Queensland's 'SuperGrid' plan to accommodate more renewables

Australia has been adding renewables to the grid at a ferocious rate, with 35% of its total electricity generation coming from renewable energy sources in 2023. Queensland is one of the states leading the way in overhauling its generation portfolio to create the grid of the future.

By Drew Robb, consultant at Robb Editorial

In Australia, solar comprised 16%, wind 12%, and hydro 6% - representing more than a doubling of renewable generation over the past decade. This level of expansion is not expected to slow down any time soon.

Queensland, in particular, is one of the states leading the way in overhauling its generation portfolio to create the grid of the future. Although the state was 70% dependent on coal generation in 2023, the plan is to aggressively transition to 70% renewables by 2032. That means that 8.1GW of coal has to be phased out and replaced by 25GW of large-scale wind and solar and 7GW of rooftop solar.

This isn’t just some idle hope or political manifesto.

The work is going on now at full speed. As well as adding massive amounts of renewables, the government of Queensland plan includes 3GW of battery storage, substantial investment in the transmission network, and 6GW of pumped hydro storage (PHS).

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Grid stability suffers

The first phase of the Queensland “SuperGrid” plan is adding 6GW of renewables and four high-voltage backbone transmission lines to connect renewable assets to distant demand centers.

But adding such a large quantity of renewables and removing so many turbines from the grid comes with its own set of challenges. Grid stability from rotating equipment is substantially lowered. Thus, Queensland planners have mandated 3GW of new gas turbine plants along with the retrofitting of aging gas turbines to operate as synchronous condensers. These actions are vitally needed to provide system strength and grid stability.

“The market has changed in Australia,” said James Lee, chief operating officer, Ratch-Australia Corp., a company that owns many renewable and non-renewable generation assets in Queensland. “The government has a big push with renewable energy.”

Ratch owns a power station in the northern Queensland town of Townsville, which is strategically located on the grid. A 500kV transmission line that is 750 km long will connect it to a new PHS facility and other energy assets to the south. Another 370 km high voltage line will connect to the massive Hughenden wind and solar complex to the west.

To support this expansion, the Australian Renewable Energy Agency (ARENA) has called for existing generators to be re-purposed as synchronous condensers to stabilise voltage, provide system strength, add inertia, provide reactive power for voltage control, and minimise fault levels.

Townsville upgrades

Townsville Power Station (TPS) consists of a 160MW Siemens Energy SGT5-2000E gas turbine, a steam turbine, and an 82MW heat recovery steam generator (HRSG). Its function is shifting due to the presence of renewables. Going forward, it will probably only provide peaking power or act as backup power in the event of a grid disturbance elsewhere. Thus, the economic future of TPS is threatened.

Coordination between the Queensland transmission operator Powerlink, Ratch, and Siemens Energy concluded that the least-cost option to address system strength services was the addition of a clutch from SSS Gears Ltd. to the shaft between the gas turbine and the generator at TPS. Siemens Energy now offers this upgrade package to turbine owners who find their revenue eroded by renewable expansion.

This is known as a hybrid rotating grid stabilizer (RGS), and the upgrade is scheduled during a major outage in 2025. Conversion of the Siemens Energy SGT5-2000E gas turbine requires modifications such as a tapered air inlet design to make room for the installation of a synchro-self-shifting (SSS) clutch between the gas turbine and its generator.

In normal peaking operation, the clutch is engaged and the gas turbine drives the generator to produce power for the grid. In synchronous condensing mode, the clutch is disengaged, allowing the generator to rotate at 3000 rpm without the gas turbine to provide stabilisation services for the network (inertia, voltage control, short circuit power).

This provides an instantaneous switch from power generation to synchronous condenser mode. When in synchronous condenser mode, the RGS unit can provide rotating inertia and short-circuit power without the need to produce power. Its short-circuit contribution will be between 350 and 400 MVA and its electrical inertia will be around 250 megawatt seconds (MW.s).

When TPS uses the SSS clutch to switch to power generation mode, it can provide about 1,000MW.s. As the plant will no longer produce as much revenue from power generation, it can remain in operation by offering grid stabilisation services, for which it will be compensated by the state.

“Power stations face many challenges going forward,” said Ratch’s Lee. “We're trying to make this business viable and monetize what we have inside. This way, we get to maintain the station, extend our business, and provide jobs to our existing staff.”

According to Siemens Energy, the RGS upgrade of an existing gas turbine is about half the cost of buying a new synchronous condenser. The lead time is much shorter, too – about 18 months for the RGS instead of three years for typical new synchronous condenser installations. Further, a new synchronous condenser only provides one revenue stream – ancillary services. Ratch-Australia can switch the unit from synchronous condensing mode to power generation mode in seconds and make money at those times when power prices soar due to lack of availability of wind or solar power.

“The hybrid RGS conversion works primarily because of the clutch that is installed between the gas turbine and the generator,” said Stanley Wilson, manager, Large Gas Turbines Service, Siemens Australia.

“This allows the unit to operate in two modes: As a generator and also as a synchronous condenser while the guest turbine is shut down, not costing money to operate. Using existing infrastructure makes the upgrade much cheaper and faster to implement.”