Definition

Congestion management refers to the measures taken by transmission system operators (TSOs) to prevent overloads of nationwide power lines, known as grid bottlenecks. Congestion is caused by excess electricity production that cannot be transported away due to insufficient capacity of the physical power lines. To avoid such congestion situations, congestion management measures are used, the most important of which is so-called "redispatch". This is a nationwide possibility to control all large generation plants and to throttle or ramp them up as needed. 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 power production, only the location of the production is changed to relieve the power line and thus avoid the bottleneck. In total, about 21 TWh (10.5 TWh feed-in curtailments & 10.5 TWh feed-in increases) were regulated under feed-in management in 2021, resulting in costs of about 2.3 billion euros. (Report Netzengpassmanagement 2021, BNetzA)
With the help of the redispatch, the operating times of the nationwide generation plants are thus intervened in order to avoid bottlenecks occurring in the grid.

What causes grid bottlenecks?

The reason for the emergence of the bottlenecks is that the current electricity infrastructure in Germany is not designed for the high volatility of renewable generation plants. Due to the high dependency on weather, electricity production from renewable sources fluctuates greatly. However, the power 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 power grid was often referred to as a nationwide "copper plate," describing a power grid that could transmit produced electricity in all directions at all times.
That this description no longer corresponds to reality can be seen in the well-known north-south bottleneck.
Due to a relatively fast expansion of onshore and offshore wind energy plants in the north of 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. Since physical grid expansion, the expansion of electricity grid capacity, lags significantly behind the expansion of renewable sources, as described in detail in our article "How the power grid must be prepared for 80% renewables." describes in detail, there are often north-south bottlenecks. As a result, renewable generation plants are shut down by redispatch and conventional power plants in the south are ramped up. In 2021, 3% of the total renewable generation capacity was already shut down. This amount of electricity could have supplied up to 2 million households. With the ambitious expansion targets of the German government to 80% renewable electricity by 2030, grid bottlenecks will occur even more frequently. For example, from 2020 to 2021 alone, the volume of congestion management measures implemented increased by 19%. (Report Grid Congestion Management 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, such 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, is also described in the article "How the power grid must be prepared for 80% renewables" .

How does redispatch work?

All redispatch measures are carried out and are the responsibility of the 4 TSOs. During the previous day, the TSOs receive both the schedules of all generation plants and the estimated consumption for the following day. With this overview, regional load flow calculations are used to forecast possible grid bottlenecks in the entire nationwide power grid. Based on this data, the TSOs determine the redispatch measures for the following day and, if necessary, correct them during the day if short-term fluctuations occur. For these measures, all generation plants with an installed capacity of 100 kW or more must be Power must be available for these measures. This limit was put into effect with the introduction of Redispatch 2.0 on 01.10.2021. Previously, the limit was 10 MW, which meant that only large-scale plants participated in the redispatch. Due to the high addition of smaller renewable generation plants, this limit was adjusted so that the TSOs have adequate intervention options in an energy system dominated by renewable energies.
In addition, Redispatch 2.0 focuses on the shutdown of conventional plants. Thus, renewable plants will only be shut down when conventional options are exhausted or the shutdown of renewables is cheaper by a factor of 10.

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, which is predicted by both the BNetzA and the Fraunhofer Institute (more information in this article), this possible more effective Redispatch 3.0 can greatly relieve the nationwide power grid. In this way, large-scale battery storage can not only avoid congestion, 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 avoided shutdowns with the help of storage, which can reduce our grid charges.