US3412200A - High voltage cable with potential gradient equalization means - Google Patents

Description

Nov. 19, 1968 LARS'GORAN VIRSBERG ET AL HIGH VOLTAGE CABLE WITH POTENTIAL GRADIENT EQUALIZATION MEANS Original Filed May 2. 1966 INVENTDR. 0am! Vmsme oRflu lIwRsLL inns Leas-6 United States Patent 3,412,200 HIGH VOLTAGE CABLE WITH POTENTIAL GRADIENT EQUALIZATION MEANS Lars-Goran Virsberg, Vasteras, and Lars-Goran Laurell,

Stockholm, Sweden, assignors to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden Continuation of application Ser. No. 546,673, May 2,

1966. This application Dec. 8, 1966, Ser. No. 600,273

Claims. (Cl. 174-102) ABSTRACT OF THE DISCLOSURE A high voltage means comprising a cable with an electrical insulation applied around the conductor and a body of conducting material surrounding the insulation and having a potential considerably different from that of the conductor. The body of conducting material leaves a portion of the insulation exposed outside an edge of the body. A coating of a material having a substantially voltage dependent resistivity is applied to the surface of the exposed portion of the insulation and in electrical contact with the body of conducting material. The coating comprises a wrapping of a tape comprising a binding material selected from the group consisting of thermoplastics and elastomers and silicon carbide particles intermixed in the binding material.

This application is a continuation of application S.N. 546,673, filed May 2, 1966, now abandoned, which is a continuation of application, S.N. 271,726, filed Apr. 9, 1963, now abandoned.

The present invention relates to a coating having a pronounced voltage dependent resistivity for equalizing the potential gradient along the surface of an electrical insulation, particularly on the surface of an exposed insulation outside a screen edge of a cable as described in the co-pending application Ser. No. 271,726, of which the present application is a continuation.

In the US. Patent No. 3,066,180 is described a method of equalizing the potential gradient along the surface of an electrical insulation, which consists in the surface being provided with a coating containing constituents which give the coating a pronounced voltage dependent resistivity. Such a coating has a strong non-linear currentvoltage characteristic so that the voltage over the active part ofthe coating, within a Wide voltage range, becomes substantially constant and independent of the current intensity in that part of the coating. The resistivity of the coatings automatically assumes the most suitable value in each part independent of the potential of the conductor, so that the coatings automatically achieve satisfactory equalization of the potential gradient even if the voltage of the conductor varies within wide limits.

In the said US. patent a thermosetting varnish containing silicon carbide is mentioned as an example of a material from which the coating can be made. -It is stated that this varnish could either be applied directly on to the surface of the insulation or could be applied first on a tape, for example of fibrous material, which is thereafter wound on to the insulation of the conductor, and the varnish subsequently cured.

Especially when the method described in the US. Patent No. 3,066,180 is used for equalizing the potential gradient along the surface of an insulation exposed outside a screen edge of a cable, certain drawbacks are involved in producing the coating with a thermosetting varnish. One drawback is that the cable cannot be mounted immediately the varnish has been applied since the varnish must dry which, particularly if no heating device is "ice available, may take a considerable time. Since joining and connecting of the cables often takes place out of doors, this drawback has considerable importance. Of course, on such occasions working with a fluid product and the inconvenience this involves when applying the product, is also associated with disadvantages, inter alia, with sedimentation problems. Apart from whether the varnish is applied directly on the surface of the insulation or first on a tape which is afterwards wound on to the insulation of the conductor, there are further problems 'with storing the product. This is particularly so if the varnish has a short curing time since its storing time will then be short.

The above-mentioned disadvantages in applying a coating having voltage dependent resistivity on the surface of an insulation, e.g. an insulation exposed outside a screen edge of a cable, are completely avoided according to the present invention. According to the invention the coating comprises .a wrapping of a tape applied around the insulation and comprising a thermoplastic or an elastomer, for example polyvinyl chloride, and silicon carbide mixed in the thermoplastic or elastomer, which gives the tape a pronounced voltage dependent resistivity. By a pronounced voltage dependent resistivity is meant that the exponent a in the equation I =C U has a value of at least about 2, where -I is the current intensity, U the voltage and C a constant.

