Connections of Three-Phase Transformer Banks

Question 1

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

Answer 1

H0, H1, H2 and H3 for high side

X0, X1, X2 and X3 for low side

Question 2

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

Answer 2

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

Question 3

What is meant by the term “neutral point”?

Answer 3

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

Question 4

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

Answer 4

0°

Question 5

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

Answer 5

30° with the high side leading the low side

Question 6

Which voltages do the phasors in voltage diagrams represent?

Answer 6

induced voltages

Question 7

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

Answer 7

the tail

Question 8

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

Answer 8

30° to the horizontal

Question 9

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

Answer 9

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

Question 10

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

Answer 10

on the right-hand side

Question 11

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

Answer 11

Yes

Question 12

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

Answer 12

Yes

Question 13

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

Answer 13

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

Question 14

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?

Answer 14

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

Question 15

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

Answer 15

H1, H2, and H3

Question 16

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

Answer 16

0°

Question 17

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

Answer 17

60° to the horizontal

Question 18

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

Answer 18

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

Question 19

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

Answer 19

On the right-hand side

Question 20

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

Answer 20

Yes

Question 21

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

Answer 21

Yes

Question 22

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

Answer 22

A mesh or delta closure test using a voltmeter

Question 23

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?

Answer 23

Yes

Question 24

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?

Answer 24

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

Question 25

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

Answer 25

X1, X2, and X3

Question 26

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

Answer 26

30°

Question 27

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

Answer 27

60° to the horizontal

Question 28

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

Answer 28

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

Question 29

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

Answer 29

On the right-hand side

Question 30

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

Answer 30

Yes

Question 31

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

Answer 31

Yes

Question 32

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

Answer 32

A mesh or delta closure test

Question 33

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?

Answer 33

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

Question 34

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?

Answer 34

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

Question 35

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

Answer 35

30° with the high side leading the low side

Question 36

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

Answer 36

60° to the horizontal

Question 37

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

Answer 37

On the right hand side

Question 38

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

Answer 38

Yes

Question 39

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

Answer 39

Yes

Question 40

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

Answer 40

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

Question 41

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

Answer 41

They are equal to each other

Question 42

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

Answer 42

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

Question 43

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?

Answer 43

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

Question 44

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?

Answer 44

To the centre tap of the B-phase transformer

Question 45

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

Answer 45

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

Question 46

Which phase does the CEC specify as the high leg?

Answer 46

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

Question 47

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

Answer 47

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

Question 48

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?

Answer 48

False

Question 49

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

Answer 49

- 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