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Design of Transformer Interview Question Answer - 1

Whether the transformer is the energy conversion device?


Describe the effect of power rating on the efficiency of the transformer.

As the power rating of the transformer increases, its efficiency also increases.

Describe the effect of leakage reactance when two windings of the transformer close to each other.

Leakage reactance
  • The leakage reactance of the transformer winding reduces as the two windings are closed to each other and vice versa.

How low voltage winding and high voltage winding wound on the core?

The low voltage winding is placed nearer to the core whereas the high voltage winding is wound over it. This type of arrangement is done in order to reduce amount of insulation.

Why the side limbs of the shell type transformer has half cross section area to that of middle limb?

Limb area
  • The side limb of the shell type transformer carries only half flux that of main or central flux therefore the cross section of the side limb is kept half to that of middle limb.

Describe the effect of short circuit on shell type transformer and core type transformer.

Effect of short circuit on
Shell type transformer
  • The winding of the shell type transformer is supported has by the core over large portion of length therefore it has greater capability to with stand against short circuit forces.

Core type transformer
  • The winding of the core type transformer is not supported or braced therefore it has lower mechanical strength.
  • Therefore the core type transformer has higher possibility to damage during short circuit than the shell type transformer.

Describe the advantages of Three Phase Shell type transformer over bank of Three Single Phase transformer.

Three single phase transformer
  • The cost of three single phase unit is approximately 15% higher as compared to three phase transformer.
  • Easily transportation is done.
  • Auxiliary apparatus requires for each transformer unit resulting higher cost.
  • High installation cost due to installation of separate unit.
  • The supply continuity is possible in the case of fault in single unit.
  • The faulty unit is disconnected from supply it continuity of supply is possible with open delta connection with 58% power capacity.

Three phase shell type transformer
  • Low cost as compared to three units of single phase transformer.
  • Lighter, occupies less space and more efficient as compared to three phase transformer.
  • Saving in cost of iron core and tank oil
  • It requires only one set of auxiliary apparatus.
  • Higher efficiency as compared to three single phase transformer.
  • Low installation cost because only one unit is install.
  • Difficult to transport because higher weight.
  • The supply continuity is interrupted during fault in the transformer.

Give reason : The efficiency of three phase transformer is higher as compared to three units of single phase transformer.

Efficiency of three phase transformer
  • The efficiency of the three phase transformer is higher because the magnetic path of the three phase transformer is short as compared to three units of single phase transformer  therefore volume of core and resulting core loss is reduced.

Describe the advantages of star connected transformer over Delta connected transformer.

Advantages of star connection transformer over Delta connection
  • The voltage per phase is 1 / √3 times the line voltage therefore number of turns in the star connection is 1 / √3 times to that of delta connection.
  • The star connection is characterized by small number of turns with large cross section area of conductor whereas the delta connection is characterized by large number of turns with smaller cross sectional area of conductor.
  • As the phase voltage is 1 / √3 times the line voltage, the major insulation between line winding to earth is reduced.
  • As the insulation plays significant role at higher voltage, the cost of transformer is reduced at higher voltage in the star connection.
  • If the neutral is earthed, the maximum voltage to the core is limited 58% to that of line voltage in the case of fault whereas the maximum voltage between winding to earth is full line voltage in the case of line to earth fault.
  • The three phase four wire supply system is possible.

Describe the applications of the Three winding transformer.

Three winding transformer
  • The power generated at the power station is transmitted to consumers at different locations.
  • The most economical transmission voltage depends upon distance between load centers to power station.
  • If the load centers are located at varying distance from power station, it is necessary to transmit power at different voltage levels.
  • The three winding transformer is necessary where some consumers require to fed power from two or independent distribution systems.

What is transformer rating in the three winding transformer?

Rating of the three winding transformer
  • The three winding transformer consists of primary winding, secondary winding and tertiary winding.
  • The rating of the three winding transformer may be unequal therefore the transformer rating is equal to largest rating out of any three winding.

Describe the function of tertiary winding.

Function of tertiary winding
  • To supply small load at different voltage level.
  • To indicate voltage in high voltage testing transformer.
  • The delta connected tertiary winding reduces the zero sequence impedance and permit fault current flows for operation of protective device in star/star or star / zig zag transformer.
  • To supply phase compensating device capacitance at different voltage level for power factor improvement.

Why the tertiary winding is also called as auxiliary winding?

Auxiliary winding
  • When the tertiary winding is used for supply small load at different voltage level, it is called as auxiliary winding.

Why the tertiary winding is also called as stabilizing winding?

Stabilizing winding
  • When the tertiary winding is used to limit the short circuit current, it is called as stabilizing winding.

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