(n-1) Load in the Swiss Electricity Grid

The Swiss transmission grid is approaching its capacity limits. More than two-thirds of the Swiss transmission grid is over 40 years old and can no longer cope with the future requirements of increasing electricity consumption as well as the fluctuating feed-in from renewable energy. Even today, power plant operators have to reduce electricity production from hydropower because Swissgrid doesn't have sufficient grid capacity to discharge the energy. The Swissgrid Control grid control room is faced with major challenges in the operation of the Swiss transmission grid due to structural bottlenecks.

Unplanned grid loads can occur in the Swiss transmission grid at any time due to unforeseen events such as outages of grid elements (lines or transformers). The electricity tries to find its way through grid elements that are still intact, for example after the outage of a line. This unplanned electricity flow can lead to an overload of other grid elements.

At least every five minutes, Swissgrid Control determines the (n-1) load of the grid elements in the entire Swiss transmission grid based on thousands of current and voltage measurements. In the process, the load values that result for the remaining grid elements are calculated after the simulated outage of any element. The simulation shows which element of the entire still intact transmission grid would be loaded the most in case of the failure of another grid element. An (n-1) load that is greater than 100% means that this could lead to an overload and to the loss of another grid element if the overload remains. Therefore, the objective of grid operations is to keep the (n-1) load of the grid elements at a maximum of 100 per cent at all times.

The (n-1) simulation therefore does not represent the actual load in the transmission grid. However, the result of the simulation is used by Swissgrid Control for the preparation and execution of measures if the possibility of overrunning the (n-1) limits exist.

The Swiss transmission and extra high voltage grid is a complex system with a total of around 6700 kilometres of lines, 140 switching substations and 41 connections to foreign countries. The grid elements with the highest (n-1) loads occur together with well-known capacity bottlenecks in the existing transmission grid. If the same grid elements are always overloaded in the (n-1) case, then this is a bottleneck in the Swiss transmission grid.

Excessively high (n-1) loads were calculated at different points in the Swiss transmission grid (380/220 kV) during calendar year 2013 as well as in the previous years. These loads can be caused by the outage of different grid elements. Swissgrid Control registers the grid element that experiences an excessively high (n-1) load due to the outage of another grid element. With this information, Swissgrid Control can initiate the necessary measures in the grid.

The following map shows the lines or transformers where the highest (n-1) loads occurred during the year 2013. In particular, the affected grid elements are heavily loaded due to a lack of transport capacity for discharging energy from hydropower plants. The illustration shows that the Swissgrid grid was subjected to major loads in 2013.

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Only as an exception do ideal conditions prevail in the operation of a transmission grid. Much more, the operation is characterised by scheduled shutdowns of grid element, maintenance stoppages of power plants, and differences between the forecast and actual electricity consumption. This is made even more difficult by grid loads from unplanned events such as, for example, the outage of a power plant or a transmission line. Swissgrid Control takes measures to restore the (n-1) security depending on the extent of a too high (n-1) load. The higher the calculated overrun of the 100% limits, the more drastic and last-minute the selected measures are. They range from topological measures (changes to the interconnection of lines), continue with interventions in the power plant production (redispatch), up to a limitation to the cross-border transits or power outages for consumers.

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