The tapes according to the invention are flexible, dry and may be stored indefinitely. They can without difficulty and without auxiliary means be applied on the insulation to a coating in the form of a tight, well fitting casing. Immediately after they have been applied, mounting of the cable may be finalized.

The thermoplastic or elastomer in the tapes may, besides the mentioned polyvinyl chloride, consist of, inter alia, polyethylene, polybutylene, polypropylene, ethyl cellulose, polyamide, natural or synthetic rubber, such as natural rubber NR, styrene-butadiene-rubber SBR (e.g. type 1502 according to ASTM), chloroprene rubber CR, neoprene GN-A, nitrite rubber NBR, chlorosulphone rubber Hypalon, butyl rubber IIR (e.g. Esso Butyl 325), stereo isoprene rubber IR (e.g. Shell IR- rubber 305), stereo butadiene rubber BR (e.g. Ameripol CB), poly-urethane rubber.

The silicon carbide may be made of a quality which is normally called electrocarbide and is used in nonlinear components, for example in lightning arresters and varistors. This quality has relatively low volume resistivity. It can also be made of a quality with high volume resistivity. By the expression silicon carbide with high volume resistivity is meant silicon carbide types 'which are electronic conductors usually on n-conducting type and have a volume resistivity of between a few ohm-cm. to several million ohm-cm. The last-mentioned type of silicon carbide is usually green and is normally used as abrasive. Silicon carbide with high volume resistivity has been found particularly suitable for use since it is then possible to use such a high carbide percentage that the occurrence of carbide grain insulated from each other is avoided and thus also the risk of corona within the coating at points with high local field intensity. By using particles with irregular shape, e.g. particles obtained through crushing, special effects are attained which do not appear when using round particles.

The percentage of silicon carbide in the tape is suitably about lO-about 60, preferably 25-50 percent by volume. The particle size of the silicon carbide is suitably smaller than particle size and preferably in the range-particle size 600 to particle size 200. These particle sizes and other particle sizes specified in the specifications are according to US. Standard Sieve Specifications. The carbide may consist of a single particle size or of mixtures of two or several particle sizes.

The tape may be manufactured in such a way that a warm mouldable mixture of the thermoplastic or the elastomer and the silicon carbide particles is shaped to a sheet-' shaped product by passing it through a rolling means with at least one pair of rolls comprising a first and a second rotating hot roll and that the shaped sheet is taken up on a cooling path in the form of a movable carrying support for the sheet, e.g. an endless movable mat or rubber, the cooling path being arranged close to the rolling means and to move with a speed equal to the speed at which the sheet is fed from the pair of rolls. It is suitable that the shaped sheet is brought to move with the second roll while lying against its envelope surface during a part of a revolution before it is taken up on the cooling path, which is arranged to lie close to the envelope surface of this roll and to move with the same speed as the peripheric speed of this roll.

According to an advantageous embodiment the shaped sheet is brought to move with the second roll while lying against its envelope surface during a part of a revolution, after which it is transferred to a third rotating roll, drum or the like, arranged with a small radial distance to the second roll, drum or the like, arranged with a small radial distance to the second roll, the third roll being kept at a lower temperature than the first and second roll and being brought to move with the same periphery speed as the second roll. The shaped sheet is then brought to move with the third roll while lying against its envelope surface during a part of a revolution, before it is taken up on the cooling path, which is arranged to lie close to the envelope surface of the third roll and to move with the same speed as the periphery speed of the third roll. By arranging in the described way the third roll serving as cooling roll, the result is obtained that the shaped sheet does not have any tendency to stick to the cooling path. This is of course of particular importance at continuous manufacture of sheets.

The invention will be explained more closely with reference to the description of some embodiments chosen as examples which are shown in the accompanying drawing, where FIGURE 1 shows a cable comprising a single conductor, FIGURE 2 one comprising three conductors, and FIGURE 3 an arrangement for manufacturing tapes used according to the invention.

