ABB Relion Technical Manual page 100

Section 6
Differential protection
94
same may be represented as YN or yn depending on whether the winding is HV
or LV.
2. The Transformer phase shift such as Yd1, Dy11, YNautod5, Yy0d5 and so on, which
introduce phase displacement between individual windings currents in multiples of
30°. Since the HV and LV winding voltages are in phase for wye/wye or Delta/Delta
transformers, the same is represented in IEC as represented as Dd0, Yy0. Polarity
reversal in one of the windings would give 180 degree phase displacement which can
be represented by clock position 6. Such transformers can thus be represented as Dd6
or Yy6. It is also possible to rename the phases ABC to CAB or BCA, giving 120 or
240 degree displacements, represented by clock positions 4 & 8 . Polarity reversals in
one of the windings would provide clock positions 10 & 2. These can all be
represented for example: Yy0, Yy2, Yy4, Dd0, Dd6. ANSI wye/Delta or Delta/wye
transformers have the HV winding leading the LV winding by 30degrees. This can be
represented by Yd1 or Dy1. Again considering polarity reversals and renaming of
phases gives rise to other clock positions 4,7,5,11
3. The Settings for elimination of zero sequence currents for the individual windings.
When the end user enters all these parameters, transformer differential function
automatically determines the matrix coefficients based on the following rules:
For the phase reference, the highest voltage wye (Y) connected winding is used. For
example, if the power transformer is a Yd1 power transformer, the HV winding (Y) is
taken as the phase reference winding. If the power transformer is a Yy0 power transformer
the HV winding (Y) is taken as the phase reference winding. If the power transformer is
a Dy1, then the LV winding (y) is taken for the phase reference. If there is no wye
connected winding, such as in Dd0 type of power transformers, then the HV delta winding
(D) is automatically chosen as the phase reference winding.
The fundamental frequency differential currents are in general composed of currents of all
sequences, that is, the positive-, the negative-, and the zero-sequence currents. If the zero-
sequence currents are eliminated (see section
then the differential currents can consist only of the positive-, and the negative-sequence
currents. When the zero-sequence current is subtracted on one side of the power
transformer , then it is subtracted from each individual phase current.
Table 29 summarizes the values of the matrices for all standard phase shifts between
windings.
1MRK 502 048-UUS A
"Elimination of zero sequence currents"),
Technical manual