Grid operations are responsible for ensuring a secure energy supply in which electricity supply and demand are balanced. This social mandate to ensure the security of the power supply is enshrined in Section 11 of the Energy Industry Act (EnWG). This results in the task of all network operators to "operate, maintain and optimize, reinforce and expand a secure, reliable and efficient energy supply network on a non-discriminatory basis in line with demand."
What are the characteristics of a grid?
A pipeline grid refers to the sum of all lines and serves as an infrastructure for the supply and disposal of the public. In this context, a grid can be used for the distribution of materials (e.g., natural gas, water), energy (electricity) and information (telecommunications). Accordingly, the power grid is used to supply electrical energy and is operated by power grid operators in specific regions. The electricity network operators are divided intotransmission system operators(TSO) and distribution system operators (DSO).
How is the German power grid structured?
The German electricity grid is operated at seven voltage levels. The first grid level is formed by the extra-high voltage grids with 380 kilovolts (kV) and 220 kV, respectively, and is used to transmit large amounts of energy over longer distances and also to establish connections between the distribution grids. In addition, large power plants with several hundred megawatts feed directly into the approximately 35,000km-long extra-high voltage grid. These lines are therefore popularly known from the media as "major electricity highways." They are operated by the transmission system operators, just like the large substations of the second grid level. All downstream network levels are operated by distribution network operators (over 97 % of the total network length belongs to the distribution network).
The third voltage level is the high-voltage network with a voltage of 110kV. This grid is mostly connected over a large area and with few direct customer connections. Only large industries with a high energy demand are connected to this voltage level. Pumped storage, gas-fired power plants and larger wind farms feed directly into this high-voltage level, which consists of over 90% overhead lines.
The medium-voltage networks, grid level five, are operated at 20kV or 10kV and predominantly as cable networks. They are used for distribution in urban areas or between villages in rural areas, up to the local network stations. Biogas plants, smaller wind farms and industrial and commercial areas are also connected to this voltage level.
The lowest voltage level, grid level seven, is the low-voltage level with 400 volts (V) or 230V (conductor-conductor-earth) and is only implemented as a cable network. Here, the house connection of private customers takes place and provides the voltage that we receive via our familiar sockets.
Grid levels two, four and six between these voltage levels are used only for transformation and are designed in the form of substations with (power) transformers or as a "small substation", a local substation.
What is the role of the transmission system operators?
As service providers, the transmission system operators, or TSOs for short, play a crucial role in the operation of the power grid. They dimension and build the transmission network, maintain it and regulate the relationship between generated electrical energy and network load. They are thus responsible for the supraregional supply and transmission of electrical energy. In addition, the transmission networks connect the German power grid with that of neighboring European countries, thus enabling the cross-border exchange of energy.
As a natural monopoly, the transmission networks in Germany are operated by four TSOs. These include Tennet TSO, 50Hertz Transmission, Amprion and TransnetBW. As independent transmission system operators, it is still their task to ensure grid security and also to provide all market participants with non-discriminatory grid access. In this context, operations are regulated and monitored by the Federal Grid Agency. As a government body, the Federal Network Agency publishes a grid development plan at intervals of a few years, with recommendations and advice for the TSOs. As a result, the power grid is to be prepared for the changes brought about by the energy transition in the future as well, in order to continue to ensure security of supply. The expansion work for the TSOs is derived from this network expansion plan.
What is the role of distribution system operators in the German power grid?
All downstream grid levels, grid levels three to seven, are operated by the distribution system operators, or DSOs for short. Divided into numerous network regions (over 900 network regions in Germany), the DSO is responsible for the construction and maintenance of the distribution network in these network regions. The DSO must also provide all end consumers, generation and storage facilities, and downstream networks with non-discriminatory access to the electricity network in return for a network fee. In addition to the electricity grid, these DSOs often also operate the regional gas grid and ensure distribution all the way to the end consumer or generator. In addition to the electrical power produced in the grid and fed into the distribution grid, the DSOs purchase power from the extra-high voltage level from the TSO. In the past, the DSOs were also the energy suppliers to the end consumers. Since the liberalization of energy supply, however, this is no longer necessarily the case. Since then, distribution and network operation have been unbundled by law, which is why an end consumer can choose the energy supplier, but not the network operator.
The development of the power grid
With the start of the energy transition, a change in the load flow in the power grid became apparent. Whereas in the past the output of large conventional power plants was fed into the extra-high voltage level and thus supplied the lower voltage levels, today there is also a supply from the lower voltage levels. The top-down grid structure is increasingly developing into a "top-down and bottom-up" supply. This is due to the increasing expansion of photovoltaic and wind power plants with a connection at the lower voltage levels.
How is the security of supply of the electricity networks ensured?
To ensure safe grid operation even in the event of unplanned shutdowns or failures of individual network components, the network is built with numerous redundancies. The principle of this security is (n-1) security. This principle forms the basic principle in network planning and states that even in the event of a failure of a component, such as a transformer or an overhead line, network security must be ensured with maximum transmission and supply tasks.Only in the distribution network are supply interruptions tolerated within limits, depending on the customer connections, provided that they can be remedied promptly.