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6.6.2
Applying the Function Parameter Settings
General
Earth Fault
Protection
Prerequisites
7SA6 Manual
C53000-G1176-C133-1
The teleprotection supplement for earth fault protection is only operational if it was set
to one of the available modes during the configuration of the device (address ).
Depending on this configuration, only those parameters which are applicable to the
selected mode appear here. If the teleprotection supplement is not required the
address is set to 7HOHSURW () = 'LVDEOHG.
The following teleprotection schemes can be selected:
− 'LU&RPS3LFNXS=Directional Comparison Scheme, as described in Sub-
− 8QEORFNLQJ
= Directional Unblocking Scheme, as described in Sub-section
− %ORFNLQJ
= Directional Blocking Scheme, as described in Sub-section
In address )&7 7HOHS () the application of teleprotection can be switched
21 or 2)).
If the teleprotection has to be applied to a three terminal line the setting in address
must be /LQH &RQILJ = 7KUHH WHUPLQDOV, if not, the setting remains 7ZR
WHUPLQDOV.
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
according to Figure 6-79, the protection at B does not recognize the fault, this would
be interpreted 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. For this
reason, the earth fault protection has an earth current stage ,R0LQ 7HOHSURW
(address ). This stage must be set more sensitive than the earth current stage
used for the teleprotection. The larger the capacitive earth current (I
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 capacitive 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. This setting can only be changed with
®
DIGSI
4 under "Additional Settings".
A
I
EA
Figure 6-79
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 6-80. In this case, the earth current flowing in
section 6.6.1.1,
6.6.1.2,
6.6.1.3.
C
E
I
EC
Functions
in Figure 6-79)
EC
B
= I
– I
I
EB
EA
EC
6-131
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