AEMO publishes final report on SA blackouts

Software settings on South Australian wind turbines were behind the statewide blackout on September 28 last year, a new report has found.

The Australian Energy Market Operator (AEMO) has today released its fourth and final report into the “Black System” event, which left the entire state in the dark.

The report confirmed the blackout was caused by a number of tornadoes with wind speeds of up to 260km/h.

“Two tornadoes almost simultaneously damaged a single circuit 275kV transmission line and a double circuit 275kV transmission line, some 170 km apart,” the report said.

According to the AEMO report, the damage to these three transmission lines caused a trip, and a sequence of faults in quick succession resulted in six voltage dips over a two-minute period around 4.16pm on September 28.

“As the number of faults on the transmission network grew, nine wind farms in the mid-north of SA exhibited a sustained reduction in power as a protection feature activated,” the report said.

“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.

“Activation of this protection feature resulted in a significant sustained power reduction for these wind farms. A sustained generation reduction of 456MW occurred over a period of less than seven seconds.

“The reduction in wind farm output caused a significant increase in imported power flowing through the Heywood Interconnector.

“Approximately 700 milliseconds 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.”

The SA power system then became separated from the rest of the NEM.

“Without any substantial load shedding following the system separation, the remaining generation was much less than the connected load and unable to maintain the islanded system frequency,” the report said.

“As a result, all supply to the SA region was lost at 4.18pm. AEMO’s analysis shows that following system separation, frequency collapse and the consequent Black System was inevitable.”

AEMO concluded in its investigations that wind turbines successfully rode through grid disturbances, but it was the action of a control setting responding to multiple disturbances that lead to the black out.

“Changes made to turbine control settings shortly after the event has removed the risk of recurrence given the same number of disturbances,” the report said.

“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.”

The report makes a total of 19 recommendations aimed at safeguarding the grid.

“The generation mix now includes increased amounts of non-synchronous (wind) and inverter-connected plant,” the report stated.

“This generation has different characteristics to conventional plant, and uses active control systems, or complex software, to ride through disturbances.

“With less synchronous generation (coal and gas) online, the system is experiencing more periods with low inertia and low available fault levels, so AEMO is working with industry on ways to use the capability of these new types of power generation to build resilience.”

Of the 19 recommendations outlined in the report, three have already been implemented which address more rigorous weather warning monitoring and improvements in System Restart Ancillary Service testing.

Work has commenced on a further eight recommendations, with the remaining items due to be completed by December 2017.