In this post we inspect measured system frequency around 08:25 on April 10, 2025.
The period is of interest due to the observed 582MW (instantaneous?) drop in ‘Market Demand’ in Victoria which delivered a large System Frequency spike.
The cause, later confirmed in a market notice, was an an unplanned outage of the APD – Heywood – Mortlake 500 kV line. The line was returned to service by 09:48.
Frequency spiked
Global-Roam’s high speed frequency logger provides measurements at 0.1 second intervals from its Brisbane location. We’ve used this to assess the impact of the outage on system frequency.
The timeseries chart of the frequency measurements shows a measured peak of 50.164 Hz at 18:25:00.809:
- Top: 30 minutes between 08:10 and 08:40.
- Bottom: A zoomed-in 10 minute series between 08:23 and 08:33.
In the zoomed-in series we observe the frequency event exhibited a very brief period outside the upper Normal Operating Frequency Band (above 50.15 Hz).
Within about 6 seconds frequency had recovered to 50.075, within the NOFB, but still above the the primary frequency control band of 50.015.
Frequency crossed 50Hz at roughly 08:32:52, approximately 8 minutes after the initial event.
Rate of Change of Frequency (RoCoF)
In the Frequency Operating Standard for the mainland, following a credible contingency event (which may be a generation event, a load event or a network event), the rate of change of frequency must not be greater than ±1Hz/s (measured over any 500ms period).
The next chart shows the peak RoCoF was below that requirement.
It appears, from the data we’ve used, the disturbance was well contained, within operating standards.
It would appear that the trippimg of the 500kV line caused the trippimg/off-loading of an aluminium potline smelter at Portland which in turn caused a drop in system demand and thus a rise in frequency. When did the smelter return to service?
Hi Robert, correct – looks like it was off-loading rather than a trip at the smelter. From this morning’s data it looks like the smelter began its return to normal loading just after 08:45. Noting that time is before the line was officially returned to service (as per the market notice, 09:48).