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Three-Phase Transformers > Connections of Three-Phase Transformer Banks > Flashcards

Connections of Three-Phase Transformer Banks Flashcards

1
Q

How are the high-voltage and low-voltage terminals of a three-phase transformer normally marked?

A

H0, H1, H2 and H3 for high side

X0, X1, X2 and X3 for low side

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2
Q

What is meant by “phase displacement between the high and low sides of a three-phase transformer”?

A

the angle between the voltage V(AN) of the high side and V(AN) of the low side where N is the neutral point on the diagram

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3
Q

What is meant by the term “neutral point”?

A

Neutral refers to a point such that the voltages between it and each of the phase conductors are equally spaced in magnitude and phase angle

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4
Q

What is the standard phase displacement for wye-to-wye and delta-to-delta transformer connections?

A

0°

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5
Q

What is the standard phase displacement for wye-to-delta and delta-to-wye transformer connections?

A

30° with the high side leading the low side

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6
Q

Which voltages do the phasors in voltage diagrams represent?

A

induced voltages

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7
Q

Which part of the phasor do the terminal markings H1 and X1 represent: the head or the tail?

A

the tail

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8
Q

In voltage diagrams for three-phase transformers, in what position is the high-side voltage Vᴀɴ always drawn?

A

30° to the horizontal

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9
Q

Why is the low-side phasor for any transformer always drawn parallel to the high-side phasor for the same transformer?

A

Since the same changing flux produces both primary and secondary induced voltages, the two voltages must be in phase

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10
Q

Where is the A-phase transformer normally located in a transformer bank when viewed from the high side?

A

on the right-hand side

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11
Q

Can an additive-polarity transformer be used in a bank with two subtractive-polarity transformers to make up a three-phase bank?

A

Yes

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12
Q

Can a subtractive-polarity transformer be used in a bank with two additive-polarity transformers to make up a three-phase bank?

A

Yes

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13
Q

Before connecting the load, what test should you perform on the secondary of a three-phase transformer bank with a wye-connected secondary?

A

a voltage test to determine that all line-to-line voltages are equal

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14
Q

If a voltmeter test on the secondary of a wye-to-wye connected transformer shows Vᴀʙ = 208V, Vʙᴄ = 120V, and Vᴄᴀ = 120V, what is indicated?

A

The C-phase transformer is connected backward on either the primary or the secondary

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15
Q

For a step-down application, would the primary lines connect to H1, H2 and H3 or to X1, X2 and X3?

A

H1, H2, and H3

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16
Q

What is the standard phase displacement for a delta-to-delta transformer connection?

A

0°

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17
Q

On voltage diagrams for three-phase delta transformers, in what position is the high-side voltage phasor Vᴀʙ always drawn?

A

60° to the horizontal

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18
Q

Why is the low-side phasor for any transformer always drawn parallel to the high-side phasor for that transformer?

A

Since the same changing flux produces both primary and secondary induced voltages, the two voltages must be in phase

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19
Q

In a transformer bank, where is the A-phase transformer normally located when viewed from the high side?

A

On the right-hand side

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20
Q

Can an additive-polarity transformer be used with two subtractive-polarity transformers to make up a three-phase, delta-to-delta bank?

A

Yes

21
Q

Can a subtractive-polarity transformer be used with two additive-polarity transformers to make up a three-phase, delta-to-delta bank?

A

Yes

22
Q

What test must be performed on the secondary of a three-phase transformer bank with a delta-connected secondary?

A

A mesh or delta closure test using a voltmeter

23
Q

The phase voltage on the delta-connected secondary of a three-phase transformer bank is 240V. The mesh or delta closure test with a voltmeter gives a reading of 73V. Is it safe to remove the meter and install the final connection?

A

Yes

24
Q

What is the most likely cause of the 73V measurement found on the delta-to-delta connected transformer secondary during the mesh or delta closure test?

A

You are likely reading the third harmonic voltage caused by the partial saturation of the transformer core/

25
Q

For a delta-to-delta, step-up application, do the primary lines connected to H1, H2, and H3 or to X1, X2 and X3?

A

X1, X2, and X3

26
Q

What is the standard phase displacement for a wye-to-delta transformer connection? Which side leads?

