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When we say "step up transformer," How the connection group to be defined: Voltage vector of HV 35kV to be taken for reference or Voltage vector LV 0.4kV.

I can not understand what is wrong, what is right:

- some people say: y/D11 is right (1)
- other people say:D/y11 is right (2)

Please be so kind to let me know what is wrong: (1) or (2) .

I will be very thankfull to your help.

Thank you and best wishes

Referring to IEC60076-1, section 6 paragraph 4:

“Letter symbols for the different windings of a transformer are noted in descending order of
rated voltage. The winding connection letter for any intermediate and low-voltage winding is
immediately followed by its phase displacement ‘clock number’ (see definition 3.10.6).”

Also, definition 3.10.6:

“NOTE The high-voltage winding phasor is taken as reference, and the displacement for any other winding is conventionally expressed by the ‘clock notation’, that is, the hour indicated by the winding phasor when the H.V. winding phasor is at 12 o’clock (rising numbers indicate increasing phase lag)”

So, the right one for a step up transformer, with delta connected primary, and star-connected (with neutral) secondary is:

YNd11

Note: 11 o'clock is the LV winding, which is the primary in a step-up transformer.

Yes, this is confusing, as you wouldn't have a clue just from a transformer vector group, whether it's a step-up or step-down.