The state-wide power outage on 28 September happened during a severe storm with high winds, thunder, lightning strikes, hail and heavy rainfall, AEMO said.
After multiple faults in a short period, 315MW of wind generation was disconnected from the system, affecting the region north of Adelaide. That increased the flow in the main state interconnector to make up the loss, but this caused the interconnector to overload. An automatic protection mechanism then kicked in to protect the interconnector, and power was lost to the whole state, AEMO said.
"While the event was triggered by extreme weather, AEMO will conduct a thorough investigation into how each component of the electricity system responded under these circumstances," it said.
Some Australian media commentators have picked up on the fact that wind power was first affected. ABC’s political editor, Chris Uhlmann asked whether the "state's heavy reliance on wind turbines might have increased the risk of a state-wide blackout" and if "renewable energy is being integrated into a national grid that was not designed to cope with it".
However, Franck Woitiez, chief executive of Neoen Australia, owner of the Hornsdale wind farm which shut down, told Australian Financial Review that the turbines behaved exactly as they were supposed to do during the storm.
"So far I am not worried about the behaviour of our wind turbines and the way they reacted to the network failure. It's exactly what they were supposed to do, remembering that the network was in a situation that was unforeseen," Woitiez told the news site.
Two gas-fired generators failed to restart gas turbines shortly after the blackout, and Woitiez said the blackout may not have been as bad if they had worked.
The company is now talking to AEMO and Siemens, the supplier of the wind turbines, about raising the tolerance of network failures to make shutdowns less likely in extreme weather, Woitiez said.
Immediately before the event, South Australia's 850,000 customers were being supplied by approximately 880 MW of state-produced wind generation, 330 MW of state gas generation, and 610 MW of electricity imports via two interconnections with the state of Victoria, AEMO said.