Underground power cables

Underground cables are generally installed out of sight and are expected to operate without maintenance for at least 40 years. They are likely to represent the most valuable asset in a utility distribution network.

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Sep 15, 2017
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Author(s): John Weatherley

Abstract

Basic cable design has changed little over more than a century since the first cables enabled the beginning of electricity distribution in the UK. The principal advances have been in materials and manufacturing processes, especially regarding insulation and protection. This has permitted the development of underground cables for ever-increasing system voltages through the 20th century.

Impregnated paper retained its monopoly position as the primary insulation of all low and high voltage cables for the first half of the century, only gradually being replaced by polymeric materials newly developed for electrical applications. Cross-linked polymeric insulation has significant advantages over impregnated paper, not least being higher operating temperature allowing increased current rating for each conductor size.

Early experience with cross-linked polyethylene cable insulation outside the UK was not all good, and this spurred research into the causes of premature failure and how to improve long-term reliability. Experience with polymeric cables in UK utility and industrial distribution networks is now approaching 40 years and the reliability record so far is good. National and international design and performance standards together with qualification testing regimes are well established.

Introduction

Cables are the backbone of electricity transmission and distribution systems. They are visible as overhead lines or unseen as underground cables.

This paper concerns underground cables; conductors that are insulated, screened and protected from the environment to make them suitable for direct burial in the ground without the need for regular inspection or maintenance. The scope of this paper includes cables commonly used by electricity distribution utilities and industrial plants. It does not include coverage of special design features of cables for applications such as fire protection or sub-sea installation. Cables for special applications follow the same basic design principles as standard cables but will have appropriately modified materials and/or enhanced mechanical properties.

Terminology and voltage ratings

Cables for alternating current (AC) operation have voltage ratings conventionally written in the form U 0/ U ( U m) where; U 0 is the RMS voltage between a phase conductor and the earth conductor; U is the RMS voltage between two phase conductors and U m is the maximum RMS voltage between two phase conductors for which the cable is designed. It is the highest voltage that can be sustained under normal conditions.

Cables and their accessories for the common UK distribution system voltages 400, 11 and 33 kV are within the voltage classes U m = 1.2 kV, U m = 12 kV and U m = 36 kV, respectively.

Use of terms such as low voltage (LV) and high voltage (HV) without the corresponding U 0/ U rating figures can be misleading since different user groups have historically adopted different conventions. Modern UK and international standards for cables and accessories tend to define LV as an upper limit of U m = 1.2 kV (with possible extension to U m = 3.6 kV) and medium voltage (MV) above LV up to U m = 42 kV. Above this limit is HV and the acronym EHV (extra-high voltage) is sometimes used for the higher transmission voltages. However, the statutory electricity safety, quality and continuity (ESQC) regulations recognise only LV ( U ≤ 1 kV) and HV ( U > 1 kV).

Transmission is the bulk movement of electrical energy, commonly from power generation sites to major HV substations, possibly followed by onward transmission at lower voltage to substations from which electricity is distributed to consumers. The boundary between transmission and distribution is set more according to the role of the network owner rather than system voltage. In the UK, National Grid is the transmission system operator and distribution is the responsibility of the distribution network operators. Underground cables are predominantly components of distribution rather than transmission networks.

Overview of design and construction

This section summarises and illustrates the main constructional features of historical and modern cables. Subsequential sections include more detail on design, individual components and their functions, choice and performance of materials, cable current rating and testing.

Organisations that generate, distribute or supply electricity must comply with the ESQC regulations. These regulations define an underground cable as any conductor surrounded by insulation which is placed below ground. There is, of course, no requirement to bury all cables, so they must be electrically safe and sufficiently robust to be installed above ground in public and private areas.

The fundamental components of underground cables that comply with UK regulations and standard practices are;

  • current-carrying conductors (copper or aluminium), including neutral conductors in LV AC cables;
  • insulation (impregnated paper or extruded polymeric) ‘…of such quality and thickness as to withstand the operating voltage of the equipment.’ (ESQC regulations);
  • protective conductor in the form of ‘…an electrically continuous metallic screen connected with earth’ (ESQC regulations). In LV combined neutral-earth (CNE) cables the protective conductor is also the neutral.

AC cables may be single-core (one current-carrying phase conductor) or multi-core. In the UK, LV distribution cables are invariably multi-core, having 2–5 conductors including the neutral. In modern CNE cables the neutral conductor is concentric, enclosing the phase conductors and functioning also as the protective earth screen. MV cables are single-core or 3-core, many factors being likely to influence choice. Not least of these is conductor size (cross-sectional area). 3-core cables with conductors larger than 300 mm 2may be impracticable in view of their weight, inflexibility and difficulty in jointing and terminating. The growth in power demand implies a trend towards single-core cables for MV distribution, with conductors as large as 1 200 mm 2 not uncommon. The largest conductors are found in HV cables and the majority of modern HV cables are single-core, with conductor sizes typically up to 2500 mm 2.

The following figures illustrate the structure and principal components of some common UK cables, both legacy paper-insulated and modern extruded polymeric insulated constructions. Successive sections of this paper include details of component materials, their functions and a summary of manufacturing processes.

Fig 1 shows an LV 4-core armoured cable typical of use in industrial applications. Cables of this type meet BS 5467 [with polyvinyl chloride (PVC) oversheath] or BS 6724 (with oversheath having low emission of smoke and corrosive gas). Cables to these standards are also suitable for use at 3.3 kV ( U m = 3.6 kV).

Fig 1: Low voltage 4-core cable with sector-shaped stranded copper conductors, XLPE insulation, steel wire armour and extruded oversheath

Components of Fig 1 cable:

  1. Stranded copper conductors (three-phase conductors + neutral).
  2. Cross-linked polyethylene (XLPE) insulation.
  3. Galvanised steel wire armour.
  4. Oversheath of PVC or a polymeric material having low emission of smoke and corrosive gas.

