ABB REG650 Technical Manual page 79

1MRK 502 034-UEN -
Technical Manual
When the end user enters all these parameters, transformer differential function
automatically calculates off-line the matrix coefficients. During this calculations
the following rules are used:
For the phase reference, the first winding with set star (Y) connection is always
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
Dy1, then the LV winding (y) is taken for the phase reference. If there is no star
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
zero sequence currents"), then the differential currents can consist only of the
positive-, and the negative-sequence currents. When the zero-sequence current is
subtracted on one power transformer side, then it is subtracted from each individual
phase current.
As it can be seen from equation
always taken for ampere level reference (current magnitudes from all other sides
are always transferred to W1 side). In other words, within the differential
protection function, all differential currents and bias current are always expressed
in HV side primary Amperes.
It can be shown that the values of the matrix A, B & C coefficients (see equation
and equation 2) can be in advanced pre-calculated depending on the relative phase
shift between the reference winding and other power transformer windings.
Table 21 summarizes the values of the matrices for all standard phase shifts
between windings.
Differential protection
1 and equation 2 the first entered winding (W1) is
Section 5
"Elimination of
1
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