Yesterday South Australia became separated from the rest of the NEM, requiring it to operate as an energy “island” for a while. All the states at the extremities of the NEM – Tasmania, South Australia and Queensland – have to operate in island mode from time to time, but South Australia’s generation mix makes it more challenging to do so than coal-rich Queensland or hydro-dominated Tasmania. AEMO took plenty of precautions, including declaring a low reserve, as they were forecasting only 130 MW “spare” capacity.
The event comes hot on the heels of a more dramatic separation event that lasted from 31 January to 17 February. On 31 January, high winds knocked down eight transmission towers west of Geelong in Victoria. This affected both the high voltage lines that flow from SA to Victoria via the Heywood interconnector and cut SA and western Victoria off from the rest of the NEM. AEMO was already managing heatwave conditions in Victoria and NSW that day, so this kind of event was the last thing they needed on their hands.
The incident was reminiscent of a similar weather event that knocked down transmission towers in the mid North region of SA in September 28, triggering a series of events on the power system that culminated in a state-wide blackout. Fortunately, 31 January turned out differently, even though the SA grid had to respond quickly to a swing in the supply demand balance of around 1000 MW. SA was exporting 500MW to Victoria until suddenly it wasn’t, and the frequency change that resulted also caused the Portland smelter to trip (500MW). It’s easier to reduce supply in such over-frequency situations than the opposite, not least because the default response of generation is to “trip” (shut down). This is indeed what happened with around 200 MW of older wind farm, and probably some rooftop PV, although the lack of visibility and controllability of this resource makes it hard to tell exactly how it responded.
It’s undesirable for too much generation to respond in this way, though, as uncontrolled tripping can result in a swing around to undersupply and ultimately make things worse. Helpfully, the newer wind farms and large scale solar delivered a more controlled reduction as did the gas generators. The three big batteries that have been installed since the system black switched from discharging to recharging. While the forensic assessment of the event is yet to take place, the early indications are that the SA grid was more resilient than in 2016, as AEMO and the industry have improved their operational responses.
Following the immediate responses, supply was restored at Portland (if not, the plant may never have re-opened…) and AEMO made sure there was sufficient gas-powered generation to keep the lights on when there was lower renewable output.
SA had to be self-sufficient in essential security services such as frequency response (FCAS) for the 17 days until temporary transmission lines could be commissioned and SA re-joined the NEM. FCAS prices jumped and remained high for much of the 17 days, as SA only has a limited supply of providers of this service, even with the recent addition of the big batteries and some demand side providers. FCAS costs are spread across both customers and generators, so the generators who don’t also supply FCAS – most of the renewable plant in the state – will have taken a financial hit as a result.