Siemens SIPROTEC 7SA522 Manual page 169

Determination of
Direction
7SA522 Manual
C53000-G1176-C155-3
2.7 Earth fault overcurrent protection in earthed systems (optional)
The time setting $GG7'(/$< (address ) allows an additional power-indepen-
dent delay time to be set.
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 3I
directional with a short or no delay time and the 3l
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 2.8, 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 = –3I
E
tion 2.7). The desired reference quantity is set in 32/$5,=$7,21 (address ):
The default setting 8  ,< RU 8 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 8  ,< 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 ZLWK ,< RQO\. This makes sense if a reliable transformer starpoint
current I is always available at the device input I
Y
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 3 I and 3 U
2
only the negative-sequence signals calculated by the device are used for direction de-
termination. In that case, the device does not require any zero-sequence signals for
direction determination.
If you are using the zero-sequence power protection (address  (DUWK )DXOW
2& = 6U LQYHUVH), it is reasonable to conduct the direction determination also via
the zero-sequence power. In this case apply the option ]HUR VHT SRZHU for
32/$5,=$7,21.
Finally, the threshold values of the reference quantities must be set. 8! (address
) determines the minimum operating voltage for direction determination with U
If U is not used for the direction determination, this setting is of no consequence. The
0
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 = | U + U + U
0 L1 L2
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 ).
, whose angle is compared with a reference quantity (Sec-
0
is connected to the device since an unconnected current
Y
, the setting ZLWK 8 DQG , is applied. In this case
2
|
L3
> stage with the same pickup
0
>>> stage could
0
as reference signal,
Y
. The direction determination is then
4
of normal sensi-
4
>> stage
0
.
4
.
0
169