Resilient Power Grids for the Energy Transition

Due to the increase in extreme weather events as a result of climate change, as well as the dangers of cyberattacks and physical sabotage, a resilient power grid is essential. The resilience of the power grid describes the extent to which the power supply can be maintained before, during, and after a disruption. In the event of major grid disturbances, the “RESIST” research project investigates the use of grid-forming inverters in the distribution grid to counteract the reduction of system inertia and work with local medium-voltage grid islands.

Initial Situation

The ongoing transformation of the energy system towards renewable, decentralized, and digitally networked structures is leading to significantly greater complexity and connectivity. This increases vulnerability to outages, disruptions, and their cascading effects – caused, for example, by technical failures, targeted cyberattacks, or increasingly frequent extreme weather events. At the same time, inertia in the energy system is decreasing because conventional power plants with their essential rotating masses are being decommissioned and replaced by converter-based generation units. This combination of lower inertia and higher complexity makes the controllability of the energy system a key technical challenge.

Objective

The RESIST research project investigated how the resilience of the power supply in Germany can be increased under the conditions of the energy transition. Five Fraunhofer institutes developed a toolbox that makes the resilience of the power grid to technical disruptions, extreme weather events, and cyberattacks measurable, monitors it in real time, and makes it usable for planning and operation. The central objectives were the development of...

• a resilience monitor for real-time analysis,
• a strategic planning tool for a resilient-by-design grid infrastructure,
• models for assessing power outages and grid stability,
• a resilience dashboard for virtual power plants,
• new approaches to cyber-resilient grid management,
• demonstrators of a real-time-capable hardware-in-the-loop medium-voltage grid model, and
• a grid-forming battery inverter used for emergency supply of an islanded grid section .

As part of “RESIST,” Fraunhofer ISE has investigated the role of grid-forming inverters in the distribution grid. These can help to increase the resilience of the power grid, counteract the reduction in system inertia, prevent potential failures in the event of major disruptions, and enable the formation of local grid islands.

Approach

With the main focus on the “natural disaster” incident, a medium-voltage grid was modeled. Within this grid, a grid-forming inverter together with a large battery storage system ensures the island grid capability of this grid area. The grid model is based on the CIGRE benchmark grid in the European configuration, which has been adapted so that the grid area is dominated by converter-based generation. The grid-forming inverter is based on the 1 MVA battery inverter developed in the NETfficient project, which has been equipped with grid-forming control.

In order not to completely neglect the dynamic influence of generators and loads at the low-voltage level, a complexity-reduced aggregated low-voltage grid was connected to the medium-voltage grid. The associated detailed low-voltage grid is also based on the CIGRE benchmark grids. Aggregation is performed using the Sensitivity Technology Control Clustered Approach. This ensures adequate reproduction of the dynamic behavior of the low-voltage grid while saving computing power.

During a grid simulation, measurement data is sent in real time to the resilience monitor developed by Fraunhofer EMI. Grid modeling therefore takes place in the real-time hardware-in-the-loop (HIL) simulation environment of the Digital Grid Lab. By calculating various fault cases, the resilient properties of the inverter control can be analyzed and validated. In addition, the resilience monitor determines the effects on the resilience of the overall system. Particular focus is placed on stable island grid formation of the medium-voltage grid, which is made possible by the resilient inverter.

Another aspect of the project is to make the communication and control of the grid-forming inverter as simple, robust, and standards-compliant as possible. The interface for transmitting measured values, alerts, and control commands therefore uses the IEC 61850 MMS protocol and is closely based on the functions of the FNN control box.

Results

Development of a grid-forming inverter

Until now, grid stability has been based on synchronous generators, which stabilize the grid through the inertia of their rotating mass. In the future, this task will increasingly have to be taken over by converter-based units. As part of the RESIST project, a battery inverter has therefore been further developed as a so-called “resilience inverter,” which provides grid-forming properties. This has been successfully implemented in a grid model.

Real-time capable distribution grid model in a hardware-in-the-loop environment

The real-time capable model of a distribution grid includes a detailed representation of the medium-voltage level, an aggregated model of the low-voltage level, a connection to a neighboring medium-voltage grid via a high-voltage grid, and renewable energy generation at the medium- and low-voltage levels. At the core of the grid model is a grid-forming battery inverter that can operate the medium-voltage grid as an island grid.

Interoperable communication interface

A hybrid MQTT/IEC 61850 interface with schedule-based control commands has been tested in several scenarios. In particular, connecting a distribution grid model in a hardware-in-the-loop simulation environment to the resilience monitor via the communication interface enables the transmission of measured values and the reception of control commands, such as charging a battery via the grid-forming inverter.

The figure shows an example scenario in which renewable generation collapses due to unfavorable weather conditions. This reduces the islanding capability (KPI 3) due to the high power demand of the medium and low-voltage grid combined with low forecast generation. In the event of a grid disconnection, the power demand would therefore have to be provided almost entirely by the grid-forming battery inverter, which would be operated at its power limit. Similarly, the possible islanding duration is greatly reduced, as the battery will discharge more quickly without additional renewable generation. The latter is reflected in the resilience monitor by a low KPI 4.

The "RESIST" project was funded by the Federal Ministry of Education and Research.