ABB RELION REX640 - Page 946

Table 906: Selection of earth-fault detection criteria
Setting Enumerator name
EF detection Mod GFC
• Io
• Io OR Uo
• Io AND Uo
• Io AND IoRef
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 EF Cur stabilization setting to "True". The stabilization increases the
residual current Gnd Op current GFC threshold setting when the maximum phase
current exceeds the nominal current. The increase is defined with the Stab slope
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 EF Cur stabilization to "False" disables the stabilization of the residual
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 Gnd Op current GFC . If the residual current is measured
from a transformer neural point, Gnd Op A Ref GFC is used. This is valid when the
setting EF detection Mod GFC is "Io AND IoRef".
3I
0
/I
n
Stab slope 1 GFC
A Ph Stab value GFC
I
maxPh
/I
n
Stab slope 2 GFC
Blocking
due to CT
saturation
Blocking due
to CT errors
1.0
Gnd Op current GFC
Gnd Op current 2 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 EF detection Mod GFC is "Io OR Uo" or EF
detection Mod GFC is "Io AND Uo". The threshold setting for the residual voltage is
Gnd Str voltage GFC .
Protection functions
1MRS759142 F
946 REX640
Technical Manual