Fig 2 shows an LV ( U m = 1.2 kV) 3-core CNE waveform cable typical of use by utilities. Cables of this type meet BS 7870-3.11 (with PVC oversheath) or BS 7870-3.12 (with oversheath having low emission of smoke and corrosive gas).

Fig 2: Low voltage waveform CNE cable with sector-shaped solid aluminium conductors, XLPE insulation, copper wire neutral/earth conductor and extruded oversheath

Components of Fig 2 cable:

  1. Sector-shaped solid aluminium phase conductors.
  2. XLPE insulation.
  3. Bedding layer.
  4. Concentric waveform copper wire neutral/earth conductor.
  5. PVC or low-smoke and corrosive gas oversheath.

Fig 3 shows an 11 kV 3-core paper insulated belted cable with corrugated aluminium sheath and PVC oversheath, commonly installed by utilities before the adoption of extruded polymeric cables. This type of cable is known by the acronym PICAS (paper insulated corrugated aluminium sheathed). The term belted refers to a construction in which a conductive earth screen (component 5) is applied over the three laid up cores rather than around individual cores. Electrically the belted design is weaker than the alternative screened construction, and for this reason the more highly stressed 33 kV paper-insulated cables are all screened design.

Fig 3: 11 kV 3-core PICAS cable with sector-shaped stranded aluminium conductors, MIND belted paper insulation, corrugated aluminium sheath and PVC oversheath

Components of Fig 3 cable:

  1. Sector-shaped stranded aluminium phase conductors.
  2. Conductive paper screen.
  3. Mass-impregnated non-draining (MIND) paper insulation.
  4. Belt insulation papers.
  5. Conductive paper insulation screen.
  6. Corrugated aluminium sheath.
  7. Bitumenised sealant layer.
  8. PVC oversheath.

Fig 4 shows an 11 kV 3-core paper insulated screened cable with lead sheath, steel wire armour and hessian serving. These cables, and the alternative belted construction, were standard cables for industrial applications and were also installed by utilities. The descriptive acronym for this cable is PILC SWA (paper-insulated lead covered and steel wire armoured).

Fig 4: 11 kV 3-core PILC SWA cable with circular stranded copper conductors, MIND screened paper insulation, lead sheath, steel wire armour and hessian serving

Components of Fig 4 cable:

  1. Circular stranded copper phase conductors.
  2. Conductive paper screen.
  3. MIND paper insulation.
  4. Metallised paper insulation screen.
  5. Circularising fillers.
  6. Lead sheath.
  7. Bitumenised hessian bedding.
  8. Galvanised steel wire armour.
  9. Bitumenised hessian serving with whitewash coating.

Fig 5 shows a 33 kV single-core cable with XLPE insulation, copper wire earth screen and medium density polyethylene (MDPE) oversheath. Cables to this design with rated voltages 11 and 33 kV meet BS 7870-4.10 and are typically installed by utilities. They are also available with a sheath having low emission of smoke and corrosive gas. A variant of this 33 kV cable has a lead sheath under the MDPE oversheath and is specified in BS 7870-4.11. Cables of the same basic design but with aluminium wire armour in place of the copper wire screen are commonly used in industrial applications.

Fig 5: 33 kV single-core cable with stranded copper conductor, XLPE insulation, copper wire screen and extruded oversheath

Components of Fig 5 cable:

  1. Stranded copper conductor.
  2. Extruded conductive screen.
  3. XLPE insulation (thickness 8 mm for 33 kV).
  4. Extruded conductive screen (semicon layer).
  5. Copper wire earth screen.
  6. MDPE or low-smoke and corrosive gas oversheath.

Fig 6 shows an 11 kV 3-core cable with XLPE insulation, copper tape earth screens, steel wire armour and MDPE oversheath. Armoured cables of this type with rated voltages 11 and 33 kV are specified in BS 6622 and are typically installed in industrial networks. Similar cables, to BS 7835, are available with sheaths having low emission of smoke and corrosive gas.

Fig 6: 11 kV 3-core cable with stranded copper conductors, XLPE insulation, copper tape screens, steel wire armour and extruded oversheath

Components of Fig 6 cable:

  1. Stranded copper conductor.
  2. Extruded conductive screen.
  3. XLPE insulation (thickness 3.4 mm for 11 kV).
  4. Extruded conductive screen (semicon layer).
  5. Copper tape earth screen.
  6. Circularising fillers.
  7. Bedding sheath.
  8. Galvanised steel wire armour.
  9. PVC or MDPE oversheath.

Through the range of MV and HV rated voltages the major design features are the same, the main differences being in the choice of metallic screening and outer protection. Primary insulation thickness must, of course, must be appropriate to the rated voltage of the cable. Insulation thicknesses for MV cables are defined in standards and have remained unchanged for many years. The standard thickness for each U m is the same across the range of International Electrotechnical Commission (IEC), CENELEC and associated national standards.

The variables determining the electric stress E in the cable insulation for each rated voltage are the diameter over the conductor screen and the insulation thickness. Stress at a radial position r within the insulation is given by

E=V/r⋅ln[r2/r1]E=V/r⋅lnr2/r1

where, V is the phase-earth rated voltage U 0; r 1 is the outer radius of the conductor screen and r 2 is inner radius of the insulation screen.

