Construction of oil filled cable

• The solid type cable is applicable only for 66 kV. 
• The solid type cable is unreliable beyond 66 kV as there is possibility of breakdown of insulation due to presence of voids. 
• The voids are completely eliminated by presence of pressure of oil that why it is called as pressure cable. 
• There are two types of pressure cable : oil filled cable and gas filled cable.

Oil filled cable

There are three types of oil filled cable

( 1 ) Single core conductor channel oil filled cable

• There is one channel is formed at the center of cable and low viscosity oil passes through that channel. 
• The oil taken from oil reservoir and maintain at a pressure above atmosphere pressure at any point along the cable. 
• As the temperature inside cable increases, the oil expands and flows towards external reservoir similarly when temperature inside cable falls, the oil flow from external reservoir to cable due to contraction of oil.
•  This will result in less chances of formation of voids inside cable paper insulation.

Disadvantages

• Jointing of cable is complicated as oil channel lies at the center of cable

( 2 ) Single core sheath channel oil filled cable

• The single core sheath channel oil filled cable is shown in the figure. 
• The oil channels are provided in the metallic sheath. 
• As the lead sheath is at earth potential, jointing of cable is simple.

( 3 ) Three core filler space oil filled cable

• The three core oil filled cable is shown in the figure. 
• The oil ducts are provided in the filler space. There channels are perforated ( composing of small holes ) and are at earth potential.

Advantages of oil filled cable

• Formation of voids and ionization is avoided
• Dielectric strength of insulation increases
• If there is a leakage of oil at any point, the defect in lead sheath is indicated and this will result in possibilities of earth fault decreases

Disadvantages

• High initial cost
• Complicated system of laying of cable

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