Until the mid-1960’s, paper-insulated cables were used all over the world for MV power circuits. At that time there were very few alternatives apart from the occasional trial installation or special application using PE or PVC insulation cable. But now the position is quite different than before. In this post we will discuss all about Paper-Insulated Cables with its construction.
There has been a worldwide trend towards XLPE cable and the UK industrial sector has adopted XLPE-insulated or EPR-insulated cable for the majority of MV power circuits. Paper-insulated cable now restricted to minor uses. It’s now use as extensions to older circuits or in special industrial locations for purpose.
Now the use of paper-insulated cables for LV power circuit has been superseded completely by polymeric cables in all sectors throughout the world. The key success of polymeric-insulated cables has been due to the much easier, cleaner, more reliable jointing and termination methods that they allow. But, large amount of paper-insulated cable still in service and its continued specification in some sectors such as the regional public supply networks for MV power circuits.
Paper-Insulated Cables Conductor:
Generally paper-insulated cables consist with copper or aluminium conductors. This cable insulated with lapped paper tapes, impregnated with insulating compound and sheathed with lead alloy or corrugated aluminium. Now the mechanical protection for the cable, lead or lead alloy sheathed cables are finished off with an armoring of steel tapes or steel wire and a covering of either bitumenized hessian tapes or an extruded PVC (Polyvinyl Chloride) or PE (Polyethylene ) over-sheath. Corrugated aluminium sheathed cables need no further metallic protection. But they are finished off with a coating of bitumen and an extruded PVC over-sheath. The intent of the bitumen in this case is to provide some additional corrosion protection.
Construction of Paper-Insulated Cables:
There are several basic types of paper-insulated cable and these are specified according to existing custom and practice as much as to meet specific needs and budgets. But the common element is the paper insulation itself. This cable is made up of many layers of paper tape and each applied with a slight gap between the turns. Purity and grade of the insulation paper is selected for best electrical properties and the thickness of the tape is selected to provide the necessary electrical strength. For achieving acceptable dielectric strength, all moisture and air is removed from the insulation and replaced by Mineral Insulating Non-Draining (MIND) compound.
MIND’s waxy nature prevents any significant migration of the compound during the lifetime of the cable at full operating temperature. Normally precautions are taken at joints and terminations to ensure that there is no local displacement of MIND compound which can cause premature failure. Paper insulation usually impregnated with MIND compound during the manufacture of the cable, immediately before the lead or aluminium sheath is applied.
A three core construction is preferred in most MV paper-insulated cables. Those three cores are used for the three different phases of the supply and there is no neutral conductor is included in the design. Now the parallel combination of lead or aluminium sheath and the armor can be used as an earth conductor, provided that circuit calculations and prove its adequacy for this purpose. Conductors of a 95 mm2 cross section and greater are sector-shaped so that when insulated they can be laid up in a compact size cable construction. Sector-shaped cable conductors are also used in lower cross sections, down to 35 mm2, 50 mm2 and 70 mm2 for cables rated at 6 kV, 10 kV and 15 kV, respectively.
But the three core 6.6 kV cables and most three core 11 kV cables are of belted design. The cores are proper insulated and laid up such that the insulation between those cable conductors is adequate for the full line-to-line voltage (6.6 kV or 11 kV). The laid-up cores then have an additional layer of insulating paper which known as the belt layer, applied and the assembly is then lead sheathed. This combination of core insulation and belt insulation is enough for phase-to-earth voltage between core and sheath (3.8 kV or 6.35 kV). A 15 kV, 22 kV and 33 kV three core cables and some 11 kV three core cables are of screened design.
Here each core has a metallic screening tape and the core insulation is adequate for the full phase-to-earth voltage. The screened cores are laid up and the lead or aluminium sheath is then applied so that the screens make contact with each other and with the sheath. The bitumenized hessian serving or PVC over-heath is primarily to protect the armor from corrosion in service and from dislocation during installation. The PVC over-sheath is now preferred because of the facility to mark cable details, and its clean surface gives a better appearance when installed. It also provides a smooth firm surface for glanding and for sealing at joints.
Ref: Electrical Power Engineers