Some of the questions personal interviews

Some of the questions personal interviews

1 -- Why cables are not used for long distance 

transmission?

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

2 -- Mention the 3 main parts of the cable?

Conductor, dielectric, sheath

3 -- What is the function of conductor?

Conductor provides the conducting path for the current.

4 - .What is the function of sheath in 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.

5 -- Mention the conductor materials in cables?

Copper, Aluminium
All occupies greater space than copper for a given conductance.
Both are high electrical conductivity.

6 -- What is the purpose of stranding of conductors?

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

7 -- 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.

8. State the properties of insulating materials.

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

9. Mention the commonly used power cables.

Impregnated paper, Polyvinyl chloride, polyethylene

10. 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.

11. State the merits of paper insulated cables.

High current carrying capacity, long life and greater reliability

12. 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.

13. State the advantages of polythene insulators.

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

14. By what materials cable sheaths are made?

Lead sheaths and Aluminums sheaths.

15. 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, withstands

the required gas pressure without reinforcement.

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Questions in Electrical Engineering

Questions in Electrical Engineering

1. Why the transmission systems are mostly overhead systems?

Because of the cost consideration, the transmission systems are mostly overhead systems.

2.Why all overhead lines use ACSR conductors?

ACSR conductors comprises of hard drawn aluminium wires stranded around a core of single or multiple strand galvanized steel wire. They provides the necessary conductivity while the steel provides the necessary mechanical strength. Has less corona loss. The breaking load is high and has less weight.

3.Why transmission lines are 3 phase 3 wire circuits while distribution lines are 3 phase 4 wire circuits?

A Balanced 3 phase circuit does not require the neutral conductor, as the instantaneous sum of the 3 line currents are zero. Therefore the transmission lines and feeders are 3 phase 3 wire circuits. The distributors are 3 phase 4 wire circuits because a neutral wire is necessary to supply the 1 phase loads of domestic and commercial consumers.

4.Why overhead line conductors are invariably stranded?

They are stranded to make them flexible during erection and while in service.

5 . Stat the advantages of interconnected systems.

Any area fed from one generating station during overload hours can be fed from another power station and thus reserved capacity required is reduced, reliability of supply is increased and efficiency is increased.

6. What is a ring distributor?

A ring distributor is a distributor which is arranged to form a closed circuit and is fed at one or more than one point.

7. State any two advantages of ring main system.

Less voltage fluctuations at consumer’s terminals.Less copper is required as each part of the ring carries less current than in radial system.

8. Mention the disadvantages of a 3 wire system

In 3 wire system a third wire is required.The safety is partially reduced.A balancer is required and therefore cost is increased.

9. What are the advantages of a 3 wire dc distribution system over a 2 wire dc distribution system?

If 3 wire system is used to transmit the same amount of power over the same distance with same efficiency with same consumer voltage we require 0.3125 times copper as required in 2 wire system.

10. Mention the differences between 3 wire and 3 phase 4 wire distribution system?

3 phase 3 wire is employed for balanced loads, but 3 phase 4 wire is employed for unbalanced loads.
3 phase 3 wire is used for transmission but 3 phase 4 wire is used for distribution of power to consumers.

 

 

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