Transformer interview Questions & Answers paper-13

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227. What should be the minimum clearance between the highest point of the conservation tank and the ceiling of the transformer room ?

Ans : Not less than 0.5 meters.

228. What arrangement should be provided where two or more transformers are installed side by side ?

Ans : If two or more transformers are installed side by side they shall be separated by fire separation walls. Fire separation walls are deemed to be adequate from fire safety point of view.

229. What type of oil draining arrangement should be provided for a transformer having more than 2000 liters of oil ?

Ans : Oil soak pit filled with sand and gravels should be provided where the oil capacity exceeds 2000 liters of oil. The capacity of the oil soak pit shall be such that to soak the entire oil content of the transformer.

230. When is it necessary to dry out a transformer ?

Ans : As a result of tests carried out if the presence of moisture is indicated or the oil does not withstand the tests as recommended by the manufacturer or the insulation resistance readings are not satisfactory it shall be necessary to dry out a transformer.

231. What are the methods of drying out transformer core and coils ?

Ans : i) Drying with core and coils in oil by short circuit method.
ii) Dering with oil removed by external heat.
iii) Drying under vacuum.

232. What are the precautions to be taken when drying out of transformers ?

Ans : (i) The transformer should never be left unattended during drying out. This must be done under continuous and competent supervision.
(ii) The temperature should be carefully observed which should not exceed 90⁰C.
(iii) Only spirit type thermometer should be used.
(iv) No naked lights and flames should be allowed in the vicinity of a transformer being dried and the fire fighting equipment should be kept ready during the dry-out period.
(v) Low voltage hand lamp should be used for inspection of the tank from inside.

233. How will you ascertain the completion of drying out ?

Ans : During the drying period the measurement of insulation resistance should be taken every 2 hours at practically constant temperature. In the course of time when the insulation resistance remains constant for a period of 24 hours at a value greater than its suggested value the drying out process is to be taken as completed.

234. What do you mean by reactivation of transformer oil ?

Ans : Reactivation of transformer oil means the reconditioning of deteriorated oil to make it active again by purifying and drying transformer oils.

235. What are the different methods of purifying and drying out transformer oils ?

Ans : The different methods are
i. Filter press.
ii. Centrifugal oil purifier.
iii. Streamline filter.

236. What do you mean by neutralization value of transformer oil ?

Ans : The neutralization value also known as total acidity is the measure of free organic and inorganic acids present together in the oil which is expressed in terms of the number of milligrams of potassium hydroxide required to neutralize the total free acids in one gram of the oil.

237. What is the maximum limit of acidity of serviceable transformer oil ?

Ans : 0.40 mg of KOH per gram of oil.

238. what is the effect of the presence of moisture and impurities in oil ?

Ans : The presence of moisture and impurities in the oil decreases the dielectric strength of the oil and also reduces the insulation resistance of the transformer.

239. What is the effect of presence of air bubbles in the transformer oil ?

Ans : When air remains in the oil the oxygen reacts on the cellulose of the insulation and the decomposition products of the cellulose lead to the formation of organic acids soluble in oil and also sludge which blocks the free circulation of the oil.

240. What is the effect of varnishes on the transformer oil ?

Ans : varnishes of the oxidizing type readily reacts with the transformer oil and precipitate sludge on the windings. But synthetic varnishes having acid inhibiting properties generally delay the natural formation of acid and sludge in the oil.

241. What is the effect of slackness of windings in the transformer ?

Ans : Slackness of winding may cause a failure due to repeated movement of coils which may be the conductor insulation at some places and lead to an inter-turn failure. The coils may also get displaced under load conditions or momentary short circuit which may cause electric and magnetic unbalance and produce even greater displacement.

242.How do you detect the presence of water in transformer oil ?

Ans : It can be detected by crackle test. For this test some quantity of oil is heated in a test tube over a silent flame or a red-hot metal rod is dipped in a cup containing oil. Any cracking sound, in either case, indicates the presence of water.

243. What type of gasket is generally used in transformer and why ?

Ans : Neoprene bonded cork gasket is generally used in between two metal surfaces in transformer assembly which is squeezed by tightening nut bolts of the flanges to make a leak-proof air-tight and oil tight joint.

244. What are the possible causes for abnormal noise in a transformer ?

Ans : External noise is caused by a loose nut-dolt on the transformer cover and its accessories and internal noise is caused by slackness of winding or capacitance discharge.

245. What are the causes of loosening of coils in the transformer ?

Ans : The causes are as following :
(i) Alternate heating and cooling.
(ii) Uneven electromagnetic forces due to defective construction.
(iii) Electric and magnetic unbalance due to abnormal load conditions or momentary short circuit.

246. What will happen if a transformer is connected to a d.c. source ?

Ans : If a d.c. supply is connected across the primary winding it will produce a steady flux in core and therefore it will not induce any voltage. Secondly, the transformer winding has more reactance and very little resistance. Since the reactance has got no effect with d.c. voltage the transformer will take high current due to low ohmic resistance and absence of self induced counter e.m.f. As a result the winding will be burnt out without transforming the electrical energy.

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