In accordance with the FIGURES 1 and 2 the metallic conductor 1 is provided with an insulation 2 which may consist, for example, of plasticized polyvinyl chloride or polyethylene, but also, inter alia, of paper. The insulated conductor has outside the conductor insulation a semiconducting layer 3 which forms an equipotential surface on the outside of the insulated conductor and may consist, for example, of a thermoplastic or an elastomer containing frame building carbon black, or of a graphitized paper. The thermoplastic or elastomer may be formed, inter alia, from polyvinyl chloride, polyethylene, natural or synthetic rubber. The layer may consist of a seamless extruded casing or of spirally running tapes. If the cable has several conductors as in FIGURE 2 these are combined into one cable and provided with a common semiconducting casing 4 which may consist of the same material and be manufactured in the same way as the semiconducting layers on the individual insulated conductors. On top of the layer 3 in the single conductor cable or the common casing 4 in the multi-conductor cable is a metallic sheath 5 which may consist of threads or tapes applied in spiral form or longitudinally applied tapes or seamlessly applied metal such as lead or aluminum. The metal sheath may be outwardly protected by a casing 6 of, for example, polyvinyl chloride.

When a cable of the above described type is being mounted, the metallic screen 5 is earthed so that the semiconducting layer 3 of the insulated conductors receives earth potential either by direct contact with the metal screen as is the case with the single conductor cable according to FIGURE 1 or by contact with the metal screen via the common casing 4, as is the case with the multi-conductor cable according to FIGURE 2. When mounting, the cable ends are scaled and the individual insulated conductors freed as shown in the FIGURES l and 2. In order to equalize the field concentration which arises during testing and operation at the screen edge, according to the invention the insulated part 2 is provided with a coating having a pronounced voltage dependent resistivity, in the form of a wrapping 7 of a tape of a thermoplastic or elastomer, for example polyvinyl chloride, containing silicon carbide. Examples of suitable compositions for the material in the tape are given later on in the description. In the embodiments shown in the figures the coating consists of a tape which is wound around the insulation of each conductor with overlap, for example, 50% overlap. In accordance wih the figures, the coating 7 overlaps the bandage 3 of the semi-conducting tape. The length of the coating is dependent upon the stress which the cable is intended to withstand. Normally a length of a few cm. is sufficient.

In certain cases it may be suitable to allow the coating 7 and the semi-conducting layer 3, whether the cable is a single conductor cable as in FIGURE 2 or a multiconductor cable as in FIGURE 2, to consist of one and the same tape, whereby thus the coating 7 constitutes a continuation of the layer 3 situated within the sheath.

In the following, some examples are given for suitable compositions for the material in the thermoplastic or elastomer tape.

EXAMPLE 1 A mixture consisting of 67 parts by weight polyvinyl chloride, 40 parts by weight dioctyl phthalate, 150 parts by Weight 400 particle size silicon carbide with high volume resistivity, 3 parts by weight of the lead compound 3PhO.PbSO .I-I O and 0.5 part by weight stearic acid are premixed in a ribbon mixer. The percentage by volume of silicon carbide in this thermoplastic mixture is about 33. This powder mixture is then kneaded and gelatined in an internal mixer, e.g. of Banbury type for 5 to 7 minutes with an end and emptying temperature of 145- 150 C. The ready-mixed mixture is then transferred to a two-roller mill, the rolls of which have a temperature of 150-160 C., where it is further Worked for about 5 minutes. From the roller mill the mixture is then successively cut off to be fed to a calender machine. The mentioned about 150 C. heat gelatined mixture, which is designated 10 in FIGURE 3 is fed into the calender machine 13 between the rolls 11 and 12. The rolls have the same periphery speed and are adjusted so that they give a sheet with the desired thickness. The roll 11 has a temperature of about 110-115" 'C. and the roll 12 a temperature of about 125-135 C. The sheet 14 formed by the passage of the pair of rolls follows the rolls 12 and is transferred from this to a cold roll 15. This roll has the same periphery speed as the other rolls and a temperature lower than 50 C., e.g. 20-25 C. It serves as a cooling roll. The distance between the middle roll 12 and the lowest roll 15 is so great that the rolled sheet goes clear between these rolls. Under the lowest roll there is a cooling path 16 in the form of an endless rubber mat or an endless steel belt or the like which follows a part of the envelope surface of the roll and which is driven by a roller 17 and goes around the rollers 18 and 19. The cooling path is driven with the same speed as the periphery speed of the lowest roll 15. The sheet is taken up and transported by the cooling path. After passage of the rollers 20, 21 and 22, the sheet is wound up on to the Winding device 23.

