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1. INTRODUCTION
The selection of cables for particular applications is best done with reference to the latest and specific manufacturer's cable data and application guides. It is not therefore appropriate to include here comprehensive tables giving cable dimensions, weight and current rating information. This Section concentrates on the properties of different types of LV, MV and HV power cables, their merits for different applications, cable sizing and loss calculations, useful installation practices and cable management systems. A section is also included on control and communication (C&C), including fiber optic cables. Technical specification details are included such that competitive quotations from leading manufacturers may be obtained. Consideration is given to the safety implications associated with gases and smoke emitted from cables under fire conditions, especially where installed in public places.
=== TABLE 1 Useful IEC Cable Standards====
IEC Standard | Brief Description and Comment
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2. CODES AND STANDARDS
TABLE 1 details some useful IEC and National cable Standards.
Standard cable nomenclature based upon IEC 60183 used to designate appropriate cable voltage ratings is as follows:
U0 =rated rms power frequency voltage, core to screen or sheath.
U =rated rms power frequency voltage, core to core.
Um =maximum rms power frequency voltage, core to core (highest core to core voltage under normal operating conditions).
Up =peak lightning impulse withstand voltage, core to screen or sheath.
A cable voltage classification may therefore be designated as U0/U (Um).
TABLE 2 Standard Power Cable Ratings
The selection of cables with the appropriate voltage rating for the particular application is dependent upon the system voltage and earthing category.
These categories are defined as follows:
Category A _ A system in which, if any phase conductor comes in con tact with earth or an earth conductor, it is automatically disconnected from the system.
Category B _ A system which, under fault conditions, is operated for a short time with one phase earthed. These conditions must not exceed 8 hours on any occasion with a total duration, during any 12 month period, not exceeding 125 hours.
Category C _ A system which does not fall into categories A and B.
Examples of cable voltage ratings are given in TABLE 2. The maximum sustained voltage should exclude transient overvoltages due to switching surges, lightning surges, fault conditions, etc. For system voltages at intermediate levels from those given in TABLE 2 the cable should be selected with the next higher rating. For example in Saudi Arabia with an MV category A or B and system voltage of 13.8 kV, an 8,700/15,000 V cable voltage rating could be selected.
3. TYPES OF CABLES AND MATERIALS
3.1 General Design Criteria
The following factors govern the design of power cables:
1. The cross-sectional area of the conductors chosen should be of the optimum size to carry the specified load current or short circuit short term current without overheating and should be within the required limits for voltage drop.
2. The insulation applied to the cable must be adequate for continuous operation at the specified working voltage with a high degree of thermal stability, safety and reliability.
3. All materials used in the construction must be carefully selected in order to ensure a high level of chemical and physical stability throughout the life of the cable in the selected environment.
4. The cable must be mechanically strong and sufficiently flexible to with stand the re-drumming operations in the manufacturer's works, handling during transport or when the cable is installed by direct burial, in trenches, pulled into ducts or laid on cable racks.
5. Adequate external mechanical and/or chemical protection must be applied to the insulation and metal or outer sheathing to enable it to withstand the required environmental service conditions.
Types of cables are detailed in TABLE 3.
After voltage selection cables tend to be specified by describing the materials and their properties from the phase conductors to the outer covering.
Manufacturers will provide a drawing showing a cross-section through the cable and the relevant technical parameters and guarantees associated with the design. A typical physical technical specification sheet for, say, 19,000/ 33,000 XLPE power cable is shown in TABLE 4.
3.2 Cable Construction
3.2.1 Conductor Materials
Copper is still the predominant conductor material in stranded, shaped, segmental, sectorial and milliken formats. Solid or stranded, shaped or segmental aluminum is also often specified on the basis of cost in the manufacturer's country at the time of tender. Aluminum is also lighter and assists with ease of handling large cables. Additional care has to be taken when jointing aluminum cables. It is necessary to ensure that the contact surfaces are free from oxide and that when connecting to copper or brass terminals no corrosion cell is formed.
TABLE 3 Types of Cables [coming soon]
3.2.2 Insulation
Paper Insulation
Oil-impregnated, paper-insulated cables have a history of satisfactory use at all voltage levels. They are nowadays rarely specified for new installations except at voltage levels of 66 kV and above or for reinforcement of existing installations where standard cable types are required throughout the network.
Until the development of XLPE or EPR cables paper tape insulation was the most stable form at high temperatures and better able to withstand the stresses occurring under short circuit conditions. However, paper insulation deteriorates rapidly because of its hygroscopic nature if exposed to moisture. In order to prevent this, the paper layers are protected against ingress of water, usually by a lead/lead alloy or corrugated aluminum alloy metal sheath.
TABLE 4 Cable Physical Parameters [coming soon]
Furthermore, during installation special attention has to be paid to the quality of joints and terminations which often require special materials and highly skilled jointers.