Maximum stress is at the surface of the conductor screen. This is the design stress of the cable. For an 11 kV cable ( U 0 = 6.4 kV) with 185 mm 2 conductor, the conductor screen diameter will be about 20 mm. With standard insulation thickness 3.4 mm the stress at working voltage is 2.2 kV/mm. This is a very modest stress for a dielectric such as XLPE which has a test breakdown strength many times this value. A short length of 11 kV XLPE cable subjected to a short-time test to breakdown might be expected to withstand up to about 80 kV test voltage, equivalent to a maximum stress in the insulation of 28 kV/mm. What has to be taken into account, however, is the very large volume of insulation in a cable network, and the length of time (hopefully at least 40 years) during which the insulation will be under stress. Both these factors influence statistical breakdown probability, which users will require to be as low as possible and within an acceptable limit.

For cables of higher rated voltage, design stresses increase. For example, the stress in a 185 mm 2 33 kV cable with standard insulation thickness 8 mm is 3.2 kV/mm. For HV cables the insulation thickness is graduated according to conductor size, to take account of the fact that maximum stress is higher for smaller conductor diameters. One manufacturer's data sheet for 132 kV cables specifies insulation thickness 18 mm for a 500 mm 2 cable. In this case the maximum stress is 6.6 kV/mm. One practical reason for accepting higher stresses in higher voltage cables is the physical size of the cable if stresses as low as 2.2 kV were maintained through the voltage range. For example, a maximum stress of 2.2 kV in the 132 kV cable necessitates an insulation thickness of 150 mm, which is clearly a practical nonsense. From the statistical point of view, the total length of installed 132 kV cable is many times less than that of 11 kV, which to some extent mitigates the effect of increase in operating stresses at higher rated voltages.

Cable components and their function

Phase conductors

From the outset of significant electricity distribution the choice of material for conductors carrying continuous current has been between copper and aluminium. Both metals are near the top of electrical conductivity tables have other physical properties well suited to applications in cables. Sodium was at one time seen as a possible alternative but did not reach the stage of commercial exploitation. The cable specifier's choice between copper and aluminium will be influenced by a number of factors, the principal of these being cost, followed by electrical conductivity (conductor size), weight of the finished cable and the attractiveness of copper to thieves.

Purchasing contracts for cables containing copper will generally have price adjustment clauses based on the prevailing London Metal Exchange price. The steep increase in the price of copper over recent years has pushed specifiers towards aluminium even where copper is the preferred choice from a technical standpoint. The reasons are not only the initial purchase price of the cable but the cost and disruption consequent on theft of cable for the scrap value of its copper content. Theft is a serious problem particularly for distribution utilities and railways where it is not possible to provide complete security for cable assets. In the UK the cables purchased by distribution utilities will normally have aluminium conductors whereas stranded copper remains preferred in cables for industrial applications.

The ratio of electrical conductivities for copper and aluminium is about 1.6, which is reflected in ratings for sustained current and short-time fault conditions. For equal conductivity, an aluminium conductor will be about 26% larger in diameter than a copper conductor of the same construction. Aluminium does, of course, have a significant weight advantage over copper. The density ratio of about 3.3:1 results in a copper conductor being twice as heavy as its aluminium equivalent. This weight difference may be significant for transportation and ease of installation of finished cable, including installations where cable is above ground and supported on racks or hangers.

The authoritative UK national standard for conductors is BS EN 60228 Conductors of insulated cables (title of the standard) which is the implementation of European standard EN 60228 and is identical to IEC 60228. This standard specifies copper and aluminium conductors of nominal cross-sectional areas 0.5 to 2500 mm 2. It includes solid conductors and stranded conductors, both types being intended for fixed installation cables. Also included are flexible (multi-strand) conductors for cables where flexibility is a key requirement.

The requirement for the 30 recognised conductor sizes is a maximum resistance in Ω/km at 20°C. For example, maximum specified resistances for 185 mm 2 conductors are 0.0991 Ω/km for copper and 0.164 Ω/km for aluminium. Conductor sizes are identified by nominal cross-sectional area but this is not measured in the standard. The most common nominal conductor sizes for LV and MV multicore distribution cables are 70, 95, 120, 150, 185, 240 and 300 mm 2. Single-core cables include these sizes together with larger cross-sections, commonly up to 1000 mm 2 but with a trend towards larger sizes to accommodate increasing power demand. The USA uses conductor size identification based on American Wire Gauge (AWG) for smaller cross-sections and kcmil for larger sizes. Reference tables give direct equivalents in square millimetres; for example, the largest AWG size 4/0 is equivalent to 107 mm 2 and 600 kcmil is equivalent to 304 mm 2.

Solid conductors are extruded in circular or shaped cross-section. Solid copper conductors larger than a few square millimetres are uncommon because the relative rigidity of copper, especially when work-hardened by bending, makes larger cables difficult to handle when connecting to equipment. Solid aluminium, on the other hand, is flexible enough to be suitable for conductors up to 300 mm 2 in LV and MV multi-core cables, and for increasingly large cross-sections for single-core cables.

Most stranded conductors are made from round wires of equal diameter twisted together and formed by passing through a shaping die. Modern stranded conductors are generally compacted by the die so that included air space is reduced to less than 10% of the total cross-section.

Conductors for 2, 3 and 4-core LV cables above 35 mm 2 are always appropriately shaped in order to minimise overall cable diameter. For MV cables with impregnated paper insulation, shaped conductors were standard but their profile had to take into account the raised electrical stress at edges. Shaped conductors in MV paper cables are therefore more rounded in profile than their LV counterparts in order to keep electrical stresses on the conductor within design limits. All conductors for modern polymeric-insulated multicore MV cables are circular.

Stranded copper conductors at the top of the size range commonly comprise a number of individual segments (generally four or six) laid up together to form a circular conductor. These conductors are Milliken type. Alternate segments may be covered with a thin insulating layer to help reduce unequal current density caused by the skin effect.

Primary insulation

Gutta Percha, a form of natural rubber with good dielectric properties, was developed as a wire insulation in the second half of the 19th century. The vulcanisation process (chemical cross-linking of long-chain molecules) made rubber a far more practical material and it remained in use well into the 20th century as the insulation of wiring cables.

