CS Energy has released a report on the technical factors that culminated in the Callide Unit C4 explosion in May 2021 that left almost half a million people without power.

The report is based on CS Energy’s assessment of the incident and work since the event with experts including Aurecon and independent investigator Dr Sean Brady.

See the technical investigation summary HERE

CS Energy chief executive officer Darren Busine said, “This was one of the most significant and complex process safety incidents in Australia and it has taken time to work through and understand the contributing factors to the event.

“The Callide Unit C4 incident was the result of the simultaneous failure of key electrical equipment and system back-ups in a complex series of events that could not have been anticipated, with some of the contributing factors being traced back to the original design of the power station.

“As we have gained a better understanding of the causes of the 2021 incident, we have not waited to act on what we have learned. 

“We have progressively improved safety for our people and plant, both at Callide and our other assets across the state.”

A fault tree developed to explain the incident, which happened after the turbine generator motored for 33 minutes without lubrication, shows that:

The turbine generator failure required the failure of the entire turbine lubrication system and back-up systems:

• failure of the ‘A’ AC lubrication oil pump (due to loss of AC from unintended tripping of arc flash protection)

• failure of the ‘B’ AC lubrication oil pump (due to loss of AC from unintended tripping of arc flash protection)

• failure of the DC lubrication oil pump (due to loss of DC from the loss of battery charger supply and no battery supply due to interlocking)

The loss of lubrication caused extreme overheating of bearings leading to mechanical failure, and the generator protection systems failed to disconnect the generator from the grid during the motoring event due to:

• failure of ‘X’ generator protection (due to loss of DC from the loss of battery charger supply and no battery supply due to interlocking)

• failure of ‘Y’ generator protection (loss of DC from the battery charger supply and no battery supply due to interlocking co-incident with the loss of the automatic backup functionality of the automatic changeover switch

These protection systems did not detect the motoring and de-energise the generator in a timely manner to prevent damage.

The loss of the DC system was initiated when the station interconnector was opened and the C4 220V battery charger failed to maintain the voltage on the C4 Unit Main Switchboard that was also supplying the C4 Unit 220V Distribution Board.

The interlocking design prevented a battery from being connected during this operation. This loss of DC initiated a loss of AC supply due to arc flap protection, which prevented the C4 220V Battery charger from recovering the DC voltage.

The unavailability of the automatic function of the backup automatic changeover switch (ACS) to operate prevented the recovery of the DC supply to the Y Generator Protection system. However, even if the ACS had operated, it would not have resulted in supply to the emergency turbine lubrication oil or the emergency sealing oil pump and therefore it is unlikely that it would have prevented the turbine bearing damage or rapid hydrogen combustion/explosion.

Unit C4 event simplified fault tree:

Mr Busine said CS Energy was eagerly awaiting the completion the investigation by forensic engineer Dr Brady.

“We expect the report will identify further opportunities for organisational improvement. We will make any further changes that improve the safety of our people and plant and will share any additional learnings with industry,” he said.