Siemens siprotec 7SA6 User Manual page 296

Functions
Direction
Determination
6-114
t
0
8! 8 LQY
0
Figure 6-69 Characteristic settings of the zero sequence voltage time dependent stage —
without additional times.
The direction of each required stage was already determined when setting the
different stages.
According to the requirements of the application, the directionality of each stage is
individually selected. If for instance a directional earth fault protection with a non-
directional back-up stage is required, this can be implemented by setting the 3I
stage directional with a short or no delay time and the 3I
up threshold but a longer delay time as directional back-up stage. The 3I
could be applied as an additional high set instantaneous stage.
If a stage is to operate with teleprotection according to Section 6.6, 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.
The direction is usually determined with the earth current I
value the angle of which is compared to a polarizing quantity (Sub-section 6.5.1). The
desired polarizing signal(s) is set in 32/$5,=$7,21 (address ). The presetting
ZLWK 8R DQG ,< generally also applies when only U
signal. If there is no transformer star-point current I
automatically only U influences the direction determination.
E
If the direction determination must be carried out using only I
setting ZLWK ,< RQO\ is applied. This makes sense if a reliable transformer star-
point current I is always available at the device input I
Y
is then not affected by disturbances in the voltage transformer secondary circuits
provided that the device is equipped with a normal sensitivity current input I
transformer star-point current is connected to I
If direction determination must be carried out using the negative sequence system
the setting ZLWK 8 DQG , is applied. In this case, only the
signals 3I and 3U
2 2
negative sequence system signals computed by the device are used for the direction
determination.
F
D
E
3 × 8LQY PLQLPDO
F
D =
3 × 8LQY PLQLPDO
.
4
>–stage with the same pick-
0
>>>–stage
0
= –3I as the measured
E 0
= 3U is used as a polarizing
E 0
connected to the device,
Y
as reference signal, the
Y
. The direction determination
4
C53000-G1176-C133-1
3U
0
>>–
0
and the
4
7SA6 Manual