A

30°

27
Q

In voltage diagrams for three-phase transformers, at what position is the high-side voltage phase Vᴀʙ always drawn?

A

60° to the horizontal

28
Q

Why is the low-side phasor for any transformer always drawn parallel to the high-side phasor for the same transformer?

A

Since the same changing flux produces both primary and secondary induced voltages, the two voltages must be in phase

29
Q

Where is the A-phase transformer normally located in a transformer bank viewed from the high side?

A

On the right-hand side

30
Q

Can an additive-polarity transformer be used with two subtractive-polarity transformers to make up a three-phase, wye-to-delta bank?

A

Yes

31
Q

Can a subtractive-polarity transformer be used with two additive-polarity transformers to make up a three-phase, wye-to-delta bank?

A

Yes

32
Q

What test must be performed on the secondary of a three-phase transformer bank with a delta-connected secondary?

A

A mesh or delta closure test

33
Q

The phase voltage on the delta-connected secondary of a three-phase transformer bank is 480V. The mesh or closed delta test gives a reading of 960V. Is it safe to remove the meter and install the final connection?

A

No. Either one of the primary connections or one of the secondary connections has been made incorrectly

34
Q

If you used the same voltage diagram for a wye-to-delta, step-up application as for a wye-to-delta, step-down application, are you still conforming to the CSA standards for three-phase transformers?

A

No. You will get the proper phase sequence and a 30° shift, but you will not have the high side leading the low side on one of the applications

35
Q

What is the standard phase displacement for a delta-to-wye transformer connection? Which side leads?

A

30° with the high side leading the low side

36
Q

On voltage diagrams for three-phase delta transformers, at what position is the high-side voltage phasor Vᴀʙ drawn?

A

60° to the horizontal

37
Q

In a transformer bank viewed from the high side, where is the A-phase transformer normally located?

A

On the right hand side

38
Q

Can an additive-polarity transformer be used with two subtractive-polarity transformers to make up a delta-to-wye, three-phase bank?

A

Yes

39
Q

Can a subtractive-polarity transformer be used with two additive-polarity transformers to make up a delta-to-wye, three phase bank?

A

Yes

40
Q

What test must be performed on the secondary of a three-phase transformer bank with a wye-connected secondary?

A

A voltage test to determine that all line-to-line voltages are equal (balanced)

41
Q

What is the relationship between the primary line voltage and primary transformer-winding voltage in a delta-to-wye, step-down transformer bank?

A

They are equal to each other

42
Q

What is the relationship between the secondary line voltage and the secondary transformer-winding voltage in a delta-to-wye, step-down transformer bank?

A

The line voltage is equal to √3 times the phase voltage

43
Q

Can the same voltage diagram be used for a delta-to-wye step-up application as for a delta-to-wye, step-down application and still conform to the CSA standards for three-phase transformers?

A

No. You will get the proper phase sequence and a 30° shift, but you will not have the high side leading the low side on one of the applications

44
Q

A three-phase, four-wire, delta-connected system is to be used for supplying both three-phase motor loads and single-phase lighting loads. Where do you normally attach the ground?

A

To the centre tap of the B-phase transformer

45
Q

If the line-to-line voltage on the three phase, four wire delta system is 480V, what will the voltages be between the following points?

a. line A to ground
b. line B to ground
c. line C to ground

A

a. 416V
b. 240V
c. 240V

46
Q

Which phase does the CEC specify as the high leg?

A

The A-phase conductor, which should be identified with red insulation colouring

47
Q

Does the CEC permit this high leg to be in the compartment of a panel board used to supply single-phase loads? Explain

A

No. The CEC wants to make sure the high leg is not accidentally used instead of one of the other legs.

48
Q

The transformer with the centre tap supplies all of the current to the single-phase load. The other two transformers supply only their share of the balanced three-phase load. True or False?

A

False

49
Q

List three applications where an open-delta transformer connection is commonly used.

A
  • The temporary supply of three-phase power when one transformer in a three-phase bank fails and is being repaired or replaced
  • The initial supply of a system that is expected to expand in the future
  • The supply of loads where the majority of the load is single-phase and only a small amount of three-phase power is required

Three-Phase Transformers (7 decks)

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