For cables up to 36 kV, mass impregnated non-draining (MIND) cables are employed. IEC 60055-2 covers the general and construction requirements of these cables but in the UK British Standard BS6480 Part 1 is generally used as a specification for these cables. Routine sample and site tests should be carried out to IEC 60055-1. The conductor screen consists of carbon paper and/or metallized paper tapes applied over the conductors which are of stranded copper or aluminum wires to IEC 60228. The insulation is made up of 6-12 layers of dry paper tapes, each layer wound in the opposite direction to the previous layer. The core screen is made up of carbon paper and/or non-ferrous metallized paper tapes applied over the insulation to a constant thickness in order to obtain a maximum electrical stress level at the conductor screen of 5 to 6 kV/mm. After construction of the insulated conductor cores, the paper is impregnated with an oil resin which has a consistency of a soft wax at 20 degrees C. After impregnation, the metal sheath is extruded onto the cable. The 'belted' type cable, which does not have individually screened cores, is used for 3 core cables up to 12 kV. Instead of individual screening, an overall belt of paper is applied round all 3 cores and this type of cable may be seen designated as 11,000/11,000 V rating, whereas screened cable of the same rating would be designated 6,350/11,000 V.
Cable types have 1, 2, 3, 3 1/2 and 4 cores with conductors to IEC 60228 for stranded copper or aluminum wires. The maximum conductor tempera ture is 60 degrees Cto70 degrees C and the power factor (dielectric loss angle) should not exceed 0.006 at 60 degrees C or 0.013 at 70 degrees C. Three- or four-core cables are generally armored for direct burial in ground with cable tests to IEC 60055-1.
Oil-filled (OF) cable is used up to 525 kV. Single core cables have a hole approximately 12 mm in diameter in the center of the conductor through which the oil may flow during expansion and contraction of the cable as it heats up and cools down. Three core oil-filled cables up to 630 mm2 have ducts between the cores to allow for the necessary oil movement at typical working pressures of between 80 kPa and 350 kPa. Reinforcement tapes, generally made of stainless steel or phosphor bronze, are applied over the lead sheath of oil-filled cables to assist withstanding abnormal oil pressures up to some 600 kPa. Maximum conductor temperatures are 85 degrees C to 90 degrees C with power factors between 0.0028 and 0.0035 for cable core-to-earth nominal voltage ratings between 66 kV and 400 kV. A typical oil-filled 132/150 kV cable has a power factor of some 0.0033. The pressurized oil in the dielectric reduces the chance of partial discharge under normal conditions; however, impulse tests are still important in order to verify the performance of the cable under lightning strike and switching surge conditions. Stress levels at the conductor vary between 8 MV/m at 33 kV and 15 MV/m at 400 kV.
Gas pressurized cables have a similar construction to mass impregnated (MI) types but because of the gas pressure the insulation thickness is less.
Dry or impregnated paper tape is used for the insulation and nitrogen gas at typical normal working pressures of up to 1400 kPa is used. Again reinforcement of the lead sheath is necessary at these pressure ratings.
Polypropylene Paper Laminate Insulation
Oil-filled cables above 200 kV are now increasingly manufactured using polypropylene paper laminate (PPL), rather than paper. This is because at higher voltages paper suffers from high dielectric losses which reduce the cable rating. PPL consists of a film of polypropylene coated on each side with a thin layer of paper. The material consists typically of 50% polypropylene and 50% paper. The physical properties of the PPL material means that it can be readily substituted for paper in the cable manufacturing process.
The PPL material has a much lower dielectric loss factor than paper (0.0021 at 90 degrees C compared to 0.0078 for paper) and hence, the heat generated within the insulation at high voltages is significantly reduced, allowing higher cur rent ratings for a given conductor size. PPL cables have a higher impulse strength compared to paper and can operate at higher stress levels reducing the amount of insulation required. They also have a lower permittivity reducing the capacitive charging current. PPL material is more expensive than paper, but this is offset against the benefits listed above.
PVC Insulation
PVC insulation is nowadays being rapidly superseded by XLPE cables with LSF (see SEC. 6) or Medium Density Polyethylene (MDPE) oversheathing. However, it is still specified and is suitable for cables rated up to 7.2 kV.
PVC has the advantage over paper insulation in that it is non-hygroscopic and does not therefore require a metallic sheath. The absence of such a sheath simplifies jointing by the elimination of plumbing operations on the lead sheath. Moreover, it is both lighter and tougher and inherently more flexible than paper. Therefore PVC-insulated cables may be bent through smaller radii than paper-insulated cables thus easing installation problems. PVC is resistant to most chemicals, although care must be taken with installations in petro chemical environments. It is a thermoplastic material which softens at high temperatures and therefore cannot withstand the thermal effects of short circuit currents as well as paper insulation. The maximum conductor tempera ture is 65 degrees C to 70 degrees C. Multicore cables are generally armored when laid direct in ground. At low temperatures PVC hardens and becomes brittle and installations should not be carried out at temperatures below 0 degrees C.
XLPE Insulation
XLPE is a thermo setting material achieved by a process akin to the vulcanization of rubber. The resulting material combines the advantages of PVC insulation (high dielectric strength, good mechanical strength, and non hygroscopic nature) with thermal stability over a wide temperature range.
