Frequency drop on Thursday evening 11th Sept 2025 with Tallawarra B trip

On Monday (via the article ‘Highlighting particular (frequency and/or AggROT) events through 2024 and 2025’) we wrote about some newer-style frequency disruptions we’re seeing increasingly that stem from:

• instances collective under-performance of all Semi-Scheduled units (which map to large positive Aggregate Raw Off-Target).
• and, much less often, collective over-performance (which map to large negative Aggregate Raw Off-Target for all these units)

Frequency drop at 17:52

‘Older-style’ frequency disruptions can and still do occur with fully Scheduled units doing unexpected things – and we spotted one of them yesterday evening (Thursday 11th September 2025) as captured here in this extract from one of our live frequency monitors, that captures frequency in Brisbane at 0.1 second cadence:

As noted here, the frequency did drop quite sharply (though not outside of the NOFB), with the lowest point being 49.877Hz at 17:52:19.7 (NEM time) on Thursday … so within the 17:55 dispatch interval.

Trip of the Tallawarra B unit

Time-Travelling ez2view back to the 17:55 dispatch interval, we quickly see the trip of the Tallawarra B unit was the largest single unexpected reduction in output – hence a primary contributor to the frequency drop in this instance:

As we can see here:

• NEMDE provided dispatch instructions for the unit to hold steady (i.e. Initial MW of 308MW to Target of 310MW)
• But the unit instead tripped to 0MW
• A Dispatch Error of +310MW (and a ‘Gen Change’ of -308MW)

No ill-effects at the unit?

Capturing some more recent data from the unit since that time via the ‘Bids & Offers’ widget in ez2view we see the following:

Worth highlighting three things:

• Rebid reasons following the trip (but cut-off at 04:00 this morning) speak to an investigation of the trip
• But we see the unit did get a run again this morning
• And the Unit Availability in PD/ST PASA is up at the MaxCapacity of the unit

… so we’re assuming no ill-effects.