The type of cable to be used at a particular location is determined by the mechanical considerations and the voltage at which it is required to operate. Usually the operating voltage determines the type of insulation and the cables are placed in various categories depending upon the voltage for which they are designed.
1. Low Tension (or LT) Cables:
These cables are meant for use up to 1,000 V. For voltages up to 6,600 V, the electrostatic stresses developed in cables are very small and thermal conductivity is also of not much importance so no special construction is required. The insulating materials used may be impregnated paper, varnished cambric, vulcanized rubber, or vulcanized bitumen.
The lt cables are of two types viz. single core and multi-core cables. The former type has the advantages of simplicity of construction and availability of larger conductor section.
The various sizes of Lt cables are given below:
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Single core cable with aluminium conductors: 1.5 – 625 mm2
Two-, three-, three-and-a-half and four core cables with aluminium conductors: 1.5 – 625 mm
Control cables up to 61 cores with copper conductors: 1.5 and 2.5 mm2.
Single core Lt cable consists of one circular core of tinned stranded copper (or aluminium) insulated by layers of impregnated paper or varnished cambric over it and a lead sheath over the insulation. The lead sheath protects the cable against ingress of moisture and mechanical handling.
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If the cables are to be buried directly then an overall serving of compounded fibrous material or hessian tape is provided over the sheath in order to protect the metallic sheath against corrosion. Single core cables are usually not provided with armouring in order to avoid excessive loss in the armour.
The multicore cables for use up to 11,000 volts are of belted type. The belted type cable consists of either circular shaped or oval or sector shaped (oval or sector shaped core improve the copper space factor) cores of stranded copper (or aluminium) conductors wrapped around by impregnated paper and then wormed together. The interstices are filled with packing to make the cable of circular cross-section.
Insultating belt of impregnated paper is provided surrounding the three cores. A lead sheath enclosing the whole is provided for its protection against the entry of moisture. The cable is provided with two layers of steel tape pre-dipped in compound. The serving over the armouring consists of a compound and then one layer of impregnated hessian tape. In order to prevent adhesion, a coating of lime wash is applied to the outside of the cable.
The belted type construction is not suitable for the cables used for voltages above 22 kV because of development of both the radial and tangential stresses. The tangential stresses act along the layers of insulation. The electric resistance, therefore, dielectric strength of the impregnated paper is much higher across the layers than along the layers.
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The leakage current on account of tangential stresses along the impregnated paper insulation causes power loss at the centre tilling and local heating resulting in breakdown at any moment. Further owing to non-homogeneity of dielectric in belted construction, when cables are loaded and unloaded some portions of the dielectric are less stressed whereas some portions are overstressed resulting in formation of vacuous spaces and voids. These vacuous spaces are ionised when voltage is applied and ultimately deteriorate the cable insulation.
The above difficulties have been overcome in the screened cables where leakage currents are conducted to earth through metallic sheaths.
2. Super-Tension Cables:
For cables above 11,000 V a special construction is adopted. For use up to 33 kV, the cables used are screened cables where leakage currents are conducted to earth through metallic sheaths.
Screened cables are of two types viz.:
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(a) H-type and
(b) S L type.
(a) H-Type Cables:
In the “H” type cable, invented by Hochstadter, no belt insulation is used, but each of the core is insulated with paper to the desired thickness and over this is provided a layer of metallized paper perforated to facilitate the process of impregnation, the coefficient of expansion and contraction is same as that of dielectric. Additional layer of the cotton tape with fine wires of copper is wrapped round all the three cores.
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Over this tape, no belt insulation is provided, but the lead sheath and armouring are provided as usual. The perforations in the metallized paper sheaths assist in the complete impregnation of cable with the compound and thus the possibility of air-duct formation is eliminated. All four screens and the lead sheath are at earth potential, with the result that the electric stresses are entirely radial and the dielectric losses are therefore reduced.
Another advantage of metal foil is that it increases the heat dissipating power and there are no sheath losses. All of these effects increase the current carrying capacity of the cable. These cables have been used up to 66 kV but are usually used up to 33 kV because pressurized cables have better performance than H type cables for voltages exceeding 33 kV.
(b) S L Type Cables:
In S L type cables each core is first insulated with an impregnated paper and then each of them, is separately lead sheathed. Now the three cores are just equivalent to three separate cables, each having its own lead sheath. The three cables are laid up with fillers in the ordinary way, armoured and served overall with impregnated hessian tape as usual. No lead covering is provided surrounding all the three cores in addition to their individual lead sheaths.
The advantages of S L type cables over H-type cables are:
(i) Bending of cable becomes possible owing to no overall lead sheath.
(ii) Less tendency for oil drainage on hilly routes owing to elimination of filler spaces containing compound.
The disadvantage of SL type cable is that the manufacturing is difficult because of thinner lead sheaths.
SL type cables can be used up to 66 kV.
