- As renewable energy sources such as solar, wind, and battery storage increasingly power South Africa’s electricity system, the way these resources connect to and interact with the grid has become critically important.
- Two key technologies define how inverter-based energy systems operate — grid-following and grid-forming inverters.
- Understanding the difference between them is essential for building a reliable, flexible, and resilient power grid.
Inverters are electronic devices that convert direct current (DC) electricity — typically produced by solar panels, batteries, or wind turbines — into alternating current (AC), which can be used by homes, businesses, and the grid. The way an inverter controls its output determines whether it is classified as grid-following or grid-forming.
Grid-following inverters: synchronising with the grid
Grid-following inverters are the most common type used today in grid-connected renewable energy systems. These inverters rely on the existing grid voltage and frequency as a reference point to inject or absorb power. Acting as current sources, they synchronise their output to the grid’s “signal,” following its voltage angle and magnitude.
Because grid-following inverters depend on a stable grid to operate, they cannot function in island or off-grid mode. They are best suited for applications such as rooftop solar systems, large-scale solar farms, energy storage systems, and electric vehicle charging stations connected to the main grid.
Advantages of grid-following inverters include:
- Simpler design and lower cost compared to grid-forming inverters.
- Faster power control and response times.
- Fewer technical and regulatory challenges related to protection, synchronisation, and coordination.
However, they also have limitations:
- Inability to operate independently from the grid.
- Lack of voltage and frequency support during grid disturbances or outages.
- Inability to provide key ancillary services such as inertia, system strength, and frequency response.
- Potential to cause grid stability issues at high levels of renewable energy penetration.
As renewable energy continues to expand, these limitations are becoming more significant, prompting a shift toward more advanced inverter technologies.
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Grid-forming Inverters: creating and stabilising the grid
Grid-forming inverters represent the next generation of inverter technology. Unlike grid-following systems, they can create and regulate voltage and frequency, acting as a voltage source rather than a current source. This means they can operate independently, from an island grid, or coordinate with other sources to maintain grid stability.
Grid-forming inverters are capable of providing voltage and frequency support, helping the grid remain stable during disturbances or even restoring it after a blackout. In essence, they behave more like traditional synchronous generators — the spinning turbines of conventional power plants that have historically provided grid inertia and stability.
Key capabilities of grid-forming inverters include:
- Operating independently or alongside other energy sources.
- Providing essential ancillary services such as inertia, system strength, voltage regulation, and frequency response.
- Supporting the grid during outages or disturbances.
- Enabling black start capability — restarting the grid after a total shutdown.
However, they also come with challenges:
- Higher cost and technical complexity.
- Slower power control and response compared to grid-following systems.
- Ongoing development of standards, regulatory frameworks, and protection systems to enable widespread adoption.
Modes and applications of grid-forming inverters
Grid-forming inverters use different control modes such as droop control, virtual synchronous machine, or hierarchical control, depending on grid conditions and desired performance. These modes allow the inverter to mimic the behaviour of synchronous generators, maintaining voltage and frequency stability even in weak or isolated grids.
Applications range from micro grids and island systems to large-scale renewable integration, where maintaining power quality and reliability is increasingly difficult as fossil-fuel-based generators retire.
Why grid-forming technology matters for the energy transition
As the global share of renewable energy grows, power systems are becoming more dependent on inverter-based resources. However, without sufficient sources of inertia and voltage support, grids risk becoming unstable. Grid-forming technology addresses this challenge by enabling renewable and storage systems to play a stabilising role once provided by conventional power plants.
Research and development efforts are underway worldwide to refine grid-forming inverter technology and create consistent technical standards. Once fully integrated, grid-forming inverters will be critical for enabling 100% renewable power systems that are both resilient and self-sustaining.
The Path Ahead
Both grid-following and grid-forming inverters are vital to today’s and tomorrow’s power systems. Grid-following inverters remain cost-effective for most grid-connected applications, but as renewable penetration increases, grid-forming inverters will become indispensable for ensuring grid stability, reliability, and flexibility.
In the evolving landscape of clean energy, the transition from grid-following to grid-forming technology represents a crucial step toward a smarter, more resilient, and fully renewable electricity grid in South Africa.
Author: Bryan Groenendaal
