Across South Africa coal-fired power plants are gradually shutting down. Either through mismanagement of their builds or age, these colossal electricity producers are not capable of meeting the country’s power needs. According to global experts, this presents the country with a unique opportunity to take a significant shift towards cleaner energy sources and begin fighting the battle against climate change.
However, the closure of these large, ageing power plants and the switch to renewable energy sources poses an unexpected challenge. These plants play a crucial role in maintaining the stability of the power grid. As more of them shut down, an alternative solution is needed to fill this gap.
The power grid is a complex network consisting of power generation systems like nuclear plants and wind turbines, as well as power storage and transmission systems, such as batteries and transmission lines. Disruptions can occur for various reasons, like fallen trees on power lines or excessive demand during heatwaves. In South Africa, the grid operates at a synchronised alternating current frequency of 50Hz, which can fluctuate if demand exceeds supply or if a significant generator goes offline. Even a slight disruption in this frequency can cause ripple effects that the grid struggles to recover from.
Large power plants are designed to enhance grid stability. Their spinning generators provide inertia, buying time in case of unexpected outages, and they adjust their power output based on the grid’s frequency, ensuring stability. In contrast, renewable energy systems, like those powered by wind and solar, rely on inverters to convert their direct current (DC) electricity into the alternating current (AC) needed for the grid. These renewable energy systems with inverters behave differently from traditional power plants, requiring a new approach to grid stability.
Stabilising a renewable grid
Shutting down the coal-powered plants in favour of renewables therefore requires researchers to find solutions to maintain grid stability. One potential answer lies in grid-forming inverters, specially programmed electrical devices positioned between power generators or storage systems, such as wind turbines, solar panels, and batteries, and the grid. These grid-forming inverters can swiftly and responsively control the flow of renewable energy into the grid, mimicking the stability provided by large power plants.
By integrating grid-forming inverters into the existing power grid, engineers can compensate for the functions lost when these large plants are retired. Grid-forming inverters offer additional benefits, including the ability to automatically restart a grid after an outage, and enhancing resilience against power disruptions caused by extreme weather events linked to climate change, such as heatwaves and floods.
Grid-forming inverters work by injecting voltage into the grid and adjusting their frequency according to the power flow, allowing other sources of electricity to synchronise with them, similar to the way power stations pulsed electricity. These inverters are available in various sizes, from smaller than a microwave to as large as a shipping container, and are capable of performing critical functions previously reserved for traditional power plants.
Manufacturers like General Electric, Siemens, Tesla, and Hitachi already produce grid-forming inverters, which have been employed in isolated power grids, especially on small islands, for many years. These inverters are now gaining global popularity as major energy companies seek to accommodate the surge in renewable energy.
Integrating grid-forming capabilities into renewable energy systems is seen as an essential initial step towards building a resilient future grid. This transition is crucial, particularly as renewable energy seeks to account for a growing percentage of the electricity generation in South Africa.
New technology
In contrast to grid-forming inverters, conventional grid-following inverters, which are commonly found in home solar panel systems and industrial-scale installations, lack the ability to adjust their frequency in real-time to stabilise the grid. Grid-forming inverters are more adaptable and responsive, helping to maintain grid stability.
While grid-forming inverters have been used in microgrids and certain applications for years, scaling up to larger power grids with substantial renewable energy contributions presents new challenges. Australia leads the way in adopting grid-forming inverters, with several operational facilities and more in development. The United Kingdom is also investing in grid-forming technologies, including a 300-megawatt facility in Scotland. South Africa is predictably lagging far behind in this regard.
Sadly, there are numerous challenges being faced around the world before future grid stability can be ensured. Determining the optimal ratio of grid-forming inverters in future grids remains a question, as it depends on factors such as the grid’s age and configuration. In addition, there are questions around the calculations when renewables and these inverters are required to work alongside nuclear or hydroelectric facilities.
These challenges do, however, come with some additional potential positives. Researchers are also exploring how grids with numerous grid-forming inverters can provide power in innovative ways. By optimizing how inverters collaborate, the system could resemble a virtual power plant, where small-scale renewable energy devices work together like a traditional power plant. Machine learning techniques may play a role in fine-tuning these operations.
Overall, the adoption of grid-forming inverters offers promise in strengthening the power grid, enhancing resilience, and promoting equity, particularly in vulnerable communities affected by power outages. Renewable energy, supported by grid-forming inverters, can play a pivotal role in addressing climate change. However, the transition to a more resilient grid requires significant effort and coordination.
