ABB RELION REX640 Technical Manual page 648

Protection functions
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.
Figure 365: Basic protection principle
The differential current I∆ (I
the formula:
I
=
d
The stabilizing current I
with the formula:
I
=
b
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
648
RED 615
I I
+
LOC REM
bias
I I
LOC REM
−
2
Communication
PROTECTED ZONE
) of the protection relay is obtained on both ends with
d
(I ) of the protection relay is obtained on both ends
b
1MRS759142 F
RED 615
(Equation 142)
(Equation 143)
REX640
Technical Manual