Almost three years after a statewide blackout plunged South Australia into darkness, the fallout is continuing.

On an afternoon in September 2016, a severe storm triggered a sequence of events that resulted in South Australia becoming cut-off from the national grid, and electricity being cut to most of the state's homes and businesses.

The Australian Energy Regulator (AER) is taking legal action against four energy companies — including AGL — over the incident.

The dispute is a technical one and will assess whether enough was done at the companies' wind farms to prepare for such an extreme event.

And the outcome of the case has the potential to reignite debate over the reliability of renewables.

Here's a look back at how things got to this point.

What actually happened?

On Wednesday, September 28, two tornadoes with wind speeds between 190 and 260 kilometres per hour tore through a single-circuit 275-kilovolts transmission line and a double-circuit 275kV transmission line, about 170km apart.

The destructive wind damage was accompanied by a sequence of faults in quick succession resulting in six voltage dips on the SA grid over a two-minute period at about 4:16pm.

Two tornadoes with wind speeds of up to 260kph ripped down transmission lines. ( ABC News: Dean Faulkner )

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.

For most, the protection settings allowed the wind turbines to withstand a pre-set number of voltage dips within a two-minute period.

When the protection feature kicked in, the output of those wind farms fell by 456 megawatts over a period of less than seven seconds.

"This was a significant loss as approximately 48 per cent of South Australia's electricity supply overall was from wind farms," the AER said in a report in December 2018, which it cited as part of its case against the energy companies.

When the wind farms unexpectedly reduced their output, the Heywood Interconnector from Victoria tried to make-up the shortfall.

About 700 milliseconds after the last wind farm powered down, the flow in the interconnector reached such a level that it activated a special protection scheme that tripped it offline.

The sudden loss of power flows across the interconnector sent the frequency in the SA grid plummeting.

How did the system respond?

South Australia has an automatic load-shedding system designed to kick-in in just such an event.

SA's generation mix before the blackout. ( Supplied: AEMO Black System SA report )

But the rate of change of the frequency was so rapid, the automatic load-shedding scheme did not work.

Without it, the remaining generation was much less than the connected load, and as a result, the entire system collapsed.

"In those circumstances, all generators connected to the grid who were trying to feed their electricity into the system on a steady basis, have certain settings that protect themselves against such a volatile situation," explained Grattan Institute energy expert Tony Wood.

"[Some] wind farms — their settings were such that the number of these voltage variances that they could deal with in a short period of time was exceeded.

"Some of them basically turned themselves off.

"That then contributed to the extent of the blackout. It wasn't caused by them but it contributed to the consequences of the blackout."

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

The Australian Energy Market Operator [AEMO] said its "analysis shows that following system separation, frequency collapse and the consequent black system was inevitable".

What happened next?

According to the AER, "technical issues" prevented power from being restored within the first hour.

Power was gradually restored to about 80 per cent of homes by midnight — but some homes, especially in regional areas, were left waiting for days.

The most iconic images were of giant electricity towers blown over by what were described as "twin tornadoes".

A map showing power outages across SA on the night of the blackout. ( SA Power Networks )

Over the coming days, weeks and months, public opinion became polarised over the root cause of the blackout.

Pro-renewables pundits blamed the storm, but critics pointed the finger at wind farms and questioned their ability to provide baseload power.

"This has got nothing to do with whether renewables are good or bad — it's all to do with the technical settings on those actual wind farms," Mr Wood said.

The blackout also became a running joke, prompting quips about South Australia's inability to keep the lights on.

For example, in 2017 then prime minister Malcolm Turnbull said SA could never host the AFL grand final unless it was in daylight, because of the state's power problems.

It also was the catalyst for the now-infamous interaction between former SA premier Jay Weatherill and Federal frontbencher Josh Frydenberg over energy security.

Sorry, this video has expired Jay Weatherill confronts Josh Frydenberg over energy security

It also resulted in Elon Musk becoming a player in South Australia's energy market.

Could it happen again?

In the years since the blackout, there have been widespread changes to South Australia's energy market to make sure a similar incident would not happen again.

In March 2017 new standards for wind farms were announced that were recommended by AEMO.

Interim conditions required new thermal and renewable generators to be capable of providing additional services, which contribute to the stability of the grid, including frequency control.

Generators must be able to ride through some voltage and frequency disturbances, and help power system restoration after a widespread blackout.

Those recommendations have since been replicated nationally.

Earlier this year, plans for an electricity connector between SA and New South Wales were proposed with the aim of lowering power costs and improve energy security.

The AER has started the process to determine whether the interconnector satisfies the Regulatory Investment Test for Transmission cost benefit analysis.

It was granted major project status by the SA Government in June.

But investment in renewables has substantially increased, and is going to increase further.

Energy Minister Dan van Holst Pellekaan last year committed to expanding the State Government's home batteries installation scheme.

However, Federal Energy Minister Angus Taylor said the Government was "deeply worried" about the prospect of blackouts in Victoria this summer.

"There are particular issues that emerge now with the amount of solar and wind that's been coming into the system in recent years," he said.

"We've had record levels, the highest levels in the world in 2018 per capita of investment in solar and wind across Australia, and that is creating its own challenges.

"We're deeply worried about Victoria, there's no question about it. South Australia is taking real steps to manage the situation.

"We need gas generators in the system to back up solar and wind."

Energy Minister Angus Taylor said the Government was concerned about Victoria's energy situation. ( ABC News: Marco Catalano )

Will the court action have any impact?

The AER's legal proceedings against subsidiaries of AGL, Neoen, Pacific Hydro and Tilt Renewables were to "send a strong signal to all energy businesses about the importance of compliance with performance standards to promote system security and reliability".

It has said it was seeking to impose penalties against the four companies.

In a statement AGL said it did "not accept the AER's conclusions" and would "strongly defend these proceedings" on the basis that it complied with the National Electricity Rules.

"We are committed to working with the regulator and stakeholders to ensure the integrity of the energy market and the ongoing stability of South Australia's electricity system," a spokesman said.