2006 Muttrah Incident Report - authority for electricity regulation, oman

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Note the decreased clearance between the bushing and the contact conductor on the ABB VCB truck. The KEMA Report also raises a number of other issues concerning the design of the retrofit VCB trucks. These other design shortcomings are not thought to be a contributing cause to the initial fault: • The construction of the cylindrical part of the moving isolating contact fingers (‘tulip’) differs from the original Brush design: it is of a different shape (not conical as in the original) and tightened with non-original bolts (metric instead of imperial thread) and additional washers. These were the parts machined at Wadi Khabir (see Sequence of Events above). It was also noted that the original (800 Amp) cylinders were silver plated: this plating has been removed by the machining. The holes in the copper conductors for mounting the cylindrical part of the isolating contacts are oversized, probably for adjustment purposes. This may affect the contact resistance and thus the current withstand capability of the isolating contacts. • The design and construction of the retrofit breaker, supplied as a prototype in 2005 and installed in the Bus Coupler bay at Al-Falaj, differs in several material respects from those delivered in 2006 e.g.: different dimensions of isolating contacts (400 A and 800 A), steel truck, VCB, isolator discs, conductors and height of the copper conductors. • During the site inspection a new retrofit VCB could not be inserted into the cubicle because the truck appeared to be stuck against vital parts of the shutter operating mechanism; pushing further would have damaged this latching mechanism. See Photograph 7 below. • An attempt was made to rack in the failed VCB truck in the Al-Harthy feeder bay. The construction appeared to be very critical, because the gap between the truck and linkage of the shutter mechanism was less than 1 mm. Racking in with force and without careful positioning could have resulted in damage to the operating mechanism of the shutters. In addition the side plate on which the bracket to activate 24
the shutter mechanism is mounted is much weaker than on the original OCB truck, which makes the operation of the shutters less reliable. • After the incident all the retrofit VCB trucks were racked out and replaced by the original OCBs. After withdrawal of the breaker truck in the Oman House feeder at Muttrah by engineers of MDEC a broken pin of the latching mechanism of one shutter was found. FIGURE 8 Interference with shutter mechanism. New VCB Truck Failed VCB Truck 6.2 The ABB Italy Report The inquiry was given access to an internal Report prepared by ABB Italy. Since the report was internal to ABB it is not possible to reproduce it here. However the inquiry is able to summarise the broad findings of the ABB Report. The ABB Italy report identifies the following design deficiencies: Contact arms a) The contact arms are not symmetrical in respect of the center of the bushing. Considering the bending tolerances the contact arms may have interference with the internal diameter of the bushing. b) The contact arms are made by a copper bar of rectangular cross section 10x50mm with the edge radius of 2mm. Such edge radius is not ideal for the heat-shrink insulation, increasing the possibility of damage of the insulation during the assembling, packaging 25
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 and 10: 2.3 Methodology In preparing its re
Page 11 and 12: outages on the distribution system
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: FIGURE 7 Truck - Cubicle Interface
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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