Advancing the world’s net zero ambitions means that utilities are transitioning to renewable energy, but this is reducing the level of spinning inertia vital for stable operation.
Kristina Carlquist, General Manager, Synchronous Condensers, ABB explains how this well-proven technology is helping to restore the balance in projects ranging from the Faroe Islands to the UK mainland grid.
In traditional power grids, based on large, centralized plant, there can be hundreds of generators working in synchronization.
This means that they are effectively locked together, rotating with the same frequency. This is the origin of the spinning inertia, or kinetic reserve, that plays a vital role in keeping the world’s power grids in balance.
Should there be a sudden change in operating conditions, such as a generator tripping offline, the cumulative energy stored by all the synchronous generators provides a fast response, available within seconds, to resist the change.
This ensures that the system frequency remains within tightly controlled limits for long enough for the grid control system to detect the issue and take appropriate action.
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The need to transition as fast as possible to decarbonized generation means that the grid penetration of wind and solar power must increase.
However, renewable resources cannot contribute real inertia to the grid. Furthermore, large synchronous power plants that were providing inertia are being taken out of service. The overall result is a significant net loss of inertia.
The situation in the UK illustrates why this loss of inertia is a concern.
In 2016, the National Grid System Operability Framework report estimated that the available system inertia ranged from a minimum of 100 to a maximum of 350 gigavolt amp seconds (GVA.s).
By 2025/26 this was expected to fall to a minimum of around 75 and a maximum of under 300 GVA.
In its Operability Strategy Report published in 2021, National Grid says that its current policy is to ensure that system inertia is always above 140 GVA.s.
However, it notes that going forward, minimum system inertia could be as low as 96 GVA.s for zero carbon operation by 2025.
Furthermore, studies have been carried out to examine the consequences of one of the largest potential losses from the system, which is 1.8 gigawatt (GW) – equivalent to a large interconnector going offline.
In that scenario, to limit the Rate of Change of Frequency (RoCoF) to the acceptable level of less than 0.5 Hz, inertia must be kept above 90 GVA.s.
The need to address the new inertia challenges are behind the renewed interest in synchronous condensers (SCs).
These are rotating devices that can put the missing inertia back into the grid by mimicking the operation of synchronous generating plant.
This offers a very cost-effective and reliable way to maintain power quality. It also provides the fault current protection essential to strengthen a weak grid.
SCs are very similar in design to large motors and generators. There are however two differences: An SC does not drive anything so is not a motor; An SC is also not a generator as it doesn’t have a prime mover.
Historically, SCs were regarded as vital elements in the power grid as they were employed to produce reactive power to balance out highly inductive loads on the grid, like electric motors. In recent years devices based on power electronics had replaced SCs. Now they are enjoying a resurgence in interest.
Helping the Faroe Islands transition to 100 percent renewable energy
SEV is the power company serving the Faroe Islands in the North Atlantic. It operates as both the TSO and DSO as well as owning power plants.
In 2014, SEV announced its vision for the Faroes to become the world’s greenest group of islands by meeting all the country’s energy needs from renewable resources by 2030.
To achieve this, SEV intends to use green electricity from hydropower, solar, wind, and potentially tidal streams.
The transition will offer important economic benefits as the Faroe Islands will no longer be dependent on expensive fossil-fuel imports.
In 2021, SEV’s electricity generation was 40% renewable, but with expansion in wind power in 2022, the share of renewable energy is expected to increase, reaching more than 50% in 2023.
A significant challenge for SEV is that decommissioning the current diesel-fueled generating plant could impact the stability of its grid.
This could affect the fish processing and aquaculture industries that are a major contributor to the Faroese GDP.
An important factor is that the country cannot call on external grid support as there are no power cables connecting the islands to neighboring countries.
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SEV is working with ABB to install SCs to keep its grid in balance. The first SC is now in operation on Suðuroy.
This electrically isolated island is the southernmost in the archipelago and relies on a hybrid power system with heavy fuel oil, hydro power, wind power and solar photovoltaics.
The SC has been installed at the 6 megawatt (MW) Porkeri Wind Farm together with a battery energy storage system.
It is now possible for wind energy to meet 100% of the island’s needs at times with good wind conditions while ensuring the stability and reliability of its grid.
SEV has followed this installation with a similar 15 MVA unit for the main grid that serves 11 islands. It is located at Sund, close to Tórshavn, the Faroese capital on the island of Streymoy.
This is scheduled to be online in 2023. Further batteries and SCs are planned for other sites in the main grid in the near future.
High-inertia SCs for Statkraft in Liverpool
Statkraft, Europe’s largest renewable energy producer, has installed two high-inertia SC systems for the Lister Drive Greener Grid project in Liverpool, England.
This innovative project will play a key role in stabilizing the local grid to handle more wind and solar power, helping National Grid meet its target of operating a zero-carbon electricity system by 2025.
The Lister Drive site was selected due to its location near to an existing substation that enables the two SCs to connect to the grid at 400 kilovolts (kV).
It is ABB’s first project anywhere in the world to feature a high-inertia SC configuration. This couples a 67 MVAr SC with a 40-tonne flywheel that increases the instantaneously available inertia by 3.5 times.
The advantage of combining a mid-size SC with a flywheel is that it multiplies the available inertia by several times.
The losses are also much lower compared to installing the whole inertia as an SC. The scheme is a cost-effective way of using two mid-sized SCs coupled together with the benefits of a high level of redundancy, increased inertia and greater controllability.
Together, the two Statkraft units in Liverpool will provide a total of more than 900 MW.s (megawatt-seconds) inertia. That means Lister Drive will provide about 1 percent of the UK’s projected minimum total inertia requirement for 2025.
Construction of the project is well advanced. The SCs have been delivered and are being commissioned with the site expected to be online in early 2023.
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To ensure round-the-clock availability for this vital system, Statkraft has signed a 10-year services contract with ABB’s UK field service team to provide a full range of maintenance services, both planned and quick response.
Digital condition monitoring solutions will be deployed to optimize performance and predict maintenance needs. By assessing real-time data with cloud-based analysis, the team will be able to plan corrective actions before issues occur, ensuring system reliability.
SCs offer a green solution to grid stability
Worldwide, the grid penetration of renewable energy is making rapid progress. This welcome news is also creating some additional challenges for grid stability.
At some times it is even necessary to shut down wind farms and operate gas power plants to keep the grid frequency within its acceptable operating limits.
Deploying synchronous condensers can help make this a thing of the past by maintaining stability without consuming fossil fuels.
The continuing success of the new generation of SCs is opening up the possibility of many further projects being developed across the world.
Located at strategic points on the power grid, they will help prevent power outages, ensure stability and most importantly contribute to the reduction of greenhouse gas emissions from energy production.
