ABB REC 561 User Manual page 386

ABB Network Partner AB
Version 1.0-00
October 1996
Function:
1 Introduction
Interlocking
Basic
The interlocking of switchgear operation can have two main purposes to:
• Avoid dangerous or damaging operation of switchgear
• Put restrictions on the configuration of the substation or total switch-
gear for operational reasons. 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 interlocking document only deals with the first point, and only with
restrictions caused by switching devices other than that one to be control-
led. 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 extension. The capacitive current is small (for example < 5A)
and power transformers with inrush current are not allowed. Capaci-
tive and magnetic voltage transformers (VT) are 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 iso-
lated 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 to prevent damage. Closing is
only interlocked against running disconnectors in the same bay, and the
bus-coupler opening is interlocked during a busbar 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 earthing switch. As an
option, a voltage indication can be used for interlocking. Take care to
avoid a dangerous enable condition at loss of VT secondary voltage, for
example, because of a blown fuse.
The switch positions used in the operational interlocking logic are
obtained from auxiliary contacts. For each position an 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 intermediate position, for example, moving.
This moving state may last a certain time, for example, 10 seconds for dis-
connectors. Should both indications stay low for a longer time, the state is
1MRK 580 151-XEN
6 -101
Page