Transformer Interview Question Part 4

Q. What are the advantages of parallel operation of transformers?

A. 1) To maximize electrical power system efficiency: Generally electrical power transformer gives the maximum efficiency at full load. If we run numbers of transformers in parallel, we can switch on only those transformers which will give the total demand by running nearer to its full load rating for that time.

2) To maximize electrical power system availability: If numbers of transformers run in parallel we can take shutdown any one of them for maintenance purpose. Other parallel transformers in system will serve the load without total interruption of power.

3) To maximize power system reliability. If any one of the transformers in a parallel system, is tripped due to fault other parallel transformers is the system will share the load hence power supply may not be interrupted.

4) To maximize electrical power system flexibility: Always there is a chance of increasing or decreasing future demand of power system. If it is predicted that power demand will be increased in future, there must be a provision of connecting transformers in system in parallel to fulfill the extra demand because it is not economical from business point of view to install a bigger rated single transformer by forecasting the increased future demand as it is unnecessary investment of money. Again if future demand is decreased, transformers running in parallel can be removed from system to balance the capital investment and its return.

Q. What are the Conditions for parallel operation of transformers?

A. a) Same voltage ratio of transformer: If two transformers of different voltage ratio, are connected in parallel with same primary supply voltage, there will be a difference in secondary voltages. If they are connected to same bus, there will be a circulating current between secondaries and therefore between primaries too. As the internal impedance of transformer is small, a small voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R loss.b) Same percentage impedance: impedance of transformers running in parallel are inversely proportional to their MVA rating

c) Same polarity: Polarity of all transformers run in parallel should be same otherwise huge circulating current flows in the transformer but no load will be fed from these transformers.

d) Same phase sequence: If they are not in same phase sequence, during the cycle, each pair of phases will be short circuited.

Q. Explain about auto transformer?

A. It is type of transformer with only one winding a portion of which is shared by both primary and secondary.

Advantages:

It is used where transformation ratio differs little from unity.
As it contains only one winding, it uses less copper and hence cheaper.
Its efficiency is more when compared with the conventional one.
Its size is relatively very smaller.
Voltage regulation of autotransformer is much better.
Lower cost
Low requirements of excitation current.
Less copper is used in its design and construction

In conventional transformer the voltage step up or step down value is fixed while in autotransformer, we can vary the output voltage as per out requirements and can smoothly increase or decrease its value as per our requirement.

Q. How does a transformer contribute towards the widespread popularity of AC system over DC?

Ans. i. High voltage of AC system can be obtained using a transformer for transmission of electrical power.

ii. Using a transformer, AC voltage can be increased or decreased without any power loss.

Q. What do you mean by power transformer?

Ans. Transformers that are used on transmission lines for the transmission and distribution of relatively large quantities of energy are called power transformers.

Q.What do you mean by distribution transformers?

Ans. When transformers are used for distributing the energy from transmission lines as well as net-works for local consumption and the secondaries are directly connected to the consumer’s load, they are called distribution transformers.

Q. What is “Class” in insulation?

A. Insulation class was the original method used to distinguish insulating materials operating at different temperature levels. Letters were used for different designations. Letter classifications have been replaced by insulation system temperatures in degrees Celsius.

Q. What is exciting current?

A. Exciting current, when used in connection with transformers, is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA. The exciting current is made up of two components, one of which is a real component and is in the form of losses or referred to as no load watts; the other is in the form of reactive power and is referred to as KVAR.