Parallel transformers - phasor diagram
Parallel transformers - phasor diagram
(OP)
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.






RE: Parallel transformers - phasor diagram
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
RE: Parallel transformers - phasor diagram
RE: Parallel transformers - phasor diagram
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
RE: Parallel transformers - phasor diagram
It all seems so simple now!!!
Regards
Peter