Congestion management means the measures taken by transmission system operators (TSOs) to prevent overloads on nationwide power lines, so-called grid bottlenecks. Congestions are caused by excess electricity production, which cannot be removed due to a lack of capacity on the physical power lines. In order to avoid such congestion situations, congestion management measures are used, the most important of which is the so-called “redispatch”. This is a nationwide option to control all large generation plants and to throttle or start up as required. If the TÖNBs predict a congestion, generation plants are throttled before the congestion and started up behind the congestion. As a result, the absolute production of electricity does not change; only the place of production is changed in order to relieve the load on the power line and thus avoid the congestion. In 2021, a total of around 21 TWh (10.5 TWh feed-in reductions & 10.5 TWh feed-in increases) were regulated as part of feed-in management, which resulted in costs of around 2.3 billion euros. (Network Congestion Management Report 2021, BNetzA)
With the help of redispatch, the operating hours of nationwide generation plants are therefore interfered with in order to avoid congestion occurring in the network.
The reason for congestions is that the current electricity infrastructure in Germany is not designed for the high volatility of renewable generation plants. As a result of severe weather dependency, the electricity production of renewable sources fluctuates strongly. However, the power grid has historically been designed for very constant, conventional large power plants and is only slowly adapting to rapidly changing electricity production. In the past, the power grid was often referred to as a nationwide “copper plate,” which describes a power grid that can conduct the electricity produced in all directions at any time.
The fact that this description no longer corresponds to reality can be seen from the well-known North-South bottleneck.
As a result of 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 network, this surplus must be transported south of Germany to major consumption centers such as industrial sites and major cities. Since physical network expansion, i.e. the expansion of power grid capacity, is significantly lagging behind the expansion of renewable sources, as described in our article”How the power grid must be prepared for 80% renewable energy“As described in detail, North-South congestions often occur. The result is a shutdown of renewable generation plants through redispatch and a ramp-up of conventional power plants in the south. 3% of the total renewable production capacity was shut down as early as 2021. Up to 2 million households could have been supplied with this amount of electricity. With the federal government's ambitious expansion goals of 80% renewable electricity by 2030, grid congestions will occur even more frequently. From 2020 to 2021 alone, the volume of congestion management measures implemented rose by 19%. (Network Congestion Management Report 2021, BNetzA)
In today's reality, the German power grid can therefore no longer be described as a nationwide “copper plate”. However, such a system is only desired to a limited extent in a renewable future. Due to the extremely high fluctuations in renewables, a significant surplus of electricity is produced at peak times. Setting up the power grid for this peak kWh production so that all electricity could be transported without congestions even at absolute maximum 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 battery storage or pumped storage. These would both limit network expansion to a realistic level and minimize redispatch measures. More about the integration of energy storage systems is also available in the article”How the power grid must be prepared for 80% renewable energy” described.
All redispatch measures are carried out and are responsible for the 4 transmission system operators. In the course of the previous day, the UDBs receive both the timetables of all generation plants and the estimated consumption for the following day. With this overview, possible grid congestions in the entire nationwide power grid are predicted through regional load flow calculations. Based on this data, the TSOs determine the redispatch measures for the following day and, if necessary, correct them over the course of the day if short-term fluctuations occur. For these measures, all generation plants of 100 kW or more must be installed power are available. This limit was put into effect with the introduction of Redispatch 2.0 on 01.10.2021. Previously, the limit was 10 MW, meaning that only large plants took part in the redispatch. As a result of the large expansion of smaller renewable generation plants, this limit has been adjusted so that the TSOs have adequate opportunities to intervene in an energy system dominated by renewable energies.
In addition, Redispatch 2.0 focused on shutting down conventional systems. For example, renewable plants are only switched off when conventional options have been exhausted or the shutdown of renewable energy is cheaper by a factor of 10.
If generation plants are instructed by the TSOs to reduce or increase their output, this will inevitably result in additional costs for the producer. 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 prices paid.
Like generation plants, storage systems such as large battery storage systems must also be available for the redispatch measures of the UENBs. If these are strategically placed at network nodes where network congestions often occur, they can even contribute particularly effectively to preventing congestions. In addition to normal redispatch measures, i.e. throttling the power of the large battery storage devices, the memories can also optionally act as consumers. In this way, large battery storage systems can not only be throttled down to zero like generation plants before bottlenecks, but can also absorb excess electricity produced. Once the congestion is avoided, the storage systems can feed this electricity back into the grid with a time delay.
With a large-scale expansion of large battery storage systems, which both the BNetzA and the Fraunhofer Institute predict, with this possible more effective Redispatch 3.0, the nationwide power grid can be significantly relieved. 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 using storage systems, which can reduce our network charges.
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