Functions
2.2 Phase Comparison Protection and Ground Differential Protection
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
63
2.2.4
Ground Fault Differential Protection in Resonant-grounded/Isolated Systems
The ground fault differential protection can be applied in power systems whose starpoint is not grounded or
grounded through an arc suppression coil (Petersen coil). It is based on the power values. This requires the
phase voltages or the 3V0 voltage (Appendix A.3, Figure A-11) to be connected to the devices at both ends of
the protected object.
2.2.4.1
Description
General
Single-phase ground faults are not detected by the short-circuit protection since no short circuit current flows.
The power system operation is not immediately affected by a ground fault (the voltage triangle is maintained,
Figure 2-13). Therefore, fast tripping is usually not required or desired. The ground fault is to be detected, in-
dicated and the affected piece of equipment is to be localized, if possible, eliminating the ground fault by initi-
ating appropriate switching operations.
The 7SD80 enables the precise localization of the piece of equipment (line) affected by the ground fault.
In resonant-grounded systems, a core balance current transformer
must
be used to detect the ground current.
Figure 2-13
Ground fault in non-grounded neutral system
Pickup
Pickup occurs when the settable threshold for the displacement voltage 3·V
0
is exceeded. To obtain steady-
state measured quantities, the ground fault detection can be delayed by a configurable time after the displace-
ment voltage has occurred.
Determination of the Phase Affected by the Ground Fault
Following pickup caused by the displacement voltage, the phase affected by the ground fault is determined first.
To do this, the individual phase-to-ground voltages are measured. If the voltage magnitude for any given phase
is below the setting value V
min
, that phase is detected as the ground faulted phase as long as the remaining
phase-to-ground voltages are simultaneously above the setting value V
max
.