Under certain circumstances it is possible to make the sheet without the use of the lowest roll 15. The cooling path 16 is then arranged on the under side of the roll 12. This can be made analogously in the manner shown in FIGURE 3 but with respect to the opposite feeding direction of this roll, the cooling path has to be turned so that its movement direction is opposite to that shown in FIGURE 3.

The manufactured sheet may be cut in its longitudinal direction to tapes 7 with desired width. The tape 7 in FIGURES 1 and 2 may for example have a width of about 20 mm. and a thickness of about 0.3 mm. and may be applied with for example 75% overlap in accordance with the figures on to a 10 kv. high voltage cable provided wtih plastic insulation so that the length of the coating is about 7 cm.

EXAMPLE 2 In the same way and using the same material given in Example 1, with the exception that instead of silicon carbide with high volume resistivity, electroca-rbide" is used, a tape is produced which is then applied in the way illustrated in Example 1.

EXAMPLE 3 67 parts by weight polyvinyl chloride, 33 parts by weight diocytl phthalate, 3 parts by weight 3 PbO'PbSO H and 100 parts by weight 230 particle size silicon carbide having high volume resistivity are mixed together. The percentage by volume of the silicon carbide in this thermoplastic mixture is about 27. In the same way as described in Example 1, a tape is produced which is then applied similarly to the described method in Example 1.

EXAMPLE 4 In the same way and using the same material as mentioned in Example 3, with the exception that instead of silicon carbide having high volume resistivity electrocarbide is used, a tape is produced which is then applied in the way illustrated in Example 1.

EXAMPLE 5 67 parts by weight polyvinyl chloride, 33 parts by weight dioctyl phthalate, 3 parts by weight 3 PbO-PbSO -H O and 150 parts by weight 600 particle size silicon carbide having high volume resistivity are mixed together. The percentage by volume of silicon carbide in this thermoplastic mixture is about 38. In the same way as described in Example 1 a tape is produced which is then applied in a similar way to that described in Example 1.

EXAMPLE 6 In the same way and using the same material as mentioned in Example 5, with the exception that instead of silicon carbide having high volume resistivity electrocarbide is used, a tape is produced which is then applied in the way illustrated in Example 1.

EXAMPLE 7 67 parts by weight polyvinyl chloride, 33 parts by weight dioctyl phthalate, 3 parts by weight 4 PhD- PbSO -H O and 200 parts by weight 400 particle size silicon carbide having high volume resistivity are mixed together. The percentage by volume of silicon carbide in this thermoplastic mixture is about 47. In the same way as described in Example 1 a tape is produced which is then applied in a similar way to the method decsribed in Example 1.

EXAMPLE 8 In the same way and using the same material as mentioned in Example 7, with the exception that instead of silicon carbide having high volume resistivity electrocarbide is used, a tape is produced which is then applied in the way illustrated in Example 1.

EXAMPLE 9 100 parts by weight Hypalon (E. I. du Pont de Nemours and Co., U.S.A.) is kneaded in an internal mixter, e.g. of Banbury type for about 4 minutes, the temperature then being raised due to developed friction heat, after which 150 parts by weight 400 particle size silicon carbide with high volume restivity is added and the mixture kneaded for about a further 4-6 minutes. The percentage by volume of silicon carbide in this mixture is about 36. The mixture is then transferred to a two-roller mill, the rolls of which have a temperature of between 60-80 C. From the roller the mixture is then succesfully cut off to be fed to the calender machine 13 according to FIGURE 3 and a sheet is produced as described in Example 1. However, the temperature of the rolls 11 and 12 is in this case lower, suitably 50-70 C. The sheet is then as described in Example 1 cut up into tapes with desired width and the tapes applied in the way described in Example 1.

