Transformer Vector Groups MCQs

Q1. What does a transformer vector group indicate?

1. Transformer rating
2. Core material
3. Phase difference between HV and LV
4. Losses in transformer

✅ Correct Answer: c) Phase difference between HV and LV

• Vector group shows phase relationship between primary and secondary.
• It compares HV and LV line voltages.
• Essential for synchronization and parallel operation.

Q2. Transformer vector groups are applicable to which transformers?

1. Single-phase
2. Auto-transformers
3. Three-phase transformers
4. Instrument transformers

✅ Correct Answer: c) Three-phase transformers

• Phase displacement exists only in three-phase systems.
• Single-phase transformers have no phase shift.
• Hence vector groups apply only to three-phase transformers.

Q3. Phase displacement in vector groups is measured in steps of

1. 15°
2. 30°
3. 45°
4. 60°

✅ Correct Answer: b) 30°

• Clock notation divides 360° into 12 equal parts.
• Each division equals 30°.
• Phase shift is always a multiple of 30°.

Q4. Phase displacement is measured from

1. LV to HV anticlockwise
2. HV to LV clockwise
3. HV to LV anticlockwise
4. LV to HV clockwise

✅ Correct Answer: b) HV to LV clockwise

• HV phasor is taken as reference.
• Phase displacement is measured from HV to LV.
• Direction is clockwise as per IEC standard.

Q5. How many main transformer vector groups are there?

1. Two
2. Three
3. Four
4. Six

✅ Correct Answer: c) Four

• Classification is based on phase displacement.
• The four groups are 0°, 180°, −30°, and +30°.

Q6. Group-1 transformers have phase displacement of

1. −30°
2. +30°
3. 180°
4. 0°

✅ Correct Answer: d) 0°

• No angular displacement between HV and LV.
• Both voltages are in phase.
• Examples: Yy0, Dd0.

Q7. Group-2 transformers have phase displacement of

1. 0°
2. 180°
3. −30°
4. +30°

✅ Correct Answer: b) 180°

• Represents half-cycle phase shift.
• Clock position corresponds to 6.

Q8. Group-3 transformers have phase displacement of

1. +30°
2. 0°
3. −30°
4. 180°

✅ Correct Answer: c) −30°

• Negative sign shows LV lags HV.
• Clock position = 1.

Q9. Group-4 transformers have phase displacement of

1. 0°
2. −30°
3. 180°
4. +30°

✅ Correct Answer: d) +30°

• Positive sign indicates LV leads HV.
• Clock position = 11.

Q10. In clock notation, HV phasor is represented by

1. Hour hand
2. Minute hand
3. Second hand
4. Dial center

✅ Correct Answer: b) Minute hand

• HV side is fixed reference.
• Always placed at 12 o’clock.

Q11. In clock notation, LV phasor is represented by

1. Minute hand
2. Dial
3. Hour hand
4. Second hand

✅ Correct Answer: c) Hour hand

• LV phasor position varies.
• Hour hand shows phase displacement.

Q12. One hour shift on clock represents

1. 15°
2. 30°
3. 45°
4. 60°

✅ Correct Answer: b) 30°

• Clock has 12 divisions.
• Each division = 360° / 12 = 30°.

Q13. If hour hand is at 12, phase displacement is

1. −30°
2. +30°
3. 180°
4. 0°

✅ Correct Answer: d) 0°

• HV and LV phasors coincide.
• No angular separation.

Q14. If hour hand is at 6, phase displacement is

1. 90°
2. 120°
3. 180°
4. 210°

✅ Correct Answer: c) 180°

• 6 × 30° = 180°.
• LV voltage is opposite to HV.

Q15. If hour hand is at 1, phase displacement is

1. +30°
2. −30°
3. 330°
4. 180°

✅ Correct Answer: b) −30°

• 1 × 30° = 30° clockwise.
• LV lags HV by 30°.

Q16. If hour hand is at 11, phase displacement is

1. −30°
2. 300°
3. 330°
4. 360°

✅ Correct Answer: c) 330°

• 11 × 30° = 330°.
• Equivalent to +30° lead.

Q17. What does the number in vector group indicate?

1. Voltage ratio
2. Current rating
3. Clock position of LV
4. Power factor

✅ Correct Answer: c) Clock position of LV

• Indicates LV phase relative to HV.
• Directly shows phase displacement.

