In the energy system, the (n-1) criterion, also known as (n-1) security, refers to the principle that in the event of a component failure in the electricity grid, redundancy ensures that the supply is still guaranteed and the failure of a system is prevented. This criterion is a fundamental principle in German grid planning and makes a decisive contribution to the high level of grid security in Germany. In the event of the failure of a component, such as a power circuit, an interruption to the supply or an extension of the disruption is avoided thanks to existing backup options. Compliance with the (n-1) rule is particularly important when the grid is at maximum capacity. 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, an (n-2) connection is even 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 occurring. 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 outage. Compliance with this principle is of crucial importance and must be taken into account by transmission system operators both in the planning and operation of electricity grids.
For this reason, overhead lines in the high and extra-high voltage range are often designed as double systems. Each pylon carries two three-phase systems per voltage level, each with three phases - one system on the left-hand side and one system on the right-hand side of the pylon. If one side fails, the other side takes over the transmission. 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 an emergency. The more transformers there are, the higher the capacity utilization of the substation. In addition, the transformers are normally only used at around 50 percent of their capacity in order to be ready for use in an emergency. This sophisticated grid 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 grid is designed in an interconnected pattern so that the power flow can be rerouted via parallel transmission lines in the event of a line failure. The only exception to the N-1 criterion is for low-voltage grids. These are often designed as beam grids, which means that the low-voltage consumers are generally not connected in an N-1-safe manner.
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