ABB Relion 620 Series - Page 503

OPR_LS
Through
fault
detection
BLK_OPR_LS
BLOCK
BLK_OPR_HS
I_A1
I_B1
OPERATE
INT_BLKD
DC
component
detection
Differential
and bias
calculation
OPR_HS
Biased low
stage
CT
saturation
based
blocking
Instan-
taneous
high stage
I_C1
I_C2
I_A2
I_B2
Figure 257: Functional module diagram
Differential and bias calculation
Differential calculation module calculates the differential current. The differential
current is the difference in current between the phase and neutral sides of
the machine. The phase currents
I
1
and
I
2
denote the fundamental frequency
components on the phase and neutral sides of the current. The amplitude of the
differential current I
d
is obtained using the equation (assuming that the positive
direction of the current is towards the machine):
I I I
d
= +
1 2
(Equation 58)
During normal conditions, there is no fault in the area protected by the function
block, so the currents
I
1
and
I
2
are equal and the differential current I
d
= 0.
However, in practice some differential current exists due to inaccuracies in the
current transformer on the phase and neutral sides, but it is very small during
normal conditions.
The module calculates the differential current for all three phases.
The low-stage differential protection is stabilized with a bias current. The bias
current is also known as the stabilizing current. Stabilization means that the
differential current required for tripping increases according to the bias current and
1MRS757644 H
Protection functions
620 series
Technical Manual
503