Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
132
Figure 2-50 Switching onto a fault
Note
When switching onto a three-pole fault with the MHO characteristic, 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 enabled.
2.5.1.4 Setting Notes
At address 1501 FCT Distance the distance protection function can be switched ON or OFF.
Minimum current
The minimum current for fault detection Minimum Iph> (address 1502) in case of impedance pickup is set
somewhat (approx. 10 %) below the minimum short-circuit current that may occur. For the other pickup modes
it is set at address 1911.
Earth fault detection
In systems with earthed starpoint, the setting 3I0> Threshold (address 1503) 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 starpoint current of the set of current transformers. In non-earthed systems the setting value is rec-
ommended to be below the earth current value for double earth faults.
The preset value 3I0>/ Iphmax = 0.10 (address 1507) is usually recommended for the slope of the 3I0 char-
acteristic. This setting can only be changed in DIGSI at Display Additional Settings.
Addresses 1504 and 1509 are only relevant for earthed power systems. In non-earthed
systems, they are
hidden.
When setting 3U0> Threshold (address 1504), care must be taken that operational asymmetries do not
cause a pickup. 3U0 is defined as the sum of the phase-to-earth voltages |U
L1-E
+ U
L2-E
+ U
L3-E
|. If the U0 cri-
terion is not required, address 1504 is set to ∞.
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