XLPE has no true melting point and remains elastic at high temperatures therefore permitting greater current carrying capacity, overload and short circuit performance in comparison with PVC- and paper-insulated cables. IEC 60502-2 covers the design and testing of these single and 3 core cables up to 36 kV. Cables with voltages between 36 kV and 150 kV are also manufactured generally in accordance with IEC 60502-2 with testing carried out to IEC 60840. Above 150 kV where single core cables are normally employed, IEC 62067 provides test methods and other requirements.
The type of curing affects the electrical strength of the insulation against partial discharges. Originally XLPE cables were steam cured for voltages in the range 24 kV to 145 kV in the USA, Scandinavia and Japan. Faults in ser vice due to partial discharges in small voids caused carbon deposits to form.
Further breakdown led to the formation of water and dust 'trees' and 'bow ties' eventually leading to full insulation breakdown. Since the early 1970s cable breakdown caused by voids and/or contaminating particles such as dust or humidity within the dielectric has been avoided by improvements in the cable insulation materials and manufacturing techniques, in particular by the introduction of the 'dry curing' process. This has also led to higher impulse withstand test results. Because of the importance of reliability and long ser vice for XLPE cables partial discharge values are extremely important.
Improvements in manufacturing techniques and good service history records allow XLPE insulation thickness to be reduced such that cable stress levels are increasing. Stress levels at the conductor of 3 to 3.5 MV/m at 36 kV- and 7 to 8 MV/m at 150 kV-rated cable voltages are typical. XLPE cables up to 36 kV are generally manufactured with water tree retardant material and do not require a metal sheath. It is recommended that cables with stress levels above 6.5 MV/m are protected by a metal sheath.
XLPE cables have greater insulation thickness than their equivalent paper-insulated cables. This results in XLPE cables having larger overall diameters and for a given cable drum size slightly less overall cable length can be transported.
The power factor of XLPE cables is very low compared to paper-insulated cables; 0.001 at the nominal system voltage to earth. Cable capacitance affects voltage regulation and protection settings. The 'star capacitance' is normally quoted on manufacturers' data sheets for 3 core screened cables operating at 6.6 kV and above, i.e. the capacitance between the conductor and screen ( FIG. 1a). Unscreened cables are only normally used at voltages less than 6.6 kV.
EPR Insulation
Ethylene propylene rubber cables have a cross-linked molecular structure like XLPE and are produced by a similar process. Both EPR and XLPE have the same durable and thermal characteristics but EPR has a higher degree of elasticity which is maintained over a wide temperature range.
This EPR flexibility characteristic is somewhat mitigated when such cables are used in conjunction with steel armoring. Between 6 and 12 ingredients are used in the production of EPR which necessitates great care to maintain purity and avoid contamination during the production process. EPR insulation is water resistant and cables are able to operate without a metallic sheath in waterlogged areas. This initially gave EPR insulation an advantage over XLPE insulation. However, this advantage has largely gone with the increasing use of water tree retardant XLPE. EPR insulation tends to be more expensive than XLPE insulation and its use has reduced in recent years with only a relative few manufacturers now offering EPR-insulated cables ( FIG. 2).
FIG. 1 (a) Three-core screened cable star capacitance. (b) Cable charging
currents with 3 core screened cable and screen earthed at one end only.
Earth current i50 because charging currents are balanced. (c) Cable charging
currents with 3 core screened cable and screen earthed at both ends. Earth
current i50 because charging currents are balanced. (d) R-phase-to-earth
fault, 3 core screened cable earthed at one end to earth fault relay. (e)
To maintain earth fault relay stability with balanced ring type CT for
a single phase-to-earth fault the setting has to be higher than that shown
in (d).
Mineral Insulation
Mineral insulated copper conductor (MICC) cables are manufactured for 600 V (light grade) and 1,000 V (heavy duty) installations which could involve high temperatures, rough mechanical handling, surface knocks or contact with oils. The cables consist of copper or aluminum conductors insulated with highly compressed magnesium oxide compound surrounded by a copper or stainless steel tube. They have a small overall diameter for a given current rating and will continue to operate continuously under fire conditions at sheath temperatures up to 250 degrees C. Such cables are specified for high security applications and in particular for use with fire alarm systems.
Particular care has to be taken during the installation and storage of such cables in order to ensure that moisture does not penetrate into the magnesium oxide material. In addition the impulse, withstand of such insulation is not as good as more conventional cable insulation.
FIG. 2 A 24 kV heat shrink sleeve termination awaiting final connections.
TABLE 5 Lead and Lead Alloys for Cable Sheaths [coming soon]
3.2.3 Sheaths
Very little lead sheathing is now specified except for special HV cables.
Lead and lead alloy sheaths have been traditionally used to prevent the ingress of moisture into paper-insulated cables or other cables installed in particularly marshy conditions. Lead corrosion and fatigue resistance properties are important and improvements are obtained by the addition of other elements. Alloy sheaths are used with unarmored cables where vibration problems might be encountered. TABLE 5 summarizes the materials normally used.
As a cheaper and nowadays far more popular alternative to lead an aluminum alloy sheath is specified. The composition is an important factor in reducing the possibility of corrosion in service. A corrugated aluminum sheath construction helps to improve overall cable flexibility.
