Quick Dips

Blame the Weather!

Warwick Bishop

This summer's record-breaking heat wave may turn out to be one of the main culprits behind the Auckland CBD power crisis.

Two of these cables have been operating for nearly 40 years with reportedly only three major faults in total. Thus the mean-time-between-failures is over 25 years per cable. For even two of them to have failed out of the blue and at the same time is statistically unbelievable. The fact that all four have failed leads one to look for another reason.

The recent spell of record hot and dry weather in Auckland has had a number of side effects -- the well publicised record rise in the temperature of the Waikato River being but just one. The National Institute of Water and Atmospheric Research says that soil temperatures in the region have risen by up to 5^oC above normal even at depths of one metre.

At the same time the ground water table has fallen to all time record lows in many parts of the city. All of the four cables are buried underground and their bedding material is designed to hold moisture which in turn helps conduct heat away from them. The prolonged high temperatures in Auckland have more than likely dried out this material turning it from a reasonably good heat conductor into a very poor one.

One commentator likened this situation to a "mini Tandoori oven".

If this occurred, it is likely that the cable will have suffered a runaway temperature increase under a high load and then failed. Whether or not this was the cause of the first failure is debatable but it is very likely to have been the cause of the other three failures.

Even a reasonable amount of rain would not have improved the situation where the cables run in ground that is sheltered by pavement, roading or motorway ramps. A similar event happened in London many years ago during an unseasonably dry spell where there were unexplained failures of 132kV underground cables being operated well within their normal load limits.

Having lost one cable to a perceived normal fault, Mercury Energy staff would have redistributed the load onto the remaining three causing their temperature to rise.

As if that was not enough, the exceptionally hot weather was itself causing an unprecedented load on the electricity distribution system. The Auckland CBD has also grown rapidly in recent years and the number of air-conditioned, high-rise buildings, with their insatiable demands for power, keeps on increasing.

Warwick Bishop is CEO of the Institute of Professional Engineers of New Zealand