South Australia has reached 100% renewable generation on an instantaneous basis even with most inverters being grid following by using spinning condensers (spinning turbines connected to the grid that don’t generate energy).
And more resources are being deployed/enabled to support longer periods of 100% renewable generation. For example the ‘Hornsdale Reserve’ Tesla battery is now providing synthetic inertia services to the grid (slowing the rate of change of frequency).
‘Virtual power plants’ consisting of large portfolios of small behind-the-meter batteries are being enabled for high-speed frequency contingency response services (rapidly responding to large changes in grid frequency).
And more true ‘grid forming’ battery energy storage systems are in development.