Australia has been a front-runner in wind and solar penetration of its electricity networks, with South Australia now routinely getting more than half its daily requirements from wind and rooftop solar panels alone. Yet this high penetration has a consequence. In Australia, securing a grid connection for a wind farm has long ceased to be a simple exercise in checking the capacity of the local grid and ensuring compliant system performance.
Over two decades, wind and solar power have displaced the big rotating machines within thermal power plants that are traditionally relied on as the 'workhorses' keeping grids stable. Wind turbines (WTGs) and inverters are thus increasingly called upon to provide not only raw energy, but also the grid support functions that the big machines provided. This includes not only the ability to restore the system after faults and to control frequency (both of which made their way into grid codes in the 2000s), but also the ability to keep everything operating at a single frequency in the first place.
The energy transition conversation in Australia (and in other island nations like Ireland, and increasingly the UK) now focusses on ensuring adequate 'system strength' and 'inertia', initially as a boost to support connections in remote areas, but also for the whole system as more large plants retire.
Synchronous condensers are being widely deployed as the traditional, proven approach to boosting system strength: essentially replacing one big rotating machine with another. Some retired thermal plants might indeed enjoy a second life as 'syncons'. But they also have their drawbacks. Like the Statcoms widely installed in the 2000s to provide fault survivability and voltage control for older wind farms, they increase the cost and complexity of grid connection. As they contain moving parts additional to the wind rotors, they add to the ongoing maintenance effort. They also add to the complexity of wider power system operation, as although physical machines boost stability they do not do so in a completely controllable manner.
An attractive alternative is to use WTGs themselves to provide these 'grid forming' services, with most first-tier suppliers now investigating grid-forming options. Grid engineers watched with interest the 2020 trial at Dersalloch wind farm in Scotland, where for several weeks the 23 direct-drive turbines were operated in grid-forming mode. This concluded that grid-forming operation did not adversely affect the wind farm in normal operation, but observed that in a large abnormal event, the wind farm alone would be unlikely to provide a substantial degree of inertia support.
At least in the short term, the main alternative to syncons will be grid-forming batteries in a hybrid installation. This is also proven technology, with the Dalrymple grid-forming battery in South Australia providing grid frequency support and seamless 'islanding' with a nearby wind farm. The challenge for such hybrid proposals is commercial. Will the firming and arbitrage benefits of local storage provide a value proposition to support the cost of grid-forming technology? And will our electricity markets find a way to value what are currently non-market system stability services?