Siemens siprotec 7SD5 User Manual page 225

Determination of The direction of each required stage was already determined when setting the differ-
Direction 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 3I
directional with a short or no delay time and the 3I
threshold, but a longer delay time as directional backup stage. The 3I
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
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
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
apply the setting with IY only. This makes sense if a reliable transformer starpoint
current I
not influenced by disturbances in the secondary circuit of the voltage transformers.
This presupposes that the device is equipped with a current input I
tivity and that the current from the transformer starpoint infeed is connected to I
If direction determination is to be carried out using exclusively the negative sequence
system signals 3I
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
If U
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
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
7SD5 Manual
C53000-G1176-C169-1
= –3I
E
Y
is always available at the device input I
Y
and 3U
2 2
is not used for the direction determination, this setting is of no consequence. The
0
3·U = |U + U + U |
0 L1 L2 L3
2.8 Earth Fault Protection in Earthed Systems (optional)
> stage with the same pickup
0
, whose angle is compared with a reference quantity. The
0
is connected to the device since an unconnected current
. The direction determination is then
4
, the setting with U2 and I2 is applied. In this case, only
>> stage
0
>>> stage could
0
as reference signal,
Y
of normal sensi-
4
.
4
.
0
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