Compact Transmission Line Design
James R. Stewart
Introduction
 The
term "compact transmission line" is used to refer to a
line, usually in the 69-230 kV range, which is built with less than
traditional phase spacing for these voltages. The opportunity for
compaction arises because early lines were designed with generous
factors of safety, partly because of the lack of knowledge of design
parameters, and partly because of lack of incentive to reduce line
size. Phase-to-phase spacing of these early lines was in excess
of ten times that required for power frequency voltage air gap flashover.
As the utility industry developed, research was directed to the
development of increasingly higher transmission voltages. With the
development of each new voltage class, increasingly sophisticated
analyses of insulator and clearance requirements were made. Clearances
were reduced closer and closer to their limiting (flashover) values.
Reduced clearances and higher voltages increased the problem of
conductor surface electric field and corona phenomena such as radio
and audible noise. These problems were addressed in turn by appropriate
research.
While new design procedures were instituted for higher (EHV) voltages,
little attention was given to application of this body of knowledge
to lower voltage lines, and lines in the 115 to 230 kV class were
still being designed and constructed according to patterns placed
decades earlier. By the 1960's, two factors appeared which called
attention to intermediate voltage transmission lines. First, increased
attention to the appearance of overhead lines brought results at
voltages where new structure concepts could be most readily implemented.
Prefabricated steel poles, laminated structures, and armless structures
first made their appearance at 115 to 138 kV. Second, the same pressures
which prompted improvements in appearance also made new rights-of-way
increasingly difficult to acquire, and led a number of utilities
to uprate existing circuits to a higher voltage class. This early
work gave dimensional constraints, which while quite reasonable
by EHV standards, were unprecedented at 115-230 kV. This showed
the feasibility of using smaller than traditional spacings at these
voltage levels. In the 1970's it became apparent that a more concerted
effort was warranted to bring EHV design technology to bear on intermediate
voltage circuits. In 1973 Siemens Power Transmission & Distribution, Inc., Power Technologies International , proposed to
an agency of the State of New York the construction of a half mile
of compact 138 kV transmission line. This, and subsequent work sponsored
by EPRI, led to the publication of a compact line design manual
and supplement. This work was rounded out with publication of a
report on phase-to-phase switching surge behavior of closely spaced
conductors. As a result of this work, a number of utilities have
constructed compact lines with good success and a few have made
compact lines their system standard.
As compact transmission line research progressed, a general study
was conducted which sought to explore the theoretical limits of
line compaction. This study confirmed an earlier idea that the optimum
use of space concerned with transmission lines was the use of high
phase order: a number of phase conductors symmetrically placed and
energized with voltages whose phasors matched the space vectors
defining the conductor locations. Subsequent research on high phase
order has not only developed this promising innovation but has added
to the body of knowledge available for the design of compact three-phase
lines. Other options remain for advancing compact transmission line
technology. One of these is the use of covered conductor, for 138
kV lines with as little as two feet between phases. The conductor
covering is insufficiently strong to withstand continual stress
at line voltage, but is able to withstand momentary contact which
may result from ice or wind induced motion. Thus conductors could
be allowed to approach within normal bare conductor flashover distance
for short times without flashover. While promising, this innovation
requires additional research and a prototype application on a utility
system.
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Compact Line Design Factors
Much of the research directed to the development of EHV transmission
involved electrical parameters. Some of this was related to line
insulation: insulator contamination performance, and insulator and
tower window phase-to-ground switching surge performance. Additional
work was related to electrical environmental effects: audible, radio,
and television noise, and electric and magnetic field coupling to
objects in proximity to the line. A considerable body of knowledge
was developed together with predictive methods and design data for
application to new line configurations.