The development of impregnated paper as an electrical insulation made possible the rapid evolution of distribution and transmission cables for ever-increasing voltage ratings. By the 1960s oil-filled cables up to 400 kV were available as the underground alternative to unsightly overhead transmission towers, though at a very high relative cost.

Special cellulose paper for cable insulation is applied as tapes helically wound (lapped) over the conductor layer-by-layer until sufficient thickness is built up. Typical tape dimensions are 20 mm width and 0.1 mm thickness. The full insulation thickness is applied in one pass through a bank of lapping machines within a factory area of controlled humidity and temperature (Fig 7). Small gaps (known as butt gaps) are left between each helical turn to enable the cable to bend without the tape edges creasing against each other. Bending of the cable without damage to the insulation by tearing or creasing of paper tapes is possible because of the advantageous mechanical properties of cellulose paper. Because butt gaps are electrically weaker than the paper layers, the gap positions are staggered with respect to underlying and overlying turns in order to maximise the dielectric strength of the bulk insulation perpendicular to the paper layers.

Fig 7: Paper lapping machine applying paper tape insulation

The cable at this stage of manufacture is vacuum-dried to reduce the moisture content of the paper. The drum of cable is then flooded with an impregnating compound, with the objective of filling all voids including those within the microscopic structure of the cellulose paper. Paper on its own has poor dielectric strength because of the included air in the fibrous mat structure. This manufacturing process is called mass impregnation and is followed by extrusion of a continuous metallic sheath to form a protective impermeable environment for the impregnated paper insulation.

Traditional impregnating compounds were based on mineral oils though in the later years synthetic compounds were developed and claimed to have improved properties. High viscosity compounds that would not drain from a cut cable were the preferred option in the UK for LV and MV cables because of clear advantages during jointing and terminating. This type of cable is known as MIND. Cables for transmission voltages were impregnated with low viscosity oils that could be pumped through the installed cable and maintained under low pressure to prevent the entry of air or contaminants in the event of damage to the cable sheath.

An alternative manufacturing process was the application of paper tapes that had been pre-impregnated with compound. Because of the inevitable presence of voids within the insulation body, this type of cable is maintained under pressure from an external source of gas, typically oxygen-free nitrogen.

The traditional metallic sheath for paper-insulated cables was lead alloy but extruded aluminium was developed as an alternative and became popular with UK utilities for MV (11 kV) and HV oil-filled transmission cables. To allow adequate flexibility, aluminium sheaths for MV and HV cables were corrugated immediately after extrusion. Until the introduction of polymeric insulation, the 11 kV distribution cable favoured by most UK utilities was a corrugated aluminium-sheathed cable (PICAS). For small diameter cables uncorrugated aluminium sheaths are sufficiently flexible and this allowed the development of a compact and economical LV distribution cable with solid aluminium conductors. This cable, known as CONSAC (concentric solid aluminium conductor), was normally used in 3-core form for CNE circuits. Unfortunately, this cable was vulnerable to unanticipated corrosion problems associated with the use of aluminium as the neutral/earth conductor protected only by a relatively thin PVC oversheath. Aluminium protects itself against corrosion by rapid formation of an oxide layer but this layer cannot form effectively in an anaerobic environment such as that of a buried cable. In the event of sheath damage, bulk corrosion can occur and lie undiscovered until it causes a fault in the cable.

Most cable-making countries ceased manufacture of paper cables some years ago because of the advance of extruded polymeric insulation for all distribution and transmission voltages. Nevertheless, a familiarity with paper cables remains important for operators of legacy LV and MV paper cable networks installed during the rapid expansion of electricity distribution in the 1950s and later decades. Repair of these cables and extension of the network requires retention of the necessary skills and the use of joints to transition between old cables and modern polymeric equivalents.

Use of synthetic polymer materials as conductor insulation dates from the 1950s. The first material of wide commercial use was PVC, which remains in use today though largely replaced by polymers with better physical and electrical properties. PVC has very good dielectric properties in its unadulterated form but is rigid and requires plasticising to be useful in cables. In its plasticised form, PVC proved practical and economical as a LV insulation and gradually replaced paper in LV distribution cables.

PVC is a thermoplastic polymer and will melt and flow when heated. When compounded as cable insulation its maximum continuous operating temperature is set at 70°C and short-time (fault conditions) temperature 160°C. The conductors of PVC-insulated cables are therefore limited to these temperatures.

The thermal characteristics of a polymer are improved if the polymer structure can be cross-linked by the formation of chemical bonds between adjacent long-chain molecules. In practical terms, the cross-linked polymer can sustain a higher temperature than its un-cross-linked form because it will no longer melt and flow when heated. PVC cannot be cross-linked to any useful degree, so its continuous operating temperature is limited to 70°C whereas XLPE, the most widely used cross-linked polymeric insulation, is suitable for maximum continuous operation at 90°C.

PVC in its plasticised form is a relatively poor dielectric in terms of breakdown strength and dielectric loss, making it unsuitable as a primary insulation in cables rated above about 3 kV. In LV distribution cables the electrical properties of PVC are more than adequate but it has now largely been superseded by XLPE. The primary reason for this change is the higher conductor current ratings achievable with a maximum conductor temperature of 90°C.

Cross-linking of XLPE takes place when active bonding sites are created on polyethylene molecules, leading to the formation of permanent chemical bonds between sites on nearby molecules. Once the polymer is cross-linked to a certain level, it is no longer thermoplastic and becomes a thermoset with changed physical properties. The other material which emerged in the 1970s as a suitable cross-linkable polymer for cable insulation is ethylene propylene rubber (EPR), with XLPE competing with EPR for technical leadership. During subsequent years, improvements in materials, manufacturing and cable design opened the way for development of MV and HV extruded cables for rated voltages eventually up to 400 kV.

