A brief recap of a large afternoon frequency wobble on the NEM mainland on Saturday 20th September 2025

We noticed yesterday (Saturday 20th September 2025) a large afternoon dip in frequency down towards (but not outside) the limits of the NOFB as shown here in this 3-hour time-range:

We can see that, whilst the sustained weakness began at approx 16:10, it was the period from 16:45 to 16:56 that saw the largest frequency deviations below 50Hz.

Now with the benefit of ‘next day public’ datasets, we can see the Target and FinalMW for each unit (i.e. Scheduled and Semi-Scheduled only) – and with the ‘Trends Engine’ within ez2view have charted the following view across the entire day (i.e. all 288 dispatch intervals):

Readers will note that all technologies display large collective Dispatch Error at times:

• Readers should remember that:
  • a positive Dispatch Error means collective under-performance relative to Target; whilst
  • a negative Dispatch Error means collective over-performance relative to Target; whilst
• This metric is an input to Aggregate Raw Off-Target:
  • which has not been calculated here
  • but is directionally consistent.
  • and which we have been trending, over time, for Semi-Scheduled units.
• Neither directions are inherently ‘bad’ … rather, which direction is ‘unhelpful’ varies over time, depending on system frequency needs.

So, with particular interest in the 16:50 and 16:55 dispatch intervals (NEM time) on Saturday 20th September 2025, we ‘zoom in’ and see the following:

The results are no surprise to us:

1)  During this period, again we see:

(a)  systemic weakness in frequency support provided by (late afternoon) Large Solar coupled with coincident weakness in Wind dragging the frequency down

(b)  despite good efforts across the coal and battery fleet to maintain the frequency at 50Hz (inside the PFR deadband).

2)  This is becoming increasingly commonplace:

… as we have explored before in ‘Highlighting particular (frequency and/or AggROT) events through 2024 and 2025’ …

3)  And it is of growing concern to us (and others) … frequent readers will recall our asking of the question ‘is the Semi-Scheduled category is sustainable or scalable?‘.