Wires and cables:Scope and Principles of power cable design.

Wires and cables

Scope

Thousands of cable types are used throughout the world. They are found in applications ranging from fibre-optic links for data and telecommunication purposes through to EHV underground power transmission at 275 kV or higher. The scope here is limited to cover those types of cable which fit within the general subject matter of the pocket book.

This chapter therefore covers cables rated between 300/500 V and 19/33 kV for use in the public supply network, in general industrial systems and in domestic and commercial wiring. Optical communication cables are included in a special section. Overhead wires and cables, submarine cables and flexible appliance cords are not included.

Even within this relatively limited scope, it has been necessary to restrict the coverage of the major metallic cable and wire types to those used in the UK in order to give the cursory appreciation which is the main aim.

Principles of power cable design

Terminology

The voltage designation used by the cable industry does not always align with that adopted by users and other equipment manufacturers, so clarification may be helpful.

A cable is given a voltage rating which indicates the maximum circuit voltage for which it is designed, not necessarily the voltage at which it will be used. For example, a cable designated 0.6/1 kV is suitable for a circuit operating at 600 V phase-to-earth and 1000 V phase-to-phase. However it would be normal to use such a cable on dis- tribution and industrial circuits operating at 230/400 V in order to provide improved safety and increased service life. For light industrial circuits operating at 230/400 V it would be normal to use cables rated at 450/750 V, and for domestic circuits operating at 230/400 V, cable rated at 300/500 V would often be used. Guidance on the cables that are suitable for use in different locations is given in BS 7540.

The terms LV (Low Voltage), MV (Medium Voltage) and HV (High Voltage) have different meanings in different sectors of the electrical industry. In the power cable industry, the following bands are generally accepted and these are used in this chapter:

LV – cable rated from 300/500 V to 1.9/3.3 kV MV – cables rated from 3.8/6.6 kV to 19/33 kV HV – cable rated at greater than 19/33 kV Multi-core cable is used in this chapter to describe power cable having two to five cores. Control cable having seven to 48 cores is referred to as multi-core control cable.

Cable insulation and sheaths are variously described as thermoplastic, thermo- setting, vulcanized, cross-linked, polymeric or elastomeric. All extruded plastic materials applied to cable are polymeric. Those which would re-melt if the temperature during use is sufficiently high are termed thermoplastic. Those which are modified chemically to prevent them from re-melting are termed thermosetting, cross-linked or vulcanized. Although these materials will not re-melt, they will soften and deform at elevated temperatures, if subjected to excessive pressure. The main materials within the two groups are as follows:

thermoplastic

– polyethylene (PE)

– medium-density polyethylene (MDPE)

– polyvinyl chloride (PVC)

thermosetting, cross-linked or vulcanized

– cross-linked polyethylene (XLPE)

– ethylene-propylene rubber (EPR)

Elastomeric materials are polymeric. They are rubbery in nature, giving a flexible and resilient extrusion. Elastomers such as EPR are normally cross-linked.

General considerations

Certain design principles are common to power cables, whether they are used in the industrial sector or by the electricity supply industry.

For many cable types the conductors may be of copper or aluminium. The initial decision made by a purchaser will be based on price, weight, cable diameter, avail- ability, the expertise of the jointers available, cable flexibility and the risk of theft. Once a decision has been made, however, that type of conductor will generally then be retained by that user, without being influenced by the regular changes in relative price which arise from the volatile metals market.

For most power cables the form of conductor will be solid aluminium, stranded aluminium, solid copper (for small wiring sizes) or stranded copper, although the choice may be limited in certain cable standards. Solid conductors provide for easier fitting of connectors and setting of the cores at joints and terminations. Cables with stranded conductors are easier to install because of their greater flexibility, and for some industrial applications a highly flexible conductor is necessary.

Where cable route lengths are relatively short, a multi-core cable is generally cheaper and more convenient to install than single-core cable. Single-core cables are sometimes used in circuits where high load currents require the use of large conductor sizes, between 500 mm2 and 1200 mm2. In these circumstances, the parallel connection of two or more multi-core cables would be necessary in order to achieve the required rating and this presents installation difficulties, especially at termination boxes.

Single-core cable might also be preferred where duct sizes are small, where longer cable runs are needed between joint bays or where jointing and termination requirements dictate their use. It is sometimes preferable to use 3-core cable in the main part of the route length, and to use single-core cable to enter the restricted space of a ter- mination box. In this case, a transition from one cable type to the other is achieved using trifurcating joints which are positioned several metres from the termination box.

Armoured cables are available for applications where the rigours of installation are severe and where a high degree of external protection against impact during service is required. Steel Wire Armour (SWA) cables are commonly available although Steel Tape Armour (STA) cables are also available. Generally, SWA is preferred because it enables the cable to be drawn into an installation using a pulling stocking which grips the out- side of the oversheath and transfers all the pulling tension to the SWA. This cannot normally be done with STA cables because of the risk of dislocating the armour tapes during the pull. Glanding arrangements for SWA are simpler and they allow full usage of its excellent earth fault capability. In STA, the earth fault capability is much reduced and the retention of this capability at glands is more difficult. The protection offered against a range of real-life impacts is similar for the two types.

