Before the end of June we should see some conclusions from the ministerial inquiry into the spectacular power supply problems that created havoc in the central business district of Auckland earlier this year. Technical reports commissioned for the inquiry (from Integral Energy Australia) have just been issued. These and the inquiry's final report will be required reading for anyone running a power supply system dependent on ageing underground cables.
Over a period of 29 days, all four 110 kV cables serving the area failed. Some of the factors involved were peculiar to the Auckland power supply system. But the repercussions of this unhappy episode are likely to be felt well beyond New Zealand.
The first two cables to fail were of a gas-filled design, with an unusual rigid aluminium sheathing. Installed in 1958, these cables, rated at 50 MVA each, had a history of unreliability and, according to the Integral Energy report had been "installed contrary to good engineering practice", with inadequate ground stability at a couple of points. It was therefore perhaps unlucky, but not a complete surprise, when the second of these older cables failed 18 days after the first.
Totally unexpected was the failure of the second pair of cables, rated at 60 MVA each, which were installed in 1974 and had operated reliably. These are oil-filled, with corrugated aluminium sheathing.
It now appears that defective installation played a role in the failure – "due to thermomechanical reasons" – of the first of these newer cables, when it was carrying 50 MVA. The second of the pair, taking on additional load, failed the following day, due to "thermal runaway". It was carrying a load close to its rating – perhaps for the first time in its life. But that is of course no excuse for failure.
The end result was that the central business district was left with about 20 MVA of capacity to cover its 170 MW load – causing disruption to businesses (some offices temporarily relocating to Australia) and requiring rapid deployment of emergency generators on a huge scale. Diesel generators were airlifted in by Antonov transport plane from abroad, clusters of diesel generators were set up on footpaths and in parking spaces, and an "instant diesel power station" appeared in the university car park. A gas turbine driven ship moored in Auckland Port was drafted in to help. The final cost to the electricity distribution company, Mercury Energy, of providing these emergency supplies and compensating customers has been estimated at well over $NZ 120 million.
Perhaps the key contributor to the debacle was that the thermal resistivity of the sand in which the cables were bedded was severely underestimated when the cables were originally laid. This meant that cable temperatures were much higher than expected for a given load. At the time of the failures Auckland was experiencing extremely hot weather and therefore high electricity demand for air conditioning.
The technical report criticises Mercury's cable maintenance approach, but it is debatable whether this was a primary factor. Neither is there much hard evidence that Mercury management's recent preoccupation with commercial matters, such as acquisition of Power New Zealand and impending flotation, had diverted it from the less glamorous issue of supply reliability, thereby contributing to the disaster. In fact, with steadily increasing load growth the company had been endeavouring for some time to strengthen supply to the central business district and in 1994 had decided to build a 10 km tunnel into the region, initially to take two 230 MVA 110 kV cables, but with space for additional capacity. Had this project been completed the fiasco could have been averted. But the tunnel has been subject to major delays, due to factors mainly beyond the control of Mercury Energy (and is now due to be completed by the end of 1999).
So what can be learned? Bryan Leyland of New Zealand based Leyland Consultants, who worked as a junior engineer on the 1958 cables and is something of an authority on the Auckland failure, suggests that "suppliers everywhere need to reassess their system security, cable ratings and emergency planning." He recommends, among other things, that if you have old cables try to work out where the hot spots might be, do some temperature measurements and check the properties of the material in which the cable is laid. That is sensible advice. Above all, avoid the attitude that "it couldn't happen here."
Linkedin