For XLPE cables above 60 kV, foil laminate sheaths have become increasingly popular. The foils are made from either copper or aluminum typically 1_2 mm thick, with a polyethylene coating. If necessary additional copper wires can be added to the design to provide for a higher sheath fault rating. The foil is applied longitudinally and folded around the cable and overlapped onto itself. Heating the polyethylene at the overlap bonds the foil to itself creating a water tight design. Although not as robust as a lead or aluminum sheath, a foil laminate cable can offer a cheaper alterative as it avoids need for expensive metal extrusion presses.
3.2.4 Insulation Levels and Screening
The correct selection of appropriate cable voltage designation depends upon the type of network and network earthing arrangements as described in SEC. 2. Generally, if the network is solidly earthed the voltage will not rise above the maximum system phase-to-neutral voltage under fault conditions. However, if under fault conditions the earthing arrangement is such as to allow the voltage to neutral to rise to the line voltage then the cable insulation must be specified accordingly.
To minimize the possibility of discharges at the inner surfaces of cable core dielectric a grading screen is introduced. This screen comprises of one or two layers of semiconducting tapes or compounds over the core insulation. Such measures are introduced at the following voltage levels:
PILC-insulated cables _ 6,350/11,000 V
PVC-insulated cables _ 7,200/12,500 V
XLPE-insulated cables _ 3,300/6,000 V
Typical three-phase screened cable systems with screens earthed at one or both ends are shown in Figs. 1b and c. During a single-phase earth fault the earth fault current will be three times the steady state per phase charging current ( FIG. 1d). Earth fault relay settings have to be sufficiently high to ensure stability for upstream single phase-to-earth faults when high cable capacitance effects due to the cable design or long lengths of cable are involved.
3.2.5 Armoring
In order to protect cables from mechanical damage such as pick or spade blows, ground subsidence or excessive vibrations cable armoring is employed. For 3 core cables, this consists of one or two layers of galvanized steel tapes, galvanized steel wire braid or galvanized steel wires helically wound over the cable. Galvanized steel wire armor (SWA) is preferred since it gives a more flexible construction, is easy to gland and gives better performance where the cable may be subjected to longitudinal stresses in ser vice. In addition, the overall cross-sectional area of steel wire armor tends to be greater than that for the equivalent steel tape armor mechanical protection and therefore SWA presents a lower impedance if the armor is used as the earth return conductor. If armoring is required on single core cables aluminum should be used instead of steel wire in order to avoid losses.
Armor protection for lead-sheathed cables was traditionally laid up on suit ably impregnated fibrous bedding material. For PVC- and XLPE-insulated cables PVC-, LSF- or MDPE-extruded bedding is now normally specified rather than the older PVC tapes.
3.2.6 Finish
One of the most important factors that can affect cable life is the degree of protection afforded by the cable finish against the harmful effects of chemical corrosion, electrolytic action, insect or rodent attack and mechanical damage. Compounded fibrous materials were originally employed but these have now been replaced by extruded MDPE or LSF outer sheaths which may be impregnated with chemicals to deter insects such as termites. The integrity of the outer sheath may be tested after installation. A graphite outer coating on the cable may be specified to allow for an electrical connection to the outside of the cable sheath.
Typical electrical XLPE cable properties could be specified in the tabular format as shown in TABLE 6 as part of an overall cable specification.
Similar formats may be used for other types of cable.
TABLE 6 Typical MV Cable Electrical Parameter [coming soon]
3.3 Submarine Cables
Submarine cables require additional tensile strength to permit laying on or under the sea or river bed under high tension conditions. Paper, PVC or XLPE insulation is used together with additional protection measures against water ingress and mechanical damage and with special sheath compositions to repel worm attack. Such cables are manufactured in the longest possible lengths in order to minimize the number of underwater cable joints. When preparing the design for submarine cables an accurate knowledge of the prevailing currents and tidal variations is essential to assist in deciding the best cable route and most favorable times for the cable laying work.
3.4 Joints and Terminations
Techniques for jointing and terminating paper-insulated cables are well established. With the trend away from paper-insulated cables the traditional practices of highly skilled jointers for soldering and plumbing and application of paper rolls and tapes have been revolutionized. Key factors in the design of cable joints and terminations include:
- safe separation between phases and between phase and earth;
- capability to avoid dielectric breakdown at the interface and around reinstated jointing insulation under normal load and impulse surge conditions;
- adequate stress control measures to avoid high fields around screen discontinuities and cable/joint interfaces.
In all cases great care should be taken to ensure dry clean conditions during the jointing process on site. Tents may be erected over the jointing area to prevent ingress of dust or moisture.
Cable conductor connections are normally achieved using compression lugs and ferrules. These provide good mechanical grip and electrical contact, are designed to avoid any oxide layer build-up in cases using aluminum cores and provide a more repeatable solution than soldered connections.
Specially designed hand-operated or hydraulic tools and dies are used.
