Interview questions of Transformer paper-9

151. What are the advantages of having the primary winding connected in delta and the secondary winding in star ?

Ans : (1) An unbalanced four wire supply may be given. (2) The secondary neutral may be earthed or it may be utilized for given a four wire supply to provide both three phase and single phase loads.
(3) Third harmonic voltages are eliminated by the circulation of third harmonic current in the primary delta.

152. Where is open delta connection applied ?

Ans : Open delta connection is applied where the three phase load is too small and when one of the transformer in a delta-delta bank is disabled so that service is continued although at reduced capacity till the faulty transformer is repaired or a new one is substituted.

153. Where is Scott connection applied ?

Ans : Teaser or Scott connection is applied to supply three phase voltage from a two phase source and vice versa.

154. An 11KV/433KV, 500KVA, ∆/Y transformer is reconnected as Y/Y with high voltage side connected in star. What will be the rating of the new connection ?

Ans : 11.√3 KV/433V, 500 KVA.

155. What are the standard terminal markings on a transformer ?

Ans : High voltages terminals are marked with capital letters, N for neutral and A,B,C for three phases. Corresponding small letters are used for the low voltage terminals. In single phase transformers, the symbols are A2 A1 and a2 a1 in descending order from left to right.

156. What do you mean by phasing out of a 3-phase transformer ?

Ans : Phasing out of a 3-phase transformer means the detection of same phase terminals from primary and secondary winding.

157. How will you phase out a 3-phase transformer ?

Ans : For phasing out of transformers all phases are short circuited except a primary and a supposedly corresponding secondary. A small direct current is circulated in the primary and a voltmeter is connected across the secondary. A momentary deflection of the voltmeter, when the primary current is made and broken, confirms that the two windings concerned belong to the same phase.

158. What are the symbolic vector groups for identification of 3-phase transformer ?

Ans : (1) Group – 1(Yy0, Dd0, Dz0, Zd0)
(2) Group – 2 (Yy6, Dd6, Dz6, Zd6)
(3) Group – 3 (Dy1, Yd1, Yz1, Zy1)
(4) Group – 4 (Dy11, Yd11, Yz11, Zy11).

159. What do you mean by the expression given in name plate of a transformer vector group Dy11 and impedance voltage 5% ?

Ans : A transformer with vector group symbol (Dy11) means a transformer having primary winding delta connected and secondary winding star connected and belonging to group-4 with +30⁰ or-330⁰ phase displacement between primary and secondary winding as referred to the standard counterclockwise vector rotation. Impedance voltage 5% means when 5% of normal voltage is applied across one winding it produces normal voltage is applied across one winding it produces normal full load current to flow through the secondary winding when its terminals are short-circuited.

160. what do you mean by parallel operation of transformers ?

Ans: To connect two or more transformers in parallel is called parallel operation of transformers.

161. What is the necessity of parallel operation ?

Ans : (1) To share the increasing demand of the load. (2) To continue the supply in case of any fault and periodical maintenance. (3) To get maximum operational efficiency.

163. What are the conditions to be fulfilled before paralleling two three phase transformers ?

Ans : The conditions are –

• The voltage transformation ratio must be same.
• The polarity must be same.
• The percentage impedance should be same.
• The phase sequence must be same.
• The vector diagram and the phase displacement must be same.
• 164. What will happen if the voltage ratios of the transformers running in parallel are not identical ?

Ans : The secondary voltage will be different for which a circulating current will flow through the two secondaries if the transformers are connected in parallel. This circulating current will act as the extra load without any useful work being done and cause heating of transformers even on no load.

165. Is it necessary for the ratings of two transformers to be the same for parallel operation ?

Ans : Not necessary but they should share the load in proportion to their ratings.

166. what will be the results of incorrect polarity and unequal percentage impedance of two transformers connected in parallel?

Ans : Incorrect polarity results in a dead short circuit and due to unequal percentage impedance the load sharing of two transformers will not be in proportion to their KVA ratings.

167. What are the effects of percentage impedance on load sharing ?

Ans : Since the percentage, impedance determines the voltage drop between no load to full load, with equal percentage impedances the transformers running in parallel will share the load in proportion to their capacities. The transformer with a lower percentage impedance will have a lesser voltage drop and therefore it will take a greater share of the load. On the contrary, the transformer with a higher percentage impedance will have a higher voltage drop and it will not take its due share of the load.

168. In parallel operation of two transformers, how will they share a load if their impedances are equal ?

Ans : Equally.

169. In parallel operation of two transformers if their ratings are equal but percentage impedances are different how will they share a load ?

Ans : If the ratings of the two transformers are equal but percentage impedances are different the loads will be shared inversely as the impedances of the transformers expressed in percent of normal voltage.

170. In parallel operation of two transformers if their ratings and also percentage impedances are different how do you calculate their load sharing ?

Ans: In the case of different percentage impedances for different ratings of transformers the percentage impedances have to be converted to the same basic KVA. Then the loads will be shared inversely as the converted impedances of the transformer.

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