Q18. In Yd11, Y indicates

1. LV star
2. HV star
3. HV delta
4. LV delta

✅ Correct Answer: b) HV star

• First letter represents HV winding.
• Y indicates star connection.

Q19. In Yd11, d indicates

1. HV delta
2. LV delta
3. HV star
4. LV star

✅ Correct Answer: b) LV delta

• Second letter represents LV winding.
• d indicates delta connection.

Q20. In Yd11, LV voltage lags HV voltage by

1. 30°
2. 330°
3. 180°
4. 0°

✅ Correct Answer: b) 330°

• Clock number 11 → 11 × 30° = 330°.
• Equivalent to −30°.

Q21. Dy11 transformer is

1. Star-Star
2. Delta-Delta
3. Delta-Star
4. Star-Delta

✅ Correct Answer: c) Delta-Star

• D → HV delta.
• y → LV star.

Q22. In Dy11, LV voltage is

1. Lagging by 30°
2. Leading by 30°
3. Lagging by 180°
4. In phase

✅ Correct Answer: b) Leading by 30°

• 330° lag equals +30° lead.
• LV leads HV.

Q23. Commonly used clock numbers are

1. 2, 4, 8, 10
2. 1, 3, 5, 7
3. 0, 1, 6, 11
4. 9, 10, 11, 12

✅ Correct Answer: c) 0, 1, 6, 11

• These represent standard phase shifts used in practice.

Q24. Which transformers can be paralleled?

1. Different ratings
2. Same vector group
3. Different vector group
4. Any type

✅ Correct Answer: b) Same vector group

• Phase displacement must be identical.
• Otherwise circulating current flows.

Q25. Δ-Δ transformer cannot be paralleled with

1. Δ-Δ
2. Y-Y
3. Y-Δ
4. Δ-Δ (same rating)

✅ Correct Answer: c) Y-Δ

• Different vector groups cause phase mismatch.
• Results in circulating current.

Q26. Main purpose of vector group is

1. Reduce losses
2. Increase efficiency
3. Enable parallel operation
4. Improve insulation

✅ Correct Answer: c) Enable parallel operation

• Ensures correct phase alignment.
• Essential for grid connection.

Q27. Phase displacement depends on

1. Core material
2. Frequency
3. Winding connection
4. Cooling system

✅ Correct Answer: c) Winding connection

• Star-Delta or Delta-Star causes phase shift.
• Other factors do not affect phase displacement.

Q28. Clock notation method is used for

1. Voltage calculation
2. Phase comparison
3. Loss measurement
4. Load analysis

✅ Correct Answer: b) Phase comparison

• Simplifies understanding of phase relation.
• Uses clock analogy.

Q29. HV phasor is always placed at

1. 3
2. 6
3. 9
4. 12

✅ Correct Answer: d) 12

• HV is fixed reference in clock notation.

Q30. Wrong vector group parallel operation causes

1. Overvoltage
2. Circulating current
3. Frequency drop
4. Heating of core

✅ Correct Answer: b) Circulating current

• Phase mismatch causes circulating current.
• Leads to heating and instability.

Q31. Which vector group has zero phase displacement?

1. Yd11
2. Dy1
3. Yy0
4. Dy11

✅ Correct Answer: c) Yy0

• Clock number 0 represents 0° phase shift.
• HV and LV voltages are in phase.
• Both windings are star-connected.

Q32. Which transformer vector group is most commonly used in power distribution?

1. Yy0
2. Dy11
3. Dd0
4. Yd1

✅ Correct Answer: b) Dy11

• Provides 30° phase shift for system stability.
• Delta on HV side suppresses harmonics.
• Star on LV side provides neutral point.

Q33. Which vector group provides a neutral on the LV side?

1. Dd0
2. Dy11
3. Yd1
4. Dd6

✅ Correct Answer: b) Dy11

• LV winding is star-connected.
• Star connection provides neutral point.
• Suitable for distribution systems.

Q34. In which vector group does LV lag HV by 30°?

1. Dy11
2. Yd1
3. Yy0
4. Dd6

✅ Correct Answer: b) Yd1

• Clock position 1 = 30° lag.
• Negative displacement indicates lagging LV voltage.

Q35. Which vector group has 180° phase displacement?

1. Yy0
2. Dy11
3. Dd6
4. Yd1

✅ Correct Answer: c) Dd6

• Clock position 6 corresponds to 180°.
• LV voltage is opposite in phase to HV.

Q36. Which vector group helps in reducing third harmonic currents?

