ABB Relion REG670, Applications Manual

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15.3 Interlocking
The main purpose of switchgear interlocking is:
• 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 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
Section 15
1MRK 502 071-UEN -
Control
514 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual