Congestion management

Congestion management refers to the measures taken by the transmission system operators (TSOs) to prevent overloading of the nationwide power lines, known as grid congestion. Congestion is caused by surplus electricity production that cannot be transported away due to a lack of capacity on the physical power lines. In order to avoid such bottleneck situations, congestion management measures are used, the most important of which is "redispatch". This is a nationwide option for controlling all large generation plants and throttling or ramping them up as required. If a bottleneck is forecast by the TSOs, generation plants are throttled before the bottleneck and ramped up after the bottleneck. This does not change the absolute electricity production, only the location of production is changed in order to relieve the power line and thus avoid the bottleneck. In 2021, a total of around 21 TWh (10.5 TWh feed-in curtailments & 10.5 TWh feed-in increases) were regulated as part of feed-in management, resulting in costs of around 2.3 billion euros. (Grid congestion management report 2021, BNetzA)
Redispatch is therefore used tointervene in the operating times of nationwide generation plants in order to avoid congestion occurring in the grid.

What causes grid bottlenecks?

The reason for the emergence of bottlenecks is that the current electricity infrastructure in Germany is not designed for the high volatility of renewable generation plants. Due to the strong dependence on the weather, electricity production from renewable sources fluctuates greatly. However, the electricity grid has historically been designed for very constant, conventional large-scale power plants and is slow to adapt to rapidly changing electricity production. In the past, the electricity grid was often referred to as a nationwide "copper plate", which describes an electricity grid that can transmit the electricity produced in all directions at all times.
The fact that this description no longer corresponds to reality can be seen in the well-known north-south bottleneck.
Due to the relatively rapid expansion of onshore and offshore wind turbines in northern Germany, a considerable surplus of electrical energy is produced there on windy and sunny days. In order not to overload the grid, this surplus must be transported to the south of Germany to large consumption centers such as industrial sites and large cities. As the physical expansion of the grid, i.e. the expansion of electricity grid capacity, is lagging well behind the expansion of renewable sources, as described in our article "How the electricity grid needs to be prepared for 80% renewable energies", north-south bottlenecks often occur. This results in the shutdown of renewable generation plants due to redispatch and the ramping up of conventional power plants in the south. In 2021, 3% of the total renewable production capacity was already shut down. This amount of electricity could have supplied up to 2 million households. With the German government's ambitious expansion targets of 80% renewable electricity by 2030, grid bottlenecks will occur even more frequently. The volume of congestion management measures implemented increased by 19% from 2020 to 2021 alone. (Grid congestion management report 2021, BNetzA)

In today's reality, the German power grid can no longer be described as a nationwide "copper plate". However, such a system is only desirable to a limited extent in a renewable future. Due to the extremely high fluctuations of renewables, a significant surplus of electricity is produced at peak times. To design the power grid for this peak kWh production, so that the entire power could be transported without bottleneck even at the absolute peak times, would be extremely uneconomical and cost many billions of euros.

An alternative to the resulting redispatch measures, which result in the shutdown of renewables, is the large-scale integration of storage systems. storage systemssuch as large-scale battery storage or pumped storage. These would both limit grid expansion to a realistic level and minimize redispatch measures. More on the integration of energy storage systems can also be found in the article "How the electricity grid must be prepared for 80% renewable energies" described.

How does redispatch work?

All redispatch measures are carried out and managed by the 4 transmission system operators. During the previous day, the TSOs receive the schedules of all generation plants as well as the estimated consumption for the following day. This overview is used to forecast possible grid bottlenecks in the entire nationwide electricity grid by means of regional load flow calculations. The TSOs use this data to determine the redispatch measures for the following day and, if necessary, correct them during the course of the day if short-term fluctuations occur. For these measures, all generation plants with an installed capacity of 100 kW or more must capacity must be available for these measures. This limit came into force with the introduction of Redispatch 2.0 on 01.10.2021. Previously, the limit was 10 MW, which meant that only large plants participated in redispatch. Due to the high expansion of smaller renewable generation plants, this limit was adjusted so that the TSOs have adequate intervention options in an energy system dominated by renewables.
Redispatch 2.0 also focuses on the shutdown of conventional plants. This means that renewable plants are only shut down when conventional options are exhausted or the shutdown of renewable energies is 10 times more cost-effective.

If generation plants are instructed by the TSOs to throttle or increase their output, this inevitably causes additional costs for the generator. These additional costs are covered by the TSOs, which in turn transfer these costs to all consumers in the form of network charges. These network charges are included in the electricity price paid.

How do large-scale battery storage systems participate in congestion management?

Like generation plants, storage systems such as large-scale battery storage must also be available for the TSOs' redispatch measures. If these are strategically placed at network nodes where network congestion frequently occurs, they can even contribute particularly effectively to congestion avoidance. In addition to normal redispatch measures, i.e., throttling the output of large-scale battery storage facilities, the storage facilities can also optionally act as consumers. Large-scale battery storage facilities can not only be throttled to zero before bottlenecks like generation plants, but can also absorb excess electricity produced. Once the bottleneck has been avoided, the storage facilities can feed this electricity back into the grid with a time delay.

With a large-scale expansion of large-scale battery storage systems, which both the BNetzA and the Fraunhofer Institute are forecasting, this potentially more effective Redispatch 3.0 can greatly relieve the nationwide electricity grid. In this way, large-scale battery storage systems can not only avoid bottlenecks, but also minimize the shutdown of renewables such as wind turbines in the north and limit grid expansion. Furthermore, the costs of congestion management measures are drastically reduced by avoiding shutdowns with the help of storage systems, which can reduce our grid fees.