Soldered connections with operating temperature limits of some 160 degrees C are not compatible with the 250 degrees C short circuit temperature rating of XLPE cables. In addition, such soldered connections require a well-trained work force if high resistance connections are to be avoided. Mechanical clamps are also used to connect cable cores together. Metal inert gas (MIG) welding is favored for aluminum conductor connections.
FIG. 3 Standard installation details at a roadway verge.
LV and MV XLPE cable joints up to about 24 kV employ two pack resin systems. The resin components are mixed just prior to pouring into the joint shell where they harden to provide good mechanical and waterproof protection. It is important to follow the manufacturer's temperature and humidity storage recommendations and to monitor the useful shelf-life of such resins.
FIG. 3 shows a resin filled through joint with LSF properties for 24 kV 3 core XLPE cable.
Modern, fully molded type plug-in connectors, pre-molded push-on sleeves and heat shrink sleeve terminations allow for repeatable and rapid terminations to be prepared. Single and three phase cable connections to SF6 GIS are described in IEC 60859 for rated voltages of 72.5 kV and above.
When connecting oil-filled cables to SF6 switchgear special barriers are introduced to prevent problems of gas and cable oil pressure differentials.
4. CABLE SIZING
4.1 Introduction
After correct cable voltage classification the following considerations apply:
- Current carrying capacity;
- Short circuit rating;
- Voltage drop;
- Earth loop impedance;
- Loss evaluation.
It should be noted that very valuable research has been carried out by the Electrical Research Association (ERA) in the UK with regard to cable cur rent carrying capacities.
Typical calculations for a 20 kV transformer feeder cable, 3.3 kV motor feeder and a 400 V distribution cable are enclosed.
At voltages of 36 kV and above, current rating calculations are normally undertaken for individual installations in accordance with IEC 60287. This standard provides detailed algorithms for calculation of current ratings taking into account details such as cable design, installation conditions (direct buried, in air, in ducts), ambient temperatures, phase spacing and arrangement, type of sheath bonding and proximity of other cables.
4.2 Cables Laid in Air
Current rating tables are generally based on an ambient air temperature of 25 degrees C ( Europe) or 40 degrees C (Japan). Separate manufacturer's tables state the factors to be applied to obtain current ratings for the particular site conditions.
A 36 kV, 3 core, 300 mm2 , Cu conductor cable is to be laid in an ambient air temperature of 35 degrees C. The rating is given in manufacturers tables as 630 A at 25 degrees C and a derating factor of 0.9 is applicable for 35 degrees C operation.
Therefore cable rating at 35 degrees C=630x0.95=67 A.
In the case of cables laid in a concrete trench, the ambient temperature in the trench would be higher than the outside ambient air temperature. In addition, the proximity to other power cables laid in the same trench will have an effect on the cable current carrying capacity. Derating factors are included in manufacturers' literature. It should also be noted that cables laid outdoors should be protected from direct sunrays with appropriate sunshields. Metallic shields should certainly not fully surround single core cables because of their effect as a closed loop magnetic circuit to stray induced currents from the cable.
4.3 Cables Laid Direct in Ground
Current rating tables are generally based upon thermal aspects and the following environmental data:
Ground thermal resistivity G=1.0 degrees C
m=W ( Japan and Scandinavia ) =1.2 degrees Cm=W (UK)
More accurate data for the particular application may be collected from site measurements. Typically values range from 0.8 to 2.5 degrees Cm/Wand occasion ally to 3.0 degrees C m/W in desert areas. Derating factors in comparison with the 1.2 degrees C m/W reference may be obtained from ERA Report 69-30. For a G-value of 2.5 degrees C m/W a derating of approximately 75% would result.
Ground temperature t=25 degrees C (Japan) 515 degrees C (Europe) Installations at variance from the standard 15 degrees C ground temperature are taken into account by suitable derating factors with values deviating from unity by approximately 1% per degrees C.
Cable laying depth d=typically 1 m -- but see typical laying arrangements in Figs. 4 - 9. Actual depths will vary according to voltage and to regulations in the territory concerned.
FIG. 4 Warning, location and identification tape (sometimes used to supplement
protective tiles, and allowed or even preferred in some countries instead
of tiles at low or medium voltage).
FIG. 5 Typical trench cross section for 132 kV cables.
FIG. 6 Cables laid in sand-filled trenches.
FIG. 7 Electrical cables in unpaved, brick-paved or tiled areas and through
roads.
Where cables are laid together in one trench the proximity will necessitate derating factors to be applied to obtain the correct current carrying capacity for the site conditions. In some cases the use of special trench back fill materials may improve the situation by improving heat transfer.
Example Two 12 kV three phase circuits comprising of single core, 500 mm2 , XLPE insulated, Al conductor cables are each laid in ground in trefoil formation in parallel at a nominal 0.7 m depth and 0.25 m apart and with their 35 mm B copper screens bonded at both ends. The 90 degrees C XLPE cable rating is 655 A. What is the rating of each circuit in this configuration?
