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Electric power will be transmitted or distributed either by the overhead system or by underground cables. The underground cables have many benefits like less susceptible to injury through storms or lightning, low maintenance price, fewer probabilities of faults, smaller free fall and higher general look.

Underground Cable

The chief use of underground cables for several years has been for distribution of electrical power in engorged urban areas at relatively low or moderate voltages. However, recent enhancements within the style and manufacture have junction rectifier to the event of cables appropriate to be used at high voltages. This has created it attainable to use underground cables for the transmission of electrical power for brief or moderate distances. during this chapter, we have a tendency to shall focus our attention on the assorted aspects of underground cables and their increasing use within the grid.

Construction of Cables:

 Cores or Conductors:
A cable might have one or over one core (conductor) relying upon the kind of service that it’s supposed. as an example, the 3-conductor cable is employed for 3-phase service. The conductors’ area unit product of canned copper or aluminum and area unit sometimes stranded so as to produce flexibility to the cable.

Each core or conductor is given an appropriate thickness of insulation, the thickness of layer relying upon the voltage to be withstood by the cable. The usually used materials for insulation area unit fertilized paper, painted cloth or rubber mineral compound.

Metallic sheath:
In order to safeguard the cable from wetness, gases or alternative damaging liquids within the soil and atmosphere, a golden sheath of lead or aluminum is provided over the insulation

Over the gold, the sheath has applied a layer of bedding that consists of fibrous material like jute or jackboot tape. the aim of bedding is to shield the gold sheath against corrosion and from mechanical injury because of armor.

Underground cable

Over the bedding, armoring is provided that consists of 1 or 2 layers of galvanized steel wire or steel tape. Its purpose is to shield the cable from mechanical injury whereas birthing it and through the course of handling. Armoring might not be worn out the case of some cables.

In order to shield armoring from region conditions, a layer of fibrous material (like jute) like bedding is provided over the armoring. this can be referred to as serving.

It may not be out of place to say here that bedding, armoring and serving square measure solely applied to the cables for the protection of conductor insulation and to shield the gold sheath from mechanical injury.

Insulating Materials for Cables:

Rubber is also obtained from the opaque sap of tropical trees or it should be created from oil product. it’s relative permittivity variable between two and three, nonconductor strength is regarding thirty kV/mm and impedance of insulation is 1017Ω cm.

Underground Cable

Vulcanized latex (V.I.R.):
It is ready by commixture pure rubber with a mineral matter like flowers of zinc, minimum, etc., and three to five of sulfur. The compound therefore fashioned is rolled into skinny sheets and take strips. The rubber compound is then applied to the conductor and is heated to a temperature of regarding 150oC.

Impregnated paper:
It consists of with chemicals pulped paper made of wood chippings and inseminated with some compound like paraffinic or naphthenic material. this sort of insulation has virtually outdated the rubber insulation. it’s as a result of it’s the benefits of low value, low capacitance, high nonconductor strength, and high insulation resistance.

Varnished material:
It is a textile inseminated and coated with varnish. this sort of insulation is additionally referred to as empire tape. The material is lapped on to the conductor within the sort of tape and its surfaces are coated with petrolatum compound to permit for the slippery of 1 flip over another because the cable is bent. because the painted material is absorbent, therefore, such cables are invariably supplied with an antimonial sheath.

Polyvinyl chloride (PVC):
This insulation could be an artificial compound. it’s obtained from the chemical action of an aliphatic compound and is within the sort of white powder. Forgetting this material as cable insulation, it’s combined with sure materials referred to as plasticizers that are liquids with high boiling purpose. The plasticizer forms a gell and renders the fabric plastic over the specified vary of temperature.

Classification of Cables:

Cables for underground service could also be classified in 2 ways that consistent with (i) the kind of insulant employed in their manufacture (ii) the voltage that they’re factory-made. However, the latter methodology of classification is mostly most well-liked, consistent with that cables will be divided into the subsequent teams :

  • low-tension (L.T.) cables — up to one thousand V
  • high-tension (H.T.) cables — up to eleven,000 V
  • Super-tension (S.T.) cables — from twenty-two kilovolt to thirty-three kilovolt
  • additional high-tension (E.H.T.) cables — from thirty-three kilovolt to sixty-six kilovolt
  • additional super voltage cables — on the far side 132 kilovolt

A cable might have one or over one core relying upon the kind of service that it’s supposed. it’s going to be

  • single-core
  • two-core
  • three-core
  • four-core etc. For a 3-phase service,

either 3-single-core cables or three-core cable will be used relying upon the in operation voltage and cargo demand.

