Parallel transformers - phasor diagram 3

[arrow123]

Hi. We occasionally find situations in our network where an overhead 3 phase transformer has been connected to the overhead line with its A terminal to Blue phase, B terminal to Yellow phase and C terminal to Red phase. Normally these connections are A - Red, B - Yellow, C - Blue. This leaves the secondaries of the two transformers with a phase difference (I think of 30 deg) but I can't think of how to draw these out to show our trainees. Can anyone tell me how to draw the phasor diagram to show how the secondary of the two Dy11 transformers would have a phase difference?? Thanks.

 

[RalphChristie]

It is a bit difficult to explain it without drawings.


Draw a triangle and mark it ABC (primary)
A-phase on top
B-phase bottom right
C-phase bottom left

For a Dyn11 transformer your a-phase (secondary) will be at the 11 o'clock position or 30° x 11 = 330° position.
Assume normal phase rotation (red, white, blue) anticlockwise. (Your red phase will first go through 0°, then white 120° behind, the blue 120° behind white, etc.)
A = red
B = white
C = blue

From this you will see a-phase (secondary) is 30° in front of A-phase (primary) Thus a is leading A with 30°


Now change
A = blue
B = white
C = red
You have changed phase-rotation from anticlockwise to clockwise.
Now A-phase will first go through 0°, the a-phase 30° behind. Thus a is now lagging A with 30°

Effectively you have created a 60° shift between the two secondaries, and, by changing phase-rotation, you have changed the Dyn11 transformer to a Dyn1 transformer.

See also FAQ238-1090

Hope it helps

Regards
Ralph

 

[arrow123]

Thanks for the quick reply Ralph. The first part of your explanantion was a very good description of how to show the Dyn11. Can you elaborate on how the second part gives the 60 deg difference. I have drawn it out and think that by having the imaginary neutral on the primary between terminal A and the imaginary centre of the Delta Triangle in both the correct way of connecting the transformer and the backwards way, I can show a leading 30 for the correct connection, but need a little clarification on the backward connection. Sorry, I have lost all my notes from a few years back (phasor diagrams were never a strong point)Cheers.

 

[RalphChristie]

Your phase-rotation is clockwise (was anticlockwise) if you change the supply connections from ABC to CBA. On the same vector-diagram (for Dyn11, standard connection), change your rotation and you'll see A-phase goes first through the 0° axis, then a-phase follows 30° later. Thus a-phase is lagging A-phase with 30° and actually you are changing the Dyn11 connection to a Dyn1 connection.(Just note that this is not a standard connection anymore)

For Dyn11 - "a" leading "A" with 30°
For Dyn1 - "a" lagging "A" with 30°
A is always your reference point

30° + 30° = 60°
There will be a 60° shift between your two secondary a-phases.

Regards
Ralph

 

[arrow123]

Thanks Ralph.
It all seems so simple now!!!

Regards

Peter