2 Functions
382
7SD5 Manual
C53000-G1176-C169-1
Figure 2-165 Voltage symmetry monitoring
Broken-wire
Monitoring
During steady-state operation the broken wire monitoring registers interruptions in the
secondary circuit of the current transformers. In addition to the hazardous potential
caused by high voltages in the secondary circuit, this kind of interruptions simulates
differential currents to the differential protection, such as those evoked by faults in the
protected object.
The broken-wire monitor scans the current of each phase and picks up when the
current drops abruptly to zero (from >0.1·I
N
), while no corresponding drop appears in
the earth current. The differential protection is blocked immediately in the relevant
phase. This blocking has an impact on all ends of the protected object. The device
issues the message „Broken wire“ indicating also the affected phase.
The blocking is cancelled as soon as the device is again supplied with current in the
relevant phase. It is also suppressed as long as a high fault current is registered by
any device of the differential protection system.
Note that electronic test devices do not simulate the correct behaviour of broken wire
so that pickup may occur.
Note
Broken wire monitoring can operate properly only when the fourth current input (II
4
) of
the relay is fed the residual current from a separate current transformer of the protect-
ed line, or no residual current at all.
Voltage Phase Se-
quence
The verification of the faulted phases and the phase preference, direction measure-
ment and polarization with quadrature voltages usually demand clockwise rotation of
the measured values. Phase rotation of measured voltages is checked by verifying the
phase sequences of the voltages
U
L1
before U
L2
before U
L3
. This check takes place if each measured voltage has a minimum magnitude of
|U
L1
|, |U
L2
|, |U
L3
| > 40 V/√3
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