1. Yy0
2. Dy11
3. Yd1
4. Yy6

✅ Correct Answer: b) Dy11

• Delta winding provides closed path for harmonics.
• Third harmonic currents circulate within delta.

Q37. Vector group selection mainly affects

1. Efficiency
2. Losses
3. Parallel operation
4. Cooling

✅ Correct Answer: c) Parallel operation

• Same vector group is mandatory for parallel transformers.
• Prevents circulating currents.

Q38. Which vector group is suitable for earthing on LV side?

1. Dd0
2. Dy11
3. Dd6
4. Dy1

✅ Correct Answer: b) Dy11

• LV star connection provides neutral.
• Neutral can be earthed easily.

Q39. Which connection introduces a 30° phase shift?

1. Star-Star
2. Delta-Delta
3. Star-Delta
4. Single-phase

✅ Correct Answer: c) Star-Delta

• Star–Delta and Delta–Star connections create phase shift.
• Pure star or delta do not introduce phase displacement.

Q40. Which vector group has LV leading HV by 30°?

1. Yd1
2. Dy11
3. Dd6
4. Yy0

✅ Correct Answer: b) Dy11

• Clock number 11 → 330° lag = +30° lead.
• LV leads HV by 30°.

Q41. Why are transformers with different vector groups not paralleled?

1. Voltage mismatch
2. Current mismatch
3. Phase mismatch
4. Frequency mismatch

✅ Correct Answer: c) Phase mismatch

• Different phase angles cause circulating currents.
• Leads to overheating and instability.

Q42. Which vector group has both windings delta-connected?

1. Yd1
2. Dy11
3. Dd0
4. Yy0

✅ Correct Answer: c) Dd0

• D indicates delta connection.
• Both HV and LV are delta-connected.

Q43. Clock notation helps to determine

1. Voltage ratio
2. Phase displacement
3. Losses
4. Efficiency

✅ Correct Answer: b) Phase displacement

• Clock analogy visually represents phase difference.
• Simplifies vector group understanding.

Q44. Vector group Yd1 belongs to which main group?

1. Group 1
2. Group 2
3. Group 3
4. Group 4

✅ Correct Answer: c) Group 3

• Clock position 1 → −30°.
• Group 3 corresponds to −30° displacement.

Q45. Which vector group has no neutral on either side?

1. Dy11
2. Yy0
3. Dd0
4. Yd1

✅ Correct Answer: c) Dd0

• Delta connection does not provide neutral.
• Both windings are delta-connected.

Q46. Vector group information is essential mainly for

1. Core design
2. Parallel operation
3. Cooling selection
4. Efficiency calculation

✅ Correct Answer: b) Parallel operation

• Correct phase alignment is mandatory.
• Prevents circulating current during parallel operation.

Q47. Which vector group is used where neutral grounding is required?

1. Dd0
2. Dy11
3. Dd6
4. Dy1

✅ Correct Answer: b) Dy11

• Star on LV side provides neutral point.
• Neutral grounding improves system protection.

Q48. Which vector group is commonly used for generator transformers?

1. Yd1
2. Yd11
3. Dy11
4. Dd0

✅ Correct Answer: b) Yd11

• HV star allows grounding.
• Delta on LV side handles harmonics.

Q49. Vector group Dd6 indicates

1. 0° displacement
2. 30° lag
3. 180° displacement
4. 30° lead

✅ Correct Answer: c) 180° displacement

• Clock position 6 × 30° = 180°.
• LV voltage is opposite to HV.

Q50. Which vector group is NOT suitable for parallel operation with Dy11?

1. Dy11
2. Yd1
3. Dy11 (same rating)
4. Dy11 (different rating)

✅ Correct Answer: b) Yd1

• Vector group must be identical for parallel operation.
• Yd1 has different phase displacement than Dy11.

📘 Transformer Vector Groups – 100 One-Line Revision Notes (GTU / Diploma / BE / PSU / GATE Foundation)

