AEMO restricts Queensland solar farms after Callide explosion, then changes mind

(Note: This story updates an earlier version after new announcement from AEMO).

The output of 11 large scale solar farms and one wind farm in north Queensland were temporarily restricted as part of an initial response to guard against any “system strength” issues emerging from the Callide coal generator explosion on Tuesday.

A notice from the Australian Energy Market Operator issued on Wednesday said the 11 solar farms, along with the Emerald wind farm, could have their output dialled back to between 50 and 80 per cent of their rated capacity depending on the number of other coal, gas or hydro units that might be operating at the time.

The affected solar farms were the Clare, Collinsville, Daydream, Hamilton, Haughton, Hayman, Kidston, Ross, Rugby Run, Sun Metals and Whitsunday projects. The wind farm affected is the Emerald wind project.

But in another market notice issued just a day later, the AEMO replaced those restrictions with a less severe advice that proposed only one scenario where the output of the solar and wind farms might be dialled back to 80 per cent.

The first constraint announcement was seen as a safety first response to the Callide event that caused multiple line and generators outages, and cut off the power to more than one million people on Tuesday.

RenewEconomy understands it was based on the assumption of zero output at Callide, which might be the case for at least another week, if not longer.

“AEMO and Powerlink are continuing to investigate system strength requirements in North Queensland,” the two organisations said in the Wednesday market notice. “Following the events in Qld on 25th May 2021 the following system strength combinations have been implemented for north Queensland generators.”

It was a disappointment to some of those solar farms, which had worked with the transmission company Powerlink to solve system strength issues that had previously limited their output by fine-tuning their inverter settings.

This was seen as a highly successful and cheaper alternative than traditional responses to system strength issues, which often involve the installation of expensive “synchronous condensers”.

One solar farm operator told RenewEconomy that their projects successfully rode through the dramatic events on Tuesday afternoon, before being instructed to ramp down to zero output as AEMO sought to “rebuild” the grid.

It is thought that were it not for the revised inverter settings these solar farms might not be operating at all while Powerlink and AEMO investigate the situation.

However, as renewable energy and battery storage operator Neoen revealed on Thursday, inverter based technologies such as the batteries at its Hornsdale Power Reserve proved they can play a key role in keeping the overall grid stable after the Callide explosion by delivering “synthetic inertia”.

See: “Virtual machine”: Hornsdale battery steps in to protect grid after Callide explosion

Those technologies responded immediately to the frequency excursions caused by the Callide explosion, showing an ability to deliver inertia that slows the rate of change of frequency, and essentially delivers more time for other equipment to respond.

Still, there are many questions about what did actually happen at Callide and the timing of subsequent events, and there will be much interest on an interim report on the incident due from AEMO within the next week.

Callide owner CS Energy said on Wednesday that the events occurred in a matter of “milliseconds”. But investigation of data by others suggest there was considerable delay (in grid terms) between some events.

For instance, this graph above suggests that the Callide B plant remained online after the initial explosion, but “oscillated” before finally giving up and was followed quickly by other coal generators. This may speak to the settings in such coal units and whether they were capable of staying on line in such an event.

“20 minutes is a long time in power system security terms. There wasn’t a sudden and instantaneous trip of 3,100MW. There was a slowish loss of  around 700MW at Callide C, and then a near instantaneous loss of  around 2400MW 20 minutes later.”

The new constraint published by AEMO on Thursday evening advised that the restrictions on the output from the 11 north Queensland solar farms and the Emerald wind farm had already been replaced by newer equations that appear to significantly relax the constraints.

One combination would cause a limit of 80 per cent of rated output, while another combination of generators would allow full output.