Underground cables

UNIT5 UNIT5

Underground Underground cablescables

IntroductionIntroduction• Since the loads having the trends towards growing density. This requires the

better appearance, rugged construction, greater service reliability and increased safety.

• An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover.

• The interference from external disturbances like storms, lightening, ice, trees etc. should be reduced to achieve trouble free service.

• The cables may be buried directly in the ground, or may be installed in ducts buried in the ground.

AdvantagesAdvantagesThe underground cables have several advantages such as,

– Better general appearance

– Less liable to damage through storms or lighting

– Low maintenance cost

– Less chances of faults

– Small voltage drops

Dis-advantageDis-advantage

The major drawback is that they have greater installation cost and introduce

insulation problems at high voltages compared with equivalent overhead

system.

Construction of Cables

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Construction of CablesConstruction of Cables

• Core or ConductorA cable may have one or more than one core depending upon the type of service for which it is intended. The conductor could be of aluminium or copper and is stranded in order to provide flexibility to the cable.

• InsulationThe core is provided with suitable thickness of insulation, depending upon the voltage to be withstood by the cable.The commonly used material for insulation are impregnated paper, varnished cambric or rubber mineral compound.

• Metallic SheathA metallic sheath of lead or aluminium is provided over the insulation to protect the cable from moisture, gases or othes damaging liquids

Core

Belted paper

Lead sheath

Bedding

Single wire armoring

Overall Serving

BeddingBedding is provided to protect the metallic sheath from corrosion and from mechanical damage due to armoring. It is a fibrous material like jute or hessian tape.

ArmouringIts purpose is to protect the cable from mechanical injury while laying it or during the course of handling. It consists of one or two layers of galvanized steel wire or steel tape.

ServingTo protect armouring from atmospheric conditions, a layer of fibrous material is provided.

Properties of Insulating MaterialProperties of Insulating Material

The insulating materials used in cables should have the following properties

High resistivity.

High dielectric strength.

Low thermal co-efficient.

Low water absorption.

Low permittivity.

Non – inflammable.

Chemical stability.

High mechanical strength.

High viscosity at impregnation temperature.

Capability to with stand high rupturing voltage.

High tensile strength and plasticity.

TYPES OF MATERIALS USED IN INSULATION.

Insulating Materials for Cables

• RubberIt can be obtained from milky sap of tropical trees or from oil products.

It has the dielectric strength of 30 KV/mm.

Insulation resistivity of 10 exp 17 ohm.cm

Relative permittivity varying between 2 and 3.

They readily absorbs moisture, soft and liable to damage due to rough handling and ages

when exposed to light.

Maximum safe temperature is very low about 38 C

• Vulcanized India Rubber

It can be obtained from mixing pure rubber with mineral compounds i-e zinc oxide, red lead

and sulphur and heated upto 150 C.

It has greater mechanical strength, durability and wear resistant property.

The sulphur reacts quickly with copper so tinned copper conductors are used.

It is suitable for low and moderate voltage cables.

• Impregnated Paper This material has superseded the rubber, consists of chemically pulped paper

impregnated with napthenic and paraffinic materials.

It has low cost, low capacitance, high dielectric strength and high insulation

resistance.

The only disadvantage is the paper is hygroscopic, for this reason paper insulation is

always provided protective covering.

• Varnished Cambric

This is simply the cotton cloth impregnated and coated with varnish.

As the varnish cambric is also hygroscopic so need some protection.

Its dielectric strength is about 4KV / mm and permittivity is 2.5 to 3.8.

• Polyvinyl chloride (PVC) This material has good dielectric strength, high insulation resistance and high

melting temperatures.

These have not so good mechanical properties as those of rubber.

It is inert to oxygen and almost inert to many alkalis and acids.

• XLPE Cables (Cross Linked Poly-ethene)

This material has temperature range beyond 250 – 300 C

This material gives good insulating properties

It is light in weight, small overall dimensions, low dielectric constant and high

mechanical strength, low water absorption.

These cables permit conductor temperature of 90 C and 250 C under normal and

short circuit conditions.

These cables are suitable up to voltages of 33 KV.

CLSSIFICATION OF CABLESCLSSIFICATION OF CABLES

• Low tension (L.T) ----- up to 1000VLow tension (L.T) ----- up to 1000V

• High tension (H.T) ----- up to 11, 000VHigh tension (H.T) ----- up to 11, 000V

• Super tension (S.T) ---- from 22KV to 33KVSuper tension (S.T) ---- from 22KV to 33KV

• Extra high tension (E.H.T) cables ------- from 33KV to 66KV Extra high tension (E.H.T) cables ------- from 33KV to 66KV

• Extra super voltage cables ------beyond 132KVExtra super voltage cables ------beyond 132KV

A cable may have one or more than one core depending upon the type of

service for application. It may be,

Single Core

Two Core

Three Core

Four Core

Extra High Tension Cable

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XLPE cable

3-core belted Cable

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3- Core Cables3- Core Cables • Solid Type CablesSolid Type Cables

1. Belted Cables

In these cables the conductors are

wrapped with oil impregnated paper,

and then cores are assembled with filler

material. The assembly is enclosed by

paper insulating belt. These can be used for voltages up to 11KV or in some cases can be

used up to 22KV. High voltages beyond 22KV, the tangential stresses becomes an

important consideration. As the insulation resistance of paper is quite small along the layer,

therefore tangential stress set up, hence, leakage current along the layer of the paper insulation.

This leakage current causes local heating, resulting breaking of insulation at any moment.

2. Screened Cables

• These can be used up to

33kv but in certain cases

can be extended up to 66kv.• These are mainly of two types H-type and S.L type cables

a. H-TYPE Cables: • Designed by H. Hochstadter. • Each core is insulated by layer of impregnated paper. • The insulation on each core is covered with a metallic screen which is usually

of perforated aluminum foil.• The cores are laid in such a way that metallic screen make contact with one

another.• Basic advantage of H-TYPE is that the perforation in the metallic screen assists

in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids in the dielectric is eliminated.

• The metallic screen increase the heat dissipation power of the cable.

b. S.L - Type: (Separate Lead)

• Each core insulation is covered by its own lead sheath.• It has two main advantages, firstly the separate sheath minimize the

possibility of core-to-core breakdown. Secondly the, bending of cables become easy due to the elimination of over all sheath.

• The disadvantage is that the lead sheaths of S.L is much thinner as compared to H-Type cables, therefore for greater care is required in manufacturing.

• Pressurized Type CablesPressurized Type Cables• In these cables pressure is maintained above atmosphere either by oil or by gas.• Gas pressure cables are used up to 275KV.• Oil filled cables are used up to 500KV.

• Oil Filled Cables• Low viscosity oil is kept under pressure and fills the voids in oil impregnated paper

under all conditions of varying load.• There are three main types of oil filled cables

a. Self-contined circular type

b. Self-contained flat type

c. Pipe Type cables

Pipe Type Cable

Sheath Channel Oil Filled 3-Core Oil filler Cable

SINGLE CORE V.I.R CABLE (BRAIDED)

TWIN CORE BELTED CABLE

3-CORE BELTED CABLE

4-CORE BELTED CABLE

3-CORE CABLE WITH STEEL WIRE ARMOUT

H & HSL TYPE CABLE

Laying of cables

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