Underground Electric Transmission Lines

Introduction

This overview contains information about electric transmission lines which are installed
underground, rather than overhead on poles or towers. Underground cables have different technical
requirements than overhead lines and have different environmental impacts. Due to their different
physical, environmental, and construction needs, underground transmission generally costs more
and may be more complicated to construct than overhead lines. Issues discussed in this pamphlet
include:
 Types of Underground Electric Transmission Cables
 Ancillary Facilities
 Construction and Operation Considerations
 Costs
 Repairs
The design and construction of underground transmission lines differ from overhead lines because
of two significant technical challenges that need to be overcome. These are: 1) providing sufficient
insulation so that cables can be within inches of grounded material; and 2) dissipating the heat
produced during the operation of the electrical cables. Overhead lines are separated from each other
and surrounded by air. Open air circulating between and around the conductors cools the wires and
dissipates heat very effectively. Air also provides insulation that can recover if there is a flashover.
In contrast, a number of different systems, materials, and construction methods have been used
during the last century in order to achieve the necessary insulation and heat dissipation required for
undergrounding transmission lines. The first underground transmission line was a 132 kV line
constructed in 1927. The cable was fluid-filled and paper insulated. The fluid was necessary to
dissipate the heat. For decades, reliability problems continued to be associated with constructing
longer cables at higher voltages. The most significant issue was maintenance difficulties. Not until
the mid-1960s did the technology advance sufficiently so that a high-voltage 345 kV line could be
constructed underground. The lines though were still fluid filled. This caused significant
maintenance, contamination, and infrastructure issues. In the 1990s the first solid cable
transmission line was constructed more than one mile in length and greater than 230 kV.

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