Manufacture of cables with XLPE or EPR insulation involves extruding the un-cross-linked compound over the conductor as it passes through an extruder head. In the process most commonly employed by cable manufacturers, cross-linking of the insulating compound takes place immediately downstream of the extruder head by the application of heat. The cross-linking reaction is promoted by chemical additives in the compound. After the cable has passed through the heating zone it is cooled and taken up on drums to await the next manufacturing stage. This well established process is known as continuous catenary vulcanisation (CCV) and takes its name from the shape of the pipe which is attached to the outlet of the extruder head and encloses the cable during the heating and cooling stages (Fig 8). Heating used to be provided by pressurised steam but the resulting moisture content of the insulation proved to be deleterious to long-term electrical performance.

Fig 8: CCV machinery for cross-linking XLPE and EPR insulation

Modern MV and HV polymeric cables are all made using the triple extrusion process, in which three concentric layers are simultaneously extruded over the conductor in a single pass through a special extruder head. These three layers comprise;

  • a thin layer of conductive polymer over the conductor, forming the conductor screen;
  • the insulation of specified thickness
  • an outer thin layer of conductive polymer forming the insulation screen.

Fig 9 shows a triple extrusion head fed by three extruder screws.

Fig 9: Triple extrusion head for applying insulation and conductive screens

The triple extrusion process is the secure way of creating smooth and contaminant-free interfaces between electrostatic screens and insulation, thus avoiding points of raised electrical stress within the insulation. This development, together with refinements in insulation compound purity and manufacturing cleanliness, paved the way for the evolution of extruded cables for higher and higher rated voltages. Lessons had been learnt from early-life failures of extruded MV cables in the USA, where enthusiasm for the adoption of extruded cables was not accompanied by necessary refinement of the manufacturing process and material quality.

The most well-known insulation failure mechanism is the growth of electrical trees or water trees at contaminant inclusions or imperfections at the screen/insulation interfaces. These defects are points of raised electrical stress which lead to localised degradation of the XLPE insulation due to partial discharge activity. The name tree derives from the branch-like structure that develops as it progresses through the insulation. Water trees are those in which water can penetrate the tree channels. Treeing is a pre-breakdown phenomenon and at some time the increased electrical stress within the insulation cannot be withstood and full breakdown occurs. Fig 10 shows a bowtie tree growing from an inclusion in the body of the insulation.

Fig 10: Bow tie tree growing from an inclusion in XLPE insulation

Protective conductors, screens, armour and metallic sheaths

The ESQC regulations stipulate that conductors of underground cables of all voltages must be protected by ‘…an electrically continuous metallic screen connected with earth.’ This screen may be in the form of wires, tapes, armour or extruded metallic sheaths. There may be more than one metallic layer in a cable, at least one of which will be capable of carrying earth fault current sufficient to operate protection equipment. This ‘protective conductor’ is defined by the ESQC regulations as a conductor which is used for protection against electric shock and which connects the exposed conductive parts of equipment to earth.

Some cables are provided with an armour layer to help protect them against damage whether installed below or above ground. Armouring also protects a cable during storage, transportation to site and installation. Metallic armouring is always earthed at one or more points in the circuit and may therefore contribute as a protective conductor to carry earth fault current.

PILC cables are normally armoured in order to protect the relatively soft underlying lead alloy sheath. Multi-core PILC cables have steel armour either as spirally-applied tapes (STA) or wires (SWA). In the UK, legacy LV PILC cables in utility networks typically have steel tape armour whereas MV cables have steel wires. It would, however, be unwise to rely on this feature to distinguish between the two when contemplating work on an exposed cable. Extruded aluminium is the alternative continuous covering for paper-insulated cores. It offers more mechanical protection than lead but is not regarded as armour.

Modern polymeric-insulated LV and MV cables designed for UK utility use are not armoured but have a concentric layer of copper wires under the outer sheath. On the other hand, cables for industrial applications are generally armoured. Where armouring is specified for MV single-core cables in non-utility applications, steel wires are not used because heat generated by magnetic losses in the armour is likely to reduce the conductor current rating. For these cables aluminium wire armour is used, and the acronym AWA is included in the cable design description. As well as for mechanical protection, AWA may be specified if the circuit is required to have a high earth fault rating. Because of the low resistance of an AWA layer care has to be taken in choosing how and where to make earth connections if the effect of high induced circulating currents is to be avoided.

Once the potentially damaging effect of moisture in XLPE had been realised, it became clear that there would be a life-time advantage in providing the same sort of moisture-proof protection as in paper cables, namely a continuous metallic sheath. Polymeric cables from 33 kV to the highest voltages are available with metallic sheaths, either as extruded lead, extruded aluminium or bonded aluminium foil laminate.

All modern cables have an extruded polymeric outer covering (oversheath) which provides both an environmental seal for the inner components and basic electrical insulation between the metallic screen/armour and the surrounding earth. For many decades all oversheaths were PVC, possibly with chemical additives to resist insect attack when buried, or to improve flame-retardant characteristics of cables installed above ground in buildings. Despite being naturally flame-retardant, PVC will burn if exposed to fire and give off noxious fumes and copious dense smoke. Following fatalities from serious fires in confined spaces, PVC is now not used as an oversheath material for cables to be installed in public buildings. It has been replaced by materials that do not easily ignite, do not propagate fire and give off only low levels of smoke and gas. These materials are based on halogen-free polymers (not containing fluorine, chlorine, bromine or iodine compounds) compounded with additives having flame-retardant and smoke-suppressant properties. The most commonly used additive material is alumina trihydrate which is compounded with a halogen-free polymer such as polyethylene or ethylene vinyl acetate.

