On Friday, 17 July, the Swiss transmission system operator (TSO) Swissgrid carried out check-operations concerning the extension of the 220kV switchgear in the Chippis substation in the canton of Valais. At 4.23 p.m., the protection equipment of the 220kV grid node Creux de Chippis was accidentally tripped, causing the switchgear to lose voltage. As a result of the technical failure at Chippis, the switchgear of the substations in Stalden, Bitsch, and Mörel were also affected by the voltage drop, which led to a regional supply interruption in the distribution system.
The result was a one-hour power outage in a total of 60 municipalities with around 112,000 households as well as trade and industry in the Swiss Sierre district and the largest part of the Upper Valais. By 5.23 p.m., all affected switchgear and lines of the TSCNET shareholder were back in regular operation. Thanks to the good cooperation between Swissgrid and the five affected distribution system operators (DSOs) of the lower voltage levels, it was possible to gradually restore the electricity supply from 5 pm onwards. By 6.15p.m. almost all customers were back on power. Swissgrid immediately initiated a detailed investigation of the incident, which is currently in progress.