cryogenic power cable applications, that is, dielectric properties and liquid impregnating characteristics. Its dissipation factor is less than 0.001% and mean breakdown strengths are 53,102, and 100 kV mm "1 to ac impulse, and dc voltage respectively.
>
y =47.4t -°'°46
60 Initial breakdown voltage
0
+' 5 0 o
40
Breakdown~
< 30
0
Nobreakdown
0.I
I
J
I
I
I0
I00
ICXDO
Applicationtime, h Fig.5
AC deterioration test result
2. The film-like screening such as coated carbon paper and metallized polyester does not reduce the dissipation factor of this insulation system. 3. Liquid nitrogen impregnated polyethylene paper insulation does not deteriorate when 1 000 shots of the same impulse voltage are applied. 4. AC deterioration tests were carried out. It seems that the ac deterioration is small, but is permissible for practical use.
In ac deterioration tests, the dissipation factor was measured about every 100 hours. The dissipation factor was less than 0.001% every time and no reduction was detected.
Considering the experimental data for liquid nitrogen impregnated wrapped insulation, only tentative hypotheses are justified until more data and operating experience have been accumulated.
It seems that the ac deterioration is small, but is permissible for practical use. In addition it seems more data and operating experience should be accumulated.
References Jefferies, M. J., Mathes, K. N. IEEE Trans Power Apparatus and Systems PAS-89 No 8 (1970)
Conclusion
Belanger, B. C. 'Lightning impulse and switching sulge breakdown of liquid nitrogen impregnated wrapped dielectrics for cryogenic cables' Submitted to the Winter Power Conference, February 1971
A liquid nitrogen impregnated insulation system was discussed and following results were derived.
PROCEEDINGS OF THE
International
CryogenicEngineering Conference As for previous conferences in the series, the ICEC5 Proceedings are to be published by IPC Science and Technology Press Ltd.
Reserve your copy now f r o m : IPC Science and Technology Press Ltd, IPC House 32 High Street, Guildford, Surrey, England GU1 3 EW.
The Proceedings, which are planned for publication in November 1974, will contain all the papers presented at Kyoto, as well as a summary of the discussion, in one A4-sized hardbacked book. The ICEC5 Proceedings will be an invaluable source of reference on the latest developments and applications in cryogenic engineering.
C R Y O G E N I C S . N O V E M B E R 1974
611