ABB RET670 Applications Manual page 622

Section 3
IED application
616
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 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.
1MRK504116-UUS C
Application manual