ABB RELION 620 Series Technical Manual page 414

Section 4
Protection functions
408
to the rated current of the machine or 3.3 times that of Low trip value, whichever is smaller.
The temporary extra limit decays exponentially from its maximum value with a time
constant of one second.
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.
Restrained differential (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
motor. In case of differential protection, a false differential current can be caused by:
• CT errors
• CT saturation at high currents passing through the motor
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 trip value setting for the stabilized stage of the function block is determined with
the equation:
I
d 1
Lowtrip value = ⋅
100%
I
n
GUID-22A3726A-4B8C-418F-B8C3-D6A5AACD165F-ANSI V1 EN
The Slope section 2 setting is determined correspondingly:
I
d 2
Slope section 2 = ⋅ 100 %
I
b 2
GUID-6707BA3B-6446-4B54-B09B-7D8D1F0DDF32 V1 EN
1MAC504801-IB E
(Equation 34)
(Equation 35)
620 series ANSI
Technical Manual