Siemens SIPROTEC 7SD610 Manual page 203

Broken wire monitoring
During steady-state operation the broken wire monitoring detects interruptions in the secondary circuit of the
current transformers. In addition to the hazard potential caused by high voltages in the secondary circuit, this
kind of interruption causes differential currents to the differential protection, such as those evoked by faults in
the protected object.
The broken wire monitoring function monitors the local phase currents of all three phases and the results of the
broken wire monitoring supplied by the devices on the other ends of the protected object. At each sampling
moment, the function checks whether there is a jump in one of the three phase currents; if there is, it generates
the „suspected wire break" signal.
There is a suspected local wire break if a jump has been detected in the affected phase and the current has
dropped to 0 A.
In case of a 1-1/2 circuit breaker arrangement, the current will not necessarily jump to 0 in case of a wire break
because the second primary current transformer will continue to measure one part of the phase current; this
means that the current of the affected phase will simply jump to a different value. For such circuit breaker ar-
rangements, parameter 2935 ΔI min is used as the criterion. This setting value specifies the difference that
must exist between the measured current values before and after the jump in order to detect a suspected local
wire break. Local wire break detection is only possible if the current amplitude after the jump is lower than the
current amplitude before the jump.
WARNING!
If the CT secondary circuits are inadvertently opened while the broken wire monitoring is activated, the differ-
ential protection is phase-selectively blocked and will be no longer able to trip!
This state may give rise to hazardous overvoltages in the open CT circuit, which will not lead to a trip because
the differential protection is blocked.
Figure 2-86 Generating the suspected local wire break information
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
Functions
2.15 Monitoring Functions
203