ABB Relion 670 Series Applications Manual page 307

1MRK 511 358-UUS A
• To avoid the dangerous or damaging operation of switchgear
• To enforce restrictions on the operation of the substation for other reasons for example, load
configuration. Examples of the latter are to limit the number of parallel transformers to a
maximum of two or to ensure that energizing is always from one side, for example, the high
voltage side of a transformer.
This section only deals with the first point, and only with restrictions caused by switching devices
other than the one to be controlled. This means that switch interlock, because of device alarms, is
not included in this section.
Disconnectors and grounding 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.
Grounding switches are allowed to connect and disconnect grounding of isolated points. Due to
capacitive or inductive coupling there may be some voltage (for example < 40% of rated voltage)
before grounding and some current (for example < 100A) after grounding 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 grounding switch is usually not fully interlocked. The operator must be convinced that the line
is not energized from the other side before closing the grounding switch. As an option, a voltage
indication can be used for interlocking. Take care to avoid a dangerous
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
wrong, and the state is again treated as
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
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
apparatus for both open and close indications. Phase discrepancies will result in an unknown
double indication state.
Application manual
unknown . If both indications stay high, something is
unknown .
enable or release conditions. When the switching state of a switching device
enable condition at the
AND-function for all three phases in each
Section 13
Control
301