1. Transformer vector group shows phase difference between primary and secondary line voltages.
2. Vector group is applicable only to three-phase transformers.
3. Vector group defines the connection of HV and LV windings.
4. Phase displacement is measured from HV to LV.
5. Phase displacement is measured in clockwise direction.
6. Phase displacement is expressed in multiples of 30°.
7. One clock hour represents 30° phase shift.
8. HV voltage phasor is taken as reference.
9. HV phasor is fixed at 12 o’clock position.
10. LV phasor is represented by the hour hand.
11. Hour hand at 12 indicates zero phase displacement.
12. Hour hand at 6 indicates 180° phase displacement.
13. Hour hand at 1 indicates −30° phase displacement.
14. Hour hand at 11 indicates 330° phase displacement.
15. Phase displacement equals clock number × 30°.
16. Transformer vector groups are divided into four main groups.
17. Group-1 transformer has 0° phase displacement.
18. Group-2 transformer has 180° phase displacement.
19. Group-3 transformer has −30° phase displacement.
20. Group-4 transformer has +30° phase displacement.
21. Vector group notation consists of letters and numbers.
22. First letter in vector group denotes HV winding connection.
23. Second letter in vector group denotes LV winding connection.
24. Capital letters indicate HV side connection.
25. Small letters indicate LV side connection.
26. The number in vector group indicates LV clock position.
27. Vector group number shows primary-secondary phase shift.
28. Clock notation method is used to represent phase displacement.
29. Clock method uses mechanical clock analogy.
30. HV side is represented by minute hand in clock method.
31. LV side is represented by hour hand in clock method.
32. In Yd11, HV winding is star connected.
33. In Yd11, LV winding is delta connected.
34. In Yd11, LV voltage lags HV voltage by 330°.
35. In Dy11, HV winding is delta connected.
36. In Dy11, LV winding is star connected.
37. In Dy11, LV voltage leads HV voltage by 30°.
38. A 330° lag is equivalent to −30° phase shift.
39. A +30° phase shift indicates LV leads HV.
40. A −30° phase shift indicates LV lags HV.
41. Vector group determines phase relationship between windings.
42. Vector group is important for parallel operation of transformers.
43. Only transformers of same vector group can be paralleled.
44. Different vector group transformers cannot be paralleled.
45. Parallel operation of different vector groups causes circulating current.
46. Circulating current leads to heating and instability.
47. Vector group selection is essential in power systems.
48. Star-Star transformer can be paralleled with Star-Star transformer.
49. Delta-Delta transformer can be paralleled with Delta-Delta transformer.
50. Delta-Delta transformer cannot be paralleled with Star-Delta transformer.
51. Vector group does not depend on voltage rating.
52. Vector group does not depend on transformer capacity.
53. Vector group depends on winding connection.
54. Vector group does not depend on core material.
55. Vector group does not depend on cooling method.
56. Vector group affects system synchronization.
57. Vector group helps in maintaining phase sequence.
58. Vector group is independent of load condition.
59. Vector group is fixed at the time of design.
60. Vector group is marked on transformer nameplate.
61. Clock position 0 represents 0° displacement.
62. Clock position 6 represents 180° displacement.
63. Clock position 1 represents −30° displacement.
64. Clock position 11 represents +30° displacement.
65. Vector group numbers commonly used are 0, 1, 6, and 11.
66. Vector group helps engineers identify phase shift quickly.
67. Vector group avoids phase mismatch in grid connection.
68. Incorrect vector group causes system disturbances.
69. Vector group ensures safe transformer operation.
70. Vector group helps in system planning.
71. Phase displacement is measured between line voltages.
72. Vector group is not applicable to single-phase transformers.
73. Vector group does not change during operation.
74. Vector group is independent of frequency.
75. Vector group is essential for load sharing.
76. Vector group notation follows HV-LV-number format.
77. Clock notation simplifies complex phasor diagrams.
78. Vector group ensures correct voltage phase relation.
79. Vector group avoids circulating currents in parallel operation.
80. Vector group improves power system reliability.
81. Phase shift occurs due to star-delta or delta-star connection.
82. Star-Star connection generally gives zero phase shift.
83. Delta-Delta connection generally gives zero phase shift.
84. Star-Delta connection produces 30° phase shift.
85. Delta-Star connection produces 30° phase shift.
86. Vector group is a standard IEC practice.
87. Clock notation is universally accepted method.
88. Vector group selection affects transformer compatibility.
89. Vector group is crucial in substations.
90. Vector group supports correct load sharing.
91. LV phasor position decides magnitude of phase shift.
92. HV phasor remains fixed in clock notation.
93. Phase displacement is always an integer multiple of 30°.
94. Vector group helps prevent phase opposition.
95. Vector group simplifies transformer identification.
96. Vector group ensures electrical safety.
97. Vector group is essential for grid interconnection.
98. Vector group is checked before parallel operation.
99. Vector group avoids abnormal circulating currents.
100. Transformer vector group ensures correct phase alignment between HV and LV.