In the past powerlines traditionally have been placed overhead on a system of pylons. As recently as the 1990s it was much more expensive and difficult to place such high voltage powerlines underground.
However there have been significant and rapid developments in this area and placing high voltage powerlines underground is now common practice throughout the world. New burial and jointing techniques have revolutionised this field. In Tokyo it has even been possible to underground a substation – under a building! Comparisons in cost between overhead and underground systems can now be as low as equal to or only twice the capital cost of overhead transmission lines. This can be seen in the increasing utilization of underground cable methods throughout the world and in Europe in particular.
It is also true that the construction times for underground lines are much shorter than the construction of an overhead system – four years as opposed to seven years for overhead. This is because of increasingly stringent environmental restrictions. And of course the delays caused by public opposition to overhead lines must also be factored in! However NIE have misleadingly suggested to landowners that it will be quicker and more convenient if this line is placed overhead.
There is general industry acceptance that underground cables are far more reliable, have a lower maintenance cost and a greater longevity. Several studies confirm reliability of underground transmission.
o North Carolina Utilities Commission (Nov. 2003) found that underground outage rates are 50% less than overhead o Maryland Public Service Commission (Feb. 2000) found that underground systems of urban utilities have lower frequency and duration of outages o Australian Government (Nov. 1998) found that high voltage underground systems had 80% less outages than overhead
As a result of the above, maintenance costs are substantially reduced to perhaps 10% of those for overhead power lines. The improvements made in the last decade in the area of undergrounding mean that power system operators can monitor underground cables through built-in temperature sensors. In the rare event of a cable fault, generally caused by external disturbance, advanced monitoring of temperature and integrity in real time will allow faults to be located immediately to within one metre and repairs to be carried out in a much shorter timeframe than in the past. However NIE continue to claim that maintenance on underground lines will be extremely difficult and time consuming to carry out – again misleading the general public.ic. ic.