A quick look at system frequency, with 2 x coal units offline overnight into Saturday 31st May 2025

Earlier on the week we saw this ‘Drop in mainland system frequency on 26 May 2025’ (briefly down outside of the NOFB) … which appears to have been in large part due to ‘Poor dispatch performance of VRE (collectively) on Monday 26th May 2025 sufficiently large to drive frequency outside of NOFB’.

… more on that later.

So it’s fitting to book-end the week with a look at frequency performance overnight (into Saturday 31st May 2025) with two coal unit outages in NSW overnight to see what happened in terms of more ‘old style’ frequency disruption …

The two coal unit outages

Let’s start with this snapshot from the ‘Generator Outages’ widget in ez2view at 10:40 Saturday morning, with the two unplanned outages flagged:

I’ve highlighted the unplanned outages at both Vales Point 6 and Mt Piper 1 – which, whilst both not planned a long time in advance, appear to have different characteristics …

VP6 offline just prior to 23:55

This was captured in this ‘Notification’ widget alert in ez2view at 23:55 (NEM time) Friday evening:

In this window from ez2view comprised on the ‘NEM Map’ widget and the ‘Unit Dashboard’ widget at 23:55 there are a couple things to note:

We see that:

1)  This outage is a short-notice outage but not a trip …

(a)  It’s come offline from only 114MW and had rebids (ready to go and) issued at 23:51 as soon as the ‘Unit shutdown complete’;

(b)  We also see (in the the ‘Generator Outages’ widget above) that the generator had allocated a few days in the MT PASA DUID Availability data prior to the shutdown for the repair process.

2)  Also at the time there’s obviously no solar, and a moderate amount of wind (3,132MW across the NEM) leaving lots of room for firming capacity, meaning a mainland inertia reading of 85,731MW.s.

MP1 offline just prior to 01:40

This was captured in this ‘Notification’ widget alert in ez2view at 01:40 (NEM time) Saturday morning:

In this window from ez2view comprised on the ‘NEM Map’ widget and the ‘Unit Dashboard’ widget at 01:40 there are also a couple things to note:

We see that:

1)  This outage is a not planned at all …

(a)  The unit was down around its minimum load overnight, but tripped from 350MW;

(b)  We also see (in the the ‘Generator Outages’ widget above) that (even at 10:40 Saturday morning) the unit was expected to make its way back online later today … or at least no repair process was noted yet in the MT PASA DUID Availability data.

2)  Similar to ~2 hours earlier:

(a)  No solar … obviously

(b)  Moderate wind (down to 2,868MW)

(c)   Level of inertia slightly lower:

i.  at this time, it was 82,976MW.s. on the mainland.

ii.  by way of comparison, at 16:15 on Monday 26th May (when the frequency dropped outside of the NOFB) it was 84,367MW.s … which is not that much lower, so we can’t say ‘less inertia’ was the reason for the frequency drop on Monday 26th May 

Four hours overnight in a frequency trace

Extracting our in-office frequency trace at the 100ms cadence and looking at the 4-hour period overnight (23:00 to 03:00 Saturday 31st May 2025) we see the following:

I’ve tried to locate the two coal unit outages in the trace, and zoomed into each 10-minute window as follows…

VP6 offline just prior to 23:55

Even zoomed in like this, it’s difficult to point to to the timing of VP6 coming offline – because the instantaneous reduction was quite small:

MP1 offline just prior to 01:35:57.1

The timing of the trip of Mt Piper 1 is much easier to locate: