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Transformer interview Questions & Answers paper-12

211. What will be the effect of open circuiting the secondary of a C.T. ?

Ans : If the secondary of a C.T. is open circuited the whole current in the primary becomes magnetizing current & a dangerously high voltage will appear across the secondary which may cause insulation breakdown, damage to the magnetic property of its iron core, overheating and also death to life. For these reasons open circuiting of a C.T. should never be permitted.

212. What is the usual limit of current and voltage above which C.T. and P.T. are generally used ?

Ans : Current limit -50 amps & voltage limit-750 volts.

213. what are the standard ratings of instrument transformers ?

Ans : The standard secondary rating of P.T. is 110 volts and that of C.T. is 5 amps.

214. What do you mean by ‘Burden’ of a P.T. or C.T. ?

Ans : The maximum load in volt amperes (VA) which may be applied across the secondary terminals of P.T. or C.T. is known as ‘burden’ of a P.T. or C.T. It depends upon the number of instruments or relays connected and their individual ratings.

215. What should be done before disconnecting an ammeter from the secondary of a C.T. ?

Ans : The secondary winding of a current transformer should always be short circuited before disconnecting an ammeter.

216. What is the basic difference between a power transformer and a potential transformer according to the operating condition ?

Ans : Though the primary winding in both power transformer and potential transformer are energized continuously at a substantially constant voltage, the secondary of a power transformer is connected to a load which may vary between zero and the maximum which the transformer is capable of carrying whereas the load or burden connected to the secondary of a potential transformer does not vary as a rule. The permissible voltage drop in potential transformer between zero and maximum burden is much smaller than the corresponding limit in a power transformer.

217. What precautions do you observe when working with instrument transformers ?

Ans : When working with C.T. the secondary should be short circuited and grounded. No fuse should be inserted in the secondary circuit of C.T. because fuse may blow and produce an open circuit. When working with P.T. the secondary should never be short circuited. One end of the secondary should be grounded and fuse should be provided both on the primary and the secondary circuit. one end of the secondary should be grounded and fuse should be provided both on the primary and the secondary circuit.

218. How are the earthing arrangements made for the following :-
a) 132 KV or 220 KV generator transformers,
b) 11Kv or 33KV substation transformers,
c) 11KV/0.433KV distribution transformers,
d) Transformer with delta winding &
e) Instrument transformer ?

Ans : (a) The metallic frame is earthed by two separate and distinct connections with earth. The neutral point is directly earthed by not less than two separate and distinct connection with earth each having its own electrode thus permitting the grading of insulation in the transformer form the terminal end to the neutral point.
(b) The frame is earthed by two separate earth electrodes as above but for neutral earthing resistance earthing not less than two numbers may be used to limit the fault current which is expected to be too high in this case.
(c) The frame is earthed by separate earth electrodes and the neutral of the secondary star winding is erathed by not less than two nos separate and distinct connections directly to earth having its own electrode.
(d) The frame is earthed by two separate earth electrodes . An earthing transformer may be used to get the star neutral point which may be earthed by two separate connections to the earth either directly or through a resistance if desired.
(e) Cases or frames of instrument transformers, the secondary winding of current transformer and one point of the secondary winding of potential transformer shall be earthed which may be done by connection to the earth bus.

219. What is earthing transformer & where is it used ?

Ans : An earthing transformer is a transformer which is intended primarily for the purpose of providing a neutral point for grounding purposes. It may be a two winding unit with a delta connected primary and a star connected secondary or a single winding three phase auto-transformer with windings in interconnected star or “Zig-Zag”. When the necessity arises for earthing the neutral of one part or another of on interconnected transmission or distribution network at a place where no natural neutral point is available, an earthing transformer is used for that purpose. The neutral point of the earthing transformer is connected to earth directly or through a current limiting impedance which the terminals are connected to the three phase lines.

