29/03/2020

Design of Transformer Interview Question Answer - 6


76
Why the transformer winding is solidly braced to core of the transformer?


If the transformer winding is not solidly braced to core of the transformer, the mechanical forces on winding may damage whole transformer during short circuit.


77
Describe the relation between radial forces and p.u. reactance in the transformer.
                  OR
Why the circular coil is used in the transformer winding?


Circular coil
  • The radial force on the winding is inversely proportional to p.u. reactance of the winding.
  • Therefore the winding should be designed such that it should withstand radial forces in any fault condition.
  • The circular coil is strongest to withstand radial forces so it is preferred for transformer winding.


78
Describe the effect of force due to winding asymmetry in the transformer winding.


Force due to winding asymmetry
  • If there is winding asymmetry in the transformer, the magnetic axis of both winding is not coincides.
  • This will produces radial and axial forces.
  • The axial forces have large value during fault condition and it cause movement of winding in axial direction.
  • Therefore the winding is solidly braced in order to prevent movement of it.


79
Why the mmf distribution of the HV winding is not uniform in the axial direction?


MMF distribution
  • The mmf distribution of the HV winding is non uniform along axial direction because the end turns of the winding are reinforced to protect against effect of surge voltages.
  • Therefore the end coils of the HV winding has lesser turns.


80
Describe the effect of change in frequency on transformer losses, leakage reactance and resistance.


Iron losses
  • If the frequency increases and supply voltage remain unaltered, there is no change in the eddy current losses.
  • However the hysteresis loss decreases as the supply frequency increases.
  • Therefore the total iron losses decrease as the supply frequency increases and supply voltage is kept constant.

Copper loss
  • The copper loss remains unchanged due to change of frequency. The copper losses increase slightly due to effective resistance of the winding.

Leakage reactance
  • As the frequency increases, the inductive reactance of the winding increases linearly.

Resistance
  • As the supply frequency increases, the effective resistance increases due to skin effect.


81
Why the plain walled transformer tank is not sufficient to dissipate heat?


Plain wall transformer tank
  • The volume and losses of the transformer increases as cube of its linear dimension whereas its dissipating surface increases as the square of the linear dimension therefore the transformer tank is sufficient to dissipated heat.


82
Describe the advantages of air blast cooling in the transformer.


Air blast cooling
  • The air blast cooling of the transformer increases the heat dissipation by 50% to 60% more than the natural cooling.


83
Describe the advantages of transformer cooling by forced oil circulation.


Transformer cooling
  • The natural oil circulation of the transformer is very slow.
  • If the velocity of the oil circulation increases by n times, the transformer output increases ( n )1/4 times with same winding temperature rise.


84
Which factors affecting the transformer insulation cellulose?


Moisture, oil purity and electric stress

85
Describe the effect of ratio of copper loss to iron loss on the overload capacity of the transformer.


As the ratio of copper loss to iron loss increase in the transformer, its overload capacity reduces.

86
On which factors the heating time constant of the transformer depends?


Heating time constant
  • The heating time constant of the transformer is directly proportional to transformer weight and specific heat whereas it is inversely proportional to specific heat dissipation.
  • As the time constant of the transformer is longer, it will take long time to temperature rise.


87
Describe the effect of weight on heating time constant of the transformer.


Heating time constant
  • As the weight of the transformer increases, longer its time constant. It is capable to withstand momentary overload than light weight transformer.


88
What is meaning of the half coils? In which type of winding it is used?


Half coils
  • The sandwich type winding is used in the shell type transformer.
  • There is one high voltage coil sandwich between two low voltage coils having half turns therefore these low voltage coils are called as half coils.


89
Describe the advantages of sandwich coil in the shell type transformer.


Advantages of sandwich coil
  • Lower leakage flux due to better coupling between two winding.
  • As the number of sections between winding increases, the leakage reactance reduces. Therefore the leakage reactance adjusted to any value by subdivision of winding.


90
Why the low voltage winding and high voltage winding is alternately placed in the transformer?


In order to reduce leakage reactance


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