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How the NSW energy grid almost went dark last week

How the NSW energy grid almost went dark last week

At 1.12pm last Saturday, while the federal government's combatants licked their wounds and new Prime Minister Scott Morrison plotted to split climate from energy policy in his ministry, Australia's biggest state came the closest to a "system black" that industry veterans can recall.

Lightning struck the Queensland-NSW Interconnector (QNI) and the high-voltage interstate transmission line which had been running heavily loaded had been tripped within seconds.

Within 50 seconds the Heywood Interconnector which connects South Australia's wind-heavy grid to Victoria's brown coal generators had also tripped, isolating South Australia as well as Queensland.

But because SA and Queensland had been sending their surplus wind and coal power to Victoria and NSW, the impact on power users was mostly felt in those states rather than in the regions at the northern and southeastern extremities of the grid.

The events that unfolded within those 50 seconds highlight some major challenges for the power grid and policymakers quite apart from the fate of the now abandoned National Energy Guarantee and Paris climate commitments.

Like most outages, this was a system security problem and not an issue with supply from any generator, whether fossil fuel or wind, solar or hydro.

But they also highlight the robustness of the grid and its in-built self-preservation mechanisms – which prevented what could have been at worst a catastrophic and potentially lethal failure of large coal or gas-fired thermal generators due to the instability of the system.

Also avoided was the very real possibility of a wide-scale blackout across large sections of the grid along the lines of the famous north-east US blackout in 2003 that left millions of households without power for two days and shut down water supplies, rail and air transport and communications.

"This was a big moment and when you have these big moments you've got to learn from them, you have to ask the question 'have we got it right?'" says Snowy Hydro chief Paul Broad, noting faces of those watching the drama unfold on Saturday went "gray" in recognition of the risks to the system.

Some 60 megawatts of Snowy's fast-start hydro capacity kicked in automatically within a few seconds, followed by another 150MW a few minutes later.

"This is just a reminder of how robust and wonderful our system is. You could argue, could we do it better over time, and yes we can."

It was also a reminder of the huge challenge the Australian Energy Market Operator and other energy regulators face in managing the transition of power from fossil fuel to clean energy amid increasing temperatures and more frequent extreme storms, which increase the risk and frequency of events like last Saturday's.

The lightning strike on QNI last Saturday was highly unusual in that it struck the middle phase of two three-phase circuits either side of the transmission towers, Transgrid chief executive Paul Italiano says.

"Lightning strikes happen every time there's a lightning storm [but] hitting the same phase on two circuits simultaneously is very rare."

When QNI tripped at 1.12pm, NSW lost about 800 megawatts of power that had been flowing from the north, according to the Wattclarity blog moderated by Brisbane-based energy analyst Paul McArdle (from which much of this is drawn).

Victoria lost about 280MW that had been flowing from SA. Tomago Aluminium, owned by Rio Tinto and CSR and the largest energy user in the country, lost about 600MW of supply and had to close two potlines, one for an hour, the other for about 40 minutes. Alcoa's Portland Aluminium smelter lost about 250MW for 50 minutes and also had to close a potline.

At Viva Energy's oil refinery in Geelong, Victoria, a protective relay system kicked in to cut power to the plant for seven hours; it took six days to get fully back online. In Tasmania, about 50MW of load was shed.

NSW distributor Ausgrid reported a loss of power to 45,000 customers across Sydney, with thousands more affected in the Hunter Valley. Delays on Sydney trains were also connected to the disruptions. But Rio's Boyne Smelters in Gladstone, Queensland, were largely unaffected.

The lightning strike put the National Electricity Market under immense pressure once again, showing the vulnerability of Australia's long and thin grid – which contrasts with denser and more heavily interconnected grids in the US, Europe and Asia – to events outside its control such as extreme weather.

When an interconnector trips and an exporting state suddenly has a surplus of power – as Queensland did last Saturday – frequency soars and generators automatically trip to protect themselves from burning out. By contrast, when an importing state suddenly has a deficit, frequency drops and generators have to fire up and increase their generation to restore frequency to safe operating levels.