(c) HSL Type Cables:
Such a cable is a combination of H-type and SL type cables. In HSL type cables each conductor is insulated, sheathed with metalized paper and in then provided with lead sheath. The three cores are then laid up and provided with filler, braided, armoured and finally served.
Advantages of Screened Type Cables over Belted Types:
(i) The metal sheathed core cable has a greater core to core thickness for a given overall diameter than a plain cable so the possibility of core to core faults is reduced to a large extent.
(ii) Electric stresses are uniformly radial in all sections of the dielectric.
(iii) As there is no worming or packing so the dielectric subjected to electric stress is only paper which is quite homogeneous and therefore possibility of formation of voids within the electric field is not there.
(iv) The metal sheaths help in dissipation of heat, so the current carrying capacity of the cables is increased.
3. Extra High Tension Cables:
The cables considered up till now are also called solid type and in such cables it was assumed that the dielectric is homogeneous and there are no voids in the layers. With this assumption it is necessary to stick to maximum safe dielectric stress of about 4 or 5 kV per mm and also to maximum operating temperature of 50° or 60°C in order to ensure safety from breakdown because the dielectric is far from homogeneous and with the normal manufacturing methods, it is almost impossible to avoid voids in the layers of dielectric.
The formation of these voids or vacuous spaces causes unequal voltage stresses (which may exceed the safe limits) and also temperature rise due to leakage current. Again, even if a new cable could be constructed free from voids owing to development of modern techniques, it would be impossible to avoid their formation under the influence of the movements of the compound and expansion and contraction of the paper during normal working.
When these voids are subjected to the electrostatic stresses, ionization takes place, which gives rise to certain chemical action, resulting in the deterioration of the chemical compound. Thus, these voids are sometimes the primary cause of a breakdown and therefore are required to be avoided.
In order to meet the increased voltage demand the extra high tension and extra super-voltage power cables, useful for 132 kV and above, have been developed. In all such cables, the voids have been eliminated by increasing the pressure of the compound and that is why such cables are also called pressure cables.
Pressure cables are of two types viz.:
(a) Oil-Filled Cables, and
(b) Gas-Pressure Cables.
(a) Oil-Filled Cables:
In this type of cable a channel is formed at the centre of the core by stranding the conductor wires around a hollow cylindrical steel spiral tape. This channel formed at the centre of core is filled with thin oil by means of oil reservoir and feeding tanks along its length and maintained at a pressure, not below atmospheric one at any point along the cable.
This oil is the same light mineral oil of very low viscosity as used for initial impregnation. The system is so designed that when the oil gets expanded on account of increase in temperature of cable, the extra oil collects in the external reservoir, which sends it back during contraction on account of fall in temperature during light load conditions.
Thus formation of voids in the dielectric is rendered impossible. Such cables are known as single core conductor channel cables. Its disadvantage is due to the fact that the channel is at the middle of the cable and is at full voltage with respect to earth, so requiring a complicated system of jointing. It is advantageous from the point of view of potential gradient due to larger diameter of conductor and its hollow construction.
The other type of single core oil-filled cable is sheath channel cable. In this type of cable, the conductor is made solid similar to that of solid cables and is paper insulated. In this type of cables the oil channels are produced either by grooving the sheath or by arranging spacers between dielectric and lead sheath.
The latter arrangement is more sounded mechanically in comparison to former one but due to high resistance to oil flow (6 to 8 times that of former type) requires large number of feeding points. In this case since the channels are at earth potential so system of joints and installation are simpler.
In 3-core oil-filled cables the oil ducts are accommodated in the hollow filler spaces. These channels are made of perforated metal-ribbon tubing and are at earth potential. While jointing such a cable great care is required to be taken.
Another design of 3-core oil-filled cable is flat type. The flat sides are reinforced with metallic tapes and binding wires so that during increase in oil pressure, due to heating, the flat side is deformed and the cable section becomes slightly elliptical.
In another construction of 3-core oil filled cables use of 3-core paper insulated cable without a lead sheath is made. The cable is pulled into a steel pipe which is then filled with oil. Pumps are then used to maintain a specified oil pressure and permit it to expand and contract with the loading cycle.
Leakage of oil in oil-filled cables is a very serious problem. Automatic signalling is provided to indicate the decrease in oil pressure in any of the phases. The pressure within the cable must not go below atmospheric and a lower limit of about 21,700 N/m2 gauge is set. The transient pressure, developed due to sudden increase of load, should not increase beyond about 8.6 × 106 N/m2 gauge. The joints in the cable are to be properly made so that there is no restriction to the oil flow.
The breakdown inception stress for oil-filled cables is 30-40 kV/mm which is much higher than that for a solid type cable. So the normal operating stresses are raised considerably. Maximum stresses up to 13 kV/mm are acceptable for 275 kV and 400 kV oil-filled cables. The current carrying capacity of oil-filled cables is determined by allowing a normal maximum conductor temperature of 85° C.
