Frequency change with Yallourn unit 2 trip

Earlier today we noted the trip of Yallourn unit 2 in Notable forced outage at Yallourn Unit 2 … due to “LP turbine issue”.

The trip happened in the 13:35 interval on the 26th August 2025.

The unit’s output at the start of the interval was 368 MW.

This is a large enough change to drive a frequency deviation of between -0.1 to -0.2 Hz, based on analysis from Frequency change associated with coal generation events.

Using the 0.1-second frequency measurements and 4-second data we inspect the impact on system frequency.

A drop of 0.1 Hz was measured by the Global-Roam frequency logger

Measurements from Brisbane indicate system frequency was 49.951 Hz before the trip and declined to 49.851 Hz over about 1 second. That’s a change of -0.1 Hz.

The fall appears to have been arrested around that (49.85 Hz) level, lasting another second before recovering. A low point of 49.838 was measured during that period.

The AEMO 4-second measurements corroborate the dip. At 4-second measurement granularity the change was from 49.958 to 49.864 across one 4-second period.

The chart from the Global-Roam loggers zoom into the event:

A large negative deviation from target in VIC

The trip appears as a large negative deviation from target in VIC. The chart below captures the deviation as a purple series using AEMO 4-second readings. 

Corresponding corrective deviations, in aggregate, appear from scheduled units in all mainland regions.

TAS units are omitted as Basslink’s target for the interval was zero, therefore not able to transfer FCAS.

No obvious material change in semi-scheduled unit deviations aligning with the event is evident in the data. A small spike appears in SA but in general aggregate deviations of semi-scheduled units were within their typical variability range for the interval.

We can also note, in the leadup to the event, how there was a general under supply of power (relative to dispatch targets) from semi-scheduled units in VIC (-180 MW) and NSW (-100 MW). And an equivalent but opposite deviation in the scheduled units, in aggregate.