220. What is tertiary winding ? what are its function ?

Ans : Tertiary winding is a delta connected additional auxiliary winding used with three phase star-star connected transformers which is wound on the same core and separated from primary and secondary main winding . The function of the tertiary winding are as follows :-
(i) It prevents the oscillation of the neutral point i.e stabilizes the neutrals of both primary and secondary winding by reducing third harmonic voltage components.
(ii) It permits the transformation of unbalance three phase loads or single phase loads only by limited the voltage unbalance.
(iii) It can supply an auxiliary load at low voltage mainly for station loads.

221.What are the important aspects related with the installation &operation of power transformers ?

Ans : The important aspects are as follows :-
i) Interchangeability of transformers & spares from the maintenance point of view.
ii) Installation layout mentioning safe clearance from the neighbouring equipments & structures.
iii) Necessary arrangements for drainage of oil.
iv) Provision for isolation the transformer for maintenance work.
v) Safety precautions.
vi) Protections of transformer.
vii) Grounding of neutral point, tank, cores etc.
viii) Provision for surge protection.
ix) Provision fire fighting arrangements.

222. What important things need to be ensured in installing a 11000/433 volts, 500KVA distributing transformer indoors ?

Ans :The most important thing to be ensured with transformer installed indoors is proper ventilation. The level of the transformer base should be higher than the highest flood and storm water level of that area. The transformer should be kept well away from the wall. The site should be so chosen that it is free from chemical fumes &damp. For indoor installation, the air inlets and outlets shall be of adequate size and so placed as to ensure proper air circulation for the efficient cooling of transformers. Minimum clearance between the highest point of the conservation tank and ceiling of the transformer room should be sufficient to remove the mounting on the transformer link the conservation. The transformer should be so installed that severe vibrations are not transmitted to its body.

223. What are the tools and equipments required for the erection of a transformer ?

Ans : General list of tools and equipments :-
i) Lifting equipment (crane or chain pulley block along with wire rope slings, D-shackles etc).
ii) Vacuum oil purifier.
iii) Vacuum pump.
iv) Oil storage tank.
v) Pressure vacuum gauge.
vi) Oil testing apparatus.
vii) 2500/1000 volts meggar.
viii) Voltmeter, milli-ammeter, low p.f. wattmeter (voltmeter of 0-500V, 0-100V, 0-5V range).
ix) AVO meter.
x) Set of spanners.
xi) Set of drum opener, crowbar, pipes, hammer etc.
xii) Set of screw drivers, cutting pliers, screw spanners and pipe wrench.
xiii) Clean cotton cloth and cotton waste.
xiv) Electric hand lamp.
xv) 12 mm venyl hose of approximate 10 meters length for using as an oil level indicator during erection.
xvi) Painting brush.
xvii) P.V.C. wires for all necessary electrical connections during testing.

224. What should be the arrangements for proper ventilation of transformer installed indoor in an enclosed room ?

Ans : For proper ventilation the transformer should be kept well away from the to make free movement of air round all the four sides. To ensure proper air circulation for the efficient cooling of the transformer a minimum area of 1 sq. meter for inlet per 1000 KVA should be provided as near the floor as possible and a minimum area of 2 sq. meters for outlet per 1000 KVA should be provided in the opposite side of air inlet as high as the building allows to enable the heated air to escape readily and be replaced by cool air.

225. In a transformer cubicle proper air circulation can not be achieved due to wall blocking. What kind of arrangement is to be provided in the cubicle for efficient dissipation of heat form the transformer ?

Ans : In addition to the provision of proper air inlet and outlet fans are provided extra for cooling the transformer.

226. What is minimum recommended spacing between the walls and transformer periphery in case (a) wall on one side, (b) wall on two sides, (c) wall on three sides and (d) wall on all sides as in an enclosed room ?

Ans : (a) 0.5 meter (b) 0.75 meter (c) 1.00 meter & (d) 1.25 meters.

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