Current rating and cable selection

Cables carrying AC power from source to load will generate heat in most of their material components. This heat will be dissipated into the environment of the cable and an equilibrium condition will be reached, assuming constant conductor current, when temperatures within the cable stabilise as the rate of heat dissipation approaches the rate of generation. The maximum continuous current rating of the conductor(s) will be calculated or measured based on the maximum temperatures that can be tolerated by the various non-metallic cable components. In practice the limiting factor is the maximum operating temperature of the conductor insulation, this being in direct contact with hottest part of the cable and the most critical non-metallic component.

Each insulating material has an agreed maximum continuous operating temperature based on characteristics such as mechanical robustness (softening etc.) and ability to withstand long-term thermal ageing. Commonly used insulation materials with the highest operating temperatures are the cross-linked polymers XLPE and EPR. These are rated at 90°C whereas thermoplastic (un-cross-linked) polymers PVC and polyethylene are restricted to 70°C. Paper insulation is ascribed several maximum temperatures dependent on the application, but all are lower than 90°C, meaning that XLPE and EPR cables have the overall current rating advantage.

The authoritative national standard for the calculation of current ratings is BS IEC 60287, which is the implementation of international standard IEC 60287. This standard is applicable to cables at all alternating voltages and to direct voltages up to 5 kV. It is published in three parts.

Part 1 of the standard includes detailed calculation of losses for all components of the cable. The major loss is due to conductor resistance, which in AC cables includes additional resistance due to skin and proximity effects. Dielectric loss in insulation materials is proportional to their loss factor tanδ and the square of the applied voltage. This loss can be ignored in LV and MV cables but becomes significant in HV cables. Impregnated paper insulation has a relatively high tanδ and much development work was done to reduce dielectric losses in the highest voltage oil-filled cables by replacing high loss cellulose paper with low loss polymeric alternatives. However, the uniquely suitable mechanical properties of cellulose paper could not successfully be matched, and the only useful outcome was a laminate of paper bonded to a film of polypropylene. This innovation yielded a useful reduction in dielectric losses but has now been overtaken by the universal adoption of XLPE insulation. A potentially significant loss, not always taken into account, results from induced circulating currents in solidly-earthed metallic screen/armour of single-core MV cables, notably those with aluminium wire armour. Current rating may be affected unless circulating currents can be avoided by special bonding arrangements such as single-point earth bonding or cross-bonding.

Part 2 of the standard provides formulae for calculating the thermal resistance of cables and the external thermal resistances for cables laid in free air, ducts or directly buried. Established values for thermal resistances of all common metallic and non-metallic constituents, together with their thickness and position within the cable, yield overall thermal resistance figures. Calculation of the thermal resistance of the cable environment is based on a similar approach together with the need to take into account the thermal effects of nearby cables. For buried cables, the thermal resistivity of the ground is an important but variable quantity depending on the nature of the ground, its moisture content and the possibility of drying out caused by heat from the cable(s).

Part 3 of the standard concerns operating conditions and economic optimisation of cable size. Selection of cable size based solely on the minimum cross-section suitable for the required current will minimise the initial investment but does not take account of the cost of losses over the anticipated life of the cable, which would typically be 40 years. Taking into account life-time total costs, a larger conductor size might be chosen. If the minimum conductor size is already large, the installation of a second parallel cable may be the economically advantageous choice. Other factors may need to be taken into account, such as prospective fault currents, voltage drop and rationalisation of conductor size for efficient purchasing. Predicting future costs and expressing them in present value carries with it much uncertainty, including trends in kWh price and general inflation. However, since only a defined range of conductor sizes is available, the case should be clear if the choice is between, for example, cables with 185 or 300 mm2 conductors. The advent of polymeric cables with 90°C maximum operating temperature offered an opportunity to reduce conductor size whilst maintaining required current rating. At least one utility in its conversion from paper to polymeric 11 kV cables reduced its standard conductor sizes from 185 to 150 mm 2 and from 300 to 240 mm2.

Fortunately, the determination of current rating for conventional cables installed in recognised environments does not normally require the cable system designer to perform the relatively complex calculations found in the standard, even though the job may be made considerably easier by specialist software. This work has already been done by the cable manufacturer or supplier and published in catalogue data sheets together with other important electrical characteristics of the cable. To produce current rating tables, certain standard values are ascribed to the characteristics of the cable environment in order to calculate the rates at which heat can be conducted away from the cable.

Cable environmental conditions vary significantly from one country or region to another. For the temperate climate and ground conditions of the UK, the following are standard values adopted for quantities that feature significantly in rating calculations.

  • Ambient air temperature 25°C.
  • Ground temperature 15°C.
  • Ground thermal resistivity 1.2 Km/W.
  • Depth of laying 0.8 m.

For tropical environments, calculations may be based on ambient air temperature of up to 50°C and ground temperature of 40°C. Ground thermal resistivity is dependent on moisture content, with values of 1 Km/W or less in moist ground and 3 Km/W in very dry ground.

Basic calculations assume that ambient conditions remain constant (including no increase in ground thermal resistivity due to drying out of the ground) and that cable circuits are thermally independent of each other. Current rating data sheets generally provide information for the three main types of installation; in free air, directly buried and laid in buried ducts. For example, one manufacturer's ratings for an 11 kV 3-core 185 mm 2 XLPE cable with steel wire armour are 385 A in air, 335 A directly buried and 290 A in buried ducts. Not surprisingly, single-core cables have higher ratings than 3-core equivalents due to separation of the heat sources. Flat spaced arrangements of single-core cables allow the highest ratings, but require more racking space and wider trenches. For single core cables not laid in close trefoil there may be a need for special earth bonding arrangements to minimise or eliminate induced currents in metallic screens or armour.

