Underground cables are generally required in urban areas where there is a high
density of population. The highest load density and the most restrictive limitations on
feeder layout occur simultaneously in the central cores of major cities. Load densities
are usually in the range of 60 to 100 MW per square mile.
Some problems associated with underground cable systems are as follows:
urban area are densely populated with/pedestrian walk ways, as well as water, sewer,
storm drain, phone and other utility systems in addition to electrical. So there is
limited room. The electrical utility must stake its claim to the routes and space
allocated to it with its own duct banks.
Secondly, duct banks are needed to protect underground cable from the constant dig-
ins of other utilities, stress form settling, and heat and moisture in this environment.
Thirdly, digging into the street in an urban area for new additions or repairs is very
expensive. Traffic control and other requirements add to cost and also digging around
all the other utilities. Thus electric utilities have no choice but to use duct banks and
cable vaults to create their own “cable tunnel system“ under the streets in dense urban
areas.
Finally branching of underground cable providing for “T” or “X” connections of
paths, while possible, is not as simple or as inexpensive as in overhead lines.
Summarizing, the underground urban area systems have the following characteristics
• Layout is restricted to street grid
• Loads are large and invariably three phase
• Fixed cost is very high
• The cost of capacity shortfall is extremely high
gregorymasila answered the question on February 20, 2018 at 16:11
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