Cornerstone for system stability with 100 % renewables

Inertia (Momentanreserve in German) is an inherent system capability of grid-forming units, which activates immediately when the balance between load and generation in the system is disturbed.

It consists of two key components:

• Inertia (Momentanreserve) that compensates immediately the disturbance through power reserve
• Inertia that limits the rate of frequency change

When a disturbance occurs (e.g., disconnection of a generation unit), it is distributed across the system to all remaining power-generating units. Grid-forming units, such as synchronous generators or grid-forming converters, respond instantly. Their output power jumps almost immediately, compensating for the disturbance collectively. However, this power does not come from the unit’s generation, but from the energy stored in its real or synthetic inertia. The rotational speed or frequency of the grid-forming unit decreases with additional power output and increases with reduced output—but the frequency does not jump due to inertia. This means the frequency gradient is reduced proportionally to the inertia.

Without additional measures, the frequency would continue to fall under solely on the effect of inertia. For small disturbances (up to approx. 3 GW in the European interconnected grid), market-based primary control can prevent this.
It is designed to maintain frequency within the quasi-steady-state range of 50 Hz ± 200 mHz. After a short delay, it reacts to the disturbance by adjusting the output of power-generating units, stabilizing the rotational speed of grid-forming units and keeping it within the defined operating range.

However, if the frequency drops too quickly due to larger disturbances, market-based primary control cannot stabilize it in time. At that point, all units in the system must participate in frequency control, using system-supporting capabilities that are currently in development by VDE FNN in the Technical Connection Rules (TCR).

Where are these requirements specified?

Primary control based on network security (PCNB) is among the system-supporting capabilities for power-generating units, storage, and continuously controllable loads. This represents the national implementation of LFSM-O/U operation, as required by the Connection Code Requirements for Generators (RfG). Primary control based on network security replaces the traditional “operation along the characteristic curve” in over- and under-frequency ranges.

As the name suggests, primary control based on network security (PCNB) is a form of primary frequency control that activates when the frequency deviates beyond the band of 50 Hz ± 200 mHz, with the goal of regulating the frequency gradient to zero. It differs significantly from the previously established P(f) control in the Technical Connection Rules (TCR), which followed a predefined characteristic curve. In contrast, primary control based on network security aligns the active power control rates of generation units with a defined inertia. For conventional power plants and grid-forming converter-based units, this is inherent inertia.

However, grid-following converter-based units do not possess their own inertia. They rely on the system’s inertia, i.e. of other grid-forming units, which the primary control based on network security coordinated to ensure stable operation.

When considering the closed control loop formed by frequency-power control and system inertia, a correctly designed system remains stable under both small-signal and large-signal disturbances. In this configuration, the unit exhibits self-stability with respect to frequency and active power behavior. This means the unit can autonomously stabilize itself during certain disturbances within its voltage and frequency limits. The collective behavior of such self-stable systems is also stable, forming a crucial foundation for managing major disturbance events, such as a system split.

If a grid-forming unit with substantial real or virtual inertia only requires a small portion of it for its own stability due to its fast control capabilities, it can then share the remaining portion of its inertia available with the grid-following units in the system, enabling their stable operation.

Frequency-controlled disconnection and reconnection of loads

Another system-supporting capability is the frequency-controlled switching of consumer installations such as heat pumps and unidirectional charging devices for electric vehicles. If primary reserves and primary control based on network security cannot restore the balance between generation and load quickly enough, the mains frequency may drop excessively and fall outside the defined operating limits.

In the event of a load shortfall, the frequency may fall below the threshold of 49.0 Hz. In such cases, the automatic under-frequency load shedding (UFLS) has traditionally been used as a last resort in the system protection plan. This involves disconnecting one or more supply areas at the distribution network level, significantly reducing power consumption.

However, due to the high penetration of renewable energy (RE) systems in the distribution grid, these RE systems would also be disconnected during UFLS events, resulting in the loss of their valuable capabilities for supporting the system during disturbances and restoring normal operation.

Therefore, selective and non-discriminatory load shedding will no longer be feasible in all network usage scenarios via UFLS. Nevertheless, UFLS remains an essential measure within the system protection plan.
Its effectiveness should be preserved through the implementation of frequency-controlled switching in the Technical Connection Rules for non-continuously controllable consumer installations (e.g., heat pumps and charging devices), as these will have a significant impact on system stability in the future.

