When the end user enters all these parameters, transformer differential function
automatically calculates 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
"Optional Elimination of
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 side of the power transformer, then it is subtracted from each
individual phase current.
As it can be seen from equation 1 and equation 2 the first entered winding (W1) is
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
1 and
equation 2) can be pre-calculated in advance depending on the relative phase shift
between the reference winding and other power transformer windings.
Table 47 summarizes the values of the matrices for all standard phase shifts between
windings.
1MRK502052-UEN B Section 6
Differential protection
117
Technical manual
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