Functions
2.15 Sensitive Earth Flt.(comp/ isol. starp.)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
299
If no displacement voltage is connected to the device, the device calculates the monitored voltage from the total
of the voltages:
3U
0
= |U
L1
+ U
L2
+ U
L3
|.
In case of a complete displacement of a healthy voltage triangle the displacement voltage also has a value that
is
√
3 times the phase-to-phase voltage.
Since, in the event of earth faults in isolated or resonant-earthed systems, the complete displacement voltage
emerges, the setting value is uncritical; it should be between approx. 25 % and 50% of the full displacement
voltage: for U
N
= 100 V therefore between 50 V and 90 V.
The earth fault is detected and reported only when the displacement voltage has applied for at least the time
T
Sens.E/F
(address
3006
). This stabilizing period is also enabled if earth fault conditions change (e.g. change
of direction).
If tripping is required for earth faults (address
3001
Sens. Earth Flt
=
ON: with Trip
), a delay time can
be set in address
3007
T 3U0>
.
For phase determination
Uph-e min
(address
3003
) is the criterion for the earth-faulted phase, when simul-
taneously the other two phase voltages have exceeded
Uph-e max
(address
3004
). The setting
Uph-e min
must be set less than the minimum phase-to-earth voltage that occurs during operation. This setting, too, is
uncritical, 40 V (default setting) should always be correct.
Uph-e max
must be greater than the maximum al-
lowable phase-to-earth voltage, but less than the minimum allowable phase-to-phase voltage. For U
N
= 100 V
that is for example at 75 V (default setting). The definite detection of the faulted phase is a further prerequisite
for alarming an earth fault.
Determination of Direction
The following is valid for determination of direction during earth faults: Pickup current
3I0>
(address
3005
)
must be set as high as possible to avoid a false pickup of the device provoked by asymmetrical currents in the
system and by current transformers (especially in a Holmgreen connection). Dependent upon the treatment of
the network star point, the magnitude of the capacitive earth fault current (for isolated networks) or the watt-
metric residual current (for compensated networks) is decisive.
In
isolated
networks, an earth fault in a cable will cause the total capacitive earth fault currents of the entire
electrically connected network to flow through the measuring point with the exception of the faulted cable itself.
This is because the latter flows directly to the fault location (i.e. not through the measuring point). It is normal
to use half the value of this earth fault current as the threshold value.
Example: A 25 kV bus-bar feeds seven cable circuits. Each circuit has a current transformer set 300 A/1 A. The
earth fault current is 2.5 A/km. The following applies for the cables circuits:
With an earth fault in cable 2, 62.5 A – 12.5 A = 50 A earth fault current will flow through the measuring point,
since 12.5 A flows directly from cable 2 into the fault. Since that cable is amongst the longest, this is the most
unfavourable case (smallest earth fault current flows through the measuring point). On the secondary side,
flows:
50 A/300 = 0.167 A.
The relay should be set to approximately half this value ,for example
3I0>
=
0.080
A.
Cable 1
Cable 2
Cable 3
Cable 4
Cable 5
Cable 6
Cable 7
3 km
5 km
2.6 km
5 km
3.4 km
3.4 km
2.6 km
7.5 A
12.5 A
6.5 A
12.5 A
8.5 A
8.5 A
6.5 A
Total
25.0 km
62.5 A
