Delta Transformer Vector Group dashed "reference" line meaning?

[arrehman1]

Hi all,

So I have a dumb question... For a DAC Wye-grnded transformer (Dyn11) with ABC positive sequence rotation - It's very easy for me to see that the secondary line current lags the primary line current by 330 degrees. Or in other words, the secondary line current leads the primary line current by 30 degrees. The phasor diagram for line current and phase currents are fairly straight forward which is evident in this video tutorial: [URL unfurl="true"]http://gpac.link/1JiMsJg[/url]

When doing the same excercise for voltage phasors - everything seems to get much more complicated. From past discussions with similar topics, users of on forum have recommended Basler's work (attached) for understanding transformer vector groups. My question is regarding the dashed "reference" line that seems to be always shown in closed-form phasor diagram for delta connected transformers. The reference line always goes through the "H1" or "U" or "R" bushing... and based on the type of transformer connection, the phasors are arranged in a particular manner to show either a leading or lagging 30 degree displacement between the W1 winding and the reference line.

On page 5 of the document attached, it says "Note the use of Y# for naming each of the configuration. The # refers to the phase angle, as viewed on a 12 hour clock, of winding W1 relative to the voltage applied to the U bushing with a balanced 3 phase positive sequence voltage (UVW or ABC). Note also the phasor diagram in Figure 3 that shows the phase angle between the W1 winding and the voltage on the U bushing. The relationship between the W1 voltage and the U bushing will become important when one needs a common reference for determining phase shift across the transformer. One finds the phase shift by lining up the W1 voltage on the different sides of a transformer and seeing how the U bushing voltage on each side compares"

Also according to the C57.12.70 (IEEE Standard for Standard Terminal Markings and Connections for Distribution and Power Transformers) - "the phase displacement number refers to the phase angle, as viewed on a 12 hour clock, of winding W1 relative to the voltage applied to the A bushing with a balanced three-phase positive sequence voltage (ABC sequence). The winding connection letter grouping shall be immediately followed by its phase displacement. A nonloadable balancing winding will be designated after the main winding group. This will be connected to the main group by a plus sign (+)"

Here is another excerpt of the "dashed" reference line: "NOTE 1 — This figure is included to illustrate the method of marking transformer terminals that are brought out of the case. Dash lines show angular displacement between high and low voltage windings. NOTE 2 — Angular displacement is the angle between a line drawn from neutral to H1 and a line drawn from neutral to X1 measured in a clockwise direction from H1 to X1.

I think both the Basler and IEEE document are telling the same story. However, i'm still confused. The first part of my dumb question is 1) what is the "dashed" reference line? How is it defined or determined or setup? And dumb question 2) When comparing the phase angle displacement between the primary and secondary voltages, are we comparing primary phase A voltage with secondary phase a voltage?? Or are we comparing the secondary phase a voltage with the said reference line?

Thanks for your patience everyone!

 

[ausphil]

Arrehman1,

The "dashed reference line" is drawn to represent an equivalent star winding (from the phase to an equivalent star point) when looking at a delta winding (HV or LV), so you can directly compare the HV to the LV side. It is not describing any actual winding in the transformer tank.

It is usually only drawn on the first phase (U or A or which other first phase letter is used for your system) so as not to clutter up the diagram, and because it doesn't matter which particular phase you use, you still get the right answer. You can draw it on all phases if you want, but it doesn't help much.

The answer to the second question is that you are comparing the angle of the secondary winding with reference to the primary, no matter which type of transformer you are looking at. You always put the primary reference phase at the 12 o'clock position, and then move around to the low voltage position. For a delta star, you are comparing the actual secondary phase winding angle with the angle of the reference line in the primary delta. For a star delta, you are comparing the secondary reference line angle (in the delta) with the actual primary phase winding angle.

Just don't think too much into it.

Ausphil