Basic Electrical Engineering

Three phase cable – they are a. Belted cable up to 11 KV, b. Screened cable 22KV to 66 KV, c. Pressure cable above 66 KV

Three phase cable

Commonly, underground cables are used to transmit three phase power supply. For three phase service, the following cables are used.

a. Belted cable up to 11 KV

b. Screened cable 22KV to 66 KV

c. Pressure cable above 66 KV

a. Belted cable up to 11 KV

The crosssection view of the belted cable. Each conductor is insulated by separate layers of impregnated paper. Thus, the insulated 3 conductors are set up as a part in the same set and then wrapped in an impregnated paper tape called a paper belt. Between insulated containers, it is filled with insulating material like jute or fiber. A lead sheath is laid on the paper bar to protect from mechanical damage and moisture. In this one or more layers are shielded and protected. These types of electric cables are used to transmit voltage up to 11 KV and sometimes voltage can be extended upto 22 KV.

b. Screened cable It is of two types

i. H – type cable (Hochstadter type)

ii. S.L. type cable (Separate lead type)

i. H Type cables 22 KV to 66 KV

The H type screened cable. Each conductor is insulated by separate layers of impregnated paper. Then it will be wrapped with metal insulation separately. The metal shield is made of thin aluminum and made to touch each other. These three containers are wrapped in copper fabric tape. There will be a lead sheath casing on it. There is one or more armors on it. These types of cables are used to transmit low and medium voltages. It is used to transmit 33 KV and sometimes to carry up to 66 KV.

ii. S.L. type cable (22KV to 66KV)

Even though this type is similar to that of H-type, each conductor is constructed with separate lead sleeves. Hence this cable can be handled easily for bending. c) Pressure cables A vacuum occurs when the solid power cord is used. It causes an electro chemical breakdown. Therefore, these pressure cables are used for distributing the voltage over 66 KV. The pressure of joint components used in these cable is increased and vacuum is reduced.

There are two types of pressure cables:

i. Oil filled cables

ii. Gas pump/pressure cables.

i. Oil filled cables

The oil under pressure is kept supplied to the channel from a tank. The oil tank is set at a distance of about 500m along the cable route. Pressure oil is prevented from vacuum by pressing the paper insulation. This type of cable is used for distributing voltages from 66 KV to 230 KV.

ii. Gas filled cable

The construction of gas pressure cable is similar to that of ordinary solid cable. It is designed in a triangular shape. The thickness of lead sheath of gas pressure cable is 75% of solid cable. The sheath is protected by a thin metal tape. The cable is kept in a gas tight steel pipe. The pipe is filled with dry nitrogen gas at 12 to 15 atmospheric pressure. The gas pressure produces radial compression and closes the voids formed between the layers of paper insulation. These cables carry more load current and operate at higher voltage than a normal cable. The nitrogen gas helps in quenching any flame. Its maintenance cost is low.

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