Download Transmission Lines Overhead AC transmission lines

Transmission Lines
Transmission lines carry electric energy from one point to
another in an electric power system. They can carry alternating
current or direct current or a system can be a combination of
both. Also, either overhead or underground lines can carry
electric current. The main characteristics that distinguish
transmission lines from distribution lines are that they are
operated at relatively high voltages, they transmit large
quantities of power and they transmit the power over large
distances.
The types of transmission lines are:
•
•
Overhead Transmission Lines
Sub transmission Lines
Overhead AC transmission lines
→Share one characteristic; they carry 3-phase
current. The voltages vary according to the
particular grid system they belong to.
Transmission voltages vary from 69 kv up to 765
kv. The following are examples of different
overhead transmission line structures in use today.
The DC voltage transmission tower has lines in
pairs rather than in threes (for 3-phase current) as
in AC voltage lines. One line is the positive current
line and the other is the negative current line.
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Figure 1. Some typical transmission line structures
Figure 2. Transmission lines crossing the San Fernando Valley
Figure 4. DC voltage transmission lines
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Figure 5. Transmission lines above - sub transmission below
Figure 6. 230 kv wood transmission line towers
Figure 7. Double set of transmission lines side by side
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Additional information:
•
The Lineman's and Cableman's Handbook,
Shoemaker, T. M., Mack, J. E., Tenth Edition
2002, McGraw-Hill.
Sub transmission lines
Carry voltages reduced from the major
transmission line system. Typically, 34.5 kv to 69
kv, this power is sent to regional distribution
substations. Sometimes the sub transmission
voltage is tapped along the way for use in
industrial or large commercial operations. Some
utilities categorize these as transmission lines.
Figure 8. Sub transmission lines.
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Figure 9. Sub transmission lines above and distribution lines below.
Figure 10. 46 kv sub transmission
Figure 11. Sub transmission lines with distribution primaries and secondaries
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Figure 12. Sub transmission lines with distribution underbuild
Secondaries
Secondaries are the conductors originating at the
low-voltage secondary winding of a distribution
transformer. Secondaries for residential service are
three-wire single-phase circuits. They extend along
the rear lot lines, alleys, or streets past customer's
premises. The secondaries can be overhead lines
or underground lines.
Overhead secondary lines are usually strung below
the primary lines and typically in a vertical plane.
When secondaries are strung in a vertical plane,
they are directly attached to the support pole one
above the other. This is in contrast to the primary
lines which are often strung on a cross bar or other
attachment in a horizontal or "V" shaped plane.
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Figure 2. Secondaries in a vertical plane
Figure 1. Cabled secondaries
Figure 3. Cabled secondaries, primaries in a "V" plane
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Steel superstructures
Steel superstructures are used to support
equipment, lines, and switches in substations as
well as transmission and distribution line towers
and poles.
Figure 1. Steel superstructure for circuit breakers
Figure 2. Substation with many steel superstructures for equipment and connection
supports
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