2006 Muttrah Incident Report - authority for electricity regulation, oman

3 BACKGROUND – RETROFITTING OF SWITCHGEAR
Many thousands of bulk-oil circuit breakers (OCB) have been in service since the early
1960s and formed an integral part of most distribution substations at the 33 KV and 11
KV levels. Many of these circuit breakers are approaching or have exceeded twenty-five
years operational service life. In general, oil-filled switchgear has a proven record of
reliability and performance. Failures are rare but, where they occur, the results may be
catastrophic. Tanks may rupture, resulting in the ejection of burning oil and gas clouds,
causing death or serious injury to persons and major damage to plant and buildings in
the vicinity of the failed equipment.
Within the Medium Voltage (MV) switchgear sector, there is now increasing interest in
the options for the economic replacement of ageing oil based switchgear with switchgear
that utilises non-flammable substances (Vacuum or SF6) as the insulating and breaking
medium.
A typical MV switchboard consists of two parts;
(i)
A fixed portion (‘the cubicle’) containing the busbars, feeder terminations,
protection equipment, voltage and current transformers etc. The cubicle also
contains the safety shutters which are operated by the action of racking in the
moveable portion.
(ii)
A moveable or withdrawable portion (‘the truck or carriage’) which contains the
circuit breaker itself together with facilities for connecting the circuit breaker to the
busbars or feeder circuits.
For the replacement of time-expired switchgear there are two options available:
a) Replace the whole switchboard with one of modern design. This involves the
replacement of both the fixed and moveable portions of the switchboard by modern
equipment. This option is expensive in terms of capital cost: it also requires extensive
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