Cross-Linked Polyethylene-Insulated Cable

The present invention relates to a cross-linked polyethylene-insulated cable, i.e., a cable with an insulating layer of cross-linked polyethylene. More particularly, the present invention relates to a high-voltage or super high-voltage cable with an insulating layer made of cross-linked polyethylene prepared by a dry cross-linking method not using high-pressure steam.

As an insulating material for use in solid material-insulated cables, cross-linked polyethylene prepared by chemical cross-linking, i.e., using cross-linking agents, is ordinarily used.

In this chemical cross-linking, dicumyl peroxide (DCP) is used as a cross-linking agent because of its high cross-linking efficiency and suitable decomposition temperature. In chemically cross-linked polyethylene-insulated cables, in particular, those cables in which the thickness of the insulating layer is large, if voltage is applied to at high temperatures (e.g., 90.degree. C.), the breakdown voltage of the cables decreases even if cables are not used in the water.

In particular, in the case of cables with a metal layer as a water-shielding layer provided around an insulating layer, the breakdown voltage abruptly decreases when voltage is applied to at much higher temperatures.

The above-described problem of the decrease in breakdown voltage is encountered particularly when cross-linking is performed by a method using high pressure steam which has been generally employed. The so-called dry cross-linking method (a method of chemically cross-linking by the use of, for example, a heat transfer medium such as silicon oil, infrared radiation, hot inert gas, or a long die) can prevent the decrease in breakdown voltage of the cable with a thin insulation (less than 6 mm).

A cross-linking system for cross-linking polyethylene insulated cables using pressurized silicone oil as a medium for heating and cooling is described in U.S. Pat. No. 3,909,177.

Systems for heating cables with infrared radiation are described in U.S. Pat. Nos. 3,588,954 and 3,513,228.

Cross-linking systems using hot inert gas are described in U.S. Pat. No. 4,069,286 and IEEE Trans. Vol. PAS-94 No. 2 (1978), T74 394-3.

The above systems have been developed as a substitute for a cross-linking system using steam in order to prevent the penetration of water into an insulating layer.

However, when voltage is applied to at high temperatures, the problem of decrease in breakdown voltage cannot be overcome satisfactorily even by employing the dry cross-linking method.

An object of the invention is to provide a cross-linked polyethylene-insulated cable in which the decrease in breakdown voltage can be prevented even if electricity is applied to at high temperatures.

As a result of extensive investigations it has been found that use of certain cross-linking agents in dry cross-linking method gives rise to cross-linked polyethylene-insulated cables to prevent the decrease in breakdown voltage when the thickness of the insulating layer thereof is at least 6 mm.

The present invention is based on the finding and provides a cross-linked polyethylene-insulated cable by dry cure method comprising a conductor core, an inner semiconductive layer, an outer semiconductive layer and an insulating layer, wherein the insulating layer has a thickness of at least 6 mm and is made of a cross-linked polyethylene cross-linked with 2,5-dimethyl-2,5-di(tertbutylperoxy)hexane or 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3 as a cross-linking agent.

In the present invention, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane or 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3 is used as a cross-linking agent. Even if voltage is applied at high temperatures, the breakdown voltage does not decrease.

2,5-Dimethyl-2',5'-di(tertiary butylperoxy)hexyne-3 is used as a cross-linking agent for cross-linking the semiconductive layer provided on a cross-linked polyethylene-insulated cable as described in U.S. Pat. No. 3,719,769. These cross-linking agents are more expensive than DCP and show a low cross-linking rate and, therefore, they are considered unsuitable for cross-linking cable insulators. Therefore, the effects of the cross-linking agents on the prevention of decrease in breakdown voltage have been unknown.

The cross-linked polyethylene prepared using the cross-linking agents as described above is particularly effective when it is applied to high voltage insulated cables with a thick insulation layer, for example, i.e., in which the thickness of the insulating layer is at least 6 mm.

The present invention can be much effectively applied to cables with a metal layer as a water-shielding layer.

Hence the cable of the invention is effective particularly when the cross-linking of polyethylene is performed by the dry cross-linking method.

In the present invention, the cross-linked polyethylene for use as the insulating layer may contain, if necessary, antioxidants and so forth.

The present invention is described in greater detail with reference to the following Examples and Comparative Examples.

The following compositions were prepared:

Each polyethylene composition was extrusion-coated around a 150 mm strand in a thickness of 3, 6, 11 or 23 mm to form an insulating layer, and the thus-formed insulating layer was then subjected to chemical cross-linking using various heating techniques to produce a cable.

Voltage was applied to the cable in the air under the conditions of temperature 90.degree. C. and voltage 7 kv/mm. The time taken for the cable to be broken, i.e., breakdown time, was measured.

The breakdown times of Comparative Examples 1, 2, 3, 4 and 9, which are control examples, are indicated by T.sub.1, T.sub.2, T.sub.3, T.sub.4 and T.sub.9, respectively. Breakdown times for other examples are shown with that of the corresponding control example (i.e., the thickness of the insulating layer is the same) as a base. For example, in Comparative Examples 5 and 7 in which the thickness of the insulating layer is the same as that of Comparative Example 1, the breakdown time is shown with T.sub.1 of Comparative Example 1 as a base.

The results are shown in the Table.

As can be seen from the Table, the cables in which the thickness of the insulating layer is 6, 11 or 23 mm and polyethylene was cross-linked with 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane or 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3 and a cross-linking agent by the use of infrared heating or a silicone bath are superior in long term dielectric breakdown characteristics to conventional cables prepared by cross-linking polyethylene with DCP.

A metal layer was provided on the cables of Comparative Example 3 and Example 2, which were further covered with a sheath of polyvinyl chloride (PVC) to prepare cables. These cables were subjected to the same test as above. The breakdown time of the cable prepared using the cable of Example 2 was about ten times that of the cable prepard using the cable of Comparative Example 3.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.