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236
Interlocking
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The interlocking of switchgear operation can have two main purposes:
•
To avoid dangerous or damaging operation of switchgear
•
To put restrictions on the operation of the substation for other reasons e.g. load con-
figuration. Examples of the latter are to limit the number of parallel transformers to
a maximum of two or to assure that energizing is always made from one side, for
example, the high voltage side of a transformer.
This document only deals with the first point, and only with restrictions caused by
switching devices other than that one to be controlled. This means that switch interlock,
because of device alarms, is not part of this document.
Disconnectors and earthing switches have a limited switching capacity. Disconnectors
are allowed to operate:
•
With basically zero current. The circuit is open at one side and has a small exten-
sion. 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 al-
lowed.
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 bus-
bar transfer.
As conditions for operational interlocking, the positions of all switching devices of a
bay and from some other bays are used. Conditions from other stations are usually not
available. So 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 earth-
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