Where are these requirements specified?

The occurrence of a disturbance event, especially a major imbalance between generation and load, is accompanied by a change in mains frequency due to the physical characteristics of the power supply system, as well as a corresponding rate of change of frequency (RoCoF).

To ensure that customer installations can support the system during and after such events, they must be designed to withstand certain RoCoFs and remain connected to the grid.

Where are these requirements specified?

Grid-forming capabilities are a fundamental prerequisite for the operation of non-grid-forming installations (converter-based Type-2 units), which are currently the state of the art. Due to the significant decline in grid-forming Type-1 units (synchronous machines in large power plants) as part of the energy transition, Type-2 units (especially renewable energy systems and storage units) must also convert into grid-forming units in the future.

A particular challenge lies in the transition period, during which a small number of grid-forming units must interact with a very large number of non-grid-forming units.The grid-forming capability essentially reflects the behavior of a voltage source with inertia. A unit with grid-forming properties:

• Defines voltage and frequency at its terminals
• Provides phase jump power (Momentanreserve in German)
• Provides inertia (Momentanreserve in German)
• Can operate at a short-circuit ratio (SCR) of zero
• Does not contribute to the reduction of the effective short-circuit ratio (ESCR) in the system

A successful system transformation toward 100% renewable energy is only possible if a sufficient number of grid-forming units are present in the power supply system.

The development of grid-forming behavior, especially in converter-based generation units, is not a simple upgrade. On the contrary, this capability requires fundamentally different conditions and requirements for generation units. The requirements that grid-forming units must meet, along with the corresponding verification procedures, are described in the FNN Guideline: “Technical Requirements for Grid-Forming Capabilities Including the Provision of Inertia.”

Compliance with the requirements and verification procedures outlined in this FNN Guideline is also the technical prerequisite for grid-forming units to participate in the future inertia market in Germany.

Where are these requirements specified?

Regarding active power control, a customer installation meets the criterion of self-stability through the correct design of primary control based on network security (PCNB). Regarding voltage control, this criterion is fulfilled by implementing fast voltage regulation.

Customer installations with this configuration are able to maintain a stable operating point consistently and to self-stabilize in response to certain disturbances, with respect to voltage and frequency limits. When combined, installations with self-stability are in also stable. This is the basis for managing major disturbance events, such as a system split.

The unintended shutdown of customer installations must be avoided after a fault has been cleared, especially during defined fault events, in order to always maintain the balance between generation and demand. This robustness requirement is also referred to as Over-Voltage or Under-Voltage Ride-Through (O-/UVRT robustness). However, to limit the spread of voltage dips, it is expected that customer installations contribute to support the grid by injecting necessary reactive power into the grid.

Where are these requirements specified?

Fast voltage control ensures a fast and stable regulation of the voltage at the unit terminals to a steady-state value. This involves requirements for the dynamic response and damping of the control system, and for stable operation even under very low short-circuit ratios at the point of connection (SCR). Fast voltage control operates in parallel with slow voltage control. With regard to the voltage at the unit terminals, thanks to fast voltage control, the unit essentially behaves like a voltage source behind an impedance.

Where are these requirements specified?

In this context, reactive power must be provided by the system so that the voltage at the upstream network can be maintained within acceptable limits. The purpose of static voltage control is to regulate slow (quasi-stationary) changes in voltage. For this, various methods are specified in the Technical Connection Rules, which must be fulfilled by power-generating units, storage systems, and demand installations.

Where are these requirements specified?

To ensure the operational capability of subnetwork operation, including overfrequency control during major disturbances, this VDE FNN Guideline published in 2021 outlines requirements and verification procedures for power-generating modules, continuously controllable storage systems, and demand installations.

The VDE FNN Infopaper gives an overview of the future requirements for system stability as developed in FNN, especially in terms of new definitions for system-supporting and grid-forming capabilities for generators as defined in the FNN Guideline. This paper informed somewhat discussions in the Expert Group on Advanced Capabilities for Grids with High Shares of Power Park Modules (EG ACPPM), which published their final report back in June 2023.