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Functions
2.15 Sensitive Earth Flt.(comp/ isol. starp.)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
295
2.15 Sensitive Earth Flt.(comp/ isol. starp.)
The earth fault detection function can be applied in power systems whose starpoint is either not earthed or
earthed through an arc suppression coil (Arc-suppression coil).
2.15.1 Method of Operation
General
Single-phase earth faults are not detected by the earth fault protection since no fault current flows. Further-
more, since network operation is not immediately affected by an earth fault (the voltage triangle is maintained,
Figure 2-148) rapid disconnection is usually not desired. It is more important that the earth fault be recognized,
indicated and, when possible, localized also. After having performed changes in the system it can finally be
cleared. But the 7SD5 enables the user to trip on directional earth fault in non-earthed systems.
Figure 2-148 Earth fault in non-earthed neutral network
Depending on the device version, the 7SD5 relay can be fitted with an earth fault detection module, which in-
cludes the following functions:
• Detection of an earth fault (pick-up) by monitoring the displacement voltage,
• Determination of the faulted phase by measuring the phase to earth voltages,
• Determination of the direction of the earth fault (residual) current by high accuracy real and reactive compo-
nent measurement.
Pickup
Pickup occurs when the settable threshold for the displacement voltage 3·U
0
is exceeded. To ensure measure-
ment of stable values, all earth fault detection functions are only released approx. 1 second (settable) after oc-
currence of the voltage displacement. Furthermore, each alteration of the earth fault conditions (e.g. change of
direction) is recognized only after this delay. Generally, the pickup is only indicated if a fault was detected for
sure by the phase determination function (see next margin heading).
Determination of the Earth-faulted Phase
After recognition of displaced voltage conditions the first objective of the device is selective detection of the
earth-faulted phase. To do this, the individual phase-to-earth voltages are measured. If the voltage magnitude
for any given phase is below the setting value U
min
that phase is detected as the earth faulted phase as long
as the remaining phase-earth voltages are simultaneously above the setting value U
max
.