Swissgrid advances 380 kV upgrade for Alpine transmission corridor

Engineering work has commenced for three major substation sites supporting the Innertkirchen to Ulrichen transmission link in Switzerland. The initiative focuses on renewing a corridor that has been in service for over 60 years. Spanning 27 km, the renewed route will feature approximately 23 km of underground cabling, laying the groundwork to elevate system voltage from 220 kV to 380 kV.

This development highlights the technical demands facing transmission system operators modernising ageing extra-high-voltage networks in complex terrain. Replacing infrastructure that has operated for over 60 years requires addressing legacy substation layouts that cannot easily accommodate higher physical clearances. Transitioning an operational corridor from 220 kV to 380 kV necessitates substations engineered either with dual-ratio transformers or with compact gas-insulated switchgear bays configured for rapid cut-over.

Furthermore, integrating 23 km of underground extra-high-voltage cable into a mountainous 27 km link presents acute system-strength and voltage-control issues. High-capacitance underground circuits generate substantial reactive power under light load conditions. Substation designers must incorporate appropriately sized variable shunt reactors, harmonic filters, and specialised switching devices to withstand transient recovery voltages and capacitive switching currents. For network operators, managing these reactive phenomena is critical to prevent severe overvoltage events across interconnected mountain valleys.

Long-distance undergrounding also shifts operational practices from overhead line inspection toward sophisticated cable monitoring, partial discharge detection, and thermal backfill management. Substations anchoring these underground sections become complex hubs where cable terminations, surge arresters, and monitoring sensors converge. Asset managers must ensure that protection schemes can rapidly identify and isolate cable faults to protect adjacent power transformers and maintain corridor stability during scheduled network transitions.

What this means for asset owners in Europe: Transmission operators and industrial grid engineers in Switzerland must plan extended outage windows to execute cable tie-ins and substation cut-overs when uprating corridors from 220 kV to 380 kV. Equipment buyers face long manufacturing lead times for extra-high-voltage shunt reactors and switchgear, requiring procurement to commence years ahead of physical civil works to avoid grid connection delays.