Swissgrid investigated the strained situation in the Swiss transmission grid from 20 May 2019. The analysis shows that different factors played a major role in the violation of certain grid elements.
On 20 May a very large amount of electrical energy totalling 12 GW was produced in Switzerland. The export to neighbouring countries of up to 4.5 GW was also very high. Likewise, the high scheduled commercial export of up to 4 GW from Switzerland to Germany was above-average. In addition, only a small amount of energy was exported to Italy, even though it typically imports large quantities from Switzerland. Moreover, Switzerland imported electrical energy from France. This type of export situation to the north, combined with these load flows with Italy and France, is potentially problematic for the Swiss transmission grid as production has to be transported north via the 220 kV grid within Switzerland. Essentially, much higher production can be exported when predominantly exporting to the south.
Besides this unusual export situation, there were also short-term increases in production and load shifts as a result of topological measures on grid levels 1 and 3 (in particular, grid separation at grid level 3 and transformer stepping operations at grid level 1) in the region of Bern and Central Switzerland. In combination, this led to an overload of a grid element and to various N-1 violations (threat of overloads) at the 220 kV level of the transmission grid.
Unscheduled flows with neighbouring countries can be ruled out as the cause with a high level of probability in this specific situation.
The prognosis tool was not able to detect the violations of the grid elements in time as grid level 3 is not mapped and the production increase took place too quickly.
Swissgrid subsequently implemented various measures to rectify the overloading of the grid element and to reduce the N-1 violations. As no national redispatch was possible in Switzerland due to the geographic distribution of production, production had to be reduced with an international redispatch.
Grid situations such as those from 20 May 2019 can occur when a grid is operated at its load limits due to specific weather, production and market conditions. In these kinds of situations small changes in absolute values, in production, load and/or import/export can lead to significant changes in the grid situation, especially to overloads of grid elements.