Grid-Forming Control Technology

A ReFlexion of Dongsheng Yang, Assistant Professor at TU/e

It’s about how we feed the electricity into the grid.

As we all know, the traditional method of generating electricity involves using generators in power plants to convert fossil fuels into electrical energy. However, due to the significant greenhouse gas emissions associated with this approach, the conventional means of electricity generation are gradually being abandoned in favour of achieving carbon neutrality.

Consequently, an increasing number of renewable energy resources are being integrated into the power system to supply electricity to the utility grid. Unlike conventional electrical machines that can independently produce and transmit power to the main grid, these green power resources, such as wind and solar energy, require the assistance of a "converter" to transport the generated electricity to the connected grid.

Converters are devices capable of converting direct current (DC) to alternating current (AC) and vice versa. For instance, solar panels generate DC power, which needs to be converted to AC power for seamless integration into the main grid. Moreover, converters consist of power electronic devices that provide controllability, which helps in dealing with the intermittent nature of renewable energy resources.

Grid integration
Figure 1: Working principle of active power transmission to a grid.

Currently, we follow the grid

So, how is active power injected from green power resources into the main grid? The converter connected to the green power source generates an AC voltage with a certain amplitude V1 and phase angle Ɵ1, which are different from those of the AC grid voltage (with magnitude V2 and phase angle Ɵ2). This is illustrated in Figure 2.

Figure 2: Simplified depiction of converter and grid voltage
Figure 2: Simplified depiction of converter and grid voltage

The working principle to transmit power between these two voltages is illustrated in Figure 3. The amount of active power that can be transmitted from the renewable energy side to the main grid is primarily determined by the phase angle difference (ΔƟ = Ɵ1 - Ɵ2). Therefore, it is crucial for the converters to have information on the grid phase angle, enabling them to adjust their own phase angle to facilitate active power transportation.

Currently, the industry's predominant method for identifying the grid phase angle is using Phase-Locked Loop (PLL) technology. The PLL monitors the grid voltage and tracks the grid phase angle, which allows the converter to follow and stay synchronised with the grid. Thus, this control philosophy for converters is known as grid-following control.

You can compare it with adaptive cruise control

Grid-following control technology can be compared with vehicles' adaptive cruise control systems. In these systems, car A uses information about the existing traffic to adjust its speed, just like the converter requires information from the grid side to realise synchronisation. Car A moves independently. Car B has cruise control, and thus, it adapts to what car A does. Car B is "tracking" or "following" what car A does and adapts to that. In this analogy, car A is the grid and car B is the grid-following converter.

However, it is worth noting that using PLL to detect the grid phase angle for grid-following control negatively impacts the system's stability. Also, since it relies on following an existing voltage phasor, a grid-following control converter alone cannot supply an electric load without an already existing bulk power system.

Grid Following Control
Figure 3 Working principle of active power transmission with a grid-following converter

Shortly, we ‘drive autonomous’ by forming the grid

In recent years, a more advanced approach named grid-forming control has gained momentum and attracted both research and industrial interests. For grid-forming technology, it is not essential to know the phase angle of the grid. This is because it can achieve self-synchronisation with the main grid by emulating the natural behaviour of a traditional synchronous machine. It can flexibly establish its own operating voltage magnitude and phase angle and even support an isolated island system without any connection to the main grid. Consequently, grid-forming control technology offers more significant benefits for system stability.

Grid-forming technology can be compared to an autonomous driving control system for vehicles due to its capability of working stand-alone, forming the grid: both cars A and B move independently, and both have autonomous driving. In a sense, it can be said that both are doing their own thing while being coordinated (to avoid collision), without the need for communication with each other. In this analogy, both cars are grid-forming converters; or one car is a grid-forming converter and the other the grid.

Grid Forming Control
Figure 4 Working principle of active power transmission with a grid-forming converter

In the end, …

As the automotive industry undergoes digitalisation, vehicle control systems evolve from adaptive cruise control to autonomous driving, ultimately towards fully self-driving systems.

Similarly, the power grid is progressively integrating more renewable resources, leading to the evolution of control technologies from grid-following to grid-forming. This ultimately culminates in creating a fully power electronics grid, eliminating the need for traditional fossil-fuel generators. In summary, grid-forming technology paves the way for enhanced flexibility and stability in integrating renewable energy, representing a milestone towards the final realisation of a "100% renewable grid" which is dominated by power electronics.