The blackout of an entire state is rare. And bad.

Having all the lights go out in a storm, even a big one, is not a sign of an electricity grid that's working well.

And the Australian Energy Market Operator's (AEMO) preliminary report has not yet determined why the whole of South Australia went to "black system" at 4.18pm local time on Wednesday September 28.

"The root cause is subject to further analysis being conducted," it says.

Once those statements would have seemed uncontroversial but not in the political storm that has raged since South Australia was shut down over a week ago.

Now to dare suggest that the state's heavy reliance on wind generation might have made its grid more vulnerable to a blackout is heresy.

So, since this column raised that heresy last week, let's examine it again, in the light of the AEMO's preliminary report on the event.

The starting point is to understand South Australia's unique energy mix.

The state now gets 40 per cent of its power from wind — a higher proportion than most other places on earth — and all coal-fired generation has been mothballed.

The rest comes from a mix of gas-fired power and two interconnectors that link it to Victoria's brown coal-fired power plants.

Wind presents two engineering problems when it is hooked up to a grid that was designed before it was viable.

First, it is intermittent so all of it has to be backed up by baseload power for those days when the wind does not blow.

Second, for an electricity network to function, demand and supply have to be kept near the perfect harmony of 50 cycles (50 hertz) every second of every day. If the frequency gets out of tune it trips the shutdown switch.

This electrical harmony is called synchronous supply, and thermal power is very good at delivering it. Wind power is asynchronous as its frequency fluctuates with the breeze, so it has to be stabilised by the give and take of other sources of demand and supply.

To ensure a reliable supply of electricity at an acceptable standard AEMO has frequency ancillary control services in place to deal with rapid changes in the ebb and flow.

Power operator knew storm was brewing

The AEMO report says that the energy market operator was expecting "severe weather" on Wednesday September 28.

Just before the power went down there was 800 megawatts of wind generation, 330MW of gas and 610MW was being imported from Victoria.

That wasn't the only thing being imported. The report says "there was no local [frequency ancillary control services] requirement pre-event, as there was no credible risk of separation of SA from the national electricity market."

That seems an extraordinary statement. The operator knew a storm was brewing and took a punt that the line to Victoria would stay up, keeping South Australia's frequency in harmony.

If the gamble fails so will South Australia's capacity to control rapid changes in supply and demand and whatever power generation is left in the state will be snuffed out.

Sorry, this video has expired SA blackout 'triggered by a weather event' Premier says

The real drama plays out in a 90 second window between 4:16:46pm and 4:18:16pm.

The weather triggers a series of transmission faults and three major 275 kilovolt lines are lost. Then, in two separate events, 315 MW of wind generation is disconnected. This unexplained, rapid loss of wind power is the event that begins the cascade towards blackout.

"In the events leading up to the SA region black system, generation reduction occurred at six wind farms," the report says. "There was no reduction in thermal generation."

Why it happened is still a mystery.

"Additional analysis is required to determine the reasons for the reduction in generation and observed voltage levels before any conclusions can be drawn," the report says.

Demand then shifts dramatically to the line with Victoria. Just before the wind generation failed the Heywood interconnector's flow was about 525 MW, well within its normal operating limit of up to 600MW.

The reduction in generation and the oscillations caused by the transmission network events drove demand to "flows between 850 to 900 MW" well in excess of its capacity. So it shut itself down.

Now the "non-credible" had become credible. At 4:18:15pm the door to Victoria slammed shut, draining 900MW of supply in a heartbeat. There was "a rapid reduction in the power system frequency" in South Australia and it "fell to zero". That tripped the two thermal power stations at Torrens Island and Ladbroke Grove and all remaining wind farms.

And the lights went out across the state.

It should be noted here that the report says that 14 of the 22 transmission towers that went down did so, "following the SA black system".

System had further flaws

In the eerie dark that followed the operator immediately began working through a pre-determined restoration plan. And that revealed more deficiencies in the system.

The operator has two contracts in South Australia for System Restart Ancillary Services (SRAS). Their identities are a secret for contractual reasons, so the report calls them SRAS 1 and SRAS 2.

Plan A was to use SRAS1 to jump start the thermal power station at Torrens Island and, at the same time, restore the interconnection with Victoria.

"This was seen as the quickest and safest way to restore supply to South Australia," it says.

In a footnote it adds, "wind farms cannot be used in the initial stages of a power system restoration due to the variable nature of their output".

Things didn't go well.

"Due to an issue currently under investigation, SRAS provider 1 was unable to supply sufficient capacity to restart any of the Torrens Island power station units," the report says.

SRAS 2 was out due to "damage caused by the storm".

Plan C was to hook up the interconnector with Victoria and use it to jump-start the state. By 6:54pm Torrens Island was restarted but it needed anther two hours before it could deliver any power.

At 6:36pm the operator was advised that the gas-fired turbines at Pelican Point could be ready in four hours. Here, it's worth noting that gas plants can't just spring into action — they need time to warm up. Pelican Point had been off-line before the storm, bid out of the market by cheap, abundant wind.

SA power system 'extremely fragile'

So what have we learned from this report?

That weather sparked a series of events that spiralled into a state-wide blackout. That it was the sudden loss of wind power that tripped the interconnector with Victoria and that loss of generation is yet to be explained.

It is also undeniable that South Australia now has an extremely fragile power system. It cannot operate with any confidence if the interconnector with Victoria is down and if the state blacks out it can't be restarted with wind power.

Politicians have said a lot of things in the wake of this outage. But judge them by what they do.

South Australia is already calling for rule changes in the national electricity market because it recognises its reliance on wind and rooftop solar has made the state's system less secure.

This won't be the last fix that South Australia will need to patch up the problems.

Finally, we know that the energy market is in transition to cleaner forms of power and that is unstoppable. In time the engineering difficulties posed by wind will be overcome.

Or they will be as long as people aren't burned as heretics for daring to point out the real and well documented problems with integrating new forms of energy into an old grid.

And, if those who claim to be friends of renewables continue to respond to any criticism with hysterics, then they will be responsible for ensuring the budding renewable industry suffers irreparable reputational damage.

Because, if the lights keep going out, people will lose faith.

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