As in many European countries, the Swiss transmission grid already today – at the beginning of the energy transition – has structural bottlenecks. Here, as elsewhere, these must be eliminated to increase security of supply and maintain it in the long term. Tense grid situations may pose problems to the connected system, as the Swiss transmission system operator (TSO) Swissgrid experienced on 17 July 2020, when the protection equipment of the 220kV grid node Creux de Chippis was accidentally tripped, which in the further course of events led to supply interruptions in the Valais distribution grid. The TSCNET shareholder immediately initiated extensive investigations into the event, the results of which have now been transparently communicated.
An extremely complex chain of technical and human factors is behind the Chippis incident. First the technical aspects: The transmission grid capacities in the region around Chippis are insufficient in summer due to the high generation of Alpine hydroelectric power plants. Because of this, Swissgrid for some time now had to completely disconnect the transmission grid in the Mörel substation in east-west direction for around 15 weeks a year to guarantee grid security and transmission capacities in the region. This reduces fault tolerance in the regional transmission system. In addition, several extension projects are currently being implemented in the Chippis area, which made adjustments to the operation of the Chippis substation necessary. These settings – also affecting the protection settings of the system – were carried out by the contracted service provider in December 2019.
As regards the human factor in the chain of events, necessary and scheduled tests of protection parameters were performed by a contracted specialised company in the Chippis substation on 17 July. During these tests, the system was not put into revision mode, which is why the system protection function unintentionally detected the test signal. This triggered the so-called busbar protection. The switchgear in the Chippis substation was subsequently disconnected and, due to the special grid situation, the switchgears in Stalden, Bitsch, Zermeiggern, and Mörel were also affected by the voltage loss, leading to a regional supply interruption in the distribution system.
The result of this combination of occurrences was a power failure in 60 municipalities in the district of Sierre and in most of the Upper Valais. After one hour, all affected switchgear and Swissgrid lines were back in regular operation. Thanks to the good cooperation between Swissgrid and the concerned distribution system operators (DSOs) of the lower voltage levels, electricity supply was gradually resumed and after less than two hours, almost all customers were back on power. Adrian Häsler, Head of Grid Infrastructure at Swissgrid, classifies she supply disruption in Valais as “an extremely rare occurrence”, but nevertheless apologises to those affected. And though the tests themselves were not negligent, the findings from the incident are incorporated into the company’s training and education of internal and external experts. Häsler also emphasises that Swissgrid drew attention to the bottlenecks in Valais some time ago: “Incidentally, we recognised these grid congestion problems in the context of ‘Strategic Grid 2025’, the TSO’s ambitious grid modernisation and extension plan, and addressed them in 2015. The grid expansion as planned by Swissgrid is essential to maintain the long-term security of supply.”