Four units trip in Victoria on Easter Saturday

My quiet, isolated (thanks to COVID-19) afternoon Easter Saturday was interrupted by my phone buzzing as a result of some ‘unit suspect trip’ alerts I have set up in an office display copy of ez2view to keep an eye on the operation of the operations of all of the 48 operational coal units* across the NEM:

*  This alert has been configured to alert only on large changes in output at the coal units for several reasons, including:
(a)  For now, the disproportionate significance of these 48 units in a NEM-wide fleet of more than 300 DUIDs of all different colours; and
(b)  Because many of those other colours are significantly more ‘noisy’ than the coal units and so alerts on change in output would quickly become unusable.

Three alerts triggered this afternoon, as seen here in this email record:

Alerts Received Brief Notes
2020-04-11-ez2viewAlerts At 13:31 we see two alerts received for unit trips at two coal units (Yallourn units 1 and unit 4).  These alerts were triggered for data published for the 13:35 dispatch interval (using the ‘INITIALMW’ SCADA snapshot reading taken just prior to 13:30 – i.e. the start of the dispatch interval).

At 13:36 we see another alert received for another unit trip (Yallourn unit 3), which I presumed to be for related reasons.

At 14:10 we receive an alert on a Market Notice that is related to the above that we will explore below.


To provide some broader context to what’s been happening across the NEM, and particularly in the Victorian region, here’s a snapshot from NEMwatch v10 taken at 14:30 this afternoon:


Winding back the clock to the 13:30 dispatch interval via Time-Travel in ez2view we see the state of the market immediately prior to the event, with dispatch prices on the mainland bumping along just around $0/MWh due to low levels of demand for Easter (perhaps assisted further by COVID-19 response) and strong supply from wind:


In the 13:35 dispatch interval we see that the two units at Yallourn (units 1 and 4) trips in the Latrobe Valley, along with a major reduction in supply from Macarthur Wind Farm out in western Victoria (500km away).  The coincidence of these events suggests that they may be related in some way, though it’s not clear at this point how:


Five minutes later (in the 13:40 dispatch interval) we see the third unit of Yallourn trip to 0MW:


At 13:45 we see a Market Notice issued relating to lightning in the area of one of the 220kV connections between the Latrobe Valley and Melbourne which affects the Yallourn units:


The text of Market Notice 75305 was as follows:


Notice ID            75305
Notice Type ID        Reclassify contingency events
Notice Type Description    MARKET
Issue Date            Saturday, 11 April 2020
External Reference        Reclassification of a Non-Credible Contingency Event: Hazelwood – Rowville No.1 220kV line and Hazelwood – Rowville No.2 220kV line in VIC1 due to Lightning.



Reclassification of a Non-Credible Contingency Event as a Credible Contingency Event due to Lightning. AEMO considers the simultaneous trip of the following circuits to now be more likely and reasonably possible. Accordingly AEMO has reclassified it as a credible contingency event.

Region: VIC1

Lines: Hazelwood – Rowville No.1 220kV and Hazelwood – Rowville No.2 220kV

Duration: 11/04/2020 13:43 until further notice

Constraint set(s) invoked:
V-HWRO_R_N-2 Constraint set contains the following interconnector(s) on the left hand side:

Auto-generated on behalf of Manager NEM Real Time Operations

This constraint set ‘V-HWRO_R_N-2’ have two equations as members of the set – each of which act to constrain down the output of the Yallourn units (three of which are already offline).  The coincidence of the AEMO action suggests that the cause might be a lightning strike on the transmission network, tripping units 1 and 4 (with Macarthur WF output drop quickly afterwards), with unit 3 shortly afterwards.

At 14:07 (in the 14:10 dispatch interval) the AEMO subsequently issued Market Notice 75304 was as follows:


Notice ID            75304
Notice Type ID        Emergency events/conditions
Notice Type Description    MARKET
Issue Date            Saturday, 11 April 2020
External Reference        Non-credible contingency event – Vic  region



Non-credible contingency event – Vic region – 11/04/2020

At Yallourn Power Station the following units tripped
At 1326 hrs – Unit W3
At 1328 hrs – Units W1 and W4

also at 1326 hrs Macarthur Wind Farm 4 collector groups (300 MW) disconnected.

AEMO has not been advised of any disconnection of bulk electrical load.

AEMO did not instruct load shedding.

The cause of this non credible contingency event is not known at this stage.

Manager NEM Real Time Operations

Note that there are some time-stamps mentioned in the AEMO notice here that don’t seem to line up with the dispatch interval data (e.g. it appears that Yallourn unit 3 was at 272MW at 13:30 based on InitialMW for the 13:35 dispatch interval – but this might have been due to timing of the SCADA snapshots into NEMDE).

That’s all I have time for today.


PS January 2021

On or around January 2021, the AEMO published this Operating Incident Report into the “Trip of Yallourn generating units 1, 3, 4 and four Macarthur Wind Farm collector groups on 11 April 2020”, which is a useful addition to the above:

About the Author

Paul McArdle
One of three founders of Global-Roam back in 2000, Paul has been CEO of the company since that time. As an author on WattClarity, Paul's focus has been to help make the electricity market more understandable.

Leave a comment

Your email address will not be published.