Overhead transmission of electric power remains one of the
most important elements of today’s electric power system. Transmission systems
deliver power from generating plants to industrial sites and to substations
from which distribution systems supply residential and commercial service.
Those transmission systems also interconnect electric
utilities, permitting power exchange when it is of economic advantage and to
assist one another when generating plants are out of service because of damage
or routine repairs. Total investment in transmission and substations is
approximately 10% of the investment in generation.
Since the beginning of the electrical industry, research has
been directed toward higher and higher voltages for transmission. As systems
have grown, higher-voltage systems have rarely displaced existing systems, but
have instead overlayed them. Economics have typically dictated that an overlay
voltage should be between 2 and 3 times the voltage of the system it is
reinforcing.
Thus, it is common to see, for example, one system using lines
rated 115, 230, and 500 kilovolts (kV). The highest ac voltage in commercial
use is 765 kV although 1100 kV lines have seen limited use in Japan and Russia.
Research and test lines have explored voltages as high as 1500 kV, but it is
unlikely that, in the foreseeable future, use will be made of voltages higher
than those already in service.
This plateau in growth is due to a corresponding plateau in
the size of generators and power plants, more homogeneity in the geographic
pattern of power plants and loads, and adverse public reaction to overhead
lines. Recognizing this plateau, some focus has been placed on making
intermediate voltage lines more compact.
Important advances in design of transmission structures as
well as in the components used in line construction, particularly insulators,
were made during the mid-1980s to mid-1990s. Current research promises some
further improvements in lines of existing voltage including uprating and now
designs for HVDC.
The fundamental purpose of the electric utility transmission
system is to transmit power from generating units to the distribution system
that ultimately supplies the loads. This objective is served by transmission
lines that connect the generators into the transmission network, interconnect
various areas of the transmission network, interconnect one electric utility
with another, or deliver the electrical power from various areas within the
transmission network to the distribution substations.
Transmission system design is the selection of the necessary
lines and equipment which will deliver the required power and quality of
service for the lowest overall average cost over the service life. The system
must also be capable of expansion with minimum changes to existing facilities.
Electrical design of ac systems involves (1) power flow
requirements; (2) system stability and dynamic performance; (3) selection of
voltage level; (4) voltage and reactive power flow control; (5) conductor
selection; (6) losses; (7) corona-related performance (radio, audible, and
television noise); (8) electromagnetic field effects; (9) insulation and
overvoltage design; (10) switching arrangements; (11) circuit-breaker duties;
and (12) protective relaying.
Mechanical design includes (1) sag and tension calculations;
(2) conductor composition; (3) conductor spacing (minimum spacing to be
determined under electrical design); (4) types of insulators; and (5) selection
of conductor hardware.
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