2 Functions
200
7SD5 Manual
C53000-G1176-C169-1
Figure 2-85
Transient blocking for permissive schemes
2.7.13 Measures for Weak and Zero Infeed
In cases where there is weak or no infeed present at one line end, the distance pro-
tection will not pick up. Neither a trip nor a send signal can therefore be generated
there. With the comparison schemes, using a permissive signal, fast tripping could not
even be achieved at the line end with strong infeed without special measures, as the
end with weak infeed does not transmit a permissive release signal.
To achieve fast tripping at both line ends in such cases, the distance protection pro-
vides special supplements for feeders with weak infeed.
To enable the line end with the weak infeed condition to trip independently, 7SD5 has
a special tripping function for weak infeed conditions. As this is a separate protection
function with a dedicated trip command, it is described in a separate section (refer to
Section 2.10.1).
Echo Function
Figure 2-86 shows the method of operation of the echo function. At address
2501
FCT
Weak Infeed
(weak infeed FunCTion) can be activated (
ECHO only
) or deactivated
(
OFF
). By means of this „switch“ the weak infeed tripping function can also be activat-
ed (
ECHO and TRIP
, refer also to Section 2.10.1). This setting is common to the tele-
protection function for the distance protection and for the earth fault protection.
If there is no fault detection, the echo function causes the received signal to be sent
back to the other line end as an „echo“, where it is used to initiate permissive tripping.
The detection of the weak infeed condition and accordingly the requirement for an
echo are combined in a central AND gate. The distance protection must neither be
switched off nor blocked, as it would otherwise always produce an echo due to the
missing fault detection. If, however, the time delayed overcurrent protection is used as
an emergency function, an echo is nevertheless possible if the distance protection is
out of service, because the fault detection of the emergency overcurrent protection re-
places the distance protection fault detection. During this mode of operation, the emer-
gency overcurrent protection must naturally not also be blocked or switched off.
Even when the emergency overcurrent protection does not pick up an echo is created
for permissive release scheme during emergency function. The time overcurrent pro-
tection at the weaker end must operate with more sensitivity than the distance protec-
tion at the end with high infeed. Otherwise the selectivity concerning 100% of the line
length is not given.
The essential condition for an echo is the absence of distance protection or overcur-
rent protection fault detection with the simultaneous reception of a signal from the tele-
protection scheme logic, as shown in the corresponding logic diagrams (Figure 2-73
or 2-77).
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