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Interview questions of Transformer paper-8

131. What will be the effect of power factor and load on the regulation of a transformer ?

Ans : For lagging power factor of load current the regulation will be positive i.e. the secondary voltage will be reduced with the increase in the load current. For leading power factor of load current, the regulation will be negative i.e. the secondary voltage will be negative i.e. the secondary voltage will be increased with the increase in the load current.

132. Why is load test performed in addition to O.C. and S.C. test ?

Ans : Load test is performed not only to find the regulation and efficiency but also to measure the temperature rise of the transformer at different loads.

133. What is percentage impedance of a transformer ?

Ans : The impedance voltage is the vector sum of resistance voltage and reactance voltage. The percentage impedance is the value of impedance voltage expressed as a percentage of rated voltage. Thus % Impedance = % Impedance voltage = (V1 Z1’)/V1 × 100
Where, I1 =Rated primary current.
Z1 =Impedance with reference to the primary.
V1 =Rated primary voltage.

134. What do you mean by tap changing ?

Ans : Tap changing is a process of changing the ratio of transformation by increasing or decreasing the number of active turns in one winding with respect to another winding for maintaining a constant voltage on a power system.

135. What are the types of tap changing arrangements ?

Ans : (1) OFF load tap changing & (2) ON load tap changing. Offload tap changing gears can be operated only when all the loads on the transformer are cut off. The voltage ratio of a transformer can be varied by about ± 10% by means of OFF load tap changing gears. ON load tap changing gears can be operated even when the load is on. The voltage ratio of a transformer can be varied by about ± 16% by means of ON load tap changing gears.

136. On which winding (H.V or L.V ) of a power transformer the tapings are normally provided and why ?

Ans : The tapings are generally provided on the high voltage side since the current loading is comparatively small on the high voltage side.

137. What is a diverter switch and why is it used ?

Ans: A diverter switch is a switching device having contacts with usually four separate make and brake units and arranged along with separate tap selectors. It is used in an on-load tap changer of a very large transformer to operate in rapid sequence and kept in a separate compartment to minimize oil pollution by carbon.

138. What is called bussing ?

Ans : Bussing is a type of porcelain or ebonite post insulator put on the top or side of the transformer tank through which connections are made to the external circuits.

139. What type of bushings is generally used according to the voltage of the transformer?

Ans : The types of bushing are as follows :- Procelain insulator bushing used up to 33KV. Oil filled bushings consisting of a hollow porcelain cylinder of special shape with a hollow tube-shaped conductor through its centre used for the voltages above 33 KV. Capacitor type bussing made to thick layers of bakelite paper alternating with thin garden layers of tin foil being covered by a porcelain rain shed and filled up with bitumen in the annular space between the rain shed and the bushing used in outdoor substation for the voltage above 33 KV.

140. What is an auto-transformer ?

Ans : An auto-transformer is a single winding transformer provided with a number of taps in which the single winding performs the functions of both the primary and the secondary.

141. What is the basic difference between distribution transformer and auto-transformer ?

Ans : The basic difference is that the distribution transformer has two separate windings and there is no electrical connection between the primary and the secondary while an auto-transformer has a single winding in which the primary and secondary are electrically connected.

142. What are the advantages of auto-transformers ?

Ans : (1) Being single winding transformer it requires less copper and becomes lighter in weight. (2) The cost is less than a two-winding transformer. (3) As it is provided with a large number of taps desired voltages can be obtained by adjusting suitable taps.

143. Why are the auto transformers not used for high voltages ?

Ans : If an auto-transformer is used in high voltage line a fault in the primary may affect the secondary side in which case the secondary load equipment may be damaged. So it is not used for high voltages.

144.What are the applications of auto-transformers ?

Ans : Auto transformers are used in case of low voltage and low transformation ratio e.g. starting equipment of three phase induction motor, control equipment of single phase and three phase electric locomotives, voltage control of power and lighting circuits, regulating transformers, boosters to raise the voltages in a.c. circuits etc.

145. What is a variable transformer ?

Ans : A variable transformer (named as Variac, Dimmerstat etc) is nothing but an auto-transformer having a continuously variable tap so that any output voltage desired could be obtained.

146. What are the different forms of connections used in primary and secondary windings in three phase transformer ?

Ans : The possible connections are as follows :- (1)Star/ Star, (2) Star/ Delta, (3) Delta/ Delta, (4) Delta/Star, (5) Open delta(Vee/Vee) & (6) Scott (Tee/Tee).

147. What is the application of Star/Star connection ?

Ans : Star/Star connection is best suited for small high voltage transformers with balanced load.

148. Where is Star/Delta connection applied ?

Ans : Star/Delta connection is applied to the transformer at sending end of the transmission line where the voltage is to be stepped up.

149. Where is Delta/Delta connection applied?

Ans : Delta/Delta connection may be used in large low voltage transformers where the insulation problem is not so urgent.

150. Where is Delta/Star connection applied ?

Ans : Delta/Star connection is generally applied in step-down distribution transformers to provide a 3-phase, 4-wire supply for three phase loads as well as single phase loads which may be balanced or unbalanced.

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