1. A three-phase transformer can be constructed by;
(A) connecting three single phase transformers in delta or star
(B) connecting three single phase transformers in series
(C) connecting three single phase transformers in parallel
(D) any of the above
2. The primary and secondary windings of a three-phase transformer are usually connected in;
(A) delta-delta
(B) star-star
(C) any of the above
(D) delta-star or star-delta
3. The voltage transformation ratio of a three-phase transformer is equal to;
(A) the ratio of the primary and secondary turns per phase
(B) the ratio of the primary and secondary phase voltages
(C) the ratio of the primary and secondary line voltages
(D) any of the above
4. The power rating of a three-phase transformer is equal to;
(A) the product of the primary line voltage and line current
(B) the product of the secondary line voltage and line current
(C) the product of the primary or secondary line voltage and line current and square root of 3
(D) the product of the primary or secondary line voltage and line current and power factor
5. The efficiency of a three-phase transformer is maximum when;
(A) the copper loss is equal to the iron loss
(B) the load power factor is leading
(C) the load power factor is lagging
(D) the load power factor is unity
6. The equivalent resistance and reactance of a three-phase transformer referred to the primary side are given by;
(A) 3 (R1 + R2’) and 3 (X1 + X2’)
(B) R1 + R2’ / 3 and X1 + X2’ / 3
(C) R1 + R2’ and X1 + X2’
(D) none of the above
7. The voltage regulation of a three-phase transformer is defined as;
(A) the change in secondary terminal voltage from no load to full load at constant primary voltage and frequency
(B) the percentage change in secondary terminal voltage from no load to full load at constant primary voltage and frequency
(C) the change in secondary terminal voltage from full load to no load at constant primary voltage and frequency
(D) the percentage change in secondary terminal voltage from full load to no load at constant primary voltage and frequency
8. The short circuit test on a three-phase transformer is used to determine;
(A) the iron loss and no-load current at normal voltage and frequency
(B) the efficiency and regulation at any load condition and power factor
(C) the equivalent resistance and reactance referred to either side
(D) none of the above
9. The open circuit test on a three-phase transformer is used to determine;
(A) the equivalent resistance and reactance referred to either side
(B) the efficiency and regulation at any load condition and power factor
(C) none of the above
(D) the iron loss and no-load current at normal voltage and frequency
10. The polarity of a three-phase transformer is determined by;
(A) applying a low voltage to one winding and observing the direction of rotation of a motor connected to another winding
(B) applying a high voltage to one winding and observing the induced voltage in another winding with respect to a common terminal
(C) applying a low voltage to one winding and observing the induced voltage in another winding with respect to a common terminal
(D) none of the above
11. The ratio of the kVA ratings of the primary and secondary windings of a three-phase transformer is equal to;
(A) the ratio of the primary and secondary line voltages
(B) the ratio of the primary and secondary phase voltages
(C) the ratio of the primary and secondary turns per phase
(D) any of the above
12. The three phase transformer connections that can provide a 30-degree phase shift between the primary and secondary line voltages are;
(A) delta-star and star-delta
(B) delta-delta and star-star
(C) delta-zigzag and zigzag-delta
(D) none of the above
13. The three phase transformer connections that can provide a zero-phase shift between the primary and secondary line voltages are;
(A) delta-delta and star-star
(B) delta-star and star-delta
(C) delta-zigzag and zigzag-delta
(D) none of the above
14. The three phase transformer connections that can provide a 180-degree phase shift between the primary and secondary line voltages are;
(A) delta-delta and star-star with reversed polarity
(B) delta-star and star-delta with reversed polarity
(C) delta-zigzag and zigzag-delta with reversed polarity
(D) any of the above with reversed polarity
15. The advantage of using a three-phase transformer over three single phase transformers is that;
(A) it has better voltage regulation and efficiency
(B) it requires less space and material
(C) it can operate even if one of the phases is open circuited
(D) all of the above
16. The disadvantage of using a three-phase transformer over three single phase transformers is that;
(A) it cannot be used for parallel operation with other transformers
(B) it is more prone to faults and failures
(C) it is more difficult to transport and install
(D) all of the above
17. The parallel operation of three phase transformers is possible when they have the same;
(A) voltage transformation ratio and polarity
(B) winding connection and phase sequence
(C) impedance per phase referred to a common base
(D) all of the above
18. The load sharing among parallel connected three phase transformers is proportional to their;
(A) impedance ratings
(B) voltage ratings
(C) kVA ratings
(D) none of the above
19. The auto-transformer connection for a three-phase transformer is obtained by;
(A) connecting the primary and secondary windings in series
(B) connecting a common winding between the primary and secondary windings
(C) connecting the primary and secondary windings in parallel
(D) none of the above
20. The advantage of using an auto-transformer connection for a three-phase transformer is that;
(A) all of the above
(B) it has lower copper loss and higher efficiency
(C) it can provide a variable output voltage by tapping the common winding
(D) it requires less winding turns and core material
21. The disadvantage of using an auto-transformer connection for a three-phase transformer is that;
(A) it has lower insulation level and higher leakage flux
(B) it has lower voltage regulation and higher short circuit current
(C) all of the above
(D) it does not provide electrical isolation between the primary and secondary circuits
22. The Scott connection for a three-phase transformer is used to;
(A) convert a three-phase system into a two-phase system
(B) none of the above
(C) balance the load on a three-phase system
(D) convert a two-phase system into a three-phase system
23. The advantage of using a Scott connection for a three-phase transformer is that;
(A) all of the above
(B) it can handle unbalanced loads on both sides without circulating currents
(C) it can provide a 90-degree phase shift between the two phase and three phase systems
(D) it requires only two single phase transformers instead of three
24. The disadvantage of using a Scott connection for a three-phase transformer is that;
(A) all of the above
(B) it has higher copper loss and lower efficiency than a three-phase transformer
(C) it cannot provide a neutral point for grounding on either side
(D) it requires a special winding called the teaser winding with 86.6% turns of the main winding
25. The tertiary winding in a three-phase transformer is used to;
(A) provide a neutral point for grounding
(B) supply an auxiliary load at a different voltage level
(C) reduce the harmonic currents and voltage distortion in the primary and secondary windings
(D) any or all of the above
26. The harmonic currents and voltage distortion in a three-phase transformer are caused by;
(A) the saturation of the magnetic core due to high flux density
(B) any or all of the above
(C) the interaction of the fluxes produced by the primary and secondary windings due to their geometrical displacement
(D) the third harmonic component of the magnetizing current due to non-linear B-H curve of the core material
27. The three-limb core type construction for a three-phase transformer is preferred over the five-limb core type because;
(A) it has lower iron loss and higher efficiency
(B) it has lower copper loss and better cooling
(C) it has lower weight and cost
(D) all of the above
28. The shell type construction for a three-phase transformer is preferred over the core type because;
(A) it has better mechanical strength and protection
(B) it has lower leakage flux and better voltage regulation
(C) all of the above
(D) it has higher output capacity and lower noise level
29. The cooling methods for a three-phase transformer are classified according to;
(A) the type of coolant used such as oil or air
(B) the mode of circulation of coolant such as natural or forced
(C) both A and B
(D) none of the above
30. The most commonly used cooling method for a large three-phase transformer is;
(A) OFAF (oil forced air forced)
(B) ONAF (oil natural air forced)
(C) ONAN (oil natural air natural)
(D) OFWF (oil forced water forced)