Through-Fault Restraint - Siemens 7UT51 Instruction Manual

7UT51 v3
Figure 6.3 Differential Protection Characteristic for a Bus
6.5 Through
CT saturation caused by a high-current internal fault
(or by long system time constants) affects the
differential current, I , and the restraining current,
diff
I , about equally; however, a high-current
rest
through-fault can cause considerable differential
current due to unequal CT saturation at the different
CT measuring locations. For this situation, the
differential protection function can be configured to
achieve the desired tripping restraint.
A high-current external fault is characterized by an
initial rise in the restraining current that is much more
rapid than the initial rise in the differential current. If
the operating point moves quickly (within 0.5 cycle)
into the through-fault-detection area shown in
Figure 6.3, the 87B function will restrained for a
configurable duration. The restraint will be released if,
for two complete cycles, the ratio of the differential
PRIM-2330C
Fault Restraint
-
Bus Differential Protection (87B)
current to the restraining current is less than 0.9 (that
is, the operating point moves up to within 90% of the
I = I line on Figure 6.3), which would indicate an
diff rest
internal fault is evolving during the external fault.
- -
1817 T SAT BLK
Maximum duration of through
of the 87B function.
250 cycles, or ∞ (until drop
–
Range: 2
Default: 8 cycles
The timer starts when the operating point enters the
fault restraint area (see Figure 6.3). Enter ∞ for the
-
through
restraint to continue until dropout of the differential
protection pickup.
-
1818 SAT RESTR.
Minimum restraint current required to activate through
-
CT saturation restraint. This value sets the left edge of the
-
through fault restraint area shown in Figure 6.3. The top of
the area is a line passing through the origin whose slope is
-
one half the slope of Slope 1 (Address 1806).
–
Range: 5.00 15.00 I
n
Default: 7.00 I
n
- -
fault CT saturation restraint
-
off of pickup)
-
fault
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