ABB RELION 620 Series Technical Manual page 499

1MRS757644 F
620 series
Technical Manual
This feature should be used in case of networks where very long time constants are
expected. The temporary sensitivity limit is higher to the set operating characteristics.
In other words, the temporary limit has superposed the unchanged operating
characteristics and temporarily determines the highest sensitivity of the protection.
The temporary sensitivity is less than the sensitivity in section 1 of the operating
characteristic and is supposed to prevent an unwanted trip during the external faults
with lower currents.
Biased low stage
The current differential protection needs to be biased because of the possible
appearance of a differential current which can be due to something else than an actual
fault in the machine. In case of differential protection, a false differential current can
be caused by:
• CT errors
• CT saturation at high currents passing through the machine
The differential current caused by CT errors increases at the same percent ratio as the
load current.
The high currents passing through the protected object can be caused by the through
fault. Therefore, the operation of the differential protection is biased with respect to
the load current. In the biased differential protection, the higher the differential current
required for the protection of operation, the higher the load current.
Based on the conditions checked from the through-fault module, the DC (component)
detection module and the CT saturation-based blocking modules, the biased low-stage
module decides whether the differential current is due to the internal faults or some
false reason. In case of detection of the TF, DC or CT saturation, the internal
differential blocking signal is generated, which in turn blocks the operating signal. In
case of internal faults, the operation of the differential protection is affected by the bias
current.
The Low operate value setting for the stabilized stage of the function block is
determined with the equation:
Low operate value I
=
d 1
GUID-22A3726A-4B8C-418F-B8C3-D6A5AACD165F V2 EN
The Slope section 2 and Slope section 3 settings are determined correspondingly:
I
d 2
Slope section 2 100
= ⋅
I
b
2
GUID-6707BA3B-6446-4B54-B09B-7D8D1F0DDF32 V1 EN
I
d 3
Slope section
3 100
= ⋅
I
b 3
GUID-ADB22AAC-F1CD-4B6F-B103-E86DBCC8069D V1 EN
%
%
Section 4
Protection functions
(Equation 59)
(Equation 60)
(Equation 61)
493