Design of Transformer Interview Question Answer - 2

Why the tertiary winding is connected in delta?

Tertiary winding in delta connection
  • The tertiary winding is connected in delta because the unbalance phase voltage may be compensated by circulating current flows in the delta during line to line or line to earth fault in the transformer primary or secondary winding.

Describe the function of the Distribution transformer.

Distribution transformer
  • It is a step down transformer which provides service voltage or transmission to distribution voltage. 
  • The rating of the distribution transformer is up to 500 kVA.

State the application of Distribution and Power transformer.

Applications of Distribution transformer
  • The distribution transformer is used for distribution of voltage from transmission to consumer end whereas the power transformer is used for step up / down voltage in the generating stations and substations.

Why the distribution transformer is designed for 50% maximum efficiency?

Efficiency of Distribution transformer
  • The distribution transformer primary is energized throughout the day whether it supplies load or not.
  • Therefore the iron loss occurs throughout the day whereas the copper losses depend upon load.
  • The iron loss is kept minimum in the distribution transformer due to above fact.
  • The design of the distribution transformer is made such that it has iron losses small as compared to full load copper losses.
  • Therefore the distribution transformer is designed for 50% efficiency.

Why the design of the power transformer is done at maximum efficiency nearly full load?

Power transformer – Efficiency
  • The power transformer connected in operation during load condition and it will disconnect during light load conditions therefore the power transformer is designed to have maximum efficiency nearly full load.

How to reduce eddy current loss in the three phase transformer?

To laminate core of the transformer

What should be thickness of lamination kept in the three phase transformer?

Thickness of lamination
  • The thickness of lamination in the three phase transformer should be 0.3 – 0.5 mm.
  • The thickness of lamination below 0.3 mm is not permitted because the lamination become mechanically weak and tends to buckle.

Why silicon is added to steel in the transformer core material?

Transformer Core Material
The transformer core material consists of steel and 3 to 5% silicon 
steel. The silicon steel has following advantages.
  • The silicon increases resistivity of the core material therefore the eddy current loss is reduced.
  • The high content of silicon results in soft iron steel having narrow hysteresis loop. This will reduce hysteresis loss also.
  • The magnetizing current is also reduced because to high permeability material.

Which type of core material allows maximum flux density in the transformer core? Hot or Cold

Maximum flux density
  • Hot rolled silicon steel allows maximum flux density of 1.45 Weber / meter2 whereas the cold rolled silicon steel allows maximum flux density of 1.8 Weber / meter2.

Describe the advantages of CRGO steel sheet transformer core material.

Advantages of CRGO transformer core material
  • The core loss during no load is low.
  • The reactive power input during no load is low.
  • Low magnetostriction
  • Higher mechanical strength and property.

Why the CRGO steel has lower hysteresis loss?

The grain orientation of the CRGO is done in the direction of magnetic field therefore it has lower hysteresis loss.

Describe the effect of magnetization on the rolling of core material.

If the magnetization is minimum in the direction of rolling whereas it is maximum at 900 direction of rolling.

What should be frequency of magnetization in the transformer? What is effect of magnetization?

Frequency of magnetization
  • The frequency of magnetization is twice the supply frequency. The magnetization produces unwanted noise in the transformer core.

Which type of coating is used in the CRGO and Hot rolled steel?

The coating used in the hot rolled steel core transformer is kaolin or varnish whereas the phosphate coating is used in the CRGO.

Which type of core area has minimum copper loss?


Explain : Circumscribing circle, Cruciform core

Circumscribing circle
  • When the numbers of steps are used for construction of the core, the corners of the steps are arranged such that they lie on a circle. This circle is known as circumscribing circle.

Cruciform core
  • The two stepped core construction is known as cruciform core.

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