Where installations do not match the conditions assumed in standard rating tables, rating factors are applied to increase the accuracy of the predicted maximum conductor current. The more comprehensive cable catalogues may include rating factor tables with multipliers (mostly <1) appropriate to condition variations including;

  • ambient temperature;
  • ground thermal resistivity;
  • ground temperature and depth of laying;
  • multiple cables, their arrangement (trefoil or flat) and spacing.

In certain circumstances a conductor size larger than that required for continuous current may be necessary in order to carry prospective short-time fault current. Insulation materials are ascribed a short-time maximum temperature in addition to their continuous operating temperatures. In the case of XLPE and EPR insulation this short circuit temperature is 250°C. Calculation of the conductor cross-section able to carry the circuit fault current starts with the assumption that the cable is carrying its full current load with a conductor temperature of 90°C. The simplest calculation is based on assumption of adiabatic conditions in which all heat generated by the fault current is retained within the conductor. This is reasonable since fault durations are typically 1 s or less. Alternatively, a slightly higher and more accurate rating will be obtained by taking into account the heat lost by the conductor during the fault. Short circuit ratings may be found in cable catalogue data, usually displayed graphically as logarithmic plots of short circuit current vs duration. For example, one manufacturer's data shows the fault capability of a 185 mm 2 copper conductor insulated with XLPE as ∼25 kA for 1 s. For a longer or shorter fault duration the current is calculated using I 2 t = constant, though accuracy is lost if the fault duration is greater than a few seconds.

Standards

International standards for cables and their accessories are developed and published by the IEC and CENELEC, the European Committee for Electrotechnical Standardisation. Work to develop CENELEC and IEC standards is done by groups of experts from member countries.

CENELEC standards are published as European Standards (prefix EN) or Harmonisation Documents (prefix HD). These standards are in turn implemented as national standards by all CENELEC member countries. ENs are published unaltered whereas HDs may have minor modifications appropriate to national requirements. Any existing conflicting national standards must be withdrawn when ENs and HDs are implemented by member countries.

In the UK, ENs are published as BS EN XXXXX and HDs are published with a BS reference number. Some EN and IEC standards carry the same reference number and in this case are identical documents. If no appropriate EN or HD exists, an IEC standard may be adopted and published as a national standard. IEC standards for test methods are commonly quoted in national standards.

Important national and international standards covering LV, MV and HV cables are listed below and some are referred to in appropriate sections of this paper. Standards documents may include clauses related to construction, dimensions, materials and testing.

  • BS 5467: LV armoured cables with copper conductors and thermosetting insulation (normally XLPE) for U = 1 kV and U = 3.3 kV. These are cables for non-utility industrial applications.
  • BS 6622: MV armoured cables with thermosetting insulation (XLPE and EPR) (see also IEC 60502-2). These are cables for non-utility industrial applications.
  • BS 6724: Same scope as BS 5467 but for cables with low emission of smoke and corrosive gases when affected by fire.
  • BS 7835: Same scope as BS 6622 but for cables having low emission of smoke and corrosive gases.
  • BS 7870-4.10 and -4.11: MV polymeric insulated cables (XLPE and EPR) for use by generation and distribution utilities (implementing applicable parts of HD 620). These are intended as utility cables but may also be specified for industrial applications.
  • BS 7912: Cables with XLPE insulation and metal sheath and their accessories for rated voltages from 66 kV ( U m = 72.5 kV) to 132 kV ( U m = 145 kV) (implementation of HD 632). See also IEC 60840.
  • BS EN 60228: Conductors, copper and aluminium, stranded and solid, with applicable size range 0.5 to 2500 mm2.
  • BS IEC 60287: Calculation of current rating:
    • Part 1: Calculation of losses.
    • Part 2: Calculation of thermal resistances.
    • Part 3: Operating conditions and economic optimisation of cable size.
  • IEC 60502: Cables with extruded insulation and their accessories:
    • Part 1: Cables with rated voltages 1 kV ( U m = 1.2 kV) and 3 kV ( U m = 3.6 kV).
    • Part 2: Cables with rated voltages 6 kV ( U m = 7.2 kV) to 30 kV ( U m = 36 kV).
  • IEC 60840: Cables with extruded insulation and their accessories for rated voltages above 30 kV ( U m = 36 kV) up to 150 kV ( U m = 170 kV).

Testing of cables

Most of the vast length of LV, MV and HV power cables are buried in the ground and the network owners will hope that there is no need to uncover them except for reasons such as diversion or renewal. Faults in underground cables are very expensive to repair, especially cables installed under roads and pavements. Repairing a fault will probably involve cutting out a length of cable either side of the fault and installing a new length together with joints to connect to the existing cable.

The wide spectrum of tests applied to cables before they are put into service cannot guarantee long life but can at least seek out design weaknesses and provide checks on material and manufacturing quality. Procedures and requirements for testing cables are written into the appropriate cable standards. Test categories and requirements applicable to polymeric cables are summarised as follows.

Tests are grouped in three categories:

  • Routine tests made by the manufacturer on each manufactured batch (normally drum length) to check that it meets the specified requirements.
  • Sample tests made by the manufacturer on samples of completed cable or components taken from a cable at a specified frequency to verify that the finished product meets the specified requirements.
  • Type tests made on a type of cable or system before supplying on a general commercial basis. They are intended to demonstrate satisfactory performance against requirements of the appropriate standard. Type tests are not normally repeated unless changes are made to materials, manufacturing process or design, which might affect performance characteristics.

In addition, there are usually Tests after installation, intended to demonstrate the integrity of the cable and accessories after installation and before use.

Routine testing of all cable lengths is done on the manufacturing site. For LV cables the procedure involves a conductor resistance test and a voltage test between each conductor and the earth components of the cable. Additional tests on MV and HV cables include a metallic screen resistance test and a partial discharge test.