EXAMPLE 10 In the same way and using the same material given in Example 9, with the exception that instead of silicon carbide with high volume resistivity, electrocarbide is used, a tape is produced which is then applied in the way illustrated in Example 1.

EXAMPLE 11 In the same way and using the material given in Example 9, with the exception that instead of parts by weight silicon carbide 200 parts by weight silicon carbide is used, a tape is produced when is then applied in the way illustrated in Example 1. The percentage by volume of silicon carbide in the tape is about 43.

EXAMPLE 12 In the same way and using the material given in Example 10, with the exception that instead of 150 parts by weight silicon carbide 200 parts by Weight silicon carbide is used, a tape is produced which is then applied in the way illustrated in Example 1. The percentage by volume of silicon carbide in the tape is about 43.

Any one of the rubbers listed in col. 2 together with Hypalon can be used instead of Hypalon in the Examples 9-12 for manufacturing the tape under the conditions stated in these examples and the tape produced can be applied as described in Example 1.

It should be clear that the invention can with advantage be utilized not only for an insulation applied outside a screen in a cable, but also in several other cases where it is necessary to equalize the potential gradient on the surface of an insulation such as, for example at the free coil ends of electrical machines and with high tension bushings.

We claim:

1. A high voltage means comprising a cable with at least one electrical conductor, an electrical insulation applied around said conductor and in contact with said conductor, a body of conducting material surrounding said insulation at least partially along its circumference and having a potential considerably different from that of the conductor, said body leaving a portion of said insulation exposed outside an edge of said body in the axial direction of the conductor, and a coating of a material having a substantial voltage dependent resistivity within the actual voltage range of the conductor applied to the surface of the exposed portion of said insulation and in contact with the surface of the exposed portion of said insulation, and in electrical contact with said body, said coating comprising a wrapping of a tape applied around the exposed portion of said insulation, said wrapping having one end electrically connected to said body and positioned substantially in the neighborhood of the edge of said body and another end positioned on the exposed portion of said insulation and said tape comprising a binding material selected from the group consisting of thermoplastics and elastomers and silicon carbide particles intermixed in the binding material.

2. A high voltage means as claimed in claim 1, in which the body of conducting material includes a metallic screen.

3. A high voltage means as claimed in claim 1, said silicon carbide particles having a volume resistivity in the range of a few ohm-cm. to several million ohm-cm.

4. A high voltage means as claimed in claim 1, the content of said silicon carbide particles being 10-65 percent by volume of said tape.

5. A high voltage means as claimed in claim 1, said silicon carbide particles having a particle size smaller than particle size 100.

6..A high voltage means as claimed in claim 1, the

content of said silicon carbide particles being 25-50 percent by volume, said silicon carbide particles having a particle size in the range of particle size 200 to particle size 600 and said binding material comprising polyvinyl chloride. 7. A high voltage means as claimed in claim 1, said coating comprising a wrapping of layers of said tape applied. around the exposed part of the insulation of said conductor in overlap arrangement.

8. A corona preventive tape comprising a binding material selected from the group consisting of thermoplastics and elastomers and intermixed in the binding material silicon carbide particles, the content of said silicon carbide particles being 1065 percent by volume of said tape, said silicon carbide particles having a particle size smaller than particle size 100.

References Cited UNITED STATES PATENTS 2,081,517 4/1937 Van Hofl'en 174--106 X 2,234,068 3/1941 Wiseman 174-106 X 2,622,152 12/1952 Rosch 1741()2 X 2,789,154 4/1957 Peterson 174-73 2,945,913 7/1960 Conangla 17473 3,066,180 11/1962 Virsberg 174-73 X 3,047,448 7/1962 Feller 174-120 X LEWIS H. MYERS, Primary Examiner.

ELLIOT A. GOLDBERG, Assistant Examiner.

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