Victoria faces the loss of another major thermal generation unit after emergency service personnel were called to an incident at the Mortlake power station that will result in one unit being closed – possibly until Christmas.

Investigations are still underway at the 550MW Mortlake power station, with owners Origin Energy currently examining telemetry data collected from the turbine, before a physical examination of the damage can be undertaken.

“The Country Fire Authority attended the site and returned operational control to Origin within three hours, with no harm to our people nor any adverse impact on the local community,” Origin Energy said in a statement.

There has been some confusion over the scale and nature of the incident, not least compounded by firefighters telling media of an “explosion” at the site.

The Country Fire Authority (CFA) attended the incident, with no injuries or harm to power station workers or the local community, with the second generation unit at the Mortlake power station continuing to remain operational.

CFA Commander Paul Marshall told radio station 3AW that there had been a “rather large explosion” at the Mortlake site, but Origin has since downplayed the scale of the incident, saying that it was an “electrical fault”.

“It was a rather large explosion, it certainly wasn’t a minor explosion, so that will obviously take a while to rectify,” Marshall told 3AW.

“There was significant damage to the generator.”

An Origin Energy spokesperson told RenewEconomy that the incident likely consisted of an “electrical arc”, but Origin Energy will undertake a physical examination of the unit to assess the extent of the damage.

Electrical arcs can be caused at facilities operating with high voltages, such as power stations or switch yards, when current effectively short circuits via the air, or when other conductive material is introduced into an environment. The unwanted transfer of energy via arcing can be hugely damaging, especially to high-precision turbine and generator equipment.

Origin Energy estimates that the damaged unit will be offline for between three to six months, informing AEMO that 20 December is the date the power station will return to full operation, but is confident that the unit will be operation again well within the six-month window.

“Origin will work with the market operator and our suppliers with the aim of making the damaged unit available ahead of the summer peak and to reduce the impact on customers,” Origin Energy’s executive general manager for energy supply and operations Greg Jarvis said.

The Mortlake power station provides peaking services to the National Electricity Market, and the loss of half of the power station’s 550MW capacity will put further strain on a Victorian grid already impacted by outages at some of the state’s largest power stations, and highlights the vulnerability of the state’s ageing thermal fleet.

A separate issue at the Loy Yang A power station will also see one of its 530MW generator units also remain offline until Christmas.

Both generators are currently scheduled to be back online in time for the peak of the Victorian summer, but following load-shedding events during the prior summer, the loss of over 750MW of generation capacity will make for nervous times for system operators.

Up to 60,000 Victorian homes were impacted by load-shedding events in January, during days of extremely high temperatures, as brown-coal generators suffered down-ratings in their available generation capacity as they too struggled in the heat.

AEMO recently completed an assessment of the adequacy of the Victorian energy system, which noted that it believes sufficient investment in new generation infrastructure should be sufficient to maintain reliable supply through the exit of the State’s ageing fleet of coal-fired generators, but additional investment will be needed to strengthen the network transmission system across the state.