Siemens SIPROTEC 7SD80 Manual page 104

Functions
2.6 Circuit-Breaker Failure Protection 50BF
In-Phase Start
Common phase initiation is used for transformer feeders or if the busbar protection trips.
If the breaker failure protection is intended to be initiated by further external protection devices, it is recom-
mended, for security reasons, to connect two starting criteria to the device. Besides the trip command of the
external relay to the binary input „>50BF Start 3p" (FNo. 1415) it is recommended to connect also the
general device pickup to binary input „>50BF release" (FNo. 1432). For Buchholz protection it is recom-
mended that the trip command is connected to the device by two separate wire pairs.
Nevertheless, it is possible to initiate the breaker failure protection in single-channel mode should a separate
release criterion not be available. The binary input „>50BF release" (FNo. 1432) must then not be assigned
to any physical input of the device during configuration.
Figure 2-38 shows the operating principle. When the trip signal appears from any internal or external feeder
protection and at least one current flow criterion (according to Figure 2-36) is present, the breaker failure pro-
tection is initiated and the corresponding time delay(s) is (are) started.
If the current criterion is not satisfied for any of the phases, the circuit-breaker auxiliary contact can be interro-
gated according to figure 2-37. After a 3-pole trip command, the circuit breaker has only operated correctly
when no current flows over any of the poles.
Figure 2-37 shows the generation of the internal signal „52 closed" (see figure 2-38 left) if at least one breaker
pole is closed.
Using binary input 1424 „>50BFSTRTonlyT2", the trip time delay 3906 50BF-2 Delay can be started. After
it has elapsed, the breaker failure protection TRIP command 1494 „50BF BusTrip" is generated.
Figure 2-37 Generation of the signal "CB aux closed"
If an internal protection function or an external protection device trips without current flow, the breaker failure
protection is initiated by the internal input „Start internal w/o I", if the trip signal comes from the internal voltage
protection or frequency protection, or by the external input „>50BF STARTw/oI". In this case the start signal
is maintained until the circuit breaker is signaled to be open by the auxiliary contact criterion.
Initiation can be blocked via the binary input „>BLOCK 50BF" (e.g. during test of the feeder protection relay).
104
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011