2 Functions
196
7SA522 Manual
C53000-G1176-C155-3
2.9 Weak-infeed tripping
In cases, where there is no or only weak infeed present at one line end, the distance
protection does not pick up there during a short-circuit on the line. The settings and
information table at “Weak Infeed” applies for the following functions.
If there is no or only a very small zero sequence current at one line end during an earth
fault, the earth fault protection can also not function.
2.9.1 Classical Tripping
2.9.1.1 Method of Operation
Transmission
Schemes
By coordinating the weak infeed function with the teleprotection in conjunction with
distance protection and/or earth fault protection, fast tripping can also be achieved at
both line ends in the above cases.
At the strong infeed line end, the distance protection can always trip instantaneously
for faults inside zone Z1. With permissive teleprotection schemes, fast tripping for
faults on 100°% of the line length is achieved by activation of the echo function (refer
to Subsection 2.6). This provides the permissive release of the trip signal at the strong
infeed line end.
The permissive teleprotection scheme in conjunction with the earth fault protection
can also achieve release of the trip signal at the strong infeed line end by means of
the echo function (refer to Subsection 2.8).
In many cases tripping of the circuit breaker at the weak infeeding line end is also de-
sired. For this purpose the device 7SA522 has a dedicated protection function with
dedicated trip command.
Pickup with Under-
voltage
In Figure 2-88 the logic diagram of the weak-infeed tripping is shown. At address 2501
)&7:HDN,QIHHG (Weak
Infeed FunCTion) can be activated ((&+2DQG75,3) or
deactivated (2))). If this “switch” is set to (&+2RQO\, the tripping is also disabled;
however the echo function to release the infeeding line end is activated (refer also to
Subsection 2.6 and 2.8). The tripping function can be blocked at any time via the
binary input ´!%/2&.:HDN,QIµ.
The logic for the detection of a weak-infeed condition is built up per phase in conjunc-
tion with the distance protection and additionally once for the earth fault protection.
Since the undervoltage check is performed for each phase, single-pole tripping is also
possible, provided the device version has the single-pole tripping option.
In the event of a short circuit, it may be assumed that only a small voltage appears at
the line end with the weak-infeed condition, as the small fault current only produces a
small voltage drop in the short-circuit loop. In the event of zero-infeed, the loop voltage
is approximately zero. The weak-infeed tripping is therefore dependent on the mea-
sured undervoltage 81'(592/7$*( which is also used for the selection of the faulty
phase.
If a signal is received from the opposite line end without fault detection by the local
protection, this indicates that there is a fault on the protected feeder. In the case of
three terminal lines when using a permissive overreach scheme a receive signal from
both ends may be present. In case of permissive underreach schemes one receive
signal from at least one end is sufficient.
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