Unit III
Part A
1. What are the factors affecting sag in a transmission line?
The two factors that affect the sag in transmission line are the length of span , the working
tensile stress which depends on the wind and the ice load. Tension and sag of a conductor is
always compensated with respect to the design.
2. Define sag.
The difference in level between the points of supports and the lowest point of the conductor is
called as sag.
3. What is a sag template?
A Sag Template is a very important tool with the help of which the position of towers on the Profile is
decided so that they conform to the limitations of vertical and wind loads on any particular tower, and
minimum clearances, as per I.E. Rules, required to be maintained between the line conductor to ground,
telephone lines, buildings, streets, navigable canals, power lines, or any other object coming under or near
the line.
4. Why are insulators used in overhead lines?
Insulators are used to support the conductors and withstand both the normal operating voltage
and surges due to switching and lightning. It also provide necessary insulation between line
conductors, tower and thus prevent any leakage from conductors to earth.
5. What are the desirable properties of insulator?
High mechanical strength.
High electrical resistance
High relative permittivity
High ratio of puncture strength
The insulator material should be non porous
6. What are the advantages of string insulators?
Cheaper than pin type insulators for voltage greater than 33 KV
Number of disc can be inserted depending upon the voltage that flows.
Failure in any one unit will not affect the entire string. Replacement can be done easily.
More flexibility.
Conductor runs below the cross arm, so line conductors are less affected by lightening.
Arrangements act as lightening arrestors.
7. What is the purpose of insulator?
The insulators provide necessary insulation between line conductors and supports and thus prevent any
leakage current from conductors to earth.
8. Define string efficiency.
The ratio of voltage across the whole string to the product of number of discs and the voltage
across the disc nearest to the conductor is known as string efficiency.
9. What are the methods of improving string efficiency in line insulators
(i) By using longer cross arms
(ii)By grading the insulators
(iii) By using a guard ring
10. What are the factors on which conductor spacing and ground clearance depend?
Nominal system Voltage
Maximum Voltage
Size of the conductor
Sag and Tension
Diameter and shape of conductor.
11. How does grading ring improve string efficiency?
This method uniforms the potential distribution across each units in insulator strings. Thereby
string efficiency improves.
12. What is mean by guard ring?
The transmission line tower a large metal ring surrounding the line unit and connected to the
metal part of the bottom of the line unit is used. Such a ring is called guard ring
Unit IV
Part A
1. 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.
2. Classify the cables used for three phase service.
o Belted cables - upto 11 kV
o Screened cables - from 22 kV to 66 kV
(i)H – Type cables
(ii)S-L Type cables
o Pressure cables - beyond 66 kV.
(i)Oil filled cables
(ii) Gas pressure cables.
3. Write down the various parts of cables.
(i)Core,
ii) Insulation,
(iii) Metallic sheath,
(iv) Bedding,
(v) Armouring,
(vi) Serving
4. Mention any four insulating materials used for underground cables.
The principal insulating materials used in cables are rubber, vulcanized Indian rubber,
impregnated paper, varnished cambric and polyvinyl chloride.
5. What are the disadvantages of oil filled cables?
1. The initial cost is very high
2. The long length is not possible
3. The laying of cable is difficult
4. Maintenance of cable is difficult
6. Define grading of cables?
The process of achieving uniform electrostatic stress in the dielectric of cables is known as
grading of cables.
7. What is the main purpose of armouring?
Over the bedding, armouring is provided which consists of one or two layers of galvanized steel
wire or steel tape. Its purpose is to protect the cable from mechanical injury while laying it and
during the course of handling.
8. What are the requirements of the cables?
The size of the conductor used must be such that it should carry the specified load without
overheating and keeping the voltage drop well within the permissible limits. At the voltage level
for which cables are designed the insulation thickness must be proper so as to provide high
degree of safety and the reliability The materials used in the manufacturing of the cables must be
such that there is complete chemical and physical stability throughout.
9. Define Capacitance Grading?
The grading done by using the layers of dielectrics having different permittivity between the core
and the sheath is called capacitance grading
10. List out the classification of underground cables
Cables can be divided into the following groups :
(i) Low-tension (L.T.) cables — upto 1000 V
(ii) High-tension (H.T.) cables — upto 11,000 V
(iii) Super-tension (S.T.) cables — from 22 kV to 33 kV
(iv) Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
(v) Extra super voltage cables — beyond 132 kV