Much of this electrical work was directly applicable to compact
intermediate voltage designs. The largest area of unknowns was in
the mechanical performance of compact lines. As spacings were considered
as small as three feet, considerations which were previously unimportant
became prominent. Among these were wind induced conductor motion,
both conductor blow out and differential swinging. Conductor motion
due to the release of ice accretions was thought to be possibly
limiting. One of the more unusual concerns was magnetic forces resulting
from through fault currents. Large current resulting from a fault
on some system component other than the compact line but carried
through the compact line causes magnetic forces which result in
conductor swinging. This swinging might cause the compact line conductors
themselves to approach within flashover distance and result in a
flashover and tripout on the compact line itself, even though it
was not involved in the initial fault. Much of the new research
directed to compact line design addressed these mechanical parameters.
Other factors considered in compact line research were lightning
performance, live-line maintenance, and code considerations. The
latter was significant in that former editions of the National Electric
Safety Code specified phase-to-phase separation in excess of that
determined to be possible by the compact line research. A subsequent
code modification was required to allow the application of compact
lines.
Not least, economic considerations of compact line design were
addressed. While compact lines are not necessarily less expensive
than conventional construction, for many applications they are competitive.
Finally, some unusual questions arose which were addressed. For
example, the question was asked if a large bird were to fly between
the phases of a 138 kV line with only three feet phase spacing,
the bird would bridge a sufficient portion of the air gap to instigate
a flashover. Analysis and testing showed that the electric fields
surrounding the line conductors are sufficiently intense that birds
would not attempt to fly between energized conductors and thus would
not cause flashovers.
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Sample Special Requirements for Compact
Lines
Some examples of specific design constraints for compact lines which
emerged from this research are:
- Insulators. Compact conductor spacing requires minimizing
conductor motions. This in turn requires use of post insulators
at the structures to eliminate insulator swinging which occurs
with suspension strings. Porcelain posts can be applied, but significant
advantages are achieved by use of synthetic insulators.
- Conductor Hardware. Conductor separations of the order
of three feet at 138 kV result in conductor surface electric fields
of the same order as EHV lines. Thus, even though the line is
operated at 138 kV, conductor hardware must be of a design which
is suitable for EHV application. Otherwise, radio noise will be
excessive. Likewise, care must be used in construction of a compact
line similar to that for an EHV line to insure that the conductor
surface will not be scratched or marred.
Some refinements are possible for special applications but are
not always necessary. One of these is the use of in-span insulating
spacers to limit conductor motion. Where galloping or ice is a problem,
perhaps long or unusually exposed spans, insulating spacers provide
an approach to retaining compaction while limiting conductor motion.
Spacers located at the 1/3 and 2/3 points of a span reduce the motion
considerably more than equivalent span reduction. When ice loads
a single conductor of a single span (to take an extreme case), the
additional weight is borne by the same conductor on adjacent spans
through deflection of the insulators and structures and by elongation
of the conductor itself. The conductor attachment points are fixed
in height above ground but have some flexibility longitudinally.
Spacers on the conductors are free to move vertically. Consequently,
an ice load on a single conductor of a single span is borne by all
three conductors by action of the spacers. Thus, when the ice is
released, some of the energy goes into bundle motion of the three
conductors as well as motion of the loaded conductor. More mechanical
modes are coupled by the spacers, resulting in each mode having
less energy than would be possible without spacers, and therefore
reduced overall motion. While it may be a novel thought, it could
be argued that in exposed locations it would be better to build
a compact line with spacers which are themselves stiff, rather than
to use generous clearances and retrofit if necessary with long flexible
spacers.
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Compact Line Design Procedure
Compact lines, because of reduced design margins, require more rigorous
analysis of insulation and mechanical parameters to ensure adequate
reliability than is required for conventional lines. Steps in the
design are:
- Consideration of alternate configurations
- Selection of phase spacing
- Power frequency air gap spacing
- Switching surge design
- Phase-to-ground
- Phase-to-phase
Radio noise (other electrical environmental effects)
Conductor motion
Selection of Insulators (and insulating spacers)
Lightning
Economics
Maintenance
Codes
These steps are interactive, and usually several iterations are
required before an acceptable solution is achieved.
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