South Australia's renewables-heavy power mix was a factor in the statewide blackout in September, a new report by the Australian Energy Market Operator (AEMO) has confirmed.

The report is the third in a series of investigations into the extraordinary loss of power during wild storms.

Previous reports confirmed a series of tornadoes severed high-voltage power lines, setting off a catastrophic chain of faults which resulted in the Heywood interconnector being cut off.

The interconnector failure left the state islanded from the national power grid and prompted a massive and rapid fluctuation of frequency, forcing all generators across South Australia to go offline.

Historically, the rate of frequency change following an interconnector failure has been managed successfully using load shedding, as demonstrated during a blackout earlier this month.

But the AEMO found in September, "the proportionally low amount of conventional generation dispatched in SA at the time of separation, and the subsequent low inertia, resulted in a higher [rate of change of frequency] than had been experienced during previous separation events".

Prior to the statewide blackout, windfarms had been providing 883 megawatts (MW) of power, the interconnector was operating close to its limit and providing 613 MW, while traditional thermal generators were providing just 330 MW of electricity.

Some of the state's biggest gas generators, including Pelican Point, were not operating.

The AEMO has subsequently ordered that two major gas-fired power stations remain online at all times in South Australia to keep the grid in a secure operating state.

A series of tornadoes severed high voltage power lines. ( Supplied: ElectraNet )

Lower resilience to extreme events

The report also made reference to strong criticisms of South Australia's energy mix in the wake of the blackout.

"The significance of the event and the intensity of review has brought to the fore a range of broader issues associated with the changing generation mix across the NEM [National Electricity Market]," it stated.

"The generation mix now includes more non-synchronous and inverter-connected plants, which has different characteristics to conventional plant and uses active control systems to ride through disturbances.

"The growing proportion of this type of generating plant within the generation portfolio is leading to more periods with low inertia and low available fault levels, hence a lower resilience to extreme events."

The report found a series of voltage disturbances during the storm prompted the fault ride-through mechanisms on nine of the state's 13 active wind farms to trip, resulting in the loss of 456 MW of power generation.

The mechanisms are pre-set to trip after a certain number of faults within a defined time period, but the AEMO said it was not aware of this feature before the entire state was plunged into darkness.

The market operator maintains the intermittent nature of renewable energy did not contribute directly to the blackout.

A final report into the massive outage, due for release in March, will examine whether the result may have been different "if there had been a 500 MW less gas-fired plant in service in SA and Northern Power Station was still in operation".

Second interconnector one option to secure SA power

On Monday, the AEMO released a separate report looking at ways to help secure the stability of Australia's power grid.

A fallen transmission tower near Melrose in South Australia's mid-north. ( ABC News: Dean Faulkner )

In the document titled, The National Transmission Network Development Plan, the AEMO said a second interconnector between South Australia and New South Wales or Victoria, could prevent a repeat of September's statewide blackout.

South Australian Treasurer Tom Koutsantonis said the construction of a new interconnector was just one option.

"There are system security issues that can be put in place like greater controls on gas-fired turbines," Mr Koutsantonis said.

He said a second interconnector into New South Wales was actually agreed to in the 1990s, and then scrapped by the then-Liberal government.

"When the former government privatised ETSA [Electricity Trust of South Australia], as part of the deal to maximise sale price, they knocked out building an interconnector to NSW," he said.

"It would have cost us $90 million back then, today the cost is a billion dollars."

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