Network operation is responsible for a secure energy supply in which electricity supply and electricity demand are balanced. This social mandate for security of electricity supply is enshrined in Section 11 of the Energy Industry Act (EnWG). This results in the task of all network operator “to operate, maintain and optimize, strengthen and expand a secure, reliable and efficient energy supply network without discrimination. ”
A pipeline network is the sum of all lines and serves as an infrastructure for the supply and disposal of the public. A network can be used for the distribution of substances (e.g. natural gas, water), energy (electricity) and information (telecommunications). Accordingly, the power grid serves to supply electrical energy and is operated by power grid operators in certain regions. The power grid operators are differentiated between transmission system operator (TSO) and Distribution system operator (VNB).
The German electricity grid is operated at seven voltage levels. The first network level consists of high-voltage networks with 380 kilovolts (kV) or 220 kV and are used to transmit large amounts of energy over longer distances in order to also establish a connection between the distribution networks. In addition, large power plants with several hundred megawatts feed directly into the approximately 35,000 km long high-voltage grid. These lines are therefore popularly known from the media as “major electricity highways.” These are operated by transmission system operators, just like the large second-level substations. All downstream network levels are operated by distribution system operators (over 97% of the total network length is part of the distribution network).
The third voltage level is the high-voltage network with a voltage of 110 kV. This network is usually connected over a large area and with a few direct customer connections. Only large industries with high energy requirements are connected at 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% of overhead lines.
The medium-voltage networks, grid level five, are operated at 20 kV or 10 kV and primarily as a cable network. They are used for distribution in urban areas or between villages in the countryside, up to local network stations. Biogas plants, smaller wind farms and industrial and commercial areas are also connected to this voltage level.
The lowest voltage level, network level seven, is the low-voltage level with 400 volts (V) or 230V (conductor-conductor/conductor-ground) and is only designed as a cable network. This is where private customers connect to their homes and provide the voltage that we receive from our well-known power outlets.
The network levels two, four and six between these voltage levels are only used for conversion and are designed in the form of substations with (power) transformers or as a “small substation”, a local network station.
As service providers, transmission system operators, TSOs for short, play a decisive role in the operation of the power grid. They size and build the transmission grid, maintain it and regulate the ratio of generated electrical energy and grid load. They are therefore responsible for the supra-regional supply and transmission of electrical energy. In addition, the transmission grids connect the German power grid with that of neighboring European countries and thus enable cross-border energy exchange.
As a natural monopoly, transmission networks in Germany are operated by four TSOs. These include Tennet TSO, 50Hertz Transmission, Amprion and TransnetBW. As independent transport network operators, it remains their task to ensure network security and also to provide all market participants with non-discriminatory network access. The operation is regulated and supervised by the Federal Network Agency. As a state body, the Federal Network Agency publishes a network development plan every few years, with recommendations and advice from the TSO. As a result, the power grid should continue to be prepared for the changes caused by the energy revolution in order to continue to ensure security of supply. This network expansion plan results in the expansion work for the TÖNBs.
All downstream network levels, grid levels three to seven, are operated by distribution system operators, or VNB for short. Divided into numerous network areas (over 900 network areas in Germany), the VNB is responsible for setting up and maintaining the distribution network in these network regions. The DSO must also provide all end users, generation and storage facilities and downstream networks with non-discriminatory access to the electricity grid in exchange for a network fee. In addition to the electricity grid, these DSOs often also operate the regional gas network and ensure distribution to the end user or producer. In addition to the electrical power that is produced in the grid and fed into the distribution grid, VNPs obtain power from the high-voltage level from the TSO. In the past, the VNB were also the energy suppliers to end users. However, since the liberalization of energy supply, this is no longer necessarily the case. Since then, sales and network operation have been legally separated, which is why an end user can choose the energy supplier but not the network operator.
With the start of the energy revolution, a change in load flow in the power grid became apparent. Whereas in the past, the power of large conventional power plants was fed in at the high-voltage level and thus supplied to the lower voltage levels, today they are also supplied from the lower voltage levels. The top-down of the network structure is increasingly becoming a “top-down and bottom-up” of supply. This is due to the increasing expansion of photovoltaic and wind power plants with connections at the lower voltage levels.
To ensure secure network 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 principle of 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 for maximum transmission and supply tasks. Depending on customer connections, supply interruptions are tolerated within limits only in the distribution network, provided that these can be resolved promptly.
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