Maintenance and expansion of the Swiss transmission grid require temporary outages of grid elements. This can result in temporary bottlenecks which affect the grid’s transport capacity. In the congestion warning process, Swissgrid anticipates such situations and calculates the maximum power output that Swiss hydropower plants can produce in this constellation. By limiting the plant output for the duration of a forecasted congestion, Swissgrid and the power plant operators jointly contribute to Switzerland's security of supply.
The congestion forecast for the following year is initially calculated one year in advance (Y-1), updated in the monthly planning (M-1) and finally validated and published one week before the expected congestion occurs (W-1). In the event of disruptions or short-term changes to outages, congestion alerts can be up-dated up to a maximum of two days in advance. All changes at shorter notice are handled by the real-time operation.