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" .