What are grid-forming properties and instantaneous reserve in type 2 generation units?

As part of my final thesis for my bachelor’s degree in electrical engineering and information technology at HAW Hamburg, I investigated the requirements for grid-forming properties and the provision of instantaneous reserve in inverter-based generation units.

In this blog article, I would like to provide an overview of the technical background and key requirements of the VDE FNN notice “Technical requirements for grid-forming properties including the provision of instantaneous reserve (Version 1.0 / July 2024)” and possible applications specifically for type 2 generation units.

Grid-forming properties: Inverters with voltage source behavior

Grid-forming inverters are characterized by their voltage source behaviour and can therefore actively contribute to the stability of the power grid. Unlike grid-connected inverters, which act as power sources that are dependent on an existing grid voltage, grid-forming systems can independently generate a grid voltage and operate a partial or isolated grid.

Control mechanisms such as the ƒ (Ρ)– and U(Q)-statics are used to dynamically adapt the frequency and voltage to the requirements of the grid.

One feature of grid-forming converters is their ability to react to phase angle jumps or frequency changes. In contrast to conventional flywheel masses, which use mechanical inertia, these converters emulate the behavior of a synchronous generator using virtual inertia in the control algorithms.

This voltage source behavior makes grid-forming converters an indispensable component of the energy transition, as they can ensure grid stability even with a high penetration of renewable energies. Particularly in situations with sudden load changes or generation outages, they make a decisive contribution to maintaining security of supply.

Instantaneous reserve: immediate support in the event of grid disruptions

The provision of instantaneous reserve is a central system service within frequency control in the electricity grid. It describes the ability to immediately provide a positive or negative power reserve in the event of a frequency change in order to compensate for an imbalance between generation and consumption.

Figure 2 – Frequency control time ranges

In conventional power plants, this reserve is provided by the kinetic energy of the turbines’ flywheels. Inverter-based systems (type 2 generation units), on the other hand, use innovative control systems to fulfill this task.

Controls can be effectively implemented using technologies such as virtual inertia in combination with grid-forming static control. One approach is to emulate the flywheel mass using control algorithms that react to frequency deviations and provide energy from storage systems. In this way, frequency curves can be stabilized in the event of a major disturbance, e.g. power plant failure, similar to conventional systems with synchronous generators and flywheel masses.

Technical requirements for grid-forming properties including the provision of instantaneous reserve

A basic distinction is made between type 1 (synchronous generators) and type 2 (everything else, e.g. converter-based generation units). In addition to the frequency-dependent requirements, the requirements also include voltage-dependent ancillary services.

The FNN notice essentially comprises the following requirements:

The new VDE FNN requirements are publicly available in detail.

Application of the new net-forming technology

The use of grid-forming converters leads to a variety of system services in the electrical energy grid:

In combination with a battery storage system, grid-forming inverters are also ideal for the following applications:

Grid-forming converters play a central role in the successful implementation of the energy transition. They enable the integration of a high proportion of renewable energies and at the same time ensure the stability and security of supply of the power grid. With their versatile application possibilities, such as the provision of instantaneous reserve, grid frequency stabilization and black start capability, they make a decisive contribution to the modernization and resilience of the energy system of the future.