2006 Auckland Blackout - Cause

Cause

The cause of the blackout was traced back to the Otahuhu sub-station, the city's main transmission switching station. A corroded shackle connecting the Otahuhu to Penrose 220 kV line's earth wire had dislodged in 90 km/h (56 mph) winds, letting the earth wire fall across the 220 kV line and the 110 kV busbar below it, tripping both the line and three sections of the busbar, disconnecting lines to Mount Roskill, Penrose and Pakuranga. The trip also disconnected Otahuhu B and Southdown power stations from the national grid. The trip left only one line, the now-dismantled Arapuni to Pakuranga 110 kV line, supplying Pakuranga, Penrose, and the central city. Eight seconds after the failure, this line tripped from overloading, leaving Penrose, Pakuranga, and central city substations without power, as well as parts of Otahuhu and Mount Roskill substations.

Investigation of this incident found that maintenance of the electricity transmission system was not adequate and that this substation had major and minor design deficiencies.