Substation Interlocking explained

Substation Interlocking explained
Interlocking schemes shall cover the following conditions:-
(a) Interlocking between circuit breakers and disconnectors to ensure disconnectors do
not operate outside their rating i.e. make or break load currents.
(b) Interlocking between disconnectors and earthing switches to ensure that earthing
switches cannot be closed on to a locally energised circuit and cannot be energised,
when closed, by operation of disconnectors.
Interlocking between disconnectors and adjacent earthing switches to permit operation
of the disconnector when earthing switches are closed on both sides of the
disconnector. Such interlocking is not required for equipment rated at 145kV and
below.
(d) To ensure correct sequence of on load busbar transfer switching operations at
multiple busbar substations.
(e) To ensure that a bus-coupler or bus-section circuit breaker is only closed with its
associated disconnectors are both open or both closed.
(f) For equipment at sites where NGET is the Occupier, to restrict access to areas of the
substation where safety clearances may be infringed unless appropriate safety
measures, such as isolation and earthing, have been taken
1.1.4 The interlocking of switching sequences involving only power operated switchgear shall be
by electrical means. The correct interlocking status shall be confirmed automatically on
initiation of an operation from any control position or from auto-switching or sequentialisolation equipment.
1.1.5 The interlocking of switching sequences involving manually operated switchgear may be by
electrical or mechanical means. The interlocking shall be designed such that the correct
interlocking status must be confirmed immediately before an operation.
1.1.6 Interlocking systems shall, where reasonably practicable, be fail-safe. They shall not be
defeated without the use of tools, clip leads etc. or a purpose designed override facility.
1.1.7 Interlock override facilities shall be lockable with a unique lock or shall be lockable by means
of a safety padlock.
1.1.8 Partial interlocking of earthing switches at circuit-entries to the substation is acceptable
where it is not reasonably practicable to extend the interlocking to the remote end
disconnectors. Any partially interlocked earthing switch shall be provided with a warning
label stating 'WARNING, THIS EARTHING SWITCH IS NOT FULLY INTERLOCKED'.
1.1.9 Interlocking shall be effective for switching and operating sequences when they are being
followed in either direction (for example; if an earthing switch must be closed before an
access gate can be opened then the gate must be secured closed before the earthing switch
can be opened).
1.1.10 Interlocking schemes shall, where reasonably practicable, provide the maximum operational
flexibility and shall not unnecessarily impose fixed operating sequences.
1.1.11 Where an interlocking scheme is being supplied for an extension to an existing substation
at the same operating voltage then, unless otherwise agreed by NGET, the interlocking
philosophy shall match that existing.
1.1.12 Interlocking for a substation extension shall be fully interfaced with the existing interlocking
scheme to achieve the functional requirements specified in this document.
1.1.13 Interlocking may, in certain circumstances, have to be by-passed by auto-reclose schemes.
The requirements for these are specified in TS 3.24.16. (RES).
1.2 Mechanical Interlocking
1.2.1 Mechanical interlocking systems shall be designed to provide a level of security and
reliability comparable with equipment specified in Clauses 1.2.2 to 1.2.6 below.
1.2.2 Mechanical interlocking shall be by key operated systems
1.2.3 Interlock keys shall be of a non-masterable design (i.e. no master key can be supplied or
manufactured). Differs shall not be repeated on the same substation site.
Note: Differ is the term for the difference in a key which prevents it being
interchangeable with another.
1.2.4 Interlock keys shall be engraved with an identifying reference which shall be unique to that
substation site. The identifier shall, where appropriate, include the system number of the
switching device where the key is located during normal operation. Key locations shall be
marked with the identifier of the required key.
1.2.5 Where key exchange boxes are provided they shall be located in convenient positions with
regard to normal substation operating sequences.
1.2.6 Where mechanical key interlocking is fitted to disconnector and earthing switch mechanisms
the requirements specified in TS 3.2.2 (RES) shall apply.
1.3 Electrical Interlocking
1.3.1 Electrical interlocking systems shall be designed to provide a level of security and reliability
comparable with equipment specified in Clauses 1.3.2 .and 1.3.3 below.
1.3.2 A facility shall be provided to allow the interlock system of each disconnector or earthing
switch to be defeated without disturbing wiring. The facility shall meet the requirements of
Clause 1.1.7

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