What are the main differences in outages & repairs between overhead lines and underground cables?

Once installed in the ground, cables are intrinsically maintenance free. Since their introduction, EHV XLPE cables have an excellent track record. It is only external influences that could damage the cable and remote monitoring of the cable circuit allows the rapid location of the fault, so that a targeted intervention can be conducted. Remote monitoring of the cable circuit also means there is no specific need for a permanently staffed route surveillance team.

Given that the majority of recent installations have only a small service track record, long term service reliability cannot yet be established but all indications from extensive testing of the cable systems and their history to date are that underground cable systems can achieve service reliability that matches OHLs.

In recent years there have been several severe weather related events that led to widespread damage to electricity pylons and conductors. This includes France in December 1999, North East USA in August 2003, Italy in September 2003 and Germany in November 2005. Most of these incidents were due to either trees falling onto conductors or damage caused by heavy winds/snow storms. In the 2005 incident in Germany, 83 pylons were damaged and three months after the event, half of the damaged pylons were still not replaced. Events such as these have led to a series of investigations and demands to place more OHLs underground.

In a recent study by Eurelectric an examination was made of four different severe weather events in Canada, France, Latvia and Sweden between 1998 and 2005 and the report concluded that underground cables can significantly decrease the impact of storms especially those accompanied by freezing rain or ice. They also recommend that when planning for investments in the electricity network, a life cost analysis approach should be used as opposed to the simpler initial investment cost.