Siemens siprotec 7SD5 User Manual

2.9 Teleprotection for Earth Fault Protection (optional)
245
7SD5 Manual
C53000-G1176-C169-1
The following mode is possible with digital transmission using the protection data in-
terface:
Dir.Comp.Pickup = Directional comparison pickup,
The send and receive signals must in this case be assigned to fast command channels
of the protection data interface (DIGSI matrix).
Earth Fault Protec-
tion Prerequisites
In the application of the comparison schemes, absolute care must be taken that both
line ends recognize an external earth fault (earth fault through-current) in order to
avoid a faulty echo signal in the case of the permissive schemes, or in order to ensure
the blocking signal in the case of the blocking scheme. If, during an earth fault accord-
ing to Figure 2-109, the protection at B does not recognize the fault, this would be in-
terpreted as a fault with single-sided infeed from A (echo from B or no blocking signal
from B), which would lead to unwanted tripping by the protection at A. Therefore, the
earth fault protection features an earth fault stage 3IoMin Teleprot (address
3105). This stage must be set more sensitive than the earth current stage used for the
teleprotection. The larger the capacitive earth current (I
EC
in Figure 2-109) is, the
smaller this stage must be set. On overhead lines a setting equal to 70 % to 80 % of
the earth current stage is usually adequate. On cables or very long lines where the ca-
pacitive currents in the event of an earth fault are of the same order of magnitude as
the earth fault currents, the echo function should not be used or restricted to the case
where the circuit breaker is open; the blocking scheme should not be used under these
conditions at all.
Figure 2-109 Possible current distribution during external earth fault
On three terminal lines (teed feeders) it should further be noted that the earth fault
current is not equally distributed on the line ends during an external fault. The most
unfavourable case is shown in Figure 2-110. In this case, the earth current flowing in
from A is distributed equally on the line ends B and C. The setting value 3IoMin
Teleprot (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 2-110 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.
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Siemens siprotec 7SD5 Specifications

General

Brand	Siemens
Model	siprotec 7SD5
Category	Relays
Language	English