Siemens siprotec 7SA6, Manual

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Functions
6-152
7SA6 Manual
C53000-G1176-C156-2
Figure 6-87 Possible current distribution during external earth fault
On three terminal lines (teed feeders) it should further be noted that the earth fault cur-
rent is not equally distributed on the line ends during an external fault. The most unfa-
vourable case is shown in Figure 6-88. In this case, the earth current flowing in from
A is distributed equally on the line ends B and C. The setting value
3IoMin Tele-
prot
(address
3105
), which is decisive for the echo or the blocking signal, must
therefore be set smaller than one half of the setting value for the earth current stage
used for teleprotection. In addition, the above comments regarding the capacitive
earth current which is left out in Figure 6-88 apply. If the earth current distribution is
different from the distribution assumed here, the conditions are more favourable as
one of the two earth currents
I
EB
or
I
EC
must then be larger than in the situation de-
scribed previously.
Figure 6-88 Possible unfavourable current distribution on a three terminal line during an
external earth fault.
Time Settings
The send signal prolongation
Send Prolong.
(address
3203
) must ensure that the
transmitted signal reliably reaches the opposite line end, even if tripping is very fast at
the sending line end and/or the signal transmission time (channel delay) is relatively
long. In the case of the permissive schemes
Dir.Comp.Pickup
and
UNBLOCKING
this signal prolongation only comes into effect if the device has already issued a trip
command. This ensures the release of the other line end even if the fault is cleared
very rapidly by a different protection function or other stage. In the case of the blocking
scheme
BLOCKING
the transmit signal is always prolonged by this time. In this case it
corresponds to a transient blocking following a reverse fault. This setting can only be
modified with DIGSI
®
4 under “Additional Settings”.
The release of the directional tripping can be delayed by means of the permissive sig-
nal delay
Release Delay
(address
3208
). This is only required for the blocking
scheme
BLOCKING
to allow sufficient transmission time for the blocking signal during
external faults. This delay only has an effect on the receive circuit of the teleprotection;
conversely tripping by the comparison protection is not delayed by the set time delay
of the directional stage.
A B
I
EC
C
E
I
EB
= I
EA
– I
EC
I
EA
A B
I
EC
I
EB
I
EA
= I
EB
+ I
EC