ABB 615 Series ANSI, Техническое руководство

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Section 4
1MAC050144-MB C
Protection functions
324 615 series ANSI
Technical Manual
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 motor 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 restrained 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 restrained
differential 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:
Equation 27
The Slope section 2 setting is determined correspondingly:
Equation 28
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
n
). Accordingly, the end of the second section End section 2 can be
set within the range of 100 percent to 300 percent (or % I
n
). The slope of the operating
characteristic for the function block varies in different parts of the range.
In section 1, where 0.0 < I
b
/I
n
< End section 1 , the differential current required for tripping
is constant. The value of the differential current is the same as the Low trip value setting
selected for the function block. The Low trip value setting allows for small inaccuracies of
the current transformers but it can also be used to influence the overall level of the
operating characteristic.
Section 2, where End section 1 < I
b
/I
n
< End section 2 , is called the influence area of the
setting Slope section 2 . In this section, variations in End section 2 affect the slope of the
characteristic, that is, how big the change in the differential current required for tripping is