In power systems undergoing a transition to net-zero carbon emissions, inverter-based resources must provide a new type of service to maintain an adequate level of security and quality of service. For this reason, in recent years, the grid control concept of so-called grid-forming (GFM) is regarded as an alternative means to ensure the security of the power system. GFM involves voltage and frequency regulation and improvement of angular stability in the grid by power electronic converters. The reduced mass inertia of the rotating component of synchronous generators should be replaced by the equivalent behaviour of grid-forming inverters, i.e. the so-called electrical or (virtual) inertia.
Highlights
Inverters are characterised by little or no energy storage capacity, on the other hand, they are actuated at much shorter time intervals than synchronous machines, which store kinetic energy depending on the moment of inertia of the rotor. Hence, the optimal control of properly built and configured inverters is considered one of the key challenges in the massive integration of renewable energy sources into existing power grids. The control of inverters can follow two different functions:
- The grid-forming function, which maintains a constant internal voltage angle and frequency, controlled to maintain synchronism with other equipment and regulate the active and reactive power of the inverter during the sub-transition interval. The function can additionally provide emergency start-up and continue operation even in the absence of synchronous generators.
- The grid-following function, which maintains a constant value of output current to control the reactive and active power injected by the inverter source into the grid during the time interval from sub-transition to transition state. These inverters are inherently dependent on the strength of the grid to which they are connected and cannot operate in island mode or provide emergency start-up.
While inverters are currently set on grid-following mode, discussions are underway to introduce obligations relating to GFM in the new Network Code on Requirements for Generators (RfG2).
Challenges and opportunities for DSOs
For an energy supply system that is 100% renewable, GFM solutions are a guarantee of grid quality and stability, as they positively affect the following factors: inertia, system resilience, short-circuit power, system restoration, power system stabilization, and power quality. The use of such functionalities could be an alternative to building new lines, reduce the need for activities of grid reinforcement and redistribution, ensure the stability of an electricity grid fully powered by clean energy, and guarantee the security of the power supply. As one of the goals of this function is frequency system control, the implementation of solutions based on grid-forming should be carried out in close cooperation between TSOs and DSOs.
E.DSO considerations
A potential topic for deeper exploration at the Association level is preparing recommendations for possible future mass application of GFM in distribution grids. Care should be taken to ensure that the provisions of the RfG2 code relating to GFM do not have unjustified costly implications for DSOs.
Potential use cases
- Creating (forming) system voltage.
- Contributing to fault level (short circuit power).
- Contributing to total system inertia.
- Supporting system survival to enable the effective operation of low-frequency demand disconnection for rare system splits.
- Acting as a sink to counter harmonics and inter-harmonics in system voltage.
- Acting as a sink to counter any unbalance in system voltage.
- Preventing adverse control system interactions.
Ongoing projects
Enedis has funded several research works on the topic of GFM:
- Arshpreet Singh PhD thesis “Stability of electrical distribution grids in the presence of renewable energies” (2023): this thesis provided a study of slow-interaction converter-driven stability in medium-voltage distribution grids with inverter-based resources, assessing the impact of both grid-following and grid-forming operation modes.
- Jane Marchand PhD thesis “Black start and stable operation of a portion of the low voltage grid in islanded mode using local renewable energy production” (2024): this thesis provided a study of solutions to achieve the re-energisation of islanded grids by using the available local renewable power generation. This requires changing the controllers of some of the currently grid-following residential photovoltaic inverters to grid-forming control mode so that they can provide black-start capabilities and impose frequency and voltage.
Last update: 14 February 2025