The Tesla battery has two connections to the grid.
One is 70MW at the connection.
It's services are contracted to the State Government for 10 years and is there to provide:
*backup to the grid to reduce the need for load shedding in the case of an unplanned event
*frequency control and ancillary services to stabilise the grid.
The event you describe is one of the reasons the battery was assembled, to provide an extra competitor to the gas generators in the FCAS market.
This was a planned outage, probably for maintenance of a network component, and the market operator called for bids to provide the service.
IIRC it's a service usually arranged for a day ahead.
The battery operator bid into the market at a lower price than gas operators previously charged.
The other part of the battery, 30MW at the connection also bid a small amount of power for that planned outage.
That 30MW connection, under the control of the adjacent windfarm, is charging and discharging all the time playing what looks like the arbitrage game.
It's activity can be seen via the widget on this page.
