19.07.2023

Series of articles on the characterisation of low-voltage grids: Low-voltage grids in Germany

Many research projects in the context of the electrification of mobility, heat and industry are investigating how new electrical consumers in combination with the ramp-up of renewable energies will affect electricity grids. A large share of the added electrical load is connected in low-voltage grids (LV grids). One approach for their integration is to make consumption behavior more flexible in order to avoid or eliminate congestions in the grid. To evaluate the concepts in terms of their impact on the grids, they are simulated with future load scenarios and grid operation strategies.

In total, there are over 500,000 LV grids in Germany, which are operated by around 800 distribution grid operators [1]. In total, these result in a line length of over 1,200,000 km [2]. It is hardly possible to simulate all grids individually. On the one hand, they are not available in a simulatable form, and on the other hand, this would require an enormous amount of computation capacity. In order to be able to make statements for different grid structures in Germany, reference grids are therefore used for simulations. These allow to draw conclusions about the grid infrastructure from geographical and structural parameters.

This series of articles shows which parameters and methods are used to create reference grids. Furthermore, data from literature is merged and a set of reference grids is created. Specifically, the following topics are addressed:

The low-voltage level

The low-voltage level (LV level) is the seventh and thus lowest voltage level of the electricity grid, which ultimately supplies the majority of consumers (households, businesses and smaller industrial companies) with electricity. Historically, this was generated by the large power plants in the higher voltage levels and transported to the final consumer via the various transformer levels. In Germany, Austria and Switzerland, a total of seven grid levels are defined, which are numbered in ascending order with descending voltage level. The numbering of the voltage levels starts with the highest voltage level and takes into account not only the individual levels (highest (1), high (3), medium (5) and low (7) voltage) but also the transformer levels (2, 4, 6) between the respective network levels. The low-voltage grids, which are the focus of this series of articles, are thus connected to the medium-voltage grid (MV grid) in the overall energy construct via one local grid transformer (LGT) each and distribute electricity to the grid connections of the end customers (cf. Figure 1).

Figure 1 Consumers and generators connected to the low-voltage grid and coupling to the medium-voltage level

Consumers and generators in low voltage grids

In the course of the energy transition and the associated ramp-up of photovoltaic systems (PV‑systems), the historical top-down supply structure is changing and the grid operation of the LV grids is being supplemented by the additional task of distributing the PV electricity generated in them. In some time periods, this ultimately also changes the direction of the electricity flow: if less electricity is consumed than generated in an LV grid, this results in a reversal of the energy flow from the LV level to the superimposed MV level in the LGT.

In addition to the transformation of the generator structure, the consumer structure also changes. For example, due to electrification in the heating and transport sectors, an increasing number of heat pumps and charging points for electric vehicles are installed in the LV grid. This increases the electrical energy to be distributed via the LV level and, due to simultaneities in consumption behaviour, results in higher load peaks in the grid [3].

These changes in the load and generation structure caused by the energy transition result in the need to upgrade the LV grids. In the classical sense, this upgrading means that the grids are supplemented or “expanded” with additional infrastructural components, which is associated with high costs. In the future energy system, the new consumers, which can help to avoid possible grid overloads through “flexible” operating behaviour, such as shifting the load over time, should also contribute to reducing the expensive grid expansion measures. The challenge of transforming the distribution grid involves various actors, processes and interfaces and has been discussed politically for years. The real implementation in the energy system is an overly complex optimisation problem that includes technical, regulatory, economic and social aspects. Energy economic/technical models address partial aspects of this problem and thus represent a valuable tool for the ultimate implementation.

In order to technically and economically evaluate potential solution options, such as the grid-serving use of flexibility in the distribution grid, know-how regarding the infrastructural conditions or the LV grid is necessary. As described at the beginning, the German LV grid consists of more than 500,000 individual topologies, whose topography is determined by the respective regional conditions [4]. In order to make holistic statements about the LV grids in Germany, simplifications and substantial assumptions are therefore also necessary in the modelling of the infrastructure.
The following contributions in this series of articles are intended to provide an insight into the diversity of the German LV grids and the possibilities for characterising them, with the following main topics being addressed:

– Characteristics of the German low-voltage grids and which factors define the different, topological characteristics.
– Methodical approaches for the typification of the low-voltage grids and the resulting clusters in the literature as well as their unification set
– Modelling of characteristic reference grid topologies, their useful application and associated limitations
– Design of low-voltage grids in the climate-neutral energy system

Further information

GridSim – Electric Grid and Energy System Model for Distribution Grids
unIT-e² – living lab for integrated e-mobility