Insulation Resistance of a Single-Cor Insulation Resistance of a Single-Core Cable:

The cable conductor is given an appropriate thickness of insulant so as to forestall outflow current. the trail for outflow current is radial through the insulation. The opposition offered by insulation to outflow current is thought as insulation resistance of the cable. For satisfactory operation, the insulation resistance of the cable ought to be terribly high.

Consider a really little layer of insulation of thickness dx at a radius x. The length through that outflow current tends to flow is dx and therefore the space of X-section offered to the current flow is 2π xl.

Cables for 3-Phase Service:

I observe, underground cables square measure typically needed to deliver 3-phase power. For the aim, either 3-core cable or three single core cables could also be used. For voltages up to sixty-six kV, 3-core cable (i.e., multi-core construction) is most popular thanks to economic reasons.

Underground Cable

  1. Belted cables — up to 11 kV
  2. Screened cables — from 22 kV to 66 kV
  3. Pressure cables — beyond 66 kV.

Belted cables:

These cables square measure used for voltages up to 11kV however in extraordinary cases, their use could also be extended to 22kV. Fig. 11.3 shows the constructional details of a three-core belted cable. The cores square measure insulated from one another by layers of fertilized paper. Another layer of fertilized paper, known as the paper belt is wound around the sorted insulated cores. The gap between the insulated cores is crammed with fibrous insulation (jute etc.) thus on provide circular crosswise to the cable.

Screened cables:

These cables square measure meant to be used up to thirty-three kV, however above all cases, their use could also be extended to operational voltages up to sixty-six kV. 2 principal styles of screened cables square measure H-type cables and S.L. kind cables.

  • H-type cables.
    This type of cable was initially designed by H. Hochstadter and thence the name. Fig. 11.4 shows the constructional details of a typical 3-core, H-type cable. every core is insulated by layers of fertilized paper. The insulation on every core is roofed with a bimetallic screen that sometimes consists of a perforated aluminum foil.
  • S.L. kind cables.
    shows the constructional details of a 3-core *S.L. (separate lead) kind cable. it’s primarily H-type cable however the screen around every core insulation is roofed by its own lead sheath. there’s no overall lead sheath however solely armoring and serving square measure provided. The S.L. kind cables have 2 main benefits over H-type cables. Firstly, the separate sheaths minimize the likelihood of a core-to-core breakdown. Secondly, the bending of cables becomes simple thanks to the elimination of overall lead sheath.

Pressure cables:

For voltages on the far side sixty-six kilovolts, solid kind cables square measure unreliable as a result of there’s a danger of breakdown of insulation thanks to the presence of voids. once the operative voltages square measure bigger than sixty-six kilovolts, pressure cables square measure used. In such cables, voids square measure eliminated by increasing the pressure of the compound and for this reason, they’re known as pressure cables. 2 styles of pressure cables viz oil-filled cables and {gas pressure|pressure|pressure level|force per unit square measurea} cables are unremarkably used.

  • Oil-filled cables.
    In such sorts of cables, channels or ducts area unit provided within the cable for oil circulation. The oil fraught (it is that the same oil used for impregnation) is unbroken perpetually provided to the channel by means that of external reservoirs placed at appropriate distances (say five hundred m) on the route of the cable. Oil fraught compresses the layers of paper insulation and is forced into any voids which will have shaped between the layers. because of the elimination of voids, oil-filled cables will be used for higher voltages, the very being from sixty-six kV up to 230 kV. Oil-filled cables area unit of 3 sorts viz., single-core conductor channel, single-core sheath channel and three-core filler-space channels.
  • Gas pressure cables.
    The voltage needed to line up ionization within a void will increase because the pressure is raised. Therefore, if the standard cable is subjected to sufficiently high, the ionization will be altogether eliminated. At an equivalent time, the raised pressure produces radial compression that tends to shut any voids. this is often the underlying principle of force per unit area cables.