Best of 2013 - Article BY FRANK ALONSO AND CAROLYN A. E.
onto Green Building Design - Architecture & Engineering
>Hurricane Sandy left many electric utility executives, their customers, local and state government leaders and regulators contemplating placing overhead power lines underground. This desire surges into prominence whenever natural disasters cause destruction on the overhead distribution and transmission networks across the country. In the past, the largest obstacle to placing overhead power lines underground has been the higher cost of installation and maintenance for underground lines. [...]
Whenever a major weather-related catastrophe occurs or land is being developed, the question of placing overhead power lines underground surges. The answer to the proverbial question, "Why can't overhead power lines be placed underground?" is, "They can be, but it's expensive."
Higher initial construction costs. According to the May 2011 paper "Underground Electric Transmission Lines" published by the Public Service Commission of Wisconsin, "The estimated cost for constructing underground transmission lines ranges from 4 to 14 times more expensive than overhead lines of the same voltage and same distance. A typical new 69 kV overhead single-circuit transmission line costs approximately $285,000 per mile as opposed to $1.5 million per mile for a new 69 kV underground line (without the terminals). A new 138 kV overhead line costs approximately $390,000 per mile as opposed to $2 million per mile for underground (without the terminals)."
These costs show a potential initial construction cost differential of more than five times for underground lines as opposed to overhead lines for construction in Wisconsin. Costs vary in other regions, but the relative difference between overhead and underground installation costs is similar from state to state.
Maintenance costs. The present worth of the maintenance costs associated with underground lines is difficult to assess. Many variables are involved, and many assumptions are required to arrive at what would be a guess at best. Predicting the performance of an underground line is difficult, yet the maintenance costs associated with an underground line are significant and one of the major impediments to the more extensive use of underground construction. Major factors that impact the maintenance costs for underground transmission lines include:
Cable repairs. Underground lines are better protected against weather and other conditions that can impact overhead lines, but they are susceptible to insulation deterioration because of the loading cycles the lines undergo during their lifetimes. As time passes, the cables' insulation weakens, which increases the potential for a line fault. [...]
Line outage durations. The durations of underground line outages vary widely depending on the operating voltage, site conditions, failure, material availability and experience of repair personnel. The typical repair duration of cross-linked polyethelene (XLPE), a solid dielectric type of underground cable, ranges from five to nine days. Outages are longer for lines that use other nonsolid dielectric underground cables such as high-pressure, gas-filled (HPGF) pipe-type cable, high-pressure, fluid-filled (HPFF) pipe-type cable, and self-contained, fluid-filled (SCFF)-type cable. In comparison, a fault or break in an overhead conductor usually can be located almost immediately and repaired within hours or a day or two at most.<