In the energy system, the (n-1) criterion, also known as (n-1) safety, describes the principle that in the event of a component in the power grid failure, the supply continues to be guaranteed through redundancies and the failure of a system is prevented. This criterion is a fundamental principle in German network planning and contributes decisively to a high level of network security in Germany. If a component, such as an electrical circuit, fails, existing alternative options prevent an interruption of the supply or an expansion of the fault. Compliance with the (n-1) rule is particularly important when the network is fully utilized. If the network is not fully utilized, even higher levels such as (n-2) can be achieved. In some networks, such as critical infrastructure networks, even an (n-2) connection is mandatory.
The principle of n-1 states that the power grid must be able to cope with the failure of a line without significant power outages. In concrete terms, this means that in the event of a faulty line, an alternative line must ensure the supply in order to prevent a power failure. Compliance with this principle is crucial and must be taken into account by transmission system operators both when planning and operating power grids.
For this reason, overhead lines in the area of high and extremely high voltage are often designed as dual systems. Each mast supports two three-phase systems per voltage level — one system on the left and one system on the right side of the mast. If one side fails, the other side takes over the transfer. In order to avoid overloads, the lines are only used at around 50 to 70 percent of their capacity during normal operation.
Substations also require components that can step in in in an emergency. The following applies: The more transformers (transformers) there are, the higher the substation can be utilized. In addition, only around 50 percent of the capacity of the transformers is normally used in order to be ready for use in an emergency. This sophisticated network design and operating practice ensure a robust and reliable power supply.
In principle, the N-1 criterion applies to all voltage levels. For this reason, the transmission network is designed in a networked pattern so that, in the event of a line failure, the power flow can be diverted via parallel transmission lines. There is only an exception to the N-1 criterion for low-voltage networks. These are often designed as radiation networks, which means that low-voltage consumers are generally not connected in an N-1 safe manner.
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