Setting Notes - Siemens SIPROTEC 7SA522 Manual

2 Functions
2.2.1.3

Setting Notes

Minimum Current
Earth Fault Detec-
tion
72
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.
At address  )&7 'LVWDQFH the distance protection function can be switched 21
or 2)).
The minimum current for fault detection 0LQLPXP ,SK! (address ) is set some-
what (approx. 10 %) below the minimum short-circuit current that may occur.
In systems with earthed star-point, the setting ,! 7KUHVKROG (address ) is
set somewhat below the minimum expected earth fault current. 3I
sum of the phase currents |I
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
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 + U + U
L1-E L2-E L3-E
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 ,! 25 8! if only one of the two criteria is valid. Select ,! $1'
+ I + I |, which equals the star-point current of the set
L1 L2 L3
criterion is not required, the address  is set
|. If the U
0
is defined as the
0
®
at Ad-
7SA522 Manual
C53000-G1176-C155-3