ABB RELION REX640 Technical Manual page 946

Protection functions
Table 906: Selection of earth-fault detection criteria
Setting
EF detection Mod GFC
If the residual current Io is derived from the phase currents either by the summation
connection of CTs, that is, the Holmgreen connection, or by internally summing
the phase currents, there is a risk of a false earth-fault detection due to apparent
residual current from the current transformer errors. This risk can be eliminated
using a stabilized residual current measurement. The stabilization is enabled by
setting the
residual current
current exceeds the nominal current. The increase is defined with the
1 GFC setting. The apparent residual current may exist during short circuits and
in inrush current situations if the CTs saturate partially. The earth-fault detection
function takes the CT saturation into account with additional stabilization settings
Gnd Op current 2 GFC, A Ph Stab value GFC and Stab slope 2 GFC .
Setting
current measurement and should be used only when the residual current is
measured with a cable current transformer. The residual current threshold setting
for earth-fault detection is
from a transformer neural point,
setting
Gnd Op current 2 GFC
Gnd Op current GFC
Figure 526: Stabilized residual current measurement for earth-fault detection
The residual voltage can also be used as a complementary criterion for earth-fault
detection. This is valid when the setting
detection Mod GFC is "Io AND Uo". The threshold setting for the residual voltage is
Gnd Str voltage GFC .
946
EF Cur stabilization setting to "True". The stabilization increases the
Gnd Op current GFC threshold setting when the maximum phase
EF Cur stabilization to "False" disables the stabilization of the residual
Gnd Op current GFC . If the residual current is measured
EF detection Mod GFC is "Io AND IoRef".
3I /I
0 n
Stab slope 1 GFC
1.0
Enumerator name
• Io
• Io OR Uo
• Io AND Uo
• Io AND IoRef
Gnd Op A Ref GFC is used. This is valid when the
Stab slope 2 GFC
Blocking
due to CT
saturation
Blocking due
to CT errors
A Ph Stab value GFC
EF detection Mod GFC is "Io OR Uo" or EF
1MRS759142 F
Stab slope
I /I
maxPh n
REX640
Technical Manual