Sample tests are carried out from time to time on lengths of cable taken from a production batch. For LV cables they are non-electrical tests to verify dimensions and constructional characteristics. MV and HV cables are subjected to more extensive testing of material components and electrical tests including AC voltage withstand and partial discharge.

Type tests are intended as qualification of a new cable design or an existing cable whose dimensions or materials have changed to an extent that might affect performance. In contrast to routine and sample tests, type tests are not intended to be repeated and there is no expiry date for the validity of the test report. This is fortunate because type testing is an expensive and lengthy procedure with MV and HV cables, especially if the job is done by one of the well-known independent testing organisations.

Type tests on LV cables are mostly non-electrical and similar to routine and sample tests. Type tests on MV and HV cables and systems are more searching. In addition to a multiplicity of non-electrical tests, the following electrical tests are found in the appropriate standards, though not necessarily all or in the same sequence. Most tests are carried out on the same cable sample with minimum length 10 m. The following summary refers to MV/HV cables up to 132 kV ( U m = 145 kV).

Bending test

Cables for fixed installations are not designed for repeated flexing but must undergo bending during manufacture, drumming and installation without damage. The bending test involves three reverse bends around a specified diameter, followed by a partial discharge test.

Partial discharge test

Partial discharges are low energy breakdowns occurring in gas-filled voids within the insulation body or at interfaces between insulation and conductive screens. Partial discharges within a cable at working voltage are a likely cause of full breakdown at some indeterminate time during the life of the cable. Measurements of partial discharge inception voltage are made at a specified test voltage above Uo using specialised equipment. The individual discharge measurement unit is pico-coulombs (pC) and typical requirements in the standards are ≤10 pC at 2 U o or ≤5 pC at 1.5 U o. The expectation with modern cables is zero partial discharge level but background electrical noise in test locations such as cable factories makes it difficult or impossible to achieve measurement sensitivities less than a few pC. Skilled operators of this test should, however, be able to distinguish partial discharges in the cable from background noise.

Measurement of loss factor ( tan δ)

Dielectric losses are low in polymeric insulation compared with impregnated paper but since there is some energy loss, limits are prescribed through a range of test voltages and conductor temperature. For 66 and 132 kV XLPE cables, BS 7912 and IEC 60840 specify a maximum loss factor of 1 × 10 −3. For MV cables higher loss factors are acceptable, with the maximum value specified in BS 6622 for XLPE insulation being 4 × 10 −3 at U o. EPR is a higher loss material which makes it less favoured for higher voltage cable insulation. For MV applications the limiting tanδ value for EPR is 20 × 10 −3.

Heating cycle voltage test

Heating cycle tests simulate the daily variation of load demand characteristic of utility networks. The well-established standard heating cycle lasts 8 h. Current is induced in the test cable in order to bring the conductor up to 5 K above maximum temperature, and this is held for 2 h, followed by a cooling period to near ambient temperature. This cycling imposes repeated thermo-mechanical stresses on the cable components. During the test the cable is energised at 2 U o. The full test is normally 20 cycles with periodic measurement of partial discharge inception voltage.

Lightning impulse voltage test

All type test regimes for cables with U m = 3.6 kV and above include a test to simulate lightning strikes. The transient surge voltage resulting from a lightning strike will be attenuated along the cable and so the more vulnerable section of cable will be close to its termination to an overhead line. Impulse generators in test laboratories are set up to deliver a single steep-front wave, normally with a front time of 1.2 μs and a tail of 50 μs to 50% voltage decay. The test cable is subjected to 10 positive and 10 negative shots at the prescribed peak voltage. In UK standards examples of impulse withstand peak voltages are 95 kV for 11 kV cables, 194 kV for 33 kV cables and 650 kV for 132 kV cables. The impulse test is normally performed with the cable at maximum working temperature.

HV test

In this test an AC voltage typically 3 U o or 4 U o is applied to the test sample for a period of 4 h. For paper cables this was a relatively searching test but no modern polymeric cable should suffer any risk of failure, given the short length of test cable and the relatively short time under stress.

Tests after installation

Given the practical difficulties of bringing large or complex test equipment to installation sites, voltage tests after installation can do little more than identify major faults associated with the installation, and these are more likely to be associated with joints and terminations rather than the cable itself. Test voltage sources are normally DC or very low frequency. Test voltage is applied between phase conductors and metallic earth components of the cable. In addition, integrity of polymeric oversheaths may be checked by applying a modest test voltage between the metallic earth components of the cable (disconnected from earth) and the surrounding earth.

Conclusions

Underground power cables have served communities and industry for more than 120 years, in most cases satisfying the demand of owners and operators for reliable networks. Cable faults cause loss of supply and are expensive to repair. This is a rational justification for the relatively slow advance of cable technology. Network operators expecting assets to give at least 40 years of trouble-free service may be hesitant to bury in the ground long lengths of cable with unproven reliability.

Fortunately, impregnated paper proved to be a highly reliable primary insulation at all distribution and transmission voltages and was only gradually replaced by extruded polymeric materials in the later decades of the last century. Research and development work to improve the short and long term electrical properties of polymeric insulation preceded widespread adoption of XLPE for higher voltages. This work paid off and evidence from the first 30 years of MV and HV polymeric cables suggests that they will continue to be highly reliable components of utility and industrial networks.

Acknowledgments

The author expresses his thanks and gratitude to past and present colleagues and friends at Pirelli Cables (now Prysmian Cables and Systems) and TE Connectivity (formerly Raychem) for sharing their knowledge, wisdom and experience over 40 years. Thanks are also due to past customers, in particular many experienced cable engineers from UK distribution network operators who have been generous with their knowledge and experience of cables and accessories.

Go to the profile of John Weatherley

John Weatherley

Consultant , Queens' College

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