Paper-insulated cables

Until the mid-1960s, paper-insulated cables were used worldwide for MV power circuits. There were at that time very few alternatives apart from the occasional trial installation or special application using PE or PVC insulation.

The position is now quite different. 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 applications, paper-insulated cable now being restricted to minor uses, such as extensions to older circuits or in special industrial locations. The use of paper-insulated cables for LV has been superseded completely by polymeric cables in all sectors throughout the world.

The success of polymeric-insulated cables has been due to the much easier, cleaner and more reliable jointing and termination methods that they allow. However, because of the 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 circuits, its coverage here is still appropriate.

Paper-insulated cables comprise copper or aluminium phase conductors which are insulated with lapped paper tapes, impregnated with insulating compound and sheathed with lead, lead alloy or corrugated aluminium. For mechanical protection, lead or lead alloy sheathed cables are finished off with an armouring of steel tapes or steel wire and a covering of either bitumenized hessian tapes or an extruded PVC or PE oversheath. Cables which are sheathed with corrugated aluminium need no further metallic protection, but they are finished off with a coating of bitumen and an extruded PVC oversheath. The purpose of the bitumen in this case is to provide additional corrosion protection should water penetrate the PVC sheath at joints, in damaged areas or by long-term permeation.

There are, therefore, 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. Particular features of paper-insulated cables used in the electricity supply indus- try and in industrial applications are described in sections 9.3.1.1 and 9.3.2.1, respectively.

The common element is the paper insulation itself. This is made up of many layers of paper tape, each applied with a slight gap between the turns. The purity and grade of the paper is selected for best electrical properties and the thickness of the tape is chosen to provide the required electrical strength.

In order to achieve acceptable dielectric strength, all moisture and air is removed from the insulation and replaced by Mineral Insulating Non-Draining (MIND) com- pound. Its waxy nature prevents any significant migration of the compound during the lifetime of the cable, even at full operating temperature. This is in contrast to oil-filled HV cables, utilizing a lower viscosity impregnant which must be pressurized through- out the cable service life to keep the insulation fully impregnated. Precautions are taken at joints and terminations to ensure that there is no local displacement of MIND compound which might cause premature failure at these locations. The paper insulation is impregnated with MIND compound during the manufacture of the cable, immediately before the lead or aluminium sheath is applied.

A 3-core construction is preferred in most MV paper-insulated cables. The three cores are used for the three phases of the supply and no neutral conductor is included in the design. The parallel combination of lead or aluminium sheath and armour can be used as an earth continuity conductor, provided that circuit calculations prove its adequacy for this purpose. Conductors of 95 mm2 cross section and greater are sector- shaped so that when insulated they can be laid up in a compact cable construction. Sector-shaped 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.

The 3-core 6.6 kV cables and most 3-core 11 kV cables are of belted design. The cores are insulated and laid up such that the insulation between 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, known as the belt layer, applied and the assembly is then lead sheathed. The combination of core insulation and belt insulation is sufficient for phase-to-earth voltage between core and sheath (3.8 kV or 6.35 kV).

A 15 kV, 22 kV and 33 kV 3-core cables and some 11 kV 3-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 oversheath is primarily to protect the armour from corrosion in service and from dislocation during installation. The PVC oversheath 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.

Polymeric cables

PVC and PE cables were being used for LV circuits in the 1950s and they started to gain wider acceptance in the 1960s because they were cleaner, lighter, smaller and easier to install than paper-insulated types. During the 1970s the particular benefits of XLPE and HEPR insulations were being recognized for LV circuits and today it is these cross-linked insulations, mainly XLPE, which dominate the LV market with PVC usage in decline for power circuits, although still used widely for low voltage wiring circuits. The LV XLPE cables are more standardized than MV polymeric types, but even so there is a choice of copper or aluminium conductor (circular or shaped), single-core or multi-core, SWA or unarmoured, and PVC or Low Smoke and Fume (LSF) sheathed. A further option is available for LV in which the neutral and/or earth conductor is a layer of wires applied concentrically around the laid-up cores rather than as an insulated core within the cable. In this case, the concentric earth conductor can replace the armour layer as the protective metal layer for the cable. This concentric wire design is mainly used in the LV electricity distribution network, whereas the armoured design is primarily used in industrial applications.

Polyethylene and PVC were shown to be unacceptable for use as general MV cable insulation in the years following the 1960s because their thermoplastic nature resulted in significant temperature limitations. The XLPE and EPR were required in order to give the required properties. They allowed higher operating and short circuit temperatures within the cable, as well as the advantages of easier jointing and terminating than for paper-insulated cables. This meant that in some applications a smaller conductor size could be considered than had previously been possible in the paper- insulated case.

