Limitations of AC underground cables

The usual type of cables used for many decades were fluid filled or mass impregnated types, either pressurised or not. Recent developments in extrusion techniques, in material handling systems and cleanliness of materials have led to the introduction of “solid” insulation cables usually based on polymeric plastic insulation.

Today Cross-linked-Polyethylene (XLPE) cables are possible at the very high voltage of 400 kV and almost a total of 2,000 km of various projects exist in various parts of Europe. The manufacturing of optimised cables based on these solid insulation materials is more efficient and when used together with standardised pre moulded accessories, allows for easier and less costly installations than those for fluid filled cables.

One of the main advantages of underground cables is the low level of Joule losses compared with equivalent overhead lines. That is the reason why operation costs are very significantly reduced in the case of underground cables.

Yet, there is a limit in EHV undergrounding due to capacitive current, requiring introduction of compensation stations for links above a certain length called the “critical length”. Usually, the maximum length between compensation stations is about 20 km for EHV cables, which is small compared with overhead lines, where smaller capacitive current allows length 10 to 20 times longer. In general, technological progress has reduced costs and space requirements in the compensation field as well. Underground cables have better performance than overhead lines regarding electro magnetic field generation. Metal screens eliminate radiated electric fields and the risk of electrical shocks. The magnetic fields can be managed both in amplitude and space and are usually less than the equivalent overhead lines at a small distance from the cable trench.