Helpful and unhelpful MW deviations on 15th January 2025

In this article we focus on the low-frequency period that commenced with Callide C units tripping at 16:55.

We chart unit deviations from each unit’s reference trajectory (target-to-target).

To aid clarity we select the top 10 contributors.

But note, there a more than 500 units in the NEM. Even if their individual deviations are relatively small the group may still collectively contribute a lot.

Unders

We know from Simultaneous with trip of Callide C3 and C4 (frequency impacts … a first look)  and Charting the 4-second data following trip of Callide C3 and C4 that frequency dropped near 16:55 and further, to 17:00.

Therefore, we initially inspect the top 10 contributors to the frequency decline, judged in order of aggregate deviation in the negative direction.

What initially stands out is that these 10 selected units were ‘off target’ by an aggregate of 1,500 MW at 17:00. Further analysis along this line, by fuel type is in  Was it the simultaneous trip of Callide C (or was it something else!) that put the most pressure on System Frequency on Thursday afternoon 15th January 2025?.

Negative deviations judged to be ‘unhelpful’ for system frequency at the time when frequency was low.

We observe Callide C units (CPP_3 and CPP_4) as being the largest contributors. Their influence lasted 2 dispatch intervals because the reference trajectory is target to target and it was the 17:05 target that was first closest to 0MW (limited by ramp rate limitations).

We also observe a number of solar farms that continued to be under performing (relative to target) after 16:55 (when the trip occurred).

Overs

Positive deviations judged to be ‘helpful’ for system frequency at the time when frequency was low.

The Western Downs BESS (stage 1) appears to have contributed more than 150 MW in the ‘raise’ direction, in the 17:00 interval. The stage 2 BESS (WDBESS2) also contributed (~75 MW eyeballing the chart).

In the 17:05 interval we find the Western Downs Green Power Hub (Solar) was above target by around 250 MW. By 17:05 interval-end it was 260 MW above target, sitting at 401 MW, relative to a max capacity of 400 MW.

Waratah BESS contributed more than 50 MW. Other batteries make the list too; the Vic Big Battery.

Coal units were mentioned in Paul’s article Was it the simultaneous trip of Callide C (or was it something else!) that put the most pressure on System Frequency on Thursday afternoon 15th January 2025? noting many units in the fleet made helpful contributions. Bayswater Unit 1 was observed to make a very clean response between 16:55 and 17:05. Bayswater Unit 4 also contributed in the 17:00 interval, in particular.

One outlier in this list is the Bouldercombe battery (BBATERY1). It was above-trajectory before the Callide event (16:55) and across the event. However by 16:56 it’s deviation was small when the system could have used more.

Further analysis on the Bouldercombe battery using ez2view’s Unit Dashboard finds the unit’s target for the 16:55 interval was 9 MW and, consistent with the chart, it had gone above that (to 50 MW, its max capacity) before the trip happened – a deviation of 41 MW. We aren’t clear of the reason for this.

Its target for the 17:00 interval (spanning the low frequency event) was 50 MW. Once it had reached that level it had no more capacity to provide in response to frequency need. This appears to be one component of the reason for the small deviation registered in the 17:00 interval after the first minute. It appears the unit was technically limited.