Two Marks Questions with Answers

Two Marks Questions with Answers

 

Q.1 Define string efficiency ?

Ans. : 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.

 

Q.2 Mention the insulating materials used in insulators.

Ans. : (i) Porcelain   (ii) Glass   (iii) Synthetic resin   iv) Steatite

 

Q.3 What are the methods of improving string efficiency ?

AU : Dec.-04, 07, 08, 16, May 13

Ans. : (i) By using longer cross arms to reduce the ratio of shunt capacitance to self capacitance (ii) By grading the insulators (iii) By using a guard ring

 

Q.4 How does guard ring improves the string efficiency ?

Ans. : The use of guard ring increases the capacitance between the metal plate of insulator and the line. These capacitances are greater for the lower units. Due to this the voltage across these units is reduced. Hence uniform voltage distribution can be obtained which improves the string efficiency.

 

Q.5 What are the advantages of suspension type insulator ?

Ans. : i) Suspension type insulators are cheaper than pin type insulators.

ii) Each unit or disc of suspension type insulator is designed for low voltage level.

iii) If any one disc is damaged the hole string does not get affected.

iv) Provide greater flexibility to the line.

v) In case of failure of any of the units, the replacement work can be done very easily and entire string need not be replaced.

vi) The increased demand can be satisfied by adding additional units to the string.

 

Q.6 What are the causes of failure of insulators ?     

Ans. : i) Due to lightning strokes

ii) Due to mechanical stresses

iii) Due to cracking of insulators caused due to uneven expansion and contraction

iv) Due to defective insulating material v) Due to porosity in the insulating material

vi) Improper glazing on the insulator surface due to which moisture sticks on the surface causing flashovers.

vii) Due to overheating caused by flashovers. viii) Due to short circuits.

 

Q.7 What are the types of insulators ?    

Ans. : i) Pin type insulators ii) Suspension insulators iii) Strain insulators iv) Shackle insulators v) Stay insulators

 

Q.8 What is the function of insulators ?

Ans. : The insulators provide necessary insulation between line conductors and supports and thus prevent any leakage current from conductors to earth.

 

Q.9 Give the importance of stay insulator.

Ans. : The stay insulators are used on stay wires to create insulation between pole and the stay clamp. The stay insulator is designed in such a way that the stay wire will not fall on the ground in case the insulator breaks down.

 

Q.10 What are the properties of insulators ?

Ans. : (i) High mechanical strength, (ii) High insulation resistance, (iii) High relative permittivity, (iv) Should not be affected by temperature changes, (v) It should be non porous, vi) It should be flexible (vii) It should be non-inflammable, (viii) It should prevent leakage current, ix) High dielectric strength, x) Must be free from internal impurities such as holes, cracks, laminations etc.

 

Q.11 Why the potential distribution across the string of insulators is not uniform ?

AU : Dec.-06

Ans. : Due to different shunt capacitors, the charging currents through them are different. Hence the potential distribution across the string of insulators is not uniform.

 

Q.12 State the advantages of synthetic resin insulators.

Ans. : i) Tensile strength is high, ii) The weight is low. iii) Comparatively cheaper.

 

Q.13 What is annealing ?

Ans. : The glass used in the insulators is made tough by heat treatment which is called annealing.

 

Q.14 What are the advantages of glass insulators ?

Ans. : (i) As transparent cracks bubbles and defects in the insulator can be easily detected by inspection,

(ii) The dielectric strength is very high

(iii) Cheaper than the porcelain,

iv) Low coefficient of thermal expansion.

(v) The resistivity is very high.

vi) Higher compressive strength than porcelain,

vii) Homogeneous and can withstand high compression stresses.

 

Q.15 What are the disadvantages of glass insulators ?

Ans. : i) Chances of moisture condensation on the surface are higher so leakage current is high, (ii) Less stronger than the porcelain iii) In high tension system the heavy mass of insulator can cause internal strain, iv) Cannot be moulded in irregular shape.

 

Q.16 What are the major limitations of synthetic resin insulators ?

Ans. : i) High leakage current, ii) Short life span.

 

Q.17 What is mean by self or mutual capacitance ?

Ans. : The porcelain portion which is an insulator is in between the two metal fittings. Thus it forms a capacitor. This is called self or mutual capacitance.

 

Q.18 What is mean by shunt capacitance ?

Ans. : In transmission line towers in addition to the self capacitance there will be capacitance between each metal fittings and the earth, i.e. tower. The air act as a insulating medium. Such a capacitance is called shunt capacitance.

 

Q.19 What are the disadvantages of suspension insulators 7

Ans. : i) The voltage distribution is not uniform.

ii) The charging currents through various mutual capacitors are varied.

iii) Voltage across bottom insulator is high.

iv) Electrical stress is high.

 

Q.20 What is guard ring 7

Ans. : In 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.

 

Q.21 What are the types of suspension type insulator ?

Ans. : (i) Cemented cap type (ii) Hewlett or inter linking type

 

Q.22 What is strain insulator ?

Ans. : When a dead end, sharp curve or a corner occurs in the path of a transmission line, it gets subjected to a higher tension. In such a case the insulators used to reduce the excessive tension are called strain insulators.

 

Q.23 Define safety factor of insulator?

Ans. : The safety factor of insulator is defined as the ratio of puncture strength to the flashover voltage.

 

Q.24 What is shackle insulator?

Ans. : The insulator used at the dead end of the aerial wire of service connection to a house or a factory to reduce the excessive tension is called shackle insulator. It is also used if distribution line changes its angle.

 

Q.25 Why string efficiency is higher in rainy season?

Ans. : Insulators are wet in rainy season which increases the value of the mutual capacitance. This decreases the value of 'k' which increases the uniformity of voltage distribution across the insulator units. Hence the string efficiency increases. 

 

Q.26 What are the tests performed on the insulators?

Ans. : The various tests performed on the insulators are,

1. Mechanical tests 2. Electrical insulation tests

3. Environmental tests 4. Temporary cycle tests 5. Corona and radio interference tests

These tests are grouped in three classes as,

a. Type tests b. Sample tests and c. Routine tests.