Practical suitability largely unknown

For safe operation, aspects which have highest priority include availability, reliability and the transmission capacity of the lines as well as protection of personnel and systems.

Although the technology of 380 kV underground cabling has been greatly developed in recent years, it has been little tested in practice and continues to be employed around the world with caution and apprehension, not least because of the lack of a sufficient quantity of empirical data for safe operation.

Swissgrid has a legal mandate to ensure the non-discriminatory, reliable and efficient operation of the transmission system. Swissgrid is also under an obligation to evaluate all measures in terms of the security of supply and to make decisions on that basis.

With regard to security of supply, various aspects have to be taken into consideration: the impact of underground cabling on the overall system, operating experience, the behaviour and complex interplay of the various components of underground cabling and the use of technology in high voltage systems.

The transmission grid is a highly interconnected network and this increases the complexity and sensitivity of the system compared to medium- and low-voltage grids. In interconnected grids, increasing the number of cable components can lead to the occurrence of effects attributable to the different physical characteristics of overhead lines and underground cabling. This can result in grid instability, which would need to be managed along with voltage stability and frequency fluctuations.

Swissgrid is committed to conducting field tests to the best of its ability to ascertain which designs are suitable for operating a cable route equivalent to an overhead line.

Technological developments

Underground cabling, particularly in the high-voltage grid up to 150 kV, has in recent years undergone tremendous technological development. This has made its use more attractive, environmentally-friendly and cost competitive.

In the high-voltage grid at levels up to 380 kV the technology still faces limitations around the world due to the high demands on capacity, availability and system functionality.

The critical elements of underground cabling are the couplings and cable-end seals. Couplings are the elements that connect two underground cables and which are required beyond a certain length of cable. Cable-end seals are used for the transition from overhead to underground cabling.