- Depth
- Temperature
- Ground thermal resistivity
- Proximity of parallel circuits (grouping)
0.7 m derating factor =1.0 25 degrees C
derating factor f1 =0.93 1.5 degrees C
m/W derating factor f2=0.91
0.25 m apart
derating factor f3 =0.86
Therefore the maximum current rating at 90 degrees C
per circuit=65= Ax1.0x0.93x0.91x0.86=477 A.
Some typical arrangements for cable installations are given in Figs. 4 - 9. For installations in roadway verges and other public areas it is very important to install to agreed standards and to maintain accurate records of the cable location (depth, lateral distance from known reference points, location of cable joints, oil tanks, etc.). Suitable cable management systems are described in SEC. 8.
FIG. 8 Cable trenches in concrete-paved area.
4.4 Cables Laid in Ducts
Cables may be installed in ducts buried in the ground with an earth, sand or concrete surround. Generally, it is good practice to install only one power cable per duct and the internal diameter of the duct should be at least 35 mm greater than the cable diameter. Cable ratings in ducts correspond to typically 80% of the direct in-ground burial rating. In order to improve the thermal conduction from the cable to the surrounding ground and improve this derating factor, the cable ducts may be filled with a bentonite slurry after cable pulling.
Bentonite is a clay-containing minerals of the montmorillonite (smectite) group which give it its characteristic swelling upon wetting and malleable nature for sealing around cables at cable duct ends. Filling the ducts has the added benefit of preventing cable thermo-mechanical movement which can lead to fatigue failure of the cable sheath and mechanical damage to joints.
FIG. 9 Typical plot plan of cable routes.
4.5 Earthing and Bonding
4.5.1 General
Sheaths and/or armoring on successive lengths of cable are bonded together and earthed to prevent stray voltages in uninsulated or lightly insulated metal in the event of a phase-to-earth fault occurring or due to the transformer action of the conductor and sheath. A mechanically sound and strong connection is essential.
When cable sheaths are bonded together, the induced voltages are short circuited but a current flows in the closed loop and this gives rise to heat loss. In addition to this loss in the sheath, a circulating eddy current due to the asymmetrical flux distribution in the sheath is also present whether the cables are bonded or not. Therefore two types of heating loss occur in the sheath:
- Sheath circuit loss (bonded sheaths only).
- Sheath eddy loss (normally small compared to loss in the bonded sheath circuit).
4.5.2 Three Core Cables
Three core cable circuits are normally solidly bonded such that the cable sheath, screen and/or armor are connected together to a grounding point at both ends. Each joint along the route is also bonded to earth (see SEC. 3.2.4).
FIG. 10 Methods of bonding and earthing cable sheaths to minimize the
effects of induced voltages.
4.5.3 Single Core Cables
Single core cable circuits require special consideration because of the voltages, which are proportional to the conductor current and frequency, being induced in the metal sheath and the introduction of circulating sheath cur rents. Single core cables may be solidly bonded (bonded at both ends) and this is the normal practice up to 36 kV with trefoil configurations. With larger conductor sizes and higher voltages specially bonded systems are more economic.
Single-point bonding over short 500 m lengths is used to keep induced voltages between the cable screen free ends within permissible limits. The sheath or screen is insulated from ground at one end and often fitted with sheath voltage limiters. The method is sometimes known as 'end point' earthing. Single-point bonding is also often employed for communications cables to prevent ground current loops.
On route lengths too long to employ end point earthing mid-point earthing may be used. In this system the cable is earthed at the mid-point of the route generally at a joint and is insulated from ground and provided with sheath voltage limiters at each termination. The maximum length of mid point-bonded circuits is about 1 km. A separate earth continuity conductor should be provided for fault currents that would normally be carried by the sheath for both single-point and mid-point-bonded circuits.
Cross-bonding or cyclic transposition is also employed to minimize the effect of induced voltages. In the cross-bonding system the cable route is split into groups of three drum lengths and all joints are fitted with insulating flanges. The cables laid in flat formation are normally transposed at each joint position. At each third joint position, the sheaths are connected together and grounded. At the other joint positions the sheaths occupying the same position in the cable trench are connected in series and connected to earth via sheath voltage limiters (see FIG. 10).
4.6 Short Circuit Ratings
Each conductor in a three phase circuit must be capable of carrying the highest symmetrical three phase short circuit through fault current at that point in the network. Ratings are normally taken over a 1 second fault duration period for a conductor temperature not exceeding 250 deg. C for XLPE-insulated cables and 160 deg. C for paper-insulated cables. This temperature must not adversely affect the conductor or the lead sheath and the armor wires (if present and being used as an earth return path). Mechanical strength to restrain the bursting forces and joint damage due to through fault currents is also a major design factor.
FIG. 11 (a) Paper-, (b) PVC- and (c) XLPE-insulated copper conductor cable short circuit ratings.
The earth fault condition affects both the phase conductors, screen wires/ metallic sheath and the armor wires. On smaller cables the short circuit rating of the phase conductor is the limiting feature but on larger sizes the effect of the fault current on the metallic sheath, screen wires and/or armoring is an overriding consideration. In the unlikely event of an internal 3 core cable fault, the intensity of the arc between the conductor and screen will normally cause rupture of the core bedding. This results in the fault current taking the least resistance along the steel tape or wire armor and will involve earth. The sheath or screen and armor must be able to carry the full specified earth fault current. On single core unarmored cables care must be taken to specify correctly the screen fault current carrying capability such that a fuse type failure of thin copper screen wires or tapes does not occur.
