Siemens siprotec 7SA522 - Page 284

Functions
6-136 7SA522 Manual
C53000-G1176-C119-2
Stub Protection A further overcurrent stage is the stub protection. It can however also be used as a
normal additional definite time overcurrent stage, as it functions independent of the
other stages.
A stub fault is a short-circuit located between the current transformer set and the line
isolator. It is of particular importance with the 1
1
/
2
–circuit breaker arrangement (Figure
6-79).
Figure 6-79 Stub fault at an 1
1
/
2
–circuit breaker arrangement
If a short circuit current I
A
and/or I
B
flows while the line isolator 1 is open, this implies
that a fault in the stub range between the current transformers I
A
, I
B
, and the line iso-
lator exists. The circuit breakers CBA and CBC which carry short-circuit current may
be tripped without delay.The two sets of current transformers are connected in parallel
such that the current sum I
A
+ I
B
represents the current flowing towards the line iso-
lator.
The stub protection is an overcurrent protection which is only in service when the state
of the line isolator indicates the open condition via a binary input “!,678%(1$%/(”.
The binary input must therefore be operated via an auxiliary contact of the isolator. In
the case of a closed line isolator, the stub protection is out of service. In Figure 6-80
shows the logic diagram.
If the stub protection stage is to be used as a normal definite time overcurrent stage,
the binary input “!%/2&.,678%”, should be left without allocation or routing (ma-
trix). The enable input “!,678%(1$%/(”, however, has to be constantly activated
(either via a binary input or via integrated logic (CFC) functions which can be config-
ured by the user.
Busbar A
busbar B
feeder 1feeder 2
stub fault
I
A
I
B
I
k
= I
A
+I
B
line isolator 1
line isolator 2
CBA
CBB
CBC