Two Marks Questions with Answers

Two Marks Questions with Answers

 

Q.1 Compare underground cables and overhead lines.

Ans. : Refer section 6.1.1.

 

Q.2 Give the relation for insulation resistance of a cable.

Ans. : The insulation resistance of a cable is given by,

R_i = (ρ / 2πl) ln (D/d)

d = Diameter of core

l = Length of cable

D = Diameter of sheath

ρ = Resistivity of the insulating material

 

Q.3 How cables are classified based on operating voltage ?

Ans. :          

1. Low Tension (L.T.) cables used up to 6.6 kV.

2. Medium and High Tension (H.T.) cables up to 66 kV.

The H.T. cables are further classified as :

i) Belted cables up to 11 kV

ii) Screened cables for 22 and 33 kV

iii) Pressure cables from 33 kV to 66 kV also called extra high tension cables

3. Super Tension (S.T.) cables for 132 kV to 275 kV which are further classified as

i) Oil filled cables and ii) Gas pressure cables

 

Q.4 What are the requirements of the cables ?

Ans. :

i) 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.

ii) 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.

iii) The materials used in the manufacturing of the cables must be such that there is complete chemical and physical stability throughout.

iv) The cables must be surrounded by number of layers of an additional insulation so as to give proper mechanical strength.

 

Q.5 List out the various parts of cables.

Ans. : i) Core or conductor

ii) Insulation        

iii) Metallic sheath

iv) Bedding

v) Armouring      

vi) Serving.

 

Q.6 What are the advantages of separate lead sheath cables ?

Ans. : i) Due to individual lead sheath core to core fault possibility gets minimized

ii) The electrical stress are radial in nature

iii) Bedding of cable is easy

iv) Increases the current carrying capacity

v) No possibility of formation of voids.

 

Q.7 Mention the limitations of S.L. cables.

Ans. : i) Due to paper used as an insulation, as temperature increases due to expansion it stretches the lead sheath.

ii) Moisture may be drawn along with the air which deteriorates the dielectric strength.

iii) Mechanical shock can cause voids.

 

Q.8 What are the types of super tension cable ?

Ans. : i) Oil filled cables ii) Gas pressure cables.

 

Q.9 What are the advantages of oil filled cables ?

Ans. : i) Thickness of insulation is less.

ii) The thermal resistance is less. 

iii) The possibility of voids is completely eliminated,

iv) Reduced earth fault,

v) Perfect impregnation is possible.

 

Q.10 What are the disadvantages of oil filled cables ?

Ans. : i) The initial cost is very high

ii) The long length is not possible

iii) The laying of cable is difficult

iv) Oil leakage is serious problem

v) Maintenance of cable is difficult.

 

Q.11 What are the advantages of gas pressure cables ?

Ans. :         

(i) Maintenance cost is small.

ii) Can carry 1.5 times the normal load current and can withstand double the voltage.

iii) The nitrogen in the steel tube helps in quenching any fire or flame

iv) No reservoirs or tanks required

v) The power factor is improved,

vi) Cable laying is easy due to steel tubes,

vii) The ionization and possibility of voids is completely eliminated.

 

Q.12 What is grading of cables ?

Ans. : The process of obtaining uniform distribution of stress in the insulation of cables is called grading of cables.

 

Q.13 Give the properties of insulating material used for the cables. |

Ans. : i) High insulation resistance. 

ii) High dielectric strength,

iii) Must be mechanically strong.     

iv) Should be flexible,

v) Should be non-hygroscopic.        

vi) Should be non-inflammable.

vii)    Should be unaffected by acids and alkalies.

viii)   Should have high temperature withstanding capability.

 

Q.14 List the insulating materials used for the cables.

Ans. :

i) Poly Vinyl Chloride (PVC)

ii) Paper

iii) Cross linked polythene

iv) Vulcanised India Rubber (VIR)

 

Q.15 What are the methods of grading the cables ?

Ans. : i) Intersheath grading and ii) Capacitance grading.

 

Q.16 Why loss angle of cable should be very small ?