The sheath or screen should be able to carry at least one-half of the specified earth fault current as sharing between cores is not always even. The general formula for calculating the allowable short circuit current ISC is:
ISC = KA / __/t (amps)
where K=a constant depending upon the conductor material and on the initial and final temperatures associated with the short circuit conditions.
T=duration of short circuit in seconds.
A=cross-sectional area of the conductor in square mm (i.e. the number of wires x cross-sectional area of each wire).
Typical values of K for paper-, PVC- and XLPE-insulated cables are given in Tables 7a-c.
TABLE 7a Paper-Insulated Cable K Values
TABLE 7b PVC-Insulated Cable K Values
TABLE 7c XLPE-Insulated Cable K Values
4.7 Calculation Examples
4.7.1 20 kV Transformer Feeder
Consider a 20/3.3 kV, 12.5 MVA transformer to be fed by direct buried, 3 core XLPE, SWA, PVC, copper conductor cable.
Cable Current Carrying Capacity
Transformer full load current = 12:5x10^6/1.73x20x10^3 =361 A
Derating Factors Manufacturers provide data sheets for cables including appropriate derating factors based upon IEC 60287 ( TABLE 8).
For a ground temperature at depth of laying of 20 degrees C, the derating factor is 0.97.
The group derating factor based upon 3 cables laid in trench at 0.45 m centers is 0.84.
Ground thermal resistivity taken as the normal of 1.2 degrees C m/W for a UK installation and 1.00 rating factor.
Cable installation depth to be 0.8 m and 1.00 rating factor.
Therefore; subsequent current rating of cable to be 361 / [0.97x0.84] =443A:
From manufacturers tables selected cable size=240 mm2.
Short Circuit Rating:
The maximum system fault level in this application is 8.41 kA. From SEC. 4.6 of this Section and IEC 60364-5-54 (Electrical Installations In Buildings _ Earthing arrangements, protective conductors and protective bonding conductors):
Isc = KA / __/t
where K=constant, 143 for XLPE cable.
A=cable cross-section, 240 mm2 based on current carrying capacity.
T=short circuit duration, for MV cables use 1 second.
=34.3kA
From manufacturers tables and/or Figs. 11a-c for working voltages up to and including 19,000/33,000 XLPE based insulated cable the selected 240 mm2 cable is just capable of this 1 second short circuit rating. Note tables are conservative and assume a fully loaded cable. At the initiation of the fault conductor temperature590 degrees C and at the end of the fault conductor temperature5250 degrees C.
TABLE 8 Derating Factors Based on IEC 60287 [coming soon]
FIG. 12 Calculation example earth loop impedance.
Voltage Drop (Vd)
Consider a 100 m route length of cable with resistance, R=0.0982 O/km and inductive reactance, XL =0.097 O/km.
At full load current, Ifl =361 A @ 0.85 pf the cable voltage drop over a 100 m cable length,
Vd =Ifl 3XL x sinf1Ifl x R x cos Φ volts
=4.87V
=0.042%
Notes:
1. At 20 kV the voltage drop is negligible over such a short length of cable.
2. IET Wiring Regulations require a voltage drop for any particular cable run to be such that the total voltage drop in the circuit of which the cable forms part does not exceed 2 1/2% of the nominal supply voltage, i.e. 10.4 volts for a three-phase 415 V supply and 6 volts for a single-phase 240 V supply.
3. Industrial plant users may use different specifications and apply 65% (or even 610%) under no load to full load conditions and perhaps 220% at motor terminals under motor starting conditions.
4. Manufacturer's data for building services installations is often expressed in terms of voltage drop (volts) for a current of 1 ampere for a 1 meter run of a particular cable size.
Earth Loop Impedance For building services work it is important with small cross-section wiring and low fault levels to ensure that sufficient earth fault current flows to trip the MCB or fuse protection. For distribution power networks with more sophisticated protection the check is still necessary and allows the calculation of the likely touch voltages arising from the earth fault. This in turn can then be checked against the allowable fault duration to avoid danger. See Section 8 for a consideration of the design criteria associated with touch and step potentials.
- Consider the earthing resistance at the source substation=0.5 ohm.
- The source substation 20 kV neutral is approximately 10 km from the 100 m cable under consideration. In addition parallel copper conductor earth cable is run to supplement and improve power cable armor resistance values from equipment back to the primary substation infeed neutral. For this example assume power and supplementary earth copper cables and armor over the 10 km distance have a combined effective resistance of 0.143 Ohm.
- The combined resistance of the 100 m, 240 mm2 , cable armor (0.028 O/ 100 m) and in parallel 2x95 mm2 copper supplementary earth cables (0.00965 Ohm/100 m)=7.18x10^-3 Ohm.
- consider the earthing resistance at the cable fault to be 0.5 Ohm.
