background image
Transmission Line Principles
ransmission Line Pr
Transmission Line Principles
The Purpose of Transmission Lines
Transmission lines act like the arteries in the human circulatory
system, moving electrical power from were it is produced by
generators to where it is consumed at load centers. And like
arteries in the human body, the loss or damage to transmission
infrastructure can have disastrous effects on the overall power
system and the customers it serves. and is measured in either
kilowatts (kW) or horsepower (Hp).
Transmission Line Construction
Transmission lines are generally built in one of two methods:
overhead, air-insulated lines, and underground cables. Other
constructions, such as Gas Insulated Lines (GIL), are extremely
Types of Transmission Lines
The most common type of transmission circuit are overhead lines,
where the energized conductors are suspended from structures
using porcelain, glass or polymer insulators. The spacing between
conductors is dependent on the voltage level the line is operated at,
and typically the conductors are located dozens of meters above
the ground for isolation and safety reasons.
Due to the weight of the suspended conductors and the distances
needed, the structures to support overhead lines are very large and
have a large geographic footprint (commonly referred to as Right-
of-Way or ROW). One of the advantages of overhead construction,
beyond the easier ability to maintain it, is the ability to use ambient
air to cool the conductors and therefore dynamically change the
rating of transmission circuits based on ambient air temperature
and wind speed.
This method of construction is typically the most common, and
usually meets the most public opposition when new construction
or upgrading of transmission infrastructure is discussed.
Underground Cables
As opposed to overhead lines, underground cables use a solid
dielectric material to isolate the energized phase conductors and
ground. Older cables made use of many layers of paper saturated
in mineral oil to insulate the phase conductors, while newer cable
technology make use of polymer dielectric materials such as
crosslinked polyethylene (XLPE). Due to the high insulation value
of polymer dielectric materials like XLPE, the conductors within
an underground cable may be placed much closer together when
compared to the air insulated overhead line. For lower voltages, all
three phases may be built into a single cable; for higher voltages
each phase conductor is in a separate cable.
Figure 1.
Overhead transmission lines
While the geographic footprint of underground cables is generally
less than that of overhead lines, the overall power capacity of
underground cables tends to be significantly less than overhead
construction. For underground cables where large power transfers
are required, cables are often installed into conduits that are filled
with mineral oil that is continuously circulated in order to provide
additional cooling. This incurs additional capital and maintenance
costs and presents additional risks for environmental incidents
should the oil leak.