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Interlocking
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ing switch. As an option, a voltage indication can be used for interlocking. Take care to
avoid a dangerous HQDEOH condition at loss of VT secondary voltage, for example, be-
cause of a blown fuse.
The switch positions used in the operational interlocking logic are obtained from aux-
iliary contacts or position sensors. For each end position (open or close) a true indica-
tion 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 intermediate position, for example, moving. This moving state may be in
progress a certain time, for example, 10 seconds for disconnectors. Should both indica-
tions stay low for a longer time, the position indication is interpreted as XQNQRZQ. If
both indications stay high, then there is something wrong, and the state is again treated
as XQNQRZQ. In both cases an alarm is given to the operator. Indications from position
sensors shall be self-checked and system faults indicated by a fault signal. In the inter-
locking logic, the signals are used to avoid dangerous HQDEOH or UHOHDVH of operation
conditions. Unknown positions are not allowed to release operation.
For switches with individual operation gear per phase, the evaluation must consider
possible phase discrepancies. This is done by an $1'IXQFWLRQ for all three phases of
respectively open and close indication of each apparatus. This leads to an unknown dou-
ble indication state in case of phase discrepancies.
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The interlocking function consists of software modules located in each control terminal.
The function is distributed and not dependent on any central function. Communication
between modules in different bays is performed via the station bus.
The basic method is the use of the reservation function (see the section of apparatus
control). This method ensures that the position of the HV apparatuses are not changed
during the time gap, which arises between the position updatings. This can occur by re-
serving all of those HV apparatuses by means of the communication system, which
might influence the interlocking condition of the intended operation. The reservation is
intended to remain until the operation is performed.
After the selection and reservation of an apparatus, the function has full information on
all positions of the apparatuses in the switchyard that are affected by the selection. This
status cannot be affected by other operators because the selection is blocked for all ap-
paratuses of which the status is important for the selected one.
The positions of the HV apparatuses are inputs to software modules distributed in the
control terminals. Each module contains the interlocking logic for a bay. The interlock-
ing logic in a module is different, depending on the bay function and the switchyard ar-
rangements, that is, double-breaker or 1 1/2 breaker bays have different modules.
237
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