- The effective earth circuit is shown in FIG. 13. The effective primary substation neutral-to-fault cable resistance=0.15 Ohm.
- The maximum earth fault current at 20 kV has to be determined.
Sometimes this is limited by a neutral earthing resistor and the maximum limited current may be taken for calculation. Maximum earth fault current for this calculation is 1,000 A. For a fault to earth at the end of the 100 m cable, 10 km from the primary power infeed the fault current, I_f=(1,000x0.15)/(110.15)=131 A. Therefore touch voltage to earth at the cable fault=131x0.5=65.3 V.
4.7.2 3.3 kV Motor Feeder
Cable current carrying capacity
Current input to a motor is given by:
I =P __/ 3 p x U x eta x cos phi (3 phase)
I =P/U x eta x cos phi (1 phase) where
P=motor shaft power output
U=phase voltage
eta=motor efficiency
phi=phase angle
Consider a 3.3 kV, 340 kW fan motor.
Full load current = 340x10x 1.73x3.3x10x 30.9 = 66A
Cable Derating Factors:
Apply a group derating factor of 0.78 based upon cables touching on trays.
Necessary cable rating= 66/0.78 =85A.
From manufacturers' data a 3 core, 16 mm2 , XLPE/SWA/PVC copper conductor cable is suitable.
Short Circuit Rating:
System fault level is 3.5 kA for 1 second.
6mm2 cable fault capability ISC(16 sqmm) = 2.23 kA.
Try next larger, 25 mm2 standard size cable, ISC(25 sqmm) = 3.6kA and therefore complies.
Voltage Drop (Vd):
Consider a 75 m route length of cable at full load running current with resistance, R50.927 O/km and inductive reactance, XL =0.094 Ohm/km.
Earth Loop Impedance:
25 mm2 cable armor resistance (from manufacturers' literature)=1.7 Ohm/km.
Earth fault current at 3.3 kV is neutral point earthing resistance limited to only 30 A. This resistance swamps all other sequence components if the motor is earthed by only the cable armor.
Touch voltage=30x1.7x75/1,000=3.83 V. This is well below the continuous allowable IEC 60364 dry condition 50 V limit.
4.7.3 400 V Distribution cable
Current Carrying Capacity:
Consider the supply to a 400 V small power and lighting distribution board with a load of 38 kW (including an allowance for future extensions. Note that low voltage switchboards should be specified with a level of equipped and unequipped spare ways to cater for such future extensions).
Full load current= 38x10^3/1.73x400 =55A
Derating Factors:
Derating factors associated with cables laid in air, touching, on cable tray apply. The current rating is based upon a specified ambient temperature (30 degrees C) shielded from direct sunlight. For XLPE cable the maximum continuous conductor operating temperature is taken as 90 degrees C and the maximum conductor short circuit temperature as 250 degrees C. For six to eight cables laid touching on horizontal tray the group derating factor based upon IEC 60287 and the table given below =0.72.
Necessary cable rating= 55/0.72 =76.5A.
From manufacturers' data sheets 16 mm2 cable (current rating=95 A) may be selected and allows a margin for power factor.
Short Circuit Rating:
For short circuits on an LV system the protective device must clear the fault limiting the maximum conductor temperature (250 degrees C for XLPE insulation).
For cables of 10 mm2 and greater cross-sectional area the maximum fault clearance time, t, is based upon t= K2 A2 I 2 SC where t=fault clearance time (s)
A=cable conductor cross-sectional area (mm2)
ISC =short circuit current (A)
K=143 for XLPE insulation
The breaking capacity of the protective device must be at least equal to the highest current produced by a short circuit at the installation location.
For a 3.58 kA fault level the maximum fault clearance time, t=1432 x162 / 358022 =0.41 s.
For protection by a 63 A MCB to IEC 157-1 (now superseded by IEC 60947-2) the fault clearance time would be 0.01 second.
TABLE 9 Touching Cables on Tray Horizontally or Vertically Installed
Voltage Drop (Vd):
For a 16 mm2 cable Vd =2.6 mV/A/m ( TABLE 10).
Therefore for a 55A full load current and 20 m cable run Vd =2.9x55x20 =3.19 V =0.8%
TABLE 10 Current Carrying Capacity and Voltage Drop _ Multicore Cable
Having Thermosetting Insulation, Non-Armored (Copper Conductors) (BS 7671
Table 4E2A and 4E2B)
Earth Loop Impedance:
For a 16 mm2 cable the loop impedance=6.42 milli Ohm/m (from manufacturer's data).
Cable armor cross-sectional area=44 mm2.
For 20 m cable length loop impedance, ZS =0.1284 Ohm.
Single phase short circuit current, ISC =V/ZS =230/0.1284=1,791 A.
ISC =KA= __/t p where K=54 for steel wire armor and XLPE insulation.
From the formula, maximum operating time of protective device,
tmax =(542 x442)/17912
=1.75 s
For 63 A MCB to IEC 157-1 (IEC 60947-2) the tripping time for this fault level will be 0.014 second.
FIG. 1 illustrates typical symmetrical short circuit current/time duration ratings for PILC-, PVC- and XLPE-insulated cables.