# Interview questions of Transformer paper-10

171. In parallel operation of transformers of different capacities what should be the capacity of the smallest transformer with respect to the largest transformer in the group ?

Ans: The rated output of the smallest transformer in the group should not be less than 331/3 percent of the rated output of the largest transformer in the group.

172. How can you parallel a transformer in group-1 having vector group symbol Yy0, Dd0, Dz0 with one in group-2 having vector group symbol Yy6, Dd6, Dz6 ?

Ans: This can be done by altering the internal connection only of all the three secondary winding of any one transformer but not both.

173. How can you parallel a transformer in a group belonging to vector symbol Dy1 or Yd1 or Yz1 with one in another group belonging to voltage symbol Dy11 or Yd11 orYz11 ?

Ans: This can be done by transposing two of the external high voltage connections of one of the transformers and also the two corresponding low voltage external connections. The change of any of the internal connections is not necessary.

174. Is it possible to operate a transformer of groups 1&2 in parallel with a transformer of groups 3&4 listed below ? If not ,why ?

Ans : Transformer in groups 1 & 2 can not be operated in parallel with those in groups 3 & 4 because there is an inherent phase angle difference which can not be corrected.

175. What will be the effect of the change in voltage on the performance of a transformer ?

Ans : Iron loss varies approximately as square of the voltage. Copper loss and regulation vary inversely as square of the voltage. Iron temperature increases and copper winding temperature decreases with increase in voltage & vice versa. Efficiency at fractional load decreases and that at full load increases with increase in voltage and vice versa.

176. What will be the effect of a change in voltage on the performance of a transformer ?

Ans: Iron loss and temperature will vary inversely as the frequency. Copper loss is independent of frequency. So total loss increases with the decrease in frequency. Accordingly, efficiency decreases with the decrease in frequency. The regulator at low power factor decreases with a decrease in frequency and vice versa.

177. Why does a transformer require protection ?

Ans : A transformer requires protection for the following hazards :-
Overloading and short circuit fault. Internal fault – (a) Loose contact, (b) Short circuit between adjacent turns & (c) Earth fault in the winding or leads. External hazards –(a) Lightning surges & (b) System overvoltage.

178. What are the protective equipment required for a large transformer ?

Ans : The protective equipments are i) Buchholz relay,
ii) Over current relay,
iii) Differential relay,
iv) Earth leakage relay &
v) Lightning arrestor.

179. What is the reason for gas formation in a transformer ?

Ans : When the heat produced by any internal fault inside the tank is intensified then it decomposes the oil and liberates gas.

180. What is the function of Buchholz relay ?

Ans : The function of Buchholz relay is to actuate an alarm in case of small amount of gas or trip the circuit breaker in case of excessive gas by making use of that gas liberated inside the transformer due to any internal fault. It also detects the accumulation of air on account of low oil level or faulty oil pumps.

181. What is the lowest capacity of the transformer on which the provisions of gas pressure type protection is obligatory ?

Ans : On transformers of rating 1000 KVA and above the provision of gas pressure type protection with Buchholz relay is obligatory as per I.E. rules.

182. Where is Buchholtz relay installed ?

Ans : The Buchholz relay is installed in the pipe connecting the transformer tank and the oil conservator.

183. what should be the angle of inclination of the connecting pipe of the Buchholz relay with the horizontal plane ?

Ans : The angle of inclination of the axis of the pipe with horizontal plane should be 10⁰ to 11⁰ .

184. What should be the size of the connecting pipe of Buchholz relay ?

Ans : The nominal pipe bore diameter should be 25 mm for transformers up to 1000 KVA, 50 mm for transformers exceeding 1000 KVA but up to 10000 KVA and 80 mm for transformers exceeding 10000 KVA capacity. The length of the pipe of the lower section between relay and transformer tank should be more than 5D and that of the upper section between relay and conservator tank should be more than 3D (where D is the internal diameter of the connecting pipe).

185. What is the function of overcurrent relay ?

Ans : The function of overcurrent relay is to protect the transformers against overload and short circuit.

186. What is the lowest capacity of the transformers on which the provision of differential protection is obligatory ?

Ans : On transformer of capacity 10 MVA and above the provision of differential protection against incipient fault is obligatory as per I.E rules.

187. What do you mean by instrument transformers and why are they used ?

Ans : An instrument transformer is a transformer designed to represent the condition of current or voltage and of phase position in the primary circuit with acceptable accuracy in the secondary circuit. It changes the voltage or current in a power circuit to values which render them convenient for measurement. Instrument transformers are used in circuits of high voltage or large current to operate measuring and recording instruments and protective relays.

188. How do the instrument transformers differ from the power transformer ?

Ans : The instrument transformers differ from the power transformers mainly by their volt-ampere rating. The instrument transformers have a very low rating of only a few tens of VA e.g 10VA, 50VA, 100VA etc. whereas the power transformers have high KVA rating e.g 25KVA, 200KVA, 500KVA, 1000KVA even up to 1000MVA.

189. What are the types of instrument transformers ?

Ans : There are two basic types, namely, current transformers and potential transformers.

190. What is called true ratio of an instrument transformer ?

Ans : The true ratio of a current or a potential transformer is the ratio of r.m.s primary current or voltage, as the case may be, to the r.m.s secondary current or voltage under specified conditions.