Transformer Interview Question Answer - 3

On which parameter the weight of the copper winding in the transformer depends?

• The weight of the copper winding material depends upon ampere turns of the winding.

What amount of power transfer inductively and conductively in the step-down transformer?

Step down auto transformer

• Power transfer inductively = (1 – k) × Input
• Power transfer conductively = k × Input

State the application of the auto transformer.

Application

• It is used as starter to start the induction motor and synchronous motor.
• It is used as variable supply in the electrical lab for single phase and three phase system.
• It compensates voltage drop by boosting supply voltage in the distribution systems.

Describe the advantages and disadvantages of auto transformer as compared to conventional transformer.

Advantages

• Saving of copper material
• Size is relatively smaller
• Low cost
• High efficiency as compared to conventional transformer
• The output voltage can step up / step down smoothly and as per requirement.
• Better voltage regulation as compared to conventional transformer

Disadvantages

• High short circuit current due to low leakage flux during fault
• Common neutral in the star – star three phase auto transformer

Give reason: The kVA rating of the auto transformer is higher than the conventional transformer.

kVA rating of the auto transformer

• The kVA rating of the auto transformer is higher than the conventional transformer due to fact that the power transfer in the auto transfer is by both conduction and induction whereas it is only by conduction in the ordinary transfer.

At what condition the parallel operation of the two transformers are required?

Necessary of parallel operation

• When the load is excess to that of load capability of the single-phase transformer.
• Maintenance of single transformer requires.
• If there is any fault in one transformer, continuity of supply is possible.

What is necessary and essential condition for parallel operation of the transformers?

Condition for parallel operation of transformers

• Both transformers must be connected with correct polarity.
• The voltage transformation ratio of the both transformers should be same.
• The percentage impedance and X / R ratio of the both transformers should be same.
• The kVA rating of the both transformers should be same.

Why two transformers must be connected in parallel with correct polarity?

• When two transformers are connected in parallel with incorrect polarity there is dead short circuit at secondary side even at no load.

In which conditions, parallel operated two transformers do not share the load directly proportional to kVA rating?

• When the impedance triangle of the both transformers is not identical in shape and size, two transformers do not share load directly proportional to kVA rating.

Comment on the statement: When two transformers are operated in parallel (a) X / R ratio of the transformers are not identical (b) two transformers have different voltage ratio

X / R ratio

• If the X/R ratio of the both transformers should not same, the parallel operation still possible.
• The two transformers operate with different power factor from the load power factor.
• The two transformers should not share the load proportional to kVA rating of the transformer.

Voltage ratio

• If the two transformers operate with different voltage ratio, the parallel operation still possible.
• Due to inequality of induced emfs in the secondary windings, some circulating current flow in the secondary circuits even at no load.
• When the secondary circuit is loaded, one transformer is under loaded whereas the second is over loaded.

Describe the advantages and disadvantages of three phase transformer over three single phase unit

Advantages of three phase transformer over three single phase unit

• Saving in iron in the three-phase shell type construction due to joint of the magnetic path.
• Low cost because it requires only single protection and switchgear equipment as compared to three single unit
• High efficiency
• Small size as compared to three combined single-phase unit.

Disadvantages of three phase transformer over three single phase unit

• Easily transportation not done
• Maintenance of single unit is not done as it is possible in the single unit.
• If there is any fault in the single-phase unit supply continuity is possible whereas it is not possible in the three-phase transformer.

What are the advantages of star neutral point of the transformer is connected to the neutral point of the generator winding?

• The star point of the transformer is connected to the star point of the generator winding to eliminate distortion of the secondary phase voltage in the transformer.

What is function of the tertiary winding?

                     OR

How to avoiding problem of floating neutral in the star connected transformer?

Function of tertiary winding

• The function of the tertiary winding is to avoiding floating neutral.
• It is low kVA rating winding and connected in the delta.

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