Transformers Multiple Choice Questions & Answers (MCQs) on “Cooling Techniques for Transformer”.
1. Which is the most common, famous and adopted method of cooling of a power transformer?
a) Air blast cooling
b) Natural air cooling
c) Oil cooling
d) Any of the above method can be used
Answer: c
Clarification: Oil acts as a best coolant material, for transformer cooling. Due to its high efficiency as a coolant it is most widely used in transformers. Not only same but Oil with suitable properties can be used for various power transformers according to their ratings.
2. Function of conservator in an electrical transformer is __________
a) Supply cooling oil to transformer in time of need
b) Provide fresh air for cooling the transformer
c) Protect the transformer from damage when oil expends due to heating
d) Cannot be determkned
Answer: c
Clarification: When transformer is loaded and when ambient temperature rises, the volume of oil inside transformer increases as oil expands. A conservator tank properly installed on transformer provides required space to this expanded transformer oil. It performs another function as a reservoir for transformer insulating oil.
3. Natural oil cooling method have some limitations due to which it is adopted for transformers up to a rating of ____________
a) 3000 kVA
b) 1000 kVA
c) 500 kVA
d) 250 kVA
Answer: a
Clarification: For the transformers n higher kVA ratings can be used with this cooling method. While transformers having capacity beyond 5 MVA, due to some improper limitations forced cooling is used. Natural cooling is based on the important phenomenon seen in fluids that when oil is heated up, moves in upward direction.
4. What is the function of spacers?
a) To insulate the coils from each other
b) To provide free passage to the cooling oil
c) To insulate coils and provide free passage
d) Cannot be determined
Answer: b
Clarification: The winding layers of transformer are separated by spacers. One or more spacers are provided here, along with at least one integrated electrical discharge barrier extending off the central body of the spacer in the vicinity of the area where the spacer is in contact with a winding.
5. Which of the following is the most important quality required for chemical in breather, so that it can be used perfectly in an electrical transformer?
a) Ionizing air
b) Absorbing moisture
c) Cleansing the transformer oil
d) Cooling the transformer oil
Answer: b
Clarification: Most of the power generation plants use silica gel breathers fitted to the conservator of oil filled transformers. The purpose of silica gel breathers is to absorb the moisture in the air sucked in by the transformer during the breathing process.
6. Which chemical is used in breather?
a) Asbestos fibre
b) Silica sand
c) Sodium chloride
d) Silica gel
Answer: d
Clarification: In order to absorb moisture from air while breathing process, breather chemical is used. So, breather chemical should possess the required ability of absorbing moisture. In all chemicals available as on today, silica gel is most perfect and best material that can be used for such process.
7. A transformer oil used in an electrical transformer must be free from ________
a) Gases
b) Odour
c) Sludge
d) Moisture
Answer: d
Clarification: Transformer oil serves the purpose of cooling and it also acts as an insulator between primary and secondary winding, thus it must be free from moisture else it will conduct electric current through it, leading to failure of a transformer.
8. On which of the following transformer, Buchholz’s relay can be fixed on?
a) Auto-transformers
b) Air-cooled transformers
c) Welding transformers
d) Oil cooled transformers
Answer: d
Clarification: Buchholz relay is used in transformers for protection against all kinds of faults. Buchholz relay is a famous and mostly used gas-actuated relay, which is installed to serve its best in oil-immersed transformers. It gives an alarm, via its electrical circuitry, if any fault occurs in the transformer.
9. Gas is liberated due to temperature limit and due to dissociation of transformer oil after ___________
a) 50°C
b) 80°C
c) 100°C
d) 150°C
Answer: d
Clarification: Gas is usually not liberated due to dissociation of transformer oil. But when the oil temperature exceeds 1500, it dissociates and liberates. It is found that hydrogen H2 and methane CH4 are produced in large quantity if internal temperature of transformer rises up to 150oC to 300oC due to abnormal thermal stresses.
10. Buchholz’s relay will give warning and protection against ___________
a) Electrical fault inside the transformer itself
b) Electrical fault outside the transformer in outgoing feeder
c) For both outside and inside faults
d) Cannot be determined
Answer: a
Clarification: Buchholz relay is used in transformers for protection against all kinds of faults which are tend to happen inside a transformer. It is most famous gas-actuated relay which is installed in an oil-immersed transformer.
11. Which of the following listed component will see and perform according to changes in volume of transformer cooling oil due to variation of atmospheric temperature during day and night?
a) Conservator
b) Breather
c) Bushings
d) Buchholz relay
Answer: a
Clarification: Conservator is an additional tank provided with transformer which stores oil when it gets expanded due to temperature rise. It also serves another important purpose that is, as a reservoir of transformer oil. Thus, at all temperature variations of day and night transformer can work without any problem.
12. What should be ideal volatility and ideal viscosity of the transformer oil?
a) Low, low
b) High, high
c) Low, high
d) High, low
Answer: a
Clarification: Transformer oil has a low viscosity, high flash point, high dielectric strength, high resistivity. It has a low pour point and low volatility with good gas absorbing properties, while It resists oxidation, sludging and emulsification with water.
13. What is the function of breather in a transformer?
a) To provide oxygen inside the tank
b) To cool the coils during reduced load
c) To cool the transformer oil
d) To arrest flow of moisture when outside air enters the transformer
Answer: d
Clarification: Most of the power generation stations use silica gel breathers fitted to conservator of oil filled transformers. The most used purpose of these silica gel breathers is to arrest the moisture when the outside air is sucked in by the transformer during the breathing process.
14. Natural air cooling method can’t be adopted because of some unavoidable effects, beyond _______
a) 1.5 MVA
b) 5 MVA
c) 15 MVA
d) 50 MVA
Answer: a
Clarification: Smaller size transformers are immersed in a tank containing transformer oil. The transformer oil because temperature properties, which is surrounding the core and windings gets heated, expands and moves upwards. It then flows downwards by the inside of tank walls which cause it to drop temperature and oil goes down to the bottom of the tank from where it rises once again completing the circulation cycle.