Functions
2.11 Measures for Weak and Zero Infeed
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
275
2.11.2
Classical Tripping
2.11.2.1 Method of Operation
Teleprotection 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 acti-
vation of the echo function (see section 2.7).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 Section 2.9).
In many cases tripping of the circuit breaker at the weak infeeding line end is also desired. For this purpose the
device 7SD5 has a dedicated protection function with dedicated trip command.
Pickup with undervoltage
In Figure 2-141, the logic diagram of the weak-infeed tripping is shown. The function can be activated (
ECHO
and TRIP
and
Echo &Trip(I=0)
) or deactivated (
OFF
) in address 2501
FCT Weak Infeed
(Weak Infeed
FunCTion). If this „switch“ is set to
ECHO only
, the tripping is also disabled; however, the echo function to
release the infeeding line end is activated (refer also to Section 2.7 and 2.9). The tripping function can be
blocked at any time via the binary input
„>BLOCK Weak Inf“
.
The logic for the detection of a weak-infeed condition is built up per phase in conjunction with the distance pro-
tection 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 measured undervoltage
UNDERVOLTAGE
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 comparison
scheme a receive signal from both ends may be present. In the case of underreach schemes one receive signal
from at least one end is sufficient.
After a security margin time of 40°ms following reception of the receive signal, the weak-infeed tripping is re-
leased if the remaining conditions are satisfied: undervoltage, circuit breaker closed and no pickup of the dis-
tance protection or of the earth fault protection.
To avoid a faulty pickup of the weak infeed function following tripping of the line and reset of the fault detection,
the function cannot pick up anymore once a fault detection in the affected phase was present (RS flip-flop in
Figure ).
In the case of the earth fault protection, the release signal is routed via the phase segregated logic modules.
Single-phase tripping is therefore also possible if both distance protection and earth fault protection or exclu-
sively earth fault protection issues a release condition.
