Presented By: GIRISH.H.MAHAJAN ROLL NO:40 CLASS: S.E Electrical

Engg

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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.

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Advantages

◦ Better general appearance

◦ Less liable to damage through storms or lighting

◦ Low maintenance cost

◦ Less chances of faults

◦ Small voltage drops

Disadvantages

◦ The major drawback is that they have greater installation

cost and introduce insulation problems at high voltages

compared with equivalent overhead system.

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

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

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

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

to 66KV

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

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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

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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.

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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.

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Pressurized 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.

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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-contained circular type b. Self-contained flat type c. Pipe Type cables

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Greater operating dielectric stresses

Greater working temperature and current carrying capacity

Better impregnation

Impregnation is possible after sheath

No void formation

Smaller size of cable due to reduced dielectric thickness

Defect can easily be detected by oil leakage

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In these cables an inert gas like nitrogen is used to exert pressure on paper dielectric to prevent void formation.

These are also termed as Compression cables

They insulated cores similar to solid type

The cable is inserted in a pressure vessel which may be a rigid steel pipe, commonly known as pipe line compression cable.

The nitrogen gas is filled in vessel at nominal pressure of 1.38 * 10 exp 6 N/ square meter with a maximum pressure of 1.725 * 10 exp 6 N/ square meter.

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In GIC cables high pressure sulphur hexaflouride (SF6), fills the small spaces in oil impregnated paper insulation and suppresses the ionization.

Most EHV and UHV lines insulated with sulphur hexaflouride (SF6) gas are being used extensively for voltages above 132 KV up to 1200 KV.

These cables are very popular for short lengths, river crossings and high way crossings.

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Gas Insulated Cables have several advantages over oil filled cables,

Efficient heat transfer hence can carry more current.

Low dielectric loss and low capacitance SF6 gas is non-toxic, chemically stable and

non-inflamable. Terminations of GIC cables are simpler and

cheaper.

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The reliability of underground cable network depends to a considerable extent upon proper laying.

There are three main methods of Laying underground cables

a. Direct Laying

b. Draw in system

c. Solid system

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Transmission: Underground may be 4-20 times Overhead.

Sub transmission: Underground may be 4-20 times Overhead

Distribution: Underground may be 2-10 times Overhead

New underground may be cheaper than overhead in special conditions and costs vary greatly from utility to utility and place to place.

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THANK YOU

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