Ans. : The dielectric loss in a cable is given by,

W = V²ω m C tan δ        where δ = Loss angle

Smaller the loss angle δ, smaller are the dielectric losses. Hence the loss angle of cable should be very small. 

 

Q.17 State the effects of unequal distribution of stress in a cable.

Ans. :

i) Greater insulation thickness is required which increases the cost and size,

ii) It may lead to breakdown of insulation.

 

Q.18 What is capacitance grading 7

Ans. : The grading done by using the layers of dielectrics having different permittivity between the core and the sheath is called capacitance grading.

 

Q.19 Define thermal resistance.

Ans. : The thermal resistance is defined as the resistance which allows the heat flow of 1 watt when a temperature difference of 1 °C is maintained. It is given by,

S = kl / A thermal ohms where k is thermal resistivity of the material,

l = Length of the path of heat flow and A = Area of section through which heat flows

 

Q.20 State the expression for the capacitance of a single core cable.

Ans. : The capacitance of a single core cable is given by,

where d = Conductor diameter and D = Total diameter with sheath

 

Q.21 What is the economical core diameter to give minimum value of maximum stress ?

Ans. : The core diameter must be 1/2.718 times the sheath diameter D so as to give the minimum value of the maximum stress.

 

Q.22 What is intersheath grading ?

Ans. : In the intersheath grading, in between the core and the lead sheath the number of metallic sheaths are placed which are called intersheaths. All these intersheaths are maintained at different potentials by connecting them to the tappings of the transformer secondary.

 

Q.23 What are the difficulties in intersheath grading of cables ?

Ans. : In intersheath grading, the intersheath has to be thin and hence there is possibility of damage to it while laying the cable. The intersheath has to carry charging current which can cause overheating of cable.

 

Q.24 What are the difficulties in capacitance grading ? 

Ans. : In capacitance grading it is necessary to select dielectrics of proper permittivity values. Practically it is difficult to get the exact values of required permittivity. The permittivity of dielectric changes with time which can cause uneven distribution of stress causing breakdown of cable at normal operating voltage.

 

Q.25 What are the possible causes of increasing the temperature of cables ?

Ans. :          

1. Heat produced within the cables.

2. Current carried by the cables.

3. The dissipation of heat up to the periphery of the cables.

4. The dissipation of heat to the surrounding medium.

5. The various load conditions like continuous, distributed, intermittent etc.

 

Q.26 Draw the thermal equivalent of a cable.

Ans. : Refer Fig. 6.11.2 from the section 6.11.1.

 

Q.27 What is dielectric stress ?

Ans. : The insulation of a cable is subjected to an electrostatic force under operating condition which is called dielectric stress.

 

Q.28 What is the role of metallic sheath in a cable ?

Ans. : It provides the mechanical protection and restricts moisture and other gases to reach to the insulation.

 

Q.29 What is bedding ?

Ans. : The metallic sheath in a cable is covered by a layer called bedding which consists of paper tape compounded with a fibrous material like jute strands or hessian tape.

 

Q.30 What is serving ? What is its function ?

Ans. : The last layer above the armouring is called serving in a cable. It is a layer of fibrous material like jute cloth which protects the armouring from the atmospheric conditions.

 

Q.31 Determine the charging current per km when the transmission line whose capacitance is 8.928 nanofarad/km is operating at 132 kV.

Ans. :         

 

Q.32 Determine the economic overall diameter of a 1-core cable metal sheathed for a working volatage of 85 kV if the dielectric strength of the insulating material is 65 kV/cm. 

Ans. :         

 

Q.33 A single core cable, 1.7 km long, has a conductor radius of 13 mm and an insulation thickness of 5.8 mm. The dielectric has a relative permittivity of 2.8. Find the capacitance per meter length of cable.

Ans. :

 

Q.34 What is a belted-cable ?

Ans. : A belted cable is the one in which the three cores of a cable are grouped together and belted with the help of paper belt. The gaps are filled with fibrous material like jute. The belt is covered with lead sheath.

 

Q.35 What is the main purpose of armouring 7

Ans. : It provides protection to the cable from mechanical injury.