Functions
2.8 Earth Fault Protection in Earthed Systems (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
243
For lines without series compensation, do not change the default setting 0 of parameter XSerCapac (address
3187
, default setting
0
). The voltage U
P
used for directional determination remains unchanged in this case.
Teleprotection with earth fault protection
The earth fault protection in the 7SD5 may be expanded to a directional comparison protection using the inte-
grated teleprotection logic. Additional information regarding the available teleprotection schemes and their
modes of operation may be obtained from Section 2.9. If this is to be used, certain preconditions must already
be observed when setting the earth current stage.
Initially, it must be determined which stage is to operate in conjunction with the teleprotection scheme. This
stage must be set directional in the line direction. If, for example, the 3
I
0
> stage should operate as directional
comparison, set address
3130
Op. mode 3I0>
=
Forward
(see above „Definite Time Stages“).
Furthermore, the device must be informed that the applicable stage functions together with the teleprotection
to allow undelayed release of the tripping during internal faults. For the 3
I
0
> stage this means that address
3133
3I0> Telep/BI
is set to
YES
. The time delay
T 3I0>
set for this stage (address
3132
) then functions
as a back-up stage, e.g. during failure of the signal transmission. For the remaining stages the corresponding
parameter is set to
NO
, therefore, in this example: address
3123
3I0>> Telep/BI
for stage 3
I
0
>>, address
3113
3I0>>> Telep/BI
for stage 3
I
0
>>>, address
3148
3I0p Telep/BI
for stage 3
I
0P
(if used).
If the echo function is used in conjunction with the teleprotection scheme, or if the weak-infeed tripping function
should be used, the additional teleprotection stage
3IoMin Teleprot
(address
3105
) must be set to avoid
unselective tripping during through-fault earth current measurement. For further information, see Section 2.9,
margin heading „Earth Fault Protection Prerequisites“.
Switching onto an earth fault
It is possible to determine with a setting which stage trips without delay following closure onto a dead fault. The
parameters
3I0>>>SOTF-Trip
(address
3114
),
3I0>> SOTF-Trip
(address
3124
),
3I0> SOTF-Trip
(address
3134
) and, if necessary,
3I0p SOTF-Trip
(address
3149
) are available for the stages and can be
set to
YES
or
NO
for each stage. Selection of the most sensitive stage is usually not reasonable as a solid short-
circuit may be assumed following switching onto a fault, whereas the most sensitive stage often also has to
detect high resistance faults. It is important to avoid that the selected stage picks up due to transients during
line energization.
On the other hand, it does not matter if a selected stage may pick up due to inrush conditions on transformers.
The switch-onto-fault tripping by a stage is blocked by the inrush stabilization even if it is set as instantaneous
switch-onto-fault stage.
To avoid a spurious pickup due to transient overcurrents, the delay
SOTF Time DELAY
(address
3173
) can
be set. Usually, the default setting
0
can be retained. In the case of long cables, where large peak inrush cur-
rents can occur, a short delay may be useful. The time delay depends on the severity and duration of the tran-
sient overcurrents as well as on which stages were selected for the fast switch onto fault clearance.
With the parameter
SOTF Op. Mode
(address
3172
) it is finally possible to determine whether the fault direc-
tion must be checked (
DIRECT.
) or not (
PICKUP
), before a switch-onto-fault tripping is generated. It
is the direction setting for each stage that applies for this direction check.
Phase current stabilization
To avoid spurious pickup of the stages in the case of asymmetrical load conditions or varying current transform-
er measuring errors in earthed systems, the earth current stages are restrained by the phase currents: as the
phase currents increase, the pickup thresholds are increased. By means of the setting in address
3104
Iph-
STAB. Slope
the preset value of 10 % for all stages can be jointly changed for all stages. This parameter can
only be changed in DIGSI at
Display Additional Settings
.
