2006 Muttrah Incident Report - authority for electricity regulation, oman

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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 10
outages on the distribution system in order to connect the existing feeders to the new switchboard; or b) Replace only the moveable portion of the switchboard (‘the truck’) containing the new circuit breaker. Only minor modifications are normally required to the fixed portion. This second option is customarily known in the switchgear industry as ‘retrofitting’. 11
Page 1 and 2: REPORT INTO THE INCIDENT AT MUTTRAH
Page 3 and 4: GLOSSARY OF TERMS The terms defined
Page 5 and 6: 1 INTRODUCTION AND SUMMARY OF FINDI
Page 7 and 8: 2 SCOPE OF INVESTIGATION AND METHOD
Page 9: 2.3 Methodology In preparing its re
Page 13 and 14: FIGURE 2 Brush OCB Truck removed Mu
Page 15 and 16: contracts were subsequently awarded
Page 17 and 18: 5 SEQUENCE OF EVENTS The sequence i
Page 19 and 20: The protection relay indications fo
Page 21 and 22: BUSHING SHUTTER FIGURE 4 Reconstruc
Page 23 and 24: FIGURE 7 Truck - Cubicle Interface
Page 25 and 26: the shutter mechanism is mounted is
Page 27 and 28: 6.3 Cause of the Incident Based on
Page 29 and 30: shortcomings in the trucks supplied
Page 31 and 32: Subsequent to the incident it emerg
Page 33 and 34: • That the design defects identif
Page 35: 10 RECOMMENDATIONS The inquiry make

2006 Muttrah Incident Report - authority for electricity regulation, oman

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