Important Short Questions and Answers: Transmission and Distribution - Insulators and Cables

86.Why cables are not used for long distance transmission?

Cables are not used for long distance transmissions due to their large charging currents.

87.Mention the 3 main parts of the cable?

Conductor ,dielectric ,sheath

88.What is the function of conductor?

Conductor provides the conducting path for the current.

89.What is the purpose of insulation in a cable?

The insulation or dielectric withstands the service voltage and isolates the conductor with other objects.

90.What is the function of sheath in a cables?

The sheath does not allow the moisture to enter and protects the cable from all external influences like chemical or electrochemical attack fire etc.

91.Mention the conductor materials in cables?

Copper , Aluminium

92.What is the purpose of stranding of conductors?

Stranding increases the resistance of the cable .It has flexibility.

93.Define the segmental conductors.

The stranded wires which are compacted by the rollers to minimize the air spaces between the individual wires are called segmented conductors .Here the conductor size is reduced for a given conductance.

94.State the properties of insulating materials.

It should have high insulation resistance ,high dielectric strength ,good mechanical properties ,non-hygroscopic, capable of being operated at high temperatures ,low thermal resistance and low power factor.

95.Mention the commonly used power cables.

Impregnated paper, Polyvinyl chloride, polyethene

96.Mention the advantages of pvc over paper insulated cables.

Reduced cost and weight, Insulation is resistant to water, Simplified jointing, Increased flexibility No plumbing required.

97. State the merits of paper insulated cables.

High current carrying capacity ,long life and greater reliability

98.            Where polythene cables are used?

  being non-hygroscopic used in cables for submarines and damp soil.

  being lighter used as aerial cables for vertical installations.

99.            State the advantages of polythene insulators.

They are non-hygroscopic, light in weight, low dielectric constant, low loss factorand low thermal resistance.

100. By what materials cable sheaths are made?

Lead sheaths and Aluminium sheaths.

101.In what way Al sheaths are superior to lead sheaths?

Al sheaths are smaller in weight, high mechanical strength , greater conducyivity, cheap,easy to manufacture and install, withstand the required gas pressure without reinforcement.

102.Where CSA sheath is used in cables ?

Corrugated seamless aluminium sheath is used in high voltage oil filled cables and telephone lines.

103.Why it is used?

It is used because it is very flexible and easily by repeated bending the sheath is not distorted and it is not damaged. It has lesser weight and reduced thickness.

104.Why protective covering is done in cables?

To protect the cables from mechanical damage , corrosion and electrolytic action when laid direct in the ground the protective covering is made.

105.By what material protective covering is made?

Bitumen &Bituminized materials, pvc and layers of fibrous materials.

106.What is meant by serving of a cable?

Layers of fibrous material permitted with waterproof compound applied to the exterior of the cable is called serving of a cable.

107.Why armouring is done in the cables ?

To protect the sheath from mechanical damage.

108.Why armouring is not done in single core cables?

The presence of magnetic material within the alternating magnetic field of a single core cable produces excessive losses. Hence single core cables are left unarmoured with non-magnetic materials like tin-bronze or silicon-bronze tapes or wires.

109.Why Al is used as an armour material?

It has non-magnetic properties, high conductivity and mechanical strength.

110.What is meant by grading of cables?

The method of equalizing the stress in the dielectric of the cable is called the grading of cables.

111.Mention the methods of cables.

Capacitance grading and inter sheath grading.

112.Why the capacitance of the cable is very high than the capacitance of the overhead lines?

The distance between the conductors are small. The distance between the cores and the earthed sheath is also small. The permittivity of the cable insulation is 3 to 5 times greater than that of air insulation.

113.What is meant by charging current of a cable?

The capacitance of a cable determines the charging current. The charging current restricts the use of cables on EHV lines. The current carrying capacity of an a.c cable is also reduced by the charging current.

114.Why power loss occurs in the dielectric of a cable?

       due to conductivity of insulation

Dielectric hysteresis or dielectric absorption.

Ionization or corona

115.       Mention the methods of laying the cables.

Direct laying , draw in system and solid system

116.       Mention the advantages of direct laying of cables.

It is simple and cheaper method. It gives the best conditions for dissipating the heat generated in the cables.

117.State any 2 disadvantages of direct laying method.

Localization of fault is difficult . It cannot be used in congested areas where excavation is inconvenient.

118.       Mention the disadvantages of pressure cables.

The cost of the pressure tube is high.

119.       Mention the types of gas pressure cables.

External and internal pressure cables.

120.       What are the types of oil filled cables?

Single core conductor channel cables , Sheath channel cables and 3 core filler space channel.

121.       What are the types of pressure cables?

Oil filled cables and gas pressure cables.

122.       What is the operating range of pressure cables?

It is greater than 66 kv.

123.       What are the advantages of SL cables over H- type cables?

The possibility of core to core breakdown decreases to a large extent. Bending of cables becomes easy owing to no overall lead sheath.

124. Mention the disadvantages of oil filled cables.

Expensive , laying and maintenance of cables is quite complicated.

125.       What are the types of screened cables?

H type and SL type cables.

126.       Why the working voltage level of belted cables is limited to 22 kv?

It is limited because beyond 22 kv tangential stresses acting along the layers of paper insulation set up large current. This current causes local heating resulting in the risk of breakdown insulation at any moment.

127. Up to what voltage range are belted cables used?

Up to 11 kv. In some extra ordinary cases they are used upto even 22 kv.

128. What are the different types of cables that are generally used for 3 phase service?

Belted cable , Screened cables and pressure cables.