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2.8 Earth Fault Protection in Earthed Systems (optional)
225
7SD5 Manual
C53000-G1176-C169-1
Determination of
Direction The direction of each required stage was already determined when setting the differ-
ent stages.
According to the requirements of the application, the directionality of each stage is in-
dividually selected. If, for instance, a directional earth fault protection with a non-direc-
tional back-up stage is required, this can be implemented by setting the 3
I
0
>> stage
directional with a short or no delay time and the 3 I
0
> stage with the same pickup
threshold, but a longer delay time as directional backup stage. The 3 I
0
>>> stage could
be applied as an additional high set instantaneous stage.
If a stage is to operate with teleprotection according to Section 2.9, it may operate
without delay in conjunction with a permissive scheme. In the blocking scheme, a
short delay equal to the signal transmission time, plus a small reserve margin of
approx. 20 ms is sufficient.
Direction determination of the overcurrent stages usually uses the earth current as
measured quantity
I
E
= –3 I
0
, whose angle is compared with a reference quantity. The
desired reference quantity is set in
POLARIZATION
(address
3160
):
The default setting
U0 + IY or U2
is universal. The device then selects automati-
cally whether the reference quantity is composed of the zero sequence voltage plus
the transformer starpoint current, or whether the negative-sequence voltage is used,
depending on which quantity prevails. You can even apply this setting when no trans-
former starpoint current
I
Y
is connected to the device since an unconnected current
does not have any effect.
The setting
U0 + IY
can also be applied with or without transformer starpoint current
connected.
If the direction determination must be carried out using only I
Y
as reference signal,
apply the setting
with IY only
. This makes sense if a reliable transformer starpoint
current I
Y
is always available at the device input I
4
. The direction determination is then
not influenced by disturbances in the secondary circuit of the voltage transformers.
This presupposes that the device is equipped with a current input
I
4
of normal sensi-
tivity and that the current from the transformer starpoint infeed is connected to I
4
.
If direction determination is to be carried out using exclusively the negative sequence
system signals 3
I
2
and 3U
2
, the setting
with U2 and I2
is applied. In this case, only
the negative-sequence signals calculated by the device are used for direction deter-
mination. The device does not require any zero-sequence signals for direction deter-
mination.
If you are using the zero-sequence power protection (address
131 Earth Fault
O/C
=
Sr inverse
), it is reasonable to conduct the direction determination also via
the zero-sequence power. In this case, apply the option
zero seq. power
for
POLARIZATION
.
Finally, the threshold values of the reference quantities must be set.
3U0>
(address
3164
) determines the minimum operating voltage for direction determination with U
0
.
If U
0
is not used for the direction determination, this setting is of no consequence. The
set threshold should not be exceeded by asymmetries in the operational measured
voltage. The setting value relates to the triple zero-sequence voltage, that is
3·U
0
= |U
L1
+ U
L2
+ U
L3
|
If the voltage dependent characteristic (U0 inverse) is used as directional stage, it is
reasonable for the minimum polarizing voltage to use a value that is equal to or below
the minimum voltage of the voltage-controlled characteristic (address
3182
).
Only if you have set in the
P.System Data 1
(see Section 2.1.2.1) the connection
of the fourth current transformer
I4 transformer
(address
220
) =
IY starpoint
,
address
3165 IY>
will appear. It is the lower threshold for the current measured in the
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