What is an electrical load center

Electric grids - how do they have to change for the energy transition?

Author: Prof. Dr.-Ing. Christian Rehtanz, ie3-Institute for Energy Systems, Energy Efficiency and Energy Economics, TU Dortmund (State: February 2015)

The electrical grids are the marketplace and the integration platform for renewable energies, conventional power plants, storage and electricity customers. The spatial balance between electricity generation and consumption must take place via the grids. It is important to ensure that the system is stable. This means that exactly as much power is fed in as is consumed. The individual fluctuations in feeders and loads balance each other out via the network. Everything that is not compensated by this has to be compensated for by controllable feeders or storage units and by load interventions.

It follows from this that large-scale network structures tend to offer better compensation options, especially for the strongly fluctuating feeders from renewable energies and thus require fewer storage facilities and compensating power plants. Ultimately, it is a political and economic decision how large-scale the electrical network structures and thus the energy balance are to be created. It is already possible today to build energy self-sufficient houses that supply themselves with electricity and heat throughout the year. However, the costs for this are exorbitantly high compared to conventional supply and therefore not economical. For the power supply, there is no reserve for technical malfunctions built in and would cause further extreme costs if the public supply of electricity were not used. The electrical energy system has been networked across Europe in order to be able to intercept failures of individual technical components. It is precisely this networking that enables a European electricity market today, which promotes the most economical use of energy sources and at the same time ensures a high level of supply reliability. For the further development of renewable energies in Germany and Europe, however, today's network structures are reaching their limits, as the network capacities are not developed enough to connect optimal locations for different renewable energies with the load centers.

If you consider such load centers, such as the Ruhr area or the greater Munich or Berlin area, then these cannot be supplied with renewable energies on their own area. Photovoltaics and waste power plants can make a significant contribution to this, but this must be supplemented by wind energy from the large surrounding area, i.e. also from other federal states, and hydropower. Balancing power plants or storage facilities must also be available for dark periods of calm.

It can be stated that Germany as a business location requires energy system structures and thus electrical grids in order to develop optimal locations for renewable energies across the region, to optimize the natural balance between the producers and also to trade and balance the energy supply with its European neighbors enable. This inevitably results in structures that, for example, combine wind energy from northern Germany with photovoltaics from southern Germany and hydropower from the Alps and also from Norway together with balancing power plants at favorable locations in an economically optimal way. If one deviates from this network structure in the sense of a stronger regionality, the need for network expansion decreases and at the same time the need for compensation increases through significantly more expensive balancing power plants and storage, so that the overall system and thus the power supply would be more expensive.

Flexibility options to stabilize the system

The balancing via the networks is ultimately always of a spatial nature. They can only compensate for a power shortage in other regions if the correct generation capacities are available somewhere. If you look at the large-scale European weather phenomena, there can be very large-scale calm situations with low temperatures in the dark winter months of up to two weeks. The generation from renewable energies is very low over a large area. Power plants in countries with electric heating, such as France, are needed there themselves. In order to enable compensation via the grids, one would have to leave the European borders and obtain electricity from North Africa, as was proposed in the DESERTEC project. Politically, however, there are certainly understandable limits here.

For such scenarios, flexibilities are required in the system that compensate for these slack periods. Storage capacities, including reservoirs, are insufficient for this. Load shifts are not possible to this extent, but can contribute to smoothing peaks to a certain extent. Today, only peak load power plants would make sense as an economic option. The power plant fleet of nuclear and coal-fired large power plants will develop more and more towards more flexible and faster controllable gas power plants. Today's very low price on the electricity exchange does not yet reflect this need. Necessary capacity mechanisms, such as the obligation for suppliers to provide compensation for the supply of renewable energy, would, however, create suitable market incentives. Such concepts are currently being evaluated and urgently need to be implemented.

Such incentives could allow all flexibility options to compete economically with one another and contribute to providing the necessary compensation.

In addition to covering performance gaps, the rate of change of renewable energies is also of particular importance for system operation. The fluctuation in output from renewable energies will continue to increase extremely over the next few years. This must also be taken into account in the overall system. It can be shown that relatively small storage units, such as those currently coming onto the market for self-supply with photovoltaic electricity in households, help to significantly reduce the gradients caused by solar feed-in in the overall system. However, these storage systems do not reduce the need for grid expansion or the need for balancing power plants, but they do reduce the requirements for the regulation speed of balancing power plants.

Expansion of the transport and distribution networks

The strong growth in wind and photovoltaic systems creates new challenges in the operation and planning of electrical networks. Depending on the structure and regional location of districts and municipalities, the distribution networks have different requirements. While the urban grids usually still have large capacity for renewable energies, they are already at their load limit in extensive rural regions. Nationwide, the transport networks must provide compensation.

If the feed-in of renewable energies is significantly greater than the peak load for which the grids were planned in the past, the grid must be expanded. Photovoltaic systems in particular require network upgrading at the low voltage level. In the medium-voltage level, larger PV systems and individual wind energy systems are also connected, so that the problem becomes even worse. If larger regions use the potential of renewable energies and these accumulate at the same time, the high-voltage grids must also be expanded. This can be seen today especially in rural federal states with high wind energy or photovoltaic potential, in which the existing network structures have to be doubled in terms of performance. Superimposed on these networks, the extra-high voltage networks ensure supraregional compensation.

In the publicly available dena distribution network study, the TU Dortmund estimated the investment requirements in the distribution networks based on real networks from over 3,000 German municipalities and scenarios for the development of the expansion of renewable energies. Depending on the scenario, there will be an additional 27 to 42 billion euros by 2030 for upgrading the distribution grids from low to high voltage. The order of magnitude is thus on a similar level as the costs for the electricity highways at the highest voltage level in the German network development plan, which is estimated at around 22 billion euros by 2025.

The future challenge will be to reduce these investments through innovative network technologies and planning methods. Intelligent control of feeders and loads can mitigate rarely occurring extreme situations in the distribution networks and thus reduce the expansion there. In certain situations, new equipment for grid control can reduce or at least delay the construction of new lines. Overall, however, it will only be possible to use the potential of renewable energies to supply the industrialized country of Germany if a correspondingly powerful network infrastructure is further developed. Without the regional and supra-regional network expansion, the energy transition will not take place as planned, or at least it will be significantly more expensive.

Further literature:

dena distribution network study "Expansion and innovation needs of electricity distribution networks in Germany by 2030" (http://www.dena.de/projekte/energiesysteme/verteilnetzstudie.html)

dena study "System Services 2030" (http://www.dena.de/projekte/energiesysteme/dena-studie-systemdienstleistungen-2030.html)