The advantages and disadvantages of oil-filled cables are given below:
Advantages:
(i) Smaller overall size and smaller weight for given voltage and kVA rating due to reduction in the thickness of dielectric required.
(ii) No ionisation, oxidation and formation of voids.
(iii) More perfect impregnation.
(iv) Possibility of increased temperature range in service.
(v) Smaller thermal resistance due to decrease in dielectric thickness, so higher current rating.
(vi) Impregnation possible after sheathing.
(vii) More maximum permissible stresses.
(viii) Detection of fault easy, as it will come to notice immediately as oil will start leaking.
Disadvantages:
(i) Greater cost.
(ii) Complicated laying of cables and maintenance.
(b) Gas Pressure Cables:
Gas pressure cables are of two types:
(i) External Pressure Cables and
(ii) Gas-Filled Cables.
(i) External Pressure Cables:
The external pressure cable, is a serious rival to the oil-filled type, and is being developed for the highest voltages. In such a cable the pressure, is applied externally and raised to such an extent that no ionisation can take place. At the same time the radial compression due to this increased pressure tend to close any voids. Also the working power factor of such’ a cable is improved.
The external pressure cable originally designed by Hochstadter, Vogel and Bowden. This cable is similar in construction to that of ordinary solid type except that it is triangular instead of circular in section and thickness of lead sheath is 75% of that of solid type cable. The triangular section reduces the weight, gives a low thermal resistance and at the same time lead sheath acts as a pressure membrane.
Also there is no bedding and serving so thermal resistance is reduced. The cable is armoured with a thin metal tape so that the formation of any abnormal ties over its surface is avoided. The core may have circular or oval section but modern practice is to employ an oval section, which gives an improved copper space factor.
The cable is laid in gas-tight metal pipes of somewhat larger section. The pipe is filled with nitrogen at a pressure of 12 to 15 atmospheres which continually compresses the cable radially from outside so that radial breathing of the cable occurs and any voids etc. are closed. The steel pipes used are coated with a special paint in order to avoid corrosion and it is further protected with an impregnated felt.
In comparison with a normal cable, such cables can carry 1.5 times the load current, double the operating voltage and thus transmit 3 times power. The maximum potential gradient is 10 kV/mm and the dielectric power factor at 15°C is 0.6 per cent. The steel pipes provide mechanical protection to the cables. The nitrogen in steel pipes help in quenching any flame. Moreover, maintenance cost of such cables is small. The disadvantage of such cables is that cost is very high.
The above cable is pipeline type. The second type is self-contained type, in which an additional reinforced lead sheath is employed, the principle being the same as that of former type. Dried nitrogen is fed into the small annular space between dielectric and sheath from gas cylinders, and finding its way along the cable maintains the whole of the dielectric at a pressure of about 14 atmospheres.
(ii) Internal Pressure Cables:
Such cables are of 3-types:
1. High-Pressure Gas-Filled Cables.
2. Gas-Cushion Cables.
3. Impregnated Pressure Cables.
1. High-Pressure Gas-Filled Cables:
In high pressure gas-filled cables spaces are provided in the dielectric itself for the gas which is inert gas like nitrogen at pressure of about 12 atmospheres for super-voltage cables and about 6 atmospheres for extra high tension cables. Pressure is retained by means of a lead sheath which in the case of single core cables has a diametral clearance of about 0.63 mm. This facilitates the axial flow of gas, which also passes along the unimpregnated strand. In-the case of multicore cables this clearance is not essential; the filler spaces and strands providing a sufficiently low resistance path for the flow of gas.
2. Gas-Cushion Cables:
In gas-cushion cable, screened space is provided all along the length of cable, in between the lead sheath and the dielectric. The screened space is sub-divided by means of barriers and thus inert gas is stored at various points along the run of a cable. This facilitates the jointing of cable without losing gas from the entire cable, because while cutting the cable, loss of gas will be local only.
The special feature of the cable is thus that no arrangement is required for the transmission of pressure to the cables from outside, so that the cable is a complete unit with its own armouring, requiring no external pipe protection whatever.
3. Impregnated Pressure Cables:
Impregnated pressure cable is similar to solid cable except that such a cable consists of mass-impregnated paper dielectric and this is maintained under a pressure of 14 atmospheres by means of nitrogen. Special reinforcement is provided to cater for the large hoop and longitudinal stresses set up. This reinforcement consists of longitudinal and circumferential metallic tapes.
The advantages of the internal pressure cables are:
(i) External accessories are eliminated.
(ii) With suitable designs the cable can be used for vertical run without any fear of drainage.
(iii) There is marked improvement in the power factor of the cable dielectric with the increased pressure.
Gas SF6, because of its good thermal characteristics and high dielectric strength, is also used for insulating the cables. Cables insulated with SF6 gas can be matched to overhead lines and can be operated corresponding to their surge impedance loading. Such cables can be employed for transmission of thousands of MVA even at ultra-high voltage while the use of conventional cables is limited to 100 MVA and 500 kV.