Uninterruptible Power Supply: Dynamic UPS Systems for the Ceneri Base Tunnel

indispensable at all times. This also applies to the Ceneri Base Tunnel, which went into operation on schedule on 13 December 2020. A sophisticated solution from the Swiss Avesco AG for uninterruptible power supply, which had already been successfully implemented in the Gotthard Base Tunnel, was used.

The New Rail Link through the Alps (NRLA) is the centrepiece of Swiss transport policy. With three base tunnels and further extensions, Switzerland has thus created an efficient railway infrastructure through the Alps. It enables shorter journey times and higher quality in passenger and freight transport.

The Lötschberg Base Tunnel has been in operation since 2007, the Gotthard Base Tunnel since 2016. With a length of 15.4 km, the Ceneri Base Tunnel is the third major structure of the NRLA. Like the Gotthard Base Tunnel, the Ceneri Base Tunnel consists of two single-track tubes with a regular centre distance of around 40 m, which are connected every 325 m by cross-passages.

Power Supply System Secured by Multiple Backups

The safety infrastructure in the tunnel includes lighting, ventilation and escape routes. These operations are powered by a multi-security power supply system, which was realised in a successful collaboration between Avesco and its partner HITEC Power Protection, the Dutch manufacturer of the dynamic rotary UPS systems (UPS = uninterruptible power supply), or DRUPS as they are sometimes called.

The tunnel tubes are divided into two separate systems: a west and an east system. In each system, the power supply is normally provided by the regular public grid. In the event of a failure of the regular power supply, there is a backup network at the north and south portals of the tunnel tubes in both the west and east sections. Each backup network is fed by a 1750 kVA DRUPS  system. This means that a total of four systems are available as backup. For comparison: 10 DRUPS systems are installed in the 57 km long Gotthard Base Tunnel.

Even the failure of one of these systems is secured: The west and east backup grids are coupled through a so-called „master-slave circuit“. This means that if one DRUPS system fails, the remaining system can supply both backup grids with emergency power without interruption. The backup grids run empty and can be used immediately at full capacity if necessary.

The “mean time between failure” (MTBF) as a measure of the safety level, i.e. the number of operating hours during which statistically a single total failure may occur, is an extremely high 4 million operating hours for the power supply of the Ceneri Base Tunnel.

This backup system, in particular the master-slave circuit, was developed for the construction of the Gotthard Base Tunnel. For the Ceneri tunnel, the plant engineers were able to benefit from this experience and use the system again with minor adjustments.

Challenges

A special challenge in this project were the exceptionally long power lines; in some cases, lines up to around 6 km long were used. Due to the extremely long and unloaded lines, there is a very high capacitive reactive power proportion, which results in additional power for the generators. Also due to the cable length, the short-circuit power of the DRUPS systems must be very high; it is 17 times the nominal power. For comparison: classic emergency power systems, as they are normally installed, have a threefold short-circuit current.

The extensive documentation requirements within the scope of the installation work were also very demanding. For each DRUPS system alone, around 800 pages of electrical diagrams had to be drawn up. In keeping with a construction project of this magnitude and importance, the awarding of the contract was preceded by comprehensive contract negotiations, and the successful project for the construction of the Gotthard Base Tunnel was a major reason for the Ceneri Tunnel being awarded as well.

Part of the project was also the training of the staff of the Swiss Federal Railways in several workshops in German and Italian language with practical and theoretical parts. Around 30 people underwent this operator training.

Support and Maintenance of the System

Even after the official opening of the Ceneri Base Tunnel in September 2020 and its commissioning the following December, work on the emergency power supply will not be completed. A framework contract has been concluded with the owner SBB for support and maintenance, which regulates the services to be provided by Avesco on all DRUPS systems in the Gotthard and Ceneri base tunnels. This ensures maxi