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:
(Equation 60)
The
Slope section 2
and
Slope section 3
settings are determined correspondingly:
Slope section
I
I
d
b
2 100
2
2
= ⋅ %
(Equation 61)
Slope section
I
I
d
b
3 100
3
3
= ⋅ %
(Equation 62)
The end of the first section
End section 1
can be set at a desired point within the
range of 0 to 100 percent (or % I
r
). Accordingly, the end of the second section
End
section 2
can be set within the range of 100 percent to 300 percent (or % I
r
).
The slope of the operating characteristic for the function block varies in different
parts of the range.
1MRS757644 H
Protection functions
620 series
Technical Manual
505
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