High-voltage cable

This is in contrast to an overhead line, which may include insulation but not fully rated for operating voltage (EG: tree wire).

In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking.

The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current.

Cable joints and terminals must be designed to control the high-voltage stress to prevent the breakdown of the insulation.

Sebastian Ziani de Ferranti was the first to demonstrate in 1887 that carefully dried and prepared kraft paper could form satisfactory cable insulation at 11,000 volts.

During World War II several varieties of synthetic rubber and polyethylene insulation were applied to cables.

The demise of PILC could be considered to be in the 1980s and 1990s as urban utilities started to install more EPR and XLPE insulated cables.

It should also be noted that rubber insulated lead-covered cable enjoyed a short period of popularity prior to 1960 in the low and medium voltage markets but was not widely used by most utilities.

Vulcanized rubber was patented by Charles Goodyear in 1844, but it was not applied to cable insulation until the 1880s when it was used for lighting circuits.

[1] Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls Power Generation project.

However, they have fallen out of favor due to the high front-end cost and massive O+M budget needed to maintain the fleet of pumping plants.

2000 KCM and larger transmission cables often include a sectored style design to reduce skin effect losses.

The function of these layers is to prevent air-filled cavities and suppress voltage stress between the metal conductors and the dielectric so that little electric discharges cannot arise and endanger the insulation material.

Further, the fusion between the insulation and these layers must be absolute;[6] any fission, air-pocket or other defect — again, even of a few μm — is detrimental to the cable.

[7] Cooperation between cable makers and manufacturers of materials has resulted in grades of XLPE with tight specifications.

The development of extruders for plastics extrusion and cross-linking has resulted in cable-making installations for making defect-free and pure insulations.

[9] Many HVDC cables are used for DC submarine connections, because at distances over approximately 100 km AC can no longer be used.

[12] On this rubber-like body R a shield electrode is applied that spreads the equipotential lines to guarantee a low electric field.

The crux of this device, invented by NKF in Delft in 1964,[13] is that the bore of the elastic body is narrower than the diameter of the cable.

This construction can further be surrounded by a porcelain or silicone insulator for outdoor use,[14] or by contraptions to enter the cable into a power transformer under oil, or switchgear under gas pressure.

Secondly, a field-free space must be created where the cut-down cable insulation and the connector of the two conductors safely can be accommodated.

[19] In this way a permanent pressure is created between the bi-manchet and the cable surface, and cavities or electrical weak points are avoided.

The technical steps of removing the outer semiconducting layer at the end of the cables, placing the field-controlling bodies, connecting the conductors, etc., require skill, cleanliness, and precision.

There can also be workmanship concerns with using a torch as the tubes must be fully recovered without scorching and any mastics used must flow into the voids and eliminate any air.

From a utility perspective, this makes it difficult to keep track of stock or retain emergency spares for critical customers.

Cold shrink is the more rapidly growing area of distribution splices and is thought to have the fewest workmanship issues with the quickest install times.

The cables are flexible, with rubber or other elastomer insulation, stranded conductors, and an outer sheath of braided copper wire.

Interpretation of measurement results can in some cases yield the possibility to distinguish between new, strongly water treed cable.

Unfortunately, there are many other issues that can erroneously present themselves as high tangent delta, and the vast majority of solid dielectric defects can not be detected with this method.

Just like with tangent delta, this method has many caveats, but with good adherence to factory test standards, field results can be very reliable.