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Transformer interview questions paper-5

71. What do you mean by oil immersed water forced type cooling ?

Ans: In oil immersed water forced cooling system the oil cooling method is further improved by the arrangement of tubes on the top of the transformer tank but below the oil level. Water is circulated through these tubes and cools the heated oil of the transformer.

72. What do you mean by oil forced water forced type cooling?

Ans: In the oil forced water forced type cooling water are circulated through the heat exchanger to exchange heat from oil to cooling water.

73. What are standard symbols used for describing different methods of cooling transformer ?

Ans: The following symbols are employed :-
A= Air cooling for dry type transformer.
N = Natural cooling by convection currents only .
G = Gas .
O = Oil immersed.
W = Water cooled .
F = Forced air or oil (oil not directed ).
S = Solid insulation used instead of mineral oil.
D = Forced (directed oil).
Practically cooling methods are expressed in two or more symbols together such as- A.N – Air natural.
A.F – Air forced.
O.N – Oil natural.
O.F – Oil forced.
W.F – Water forced.
O.N.A.N – Oil natural air natural.
O.N.A.F- Oil natural air forced.
O.F.A.N – Oil forced air natural.
O.F.A.F –Oil forced air forced.
O.F.W.F – Oil forced water forced.
O.N.W.F – Oil natural water forced.
O.D.A.F – Directed oil forced and air forced.
O.D.W.F – Directed oil forced and water forced.

74. What do you mean by power oil? Given an example.

Ans: The oil used in electrical apparatus for cooling purpose as well as quenching of arc is called as power oil.

75. What is transformer oil?

Ans: Transformer oil is a variety of mineral oil obtained by refining crude petroleum which is an excellent insulator and its loss by evaporation is negligible small.

76. Why are power transformers filled with oil?

Ans: Power transformer are filled with oil to provide cooling by dissipating heat from coils and core to the outer casing and also to increase the dielectric strength of the insulation.

77.What do you mean by flash point and fire point of transformer oil?

Ans : The flash point of transformer oil is the ignition temperature of oil vapor i.e. the lowest temperature at which the oil gives off sufficient inflammable vapour to produce momentary flash on the application of a small flame . It is about 140⁰C.

78 .What is the unit of measuring dielectric strength of the transformer oil?

Ans: The measuring unit of dielectric strength is Kilo-Volt /millimetre.

79 Which factors affect the dielectric strength of the transformer oil?

Ans : The moisture , dust and temperature affect the dielectric strength.

80. What is the effect of moisture on transformer oil?

Ans: The moisture decreases the dielectric strength.

81. What do you understand by the slugging in the transformer oil?

Ans: Sludging means the slow formation of semi-solid hydrocarbons in the transformer oil and deposition on the surface of the winding and the tank walls.

82.What do you mean by “Askarel” and where is it used?

Ans: “Askarel” means a group of synthetic liquids of high dielectric strength which overcomes the disadvantage of mineral insulated oil e.g. susceptibility to catching and propagate fire. This synthetic oil named as “Askarel” is used as an insulating and cooling medium in the electrical equipment where there is a necessity to reduce the risked of fire and explosion.

83. What are the fittings and accessories which should be provided on a transformer ?

Ans: The standard fittings and accessories - (1) Cable boxes, (2)Oil conservator, (3) Breather ,(4) Oil gauge, (5) Relief and Explosion vent,(6) Tapping switch, (7) Buchholz relay, (8)Radiator, (9)Bushings, (10) Oil level indicator, (11) Drain valve with plug, (12) Lifting hook, (13) Earthing terminal, (14) Nameplate, (15) Oil filling hole and cap, (16) Thermometer pocket & (17) Rollers.

84. What precautions do manufacturers take in the form of accessories to prevent transformer oil from deteriorating?

Ans: The following accessories are fitted as the precautionary measure against deterioration of transformer oil :- (1) Oil conservator, (2) Breather, (3)Buchholz relay & (4) Oil level indicator

85.What is a conservator ?

Ans: Conservator is a cylindrical tank placed on the top of the transformer connected to the main transformer oil tank by a short length pipe. It is partly filled with oil.

86. Why are oil conservators fitted to the transformer ?

Ans: Oil conservators are fitted to the transformers to take up the expansion and contraction of oil during running operation and also to observe the full oil filling of transformer tank through its oil level indicator.

87. Why is the conservation tank not completely filled with oil ?

Ans : During running condition the oil volume is increased with the increase of temperature . So the conservator tank is not completely filled with oil to absorb the increased volume of oil in its upper air space.

88. What is the lowest capacity of transformers on which the provision of conservators is obligatory ?

Ans: On transformers of capacities 50 KVA and above the provision of conservators is obligatory . On transformers of rating 40 KVA and below conservators shall not be fitted.

89.What is a breather ?

Ans: Breather is a small cylindrical chamber containing silica gel and connected to the top of the conservator by an air tight extended pipe. It allows moisture free air to the conservator tank.

90. What is the function of a breather ?

Ans: The function of a breather is to extract moisture from the ingoing air in order to prevent any contamination of the transformer oil by the moisture which is very much present in the open air.

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