2 Functions
72
7SA522 Manual
C53000-G1176-C155-3
Figure 2-17 Circuit breaker closure onto a fault
Note
When switching onto a three-pole fault with the MHO circle, there will be no voltage in
the memory or unfaulted loop voltage available. To ensure fault clearance when
switching onto three-pole close-up faults, please make sure that in conjunction with
the configured MHO characteristic the instantaneous tripping function is always en-
abled.
2.2.1.3 Setting Notes
At address )&7'LVWDQFH the distance protection function can be switched 21
or 2)).
M i n i m u m C u r r e n t The minimum current for fault detection 0LQLPXP,SK! (address ) is set some-
what (approx. 10 %) below the minimum short-circuit current that may occur.
Earth Fault Detec-
tion
In systems with earthed star-point, the setting ,!7KUHVKROG (address ) is
set somewhat below the minimum expected earth fault current. 3I
0
is defined as the
sum of the phase currents |I
L1
+ I
L2
+ I
L3
|, which equals the star-point current of the set
of current transformers. In non-earthed systems the setting value is recommended to
be below the earth current value for double earth faults.
The preset value ,!,SKPD[ = 0.10 (Address ) usually is recommended for
the slope of the 3I0 characteristic. This setting can only be changed via DIGSI
®
at Ad-
ditional Settings.
Addresses and are only relevant for earthed power systems. In non-
earthed systems this setting is not relevant and therefore not accessible.
When setting 8!7KUHVKROG (address ), care must be taken that operational
asymmetries do not cause a pickup. 3U0 is defined as the sum of the phase-earth volt-
ages |U
L1-E
+U
L2-E
+U
L3-E
|. If the U
0
criterion is not required, the address is set
to ∞.
In earthed power systems the earth fault detection can be complemented by a zero
sequence voltage detection function. You can determine whether an earth fault is de-
tected when a zero sequence current
or a zero sequence voltage threshold is sur-
passed or when both criteria are met. (default setting) applies at address ()
UHFRJQLWLRQ ,!258! if only one of the two criteria is valid. Select ,!$1'
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