The MV polymeric cables comprise copper or aluminium conductors insulated with XLPE or EPR and covered with a thermoplastic sheath of MDPE, PVC or LSF material. Within this general construction there are options of single-core or 3-core types, individual or collective screens of different sizes and armoured or unarmoured construction. Single-core polymeric cables are more widely used than single-core paper-insulated cables, particularly for electricity supply industry circuits. Unlike paper-insulated cables, MV polymeric 3-core cables normally have circular-section cores. This is mainly because the increases in price and cable diameter are usually outweighed in the polymeric case by simplicity and flexibility of jointing and termination methods using circular cores.

Screening of the cores in MV polymeric cables is necessary for a number of reasons, which combine to result in a two-level screening arrangement. This comprises extruded semiconducting layers immediately under and outside the individual XLPE or EPR insulation layer, and a metallic layer in contact with the outer semiconducting layer. The semiconducting layers are polymeric materials containing a high proportion of carbon, giving an electrical conductivity well below that of a metallic conductor, but well above that required for an insulating material.

The two semiconducting layers must be in intimate contact in order to avoid partial discharge activity at the interfaces, where any minute air cavity in the insulation would cause a pulse of charge to transfer to and from the surface of the insulation in each half-cycle of applied voltage. These charge transfers result in erosion of the insulation surface and premature breakdown. In order to achieve intimate contact, the insulation and screens are extruded during manufacture as an integral triple layer and this is applied to the individual conductor in the same operation. The inner layer is known as the conductor screen and the outer layer is known as the core screen or dielectric screen.

When the cable is energized, the insulation acts as a capacitor and the core screen has to transfer the associated charging current to the insulation on every half-cycle of the voltage. It is therefore necessary to provide a metallic element in contact with the core screen so that this charging current can be delivered from the supply. Without this metallic element, the core screen at the supply end of the cable would have to carry a substantial longitudinal current to charge the capacitance which is distributed along the complete cable length, and the screen at the supply end would rapidly overheat as a result of excessive current density. However the core screen is able to carry the current densities relating to the charging of a cable length of say 200 mm, and this allows the use of a metallic element having an intermittent contact with the core screen, or applied as a collective element over three laid-up cores. A 0.08-mm thick copper tape is adequate for this purpose.

The normal form of armouring is a single layer of wire laid over an inner sheath of PVC or LSF material. The wire is of galvanized steel for 3-core cables and aluminium for single-core cables. Aluminium wire is necessary for single-core cables to avoid magnetizing or eddy-current losses within the armour layer. In unarmoured cable, the screen is required to carry the earth fault current resulting from the failure of any equipment being supplied by the cable or from failure of the cable itself. In this case, the copper tape referred to previously is replaced by a screen of copper wires of cross section between say 6 mm2 and 95 mm2, depending on the earth fault capacity of the system.

Low Smoke and Fume (LSF) and fire performance cables

Following a number of fire disasters in the 1980s, there has been a strong demand for cables which behave more safely in a fire. Cables have been developed to provide the following key areas of improvement:

● improved resistance to ignition

● reduced flame spread and fire propagation

● reduced smoke emission

● reduced acid gas or toxic fume emission

An optimized combination of these properties is achieved in LSF cables, which provide all of the above-mentioned characteristics.

The original concept of LSF cables was identified through the requirements of underground railways in the 1970s. At that time, the main concern was to maintain sufficient visibility such that orderly evacuation of passengers through a tunnel could be managed if the power to their train were interrupted by a fire involving the supply cables. This led to the development of a smoke test known as the ‘3-metre cube’, this being based on the cross section of a London Underground tunnel. This test is now defined in BS EN 50268. The reduced emissions of the toxic fumes also ensured passengers escape was not impaired. PVC sheathed cables can, by suitable use of highly flame retarded PVC materials, be designed to provide good resistance to ignition and flame spread, but they produce significant volumes of smoke and toxic fumes when burning. On this basis the LSF materials become specified for underground applications. The tests for reduced flame propagation are defined in BS EN 50265 (single cable) and BS EN 50266 (grouped cables), with tests for acid gas emission being defined in BS EN 50267.

The demand for LSF performance has since spread to a wide range of products and applications and LSF now represents a generic family of cables. Each LSF cable will meet the 3-metre cube smoke emission, ignition resistance and acid gas emission tests, but fire propagation performance is specified as appropriate to a particular product and application. For instance, a power cable used in large arrays in a power station has very severe fire propagation requirements, while a cable used in individual short links to equipment would have only modest propagation requirements. Hence BS EN 50265 would be appropriate to assess the single cable, but BS EN 50266 comprises of a number of categories to cater for varying numbers of cables grouped together.

Additionally, there has been a significant growth in demand for cables that are expected to continue to function for a period in a fire situation, enabling essential services to continue operation during the evacuation of buildings or during the initial fire fighting stages. In these cables, in addition to these LSF properties, the insulation is expected to maintain its performance in a fire. This insulation may be achieved by use of a compacted mineral layer, mica/glass tapes or a ceramifiable silicone layer. Specific designs are described in section 9.3.3.

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