Three phase transformer windings
TZDZ is correct that the capitalized letter always stands for the HV winding.

However the letter symbols are always given in descending order of voltage. It is not correct to write dY1 for a Yd11 transformer.

The following citation from IEC 60076.1 (AS 60076.1:2014) should clarify:


  7.1 Connection and phase displacement symbols for three-phase transformers and for single phase transformers connected in a three phase bank
  
  7.1.1 Connection symbol
  
  The star, delta, or zigzag connection of a set of phase windings of a
  three-phase transformer or of windings of the same voltage of
  single-phase transformers associated in a three-phase bank shall be
  indicated by the capital letters Y, D or Z for the high-voltage (HV)
  winding and small letters y, d or z for the intermediate and
  low-voltage (LV) windings.
  
  ...
  
  Letter symbols for the different windings of a transformer are noted in descending 
  order of rated voltage independently of the intended power flow. The
  winding connection letter for any intermediate and low-voltage winding
  is immediately followed by its phase displacement 'clock number' (see
  3.10.6).