Important Two Marks & 16 Marks Question and Answers

TWO MARKS QUESTION AND ANSWERS

1. What is step down transformer?

The transformer used to step down the voltage from primary to secondary is called as step down transformer (Ex: 220/110 V).

2. Draw the no load phasor diagram of a single phase transformer.

3. Why is an auto-transformer not used as a distribution transformer?

The autotransformer cannot provide isolation between HV and LV side. Due to open circuit in the common portion, the voltage on the load side may shoot up to dangerously high voltage causing damage to equipment. This unexpected rise in the voltage on LV side is potentially dangerous. Hence the autotransformer cannot be used as distribution transformer.

4. Give the basic principle behind the working of transformer.

The transformers works in the principle of mutual induction between two coils which are electrically isolated but magnetically coupled.

5. What are the conditions for parallel operation of transformer?

In order that the transformers work satisfactorily in parallel, the following conditions should be satisfied:

i. Transformers should be properly connected with regard to their polarities.

ii. The voltage ratings and voltage ratios of the transformers should be the same.

iii. The per unit or percentage impedances of the transformers should be equal.

iv. The reactance/resistance ratios of the transformers should be the same.

6. What are the no load losses in a two winding transformer? And state the reasons for such losses.

The no load losses in transformer are:

i. Hysteresis Loss: To establish the magnetic circuit in the transformer core.

ii. Eddy Current Loss: Due to circulation of current induced in the core due to induction.

7. Why is transformer rated in KVA?

The copper loss of a transformer depends on current and iron loss on voltage. Hence, total transformer loss depends on volt-ampere (VA) and not on phase angle between voltage and current. i.e., it is independent of load power factor. That's why rating of transformers are in kVA and not in kW.

8. Compare two winding transformer and auto-transformer.

9. Classify the transformer according to the construction.

Depending upon the manner in which the primary and secondary are wound on the core, transformers are of two types viz., (i) core-type transformer and (ii) shell-type transformer.

10. What is transformation ratio?

It is the ratio in which the voltage to be transformed (stepped up or down) from primary to secondary of a transformer. It is given by:

V₂ / V₁ = N₂ / N₁ = K

where, K is the transformation ratio.

11. Draw the exact equivalent circuit of a transformer.

12. What are the advantages of auto-transformer over ordinary transformer?

i. The autotransformer is lesser size than ordinary two winding transformer for the same rating. Hence the cost reduced.

ii. Auto-transformer operates at higher efficiency.

iii. Superior voltage regulation.

13. Mention the properties of oil used in transformers.

The following are the desirable properties of transformer oil:

i. It should be free from moisture.

ii. It should have high dielectric strength.

iii. It should have thermally stability and higher thermal conductivity.

iv. It should be contaminated by temperature rise.

14. Define voltage regulation of transformer.

The voltage regulation of a transformer is the arithmetic difference (not phasor difference) between the no-load secondary voltage (₀V₂) and the secondary voltage V_2oh load expressed as percentage of no-load voltage.

15. Write down the volt-ampere transferred inductivity and volt-ampere transferred conductivity in an auto-transformer.

Volt-amperes transferred inductively: V₂(I₂ – I₁)

Volt-amperes transferred conductively: KV₁I₁.

16. What are the properties of an ideal transformer?

An ideal transformer has the following properties:

i. No winding resistance

ii. No flux leakage

iii. No coreloss

iv. Magnetize at zero current.

17. Mention the applications of auto-transformer.

The autotransformers are used in the following applications:

i. To give small boost to a distribution cable to correct the voltage drop.

ii. As auto transformer starter to give upto 50% to 60% of full voltage to an induction motor during starting.

iii. As furnace transformers for getting a convenient supply to suit the furnace winding from a 230 V supply.

iv. As interconnection transformers in 132 kV/330 kV system.

v. In control equipment for single phase and three phase electrical locomotives.

18. Why V₁ : V₂ ≠ N₁ : N₂ in a real (practical) transformer?

In practical transformers, the terminal output depends on the resistive drop and magnetic leakages. Hence the ratio of turns do not match equal with the ratio of terminal voltages.

19. Explain the term percentage impedance as applied to transformer.

The percentage impedance is the per-unit impedance expressed as a percentage on a certain MVA and voltage base.

20. What are the various types of three phase transformer connections?

The most common types of transformer connections are:

i. Star-Star (Y-Y)

ii. Delta-Delta (A-A)

iii. Star-Delta (Y-A)

iv. Delta-Star (A-Y)

v. Open Delta (V-V)

vi. Scott Connection (T-T)

21. What is an ideal transformer?

The transformer has the following properties is said to be an ideal transformer:

i. No winding resistance

ii. No flux leakage

iii. No coreloss

iv. Magnetize at zero current.

In practical, it is difficult to satisfy all the above properties and the concept of ideal transformer is only an imaginative.

22. What are the two components of noload current in transformer?

The noload current contains two components as follows:

1. Loss component (I_W)

2. Magnetizing component (I_m).

23. What is all day efficiency?

The ratio of output in kWh to the input in kWh of a transformer over a 24-hour period is known as all-day efficiency.

24. Define regulation and efficiency of a transformer.

The voltage regulation of a transformer is the arithmetic difference (not phasor difference) between the no-load secondary voltage (₀V₂) and the secondary voltage V₂ on load expressed as percentage of no-load voltage.

The efficiency of a transformer is defined as the ratio of output power (in watts or kW) to input power (watts or kW).

25. Mention the different losses in transformer.

The losses that occur in a transformer are:

(a) Core Losses - eddy current and hysteresis losses.

(b) Copper Losses.

PART B QUESTIONS

1. What is a transformer? Explain the function it fulfills as an element of a power system.

2. Explain the constructional differences between core and shell-type transformers.

3. Explain briefly the ideal transformer as a circuit element. Can voltage and current ratios be adjusted independently?

4. Explain the operation and application of the impedance transforming property of an ideal transformer.

5. State how the LV and HV windings are arranged in a core-type transformer. Explain the reason.

6. What is the phase relationship between the core flux, the magnetizing current and the induced emfs in the primary and secondary winding of a transformer? Draw the phasor diagram.

7. What is the transformation ratio of a transformer? Why is it not identical to voltage ratio of a transformer?

8. What determines the maximum value of flux in a transformer core when excited from the primary side? Does the value of flux change substantially when the secondary is loaded? Explain the reason.

9. Explain how the OC/SC tests separate out the core loss and copper loss.

10. A transformer is excited from the primary side at rated voltage but with secondary open. Would it draw any current? If so, what is action of this current and its components?

11. Why cannot the SC test separate out the primary and secondary resistances and leakage inductances?

12. Justify that under SC test the core loss is negligible.

13. Prove that in the system, if the voltage bases are selected in the ratio of transformation, the pu impedance of the transformer is same as either side.