Photo: Morne de Klerk/Getty Images

An investigation into a major blackout in South Australia six months ago, which sparked a war of words between the state and federal governments over renewable energy, has found that technical issues led to the network’s breakdown.

The 271-page fourth and final report by the Australian Energy Market Operator (AEMO) made 19 recommendations to improve network stability as the system transitions to include an increased reliability on renewables. But it blamed the blackout on September 28 primarily on overly sensitive protection settings on nine wind farms that triggered further problems and left the state cut off from the rest of the grid.

The AEMO said that on the day, two tornadoes with wind speeds in the range of 190–260km/h simultaneously damaged a single circuit 275 kilovolt (kV) transmission line and a double circuit 275kV transmission line, 170km apart.

The damage to those three lines caused them to trip, and a sequence of faults in quick succession resulted in six voltage dips on the SA grid over a two-minute period around 4.16pm.

As the faults grew, protection settings on nine wind farms in the state were triggered, and the subsequent drop of 456 megawatts (MW) in power led to a surge on the interconnector with Victoria.

“For eight of these wind farms, the protection settings of their wind turbines allowed them to withstand a pre-set number of voltage dips within a two-minute period,” the AEMO report says.

“Approximately 700 milliseconds (ms) after the reduction of output from the last of the wind farms, the flow on the Victoria–SA Heywood Interconnector reached such a level that it activated a special protection scheme that tripped the interconnector offline.”

At that point, the state was cut off from the rest of the grid around 4.18pm. It took just 87 seconds before system shut down. Around 850,000 SA customers lost electricity supply.

“AEMO’s analysis shows that following system separation, frequency collapse and the consequent Black System was inevitable,” the report says.

A system restart plan began at 4.30pm. The first customers had power restored by 7pm, around 40% of the load in SA was back by 8.30pm, and 80-90% restored by midnight. But some customers had to wait until October 11 before supply was restored.

The report says “valuable lessons” were learnt as Australia makes the transition to high levels of renewable energy sources. While the wind turbines successfully rode through grid disturbances “it was the action of a control setting responding to multiple disturbances that led to the Black System,” AEMO says.

“Had the generation deficit not occurred, AEMO’s modelling indicates SA would have remained connected to Victoria and the Black System would have been avoided. AEMO cannot rule out the possibility that later events could have caused a black system, but is not aware of any system damage that would have done this.”

AEMO says changes made to turbine control settings shortly after the event “removed the risk of recurrence given the same number of disturbances”.

The organisation said it investigated a number of other factors and concluded they had “little or no material effect on the event”, including trips of wind turbines due to high wind speeds.

The failure of the Quarantine Power Station was due to the switching sequence used. AMEO says measures have been put in place and tested to remedy this. It adds that other problems, such as the Mintaro emergency diesel generator tripped soon after starting, have also been addressed.

Of the 19 recommendations AEMO made, three have already been implemented, including more rigorous weather warning monitoring and improvements in System Restart Ancillary Service testing. The remainder are expected to be in place by the end of 2017, although two relate to potential changes to the National Electricity Rules and others will involve review, consultation and engagement processes with other organisations and bodies.

The full report is here.

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