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Section 3
IED application
1MRK504089-UEN C
Disconnectors and earthing switches have a limited switching capacity.
Disconnectors may therefore only operate:
•
•
With basically zero current. The circuit is open on one side and has a small
extension. The capacitive current is small (for example, < 5A) and power
transformers with inrush current are not allowed.
To connect or disconnect a parallel circuit carrying load current. The switching
voltage across the open contacts is thus virtually zero, thanks to the parallel
circuit (for example, < 1% of rated voltage). Paralleling of power transformers
is not allowed.
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.
For switches with an individual operation gear per phase, the evaluation must
consider possible phase discrepancies. This is done with the aid of an ANDfunction for all three phases in each apparatus for both open and close indications.
Phase discrepancies will result in an unknown double indication state.
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Application manual