Connections of Three-Phase Transformer Banks Flashcards
How are the high-voltage and low-voltage terminals of a three-phase transformer normally marked?
H0, H1, H2 and H3 for high side
X0, X1, X2 and X3 for low side
What is meant by “phase displacement between the high and low sides of a three-phase transformer”?
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
What is meant by the term “neutral point”?
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
What is the standard phase displacement for wye-to-wye and delta-to-delta transformer connections?
0°
What is the standard phase displacement for wye-to-delta and delta-to-wye transformer connections?
30° with the high side leading the low side
Which voltages do the phasors in voltage diagrams represent?
induced voltages
Which part of the phasor do the terminal markings H1 and X1 represent: the head or the tail?
the tail
In voltage diagrams for three-phase transformers, in what position is the high-side voltage Vᴀɴ always drawn?
30° to the horizontal
Why is the low-side phasor for any transformer always drawn parallel to the high-side phasor for the same transformer?
Since the same changing flux produces both primary and secondary induced voltages, the two voltages must be in phase
Where is the A-phase transformer normally located in a transformer bank when viewed from the high side?
on the right-hand side
Can an additive-polarity transformer be used in a bank with two subtractive-polarity transformers to make up a three-phase bank?
Yes
Can a subtractive-polarity transformer be used in a bank with two additive-polarity transformers to make up a three-phase bank?
Yes
Before connecting the load, what test should you perform on the secondary of a three-phase transformer bank with a wye-connected secondary?
a voltage test to determine that all line-to-line voltages are equal
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?
The C-phase transformer is connected backward on either the primary or the secondary
For a step-down application, would the primary lines connect to H1, H2 and H3 or to X1, X2 and X3?
H1, H2, and H3
What is the standard phase displacement for a delta-to-delta transformer connection?
0°
On voltage diagrams for three-phase delta transformers, in what position is the high-side voltage phasor Vᴀʙ always drawn?
60° to the horizontal
Why is the low-side phasor for any transformer always drawn parallel to the high-side phasor for that transformer?
Since the same changing flux produces both primary and secondary induced voltages, the two voltages must be in phase
In a transformer bank, where is the A-phase transformer normally located when viewed from the high side?
On the right-hand side
Can an additive-polarity transformer be used with two subtractive-polarity transformers to make up a three-phase, delta-to-delta bank?
Yes
Can a subtractive-polarity transformer be used with two additive-polarity transformers to make up a three-phase, delta-to-delta bank?
Yes
What test must be performed on the secondary of a three-phase transformer bank with a delta-connected secondary?
A mesh or delta closure test using a voltmeter
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?
Yes
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?
You are likely reading the third harmonic voltage caused by the partial saturation of the transformer core/
For a delta-to-delta, step-up application, do the primary lines connected to H1, H2, and H3 or to X1, X2 and X3?
X1, X2, and X3
What is the standard phase displacement for a wye-to-delta transformer connection? Which side leads?
30°
In voltage diagrams for three-phase transformers, at what position is the high-side voltage phase Vᴀʙ always drawn?
60° to the horizontal
Why is the low-side phasor for any transformer always drawn parallel to the high-side phasor for the same transformer?
Since the same changing flux produces both primary and secondary induced voltages, the two voltages must be in phase
Where is the A-phase transformer normally located in a transformer bank viewed from the high side?
On the right-hand side
Can an additive-polarity transformer be used with two subtractive-polarity transformers to make up a three-phase, wye-to-delta bank?
Yes
Can a subtractive-polarity transformer be used with two additive-polarity transformers to make up a three-phase, wye-to-delta bank?
Yes
What test must be performed on the secondary of a three-phase transformer bank with a delta-connected secondary?
A mesh or delta closure test
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?
No. Either one of the primary connections or one of the secondary connections has been made incorrectly
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?
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
What is the standard phase displacement for a delta-to-wye transformer connection? Which side leads?
30° with the high side leading the low side
On voltage diagrams for three-phase delta transformers, at what position is the high-side voltage phasor Vᴀʙ drawn?
60° to the horizontal
In a transformer bank viewed from the high side, where is the A-phase transformer normally located?
On the right hand side
Can an additive-polarity transformer be used with two subtractive-polarity transformers to make up a delta-to-wye, three-phase bank?
Yes
Can a subtractive-polarity transformer be used with two additive-polarity transformers to make up a delta-to-wye, three phase bank?
Yes
What test must be performed on the secondary of a three-phase transformer bank with a wye-connected secondary?
A voltage test to determine that all line-to-line voltages are equal (balanced)
What is the relationship between the primary line voltage and primary transformer-winding voltage in a delta-to-wye, step-down transformer bank?
They are equal to each other
What is the relationship between the secondary line voltage and the secondary transformer-winding voltage in a delta-to-wye, step-down transformer bank?
The line voltage is equal to √3 times the phase voltage
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?
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
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?
To the centre tap of the B-phase transformer
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. 416V
b. 240V
c. 240V
Which phase does the CEC specify as the high leg?
The A-phase conductor, which should be identified with red insulation colouring
Does the CEC permit this high leg to be in the compartment of a panel board used to supply single-phase loads? Explain
No. The CEC wants to make sure the high leg is not accidentally used instead of one of the other legs.
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?
False
List three applications where an open-delta transformer connection is commonly used.
- 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