On Saturday, the frequency of the grid, always maintained very close to 50 Hertz, dived spectacularly to 49.0 Hz, a level one industry insider describes as "very dodgy" in terms of the stability of some of the big thermal generators. At risk is a major plant failure or even an explosion, setting off a domino effect – known as a cascade failure of the power system – unless the system can rapidly rebalance.

"A little bit of variation from 50.0 from the second decimal point is fine, but once you get down to a whole digit that's a real worry," the insider said, noting that even in the worst Sydney bushfires last decade the low point was 49.5 Hz.

Kogan Creek and Gladstone coal power stations in Queensland quickly reduced their output, while in NSW AGL Energy's Bayswater power station's number-three turbine raised its output sharply and in Tasmania Hydro Tassie's Gordon Dam power station powered up. This was all as expected.

But some generators didn't perform entirely as expected. Bayswater number-two turbine raised its output but then dropped it just as quickly, while at Engie's Loy Yang A power station in Victoria's Latrobe Valley the output from units one and four spiked, dropped and then recovered to the original level all within eight seconds. SA's Hornsdale Power Reserve (the Tesla battery) switched from drawing about 40MW from the grid (charging) to discharging 40MW into the grid, helping to avert any load loss in SA.

Prices responded to the wild fluctuations caused by large chunks of the grid being "islanded". In SA, which had a sudden surplus of wind power (even though it was heading down at the time) the NEM (wholesale) price plummeted to the minus $1000 per megawatt hour floor. Negative prices occur - typically in the wind-heavy SA market - when there is surplus supply. In Victoria and NSW, which remained connected, prices were well behaved at around $84-$90/MWh, suggesting the load shedding at the smelters achieved its purpose, the stress on the assets and owners notwithstanding.

But in Queensland, which should have had a large surplus, the price jumped and was still at $1411.69 at 1.40pm. As well as Kogan Creek and Gladstone power stations, the Clare, Kidston, Hamilton and Whitsunday solar farms all cut their output in response to the increased frequency.

Two other intermittent plants – the North Brown Hill Wind Farm in SA and the Royalla Solar Farm in NSW – appeared to trip completely with QNI. In SA rooftop solar systems also tripped because of the volatile frequency.

The QNI was fully restored by 2.33pm, according to Powerlink, the Queensland transmission company. Heywood was restored earlier, by about 1.40pm, and according to AEMO all customer loads were restored by 3.28pm.

It wasn't until Sunday evening that AEMO felt able to categorically attribute the outages to lightning and withdraw all Lack of Reserve 2 (LOR2) notices. These are calls on grid suppliers to dispatch extra power in response to expected shortfalls.

AEMO has four weeks from the incident to gather information from suppliers and customers and report its preliminary findings and isn't saying much more for now.

Even so, questions abound. McArdle says the grid data on Queensland frequency – vital to managing the grid safely – seemed to be wrong.

He is also unsure exactly why Heywood tripped. The wind and solar farms that dropped their output weren't registered to supply FCAS – or grid stabilising services – to the grid, raising the possibility of "some form of technical fault".

Italiano says the grid had operated as intended but NSW is relying more on power imports from Queensland, increasing the risk of such events. Since January 1, NSW has imported about 3100GWh of energy via the QNI, which has operated at more than 90 per cent of capacity on more than 150 occasions.

"That's only likely to increase because energy demand in NSW has been growing," Italiano says. "We think as the system continues to operate like this we are going to see more events like this."

Others question AEMO's strategy for enabling FCAS services from generators and other loads that would come into action to stabilise the system – but at an ongoing cost that would flow directly to customers. Or they argue for a better interconnected grid system that would spread the impact of the failure of one major cable more broadly throughout the system – also with an impact on power bills.

"You can't leap to a conclusion and say, 'oh my god, this means we can't depend on transmission, nor does it say we can't depend upon other generation," says Grattan Institute's Tony Wood.

"This is really illustrating how we have to fundamentally think about and decide what level of reliability at what level of cost we are going to build into our system and understand the various tradeoffs.

"This is not a warning shot but it means there are some serious discussions to be had. While everyone is talking about [new federal energy minister] Angus Taylor being the minister for lower prices, he is also the minister for reliability as well."

Australian Financial Review

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