Earthing switches are allowed to connect and disconnect earthing of isolated points.
Due to capacitive or inductive coupling there may be some voltage (for example <
40% of rated voltage) before earthing and some current (for example < 100A) after
earthing of a line.
Circuit breakers are usually not interlocked. Closing is only interlocked against
running disconnectors in the same bay, and the bus-coupler opening is interlocked
during a busbar transfer.
The positions of all switching devices in a bay and from some other bays determine the
conditions for operational interlocking. Conditions from other stations are usually not
available. Therefore, a line earthing switch is usually not fully interlocked. The
operator must be convinced that the line is not energized from the other side before
closing the earthing switch. As an option, a voltage indication can be used for
interlocking. Take care to avoid a dangerous enable condition at the loss of a VT
secondary voltage, for example, because of a blown fuse.
The switch positions used by the operational interlocking logic are obtained from
auxiliary contacts or position sensors. For each end position (open or closed) a true
indication is needed - thus forming a double indication. The apparatus control function
continuously checks its consistency. If neither condition is high (1 or TRUE), the
switch may be in an intermediate position, for example, moving. This dynamic state
may continue for some time, which in the case of disconnectors may be up to 10
seconds. Should both indications stay low for a longer period, the position indication
will be interpreted as unknown. If both indications stay high, something is wrong, and
the state is again treated as unknown.
In both cases an alarm is sent to the operator. Indications from position sensors shall
be self-checked and system faults indicated by a fault signal. In the interlocking logic,
the signals are used to avoid dangerous enable or release conditions. When the
switching state of a switching device cannot be determined operation is not permitted.
10.5.3 Configuration guidelines
The following sections describe how the interlocking for a certain switchgear
configuration can be realized in the IED by using standard interlocking modules and
their interconnections. They also describe the configuration settings. The inputs for
delivery specific conditions (Qx_EXy) are set to 1=TRUE if they are not used, except
in the following cases:
• QB9_EX2 and QB9_EX4 in modules BH_LINE_A and BH_LINE_B
• QA1_EX3 in module AB_TRAFO
when they are set to 0=FALSE.
10.5.4 Interlocking for busbar earthing switch BB_ES
Section 10 1MRK 505 291-UEN A
Control
238
Application manual
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