Sequence of Events on 25 May – A Very Short & Partial Visual Guide

Normally this would be part of a much more complete article adding to WattClarity’s growing coverage of the major power system events in Queensland following the incident at Callide C Power Station.

However I for one find it easier to start understanding events like this with at least a rough idea of “what happened where?”, so with the help of a Powerlink network diagram, I’ve sketched out the location and sequence of the major events leading to the load-shedding on that afternoon:


This is a schematic diagram so doesn’t represent the geography in any realistic way, but it does show electrically how the different pieces of the puzzle are related.

In summary, the sequence of key events seems to have been:

  • 13:45: fire in Callide C Power Station turbine hall leading to trip of both units (C3 and C4) and evacuation of the station
  • 14:06: trips of all 275kV lines in and out of the nearby Calvale terminal station, reported by AEMO
  • 14:06-07: trips of units at Callide B, Stanwell, Gladstone and Yarwun Power Stations – not clear whether any of these preceded the 275kV line trips

The latter loss of generation was three times as large as the initial loss of generation from Callide C, and the event that precipitated the widespread under-frequency load shedding in Queensland and NSW.

The causes of all these events as well as their exact sequencing remains unclear.

More to follow on other aspects of the event, including how the rest of the market reacted, but that’s all for this update.

About our Guest Author

Allan O'Neil Allan O’Neil has worked in Australia’s wholesale energy markets since their creation in the mid-1990’s, in trading, risk management, forecasting and analytical roles with major NEM electricity and gas retail and generation companies.

He is now an independent energy markets consultant, working with clients on projects across a spectrum of wholesale, retail, electricity and gas issues.

You can view Allan’s LinkedIn profile here.

Allan will be sporadically reviewing market events here on WattClarity

Allan has also begun providing an on-site educational service covering how spot prices are set in the NEM, and other important aspects of the physical electricity market – further details here.

6 Comments on "Sequence of Events on 25 May – A Very Short & Partial Visual Guide"

  1. Philip Wong Too | Friday, May 28 2021 at 7:14 am | Reply

    Looking at the diagram it must’ve got very close to splitting Queensland into two electrical islands. It will be interesting to find what caused the tripping of all circuits out of Calvale.

    • Exactly right Philip. Understandably it’s the incident at Callide C that has attracted a lot of attention, but it was the Calvale trip and its apparent impacts on other generators that had the most widespread effects. It’s not at all clear whether the Calvale trip was the direct result of what happened at Callide C or some other cause.

  2. Hi Allan, Nice to see some details. In what looks a related incident, Wivenhoe was pumping at the time of the trip of the Callide units, and from 5 minute resolution it looks to me that that pumping load tripped pretty much around then. Perhaps that helped to stabilise that first trip. Maybe that was the low hanging fruit to arrest a frequency collapse.
    I look forward to your further exploration of this event.

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  4. I had a look for diagrams and from an old AEMO incident report it seems that Calvale is a breaker and a half substation, so even a bus fault or a CB fail shouldn’t have resulted in the loss of all circuits from Calvale.

    Diagram on page 5 of (

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