ABB RELION REX640 - Page 648

Figure 364: Inrush current detection logic
Differential calculation
The operating principle is to calculate on both ends differential current from
currents entering and leaving the protection zone by utilizing the digital
communication channels for data exchange. The differential currents are almost
zero on normal operation. The differential protection is phase segregated and the
differential currents are calculated separately on both ends.
Communication
INCOMING
INCOMING
PROTECTED ZONE
RED 615 RED 615
Figure 365: Basic protection principle
The differential current I∆ (I
d
) of the protection relay is obtained on both ends with
the formula:
I I I
d
LOC REM
= +
(Equation 142)
The stabilizing current I
bias
(I
b
) of the protection relay is obtained on both ends
with the formula:
I
I I
b
LOC REM
=
−
2
(Equation 143)
Depending on the location of the star points of the current transformers, the
polarity of the local end remote currents may be different causing malfunction of
the calculation algorithms. The CT transformation ratio may be different and this
Protection functions
1MRS759142 F
648 REX640
Technical Manual