The right momentum for grid stability

By Frank Krull

In Robertstown, South Australia, which lies 90 minutes north of the port city of Adelaide by car, people enjoy advertising that it’s “at Worlds End.” The landscape there really is truly very isolated and is defined by the picturesque contrast between farmland and a drought-stricken region that stretches into the distance. There is, however, another reason for promoting the area as “Worlds End.” It’s due to a nearby nature reserve of the same name that’s very popular with tourists.

Robertstown can’t claim the superlative “Worlds End” for itself, however. Due to the boom in solar and wind farms in South Australia, the location has become a hub for the region’s energy supply system. In addition to a central substation, a high-tech grid stabilization plant has been in operation here since September 30 of this year. The high proportion of electricity from renewables in the South Australian grid, which already exceeds 50 percent and is continuing to rise, makes this step essential. “Robertstown is the perfect location for the plant,” confirms Simon Emms, in charge of Network Services in the executive team of local grid operator ElectraNet.

If electricity supply and demand don’t match in an AC grid – because electricity producers drop out or new major consumers demand electricity – the grid frequency will also drop. To ensure that the actual frequency doesn’t deviate significantly from the rated value of 50 Hz, more electricity has to be fed into the grid as quickly as possible to mitigate the risk of a blackout. If the frequency drops below a certain threshold, parts of the grid need to be switched off to prevent damage to the generators at the power plants; these drops can cause destructive vibration.

Since more and more fossil-based power stations are being replaced by solar and wind farms, grids increasingly lack the inertia created by the large rotating masses in the generators and turbines at fossil-based power stations. “These rotating masses are extremely important in the event of a drop in frequency,” explains Peter Luijmers from Siemens Energy, who’s in charge of constructing the grid stabilization plant in Robertstown. “They store huge amounts of energy that’s available at all times with no delay to cover the initial fractions of seconds of a frequency drop. This slows the drop for long enough to feed more electricity into the grid using batteries, supercapacitors, quick-start gas motors, or other fossil-based power stations in standby to restore the frequency to its rated value again.”

Solar and wind farms don’t have large masses that rotate at grid frequency, and therefore they can’t contribute inertia to stabilize grids. Grid operators like ElectraNet, whose duties also include maintaining grid stability, are aware of this problem and are installing countermeasures. Using synchronous condensers with flywheels, they can restore the lost inertia in their grids. This is exactly the reason for the expansion of the substation in Robertstown, where two of these combinations were installed.

The solution is obvious. Synchronous condensers have long been used to stabilize AC grids: These are large generators incorporated into the grid, which rotate at the same frequency as the grid. Although they don’t generate active current that could be used to operate air-conditioning equipment, industrial plants, or to charge smartphones, for example, they still have a very important role, because they help balance what is known as reactive current. This current is a characteristic of AC grids, because the active current is transmitted using magnetic fields that are generated by the reactive current.

Due to their large rotating masses, synchronous condensers themselves contribute a reasonable amount of inertia, but many grids need more. This is where the opportunity to multiply the inertia comes in by adding flywheels. Although flywheels mean that synchronous condensers need to draw more current from the grid to remain in rotation and perform their function, innovations in recent years have made solutions available that consume only minimal power. “For us, electricity consumption is a key factor in selecting a solution,” emphasizes Emms from ElectraNet. “Every kilowatt we save counts for us, both economically and environmentally.”

The developers at Siemens Energy have ensured that power losses for operating flywheels are kept as low as possible. The flywheels used in Robertstown rotate in an underpressure close to a perfect vacuum. This reduces frictional heat and therefore energy losses – which would occur even if a surface as smooth as a mirror were turning in air – by 90 percent. “That’s a peak value that we can achieve only because we use special seals at the point where the flywheel rotor exits the underpressure housing,” explains Philipp Büttner, in charge of flywheel development for Siemens Energy at the Center for Energy Transition Technologies in Mülheim an der Ruhr, Germany.

He and his team have also developed a special cooling system for the flywheels to reliably dissipate the frictional heat that develops on the flywheel surface even in underpressure operation. The solution is a water-cooled outer sheath connected to the plant cooling system. In contrast to air-cooling it does not require an emergency mechanism to ensure cooling at all times. This is especially important in the event of a power failure, where the underpressure is broken and the frictional heat rises abruptly. A water-cooled system like the one used with the flywheels in Robertstown compensates this effect due to the ability of the water to absorb a lot of heat, and to buffer sudden temperature changes.

Despite the water cooling system, the flywheels from Mülheim have a very small footprint thanks to their optimized design. In Robertstown, they each multiply the inertia of the synchronous condensers by a factor of three. But even so, they’re just under six meters long. The length of the synchronous condensers is almost double that. “This creates a huge amount of space in the machine room that we can put to good use in other ways,” says ElectraNet’s Emms.

“Our design, however, also allows us to grow very flexibly in length when more inertia is needed,” Büttner adds. “We’re currently building a flywheel with a total mass of about 180 metric tons for Irish grid operator ESB Networks to help stabilize the grid in Moneypoint in Southwest Ireland. That will be a new world record for flywheels used in grid stabilization.”

With the two flywheels in Robertstown, ElectraNet has covered its requirements for inertia in South Australia for now, at least. But if the number of solar and wind farms there continues to grow as quickly as it has in recent years, they certainly won’t remain the only ones, nor are they likely to be the largest.

November 22, 2021

Frank Krull is a physicist and journalist who works in Communications at Siemens Energy.

Combined picture and video credits: ElectraNet, Siemens Energy