Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TRANSMISSION CROSS ARM
Technical Field
[0001]The present embodiments relate to the field of cross arms for use in the
power transmission industry.
Background
[0002]Cross arms are used throughout the world as structural elements to
support
electrical power transmission lines above the ground. These transmission cross
arms, normally between 6 to 14 m in length, can be made of a variety of
materials,
the most common of which is treated wood.
[0003]The service life of cross arms is a very important factor. Given the
difficulties
of reaching and replacing the cross arms (which may be in very remote
locations),
the cost of replacing a cross arm often exceeds that of the cost of the cross
arm,
itself.
[0004]The use of timber cross arms poses certain challenges. Good quality
timber
for use in the cross arm is becoming increasing difficult to obtain given
diminishing
old growth forests, which is the prime timber source, as well as the impact of
modern
environmental laws.
[00051 Timber cross arms also have a limited life span (typically about 25
years) and
decay naturally. The life span of timber cross arms may be enhanced by the use
of
wood preservatives, such as Creosote, Penta and CCA, however, these
preservatives are not environmentally friendly, and may be toxic. In
particular, many
wood preservative treated wood products are banned for use in certain areas or
industries.
[0006]Further, it is difficult to determine the state of a timber cross arm in
service
and assess the remaining life through visual inspection. Defect in timber due
to
insects and pests, moistness and/or temperature of the ambient surroundings of
the
timber, may be hidden and lead to costly asset failures and electrical system
outages.
[0007]Timber cross arms are combustible and propagate fire rapidly in forest
fires;
they are attractive to woodpeckers; and, under certain weather conditions,
such as
lightening, for example, they can initiate a pole top fire leading to
electrical system
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outages. Timber cross arms also creep (e.g. deflect) under heavy loads
sustained
for long periods of time.
[0008] There have been several attempts to overcome these difficulties by
substituting timber with other materials. Despite these attempts, timber
remains the
primary source of cross arms in the power transmission industry.
[0009] Metal, particularly galvanized steel cross arms, have been used in
order to
overcome some of the disadvantages of timber. The primary disadvantage of
using
a metal cross arm is its electrical conductivity, which makes the cross arm
very
dangerous for transmission line technicians (or linemen) to work with on
energized
live lines. The galvanized coating of such cross arms has a life expectancy of
about
25 years, after which the cross arm is susceptible to corrosion. In addition
the
commonly used steel sections are heavy and require significant lifting
capacity in the
field to install them. For these reasons, metal cross arms are not widely
used.
[0010] Laminated timber has also been used for cross arms. Laminated timber is
coated with a protective coating in order to generally prevent moisture
penetration
and increase the life expectancy of the cross arm. Some coatings are
environmentally unfriendly, and may leach into the surrounding environment.
Further, moisture and cracks may cause delamination of the timber. Under many
circumstances, such cross arms may have a lower life expectancy than untreated
timber.
[0011]Concrete, while commonly used as a building material, has not proven
suitable for use as a cross arm. Concrete has large capillarity porosity,
which allows
water to penetrate and can cause the concrete to crack in freezing and thawing
cycles. Unreinforced concrete will crack under tension stress. Regular
concrete
without reinforcement is quite brittle, and lacks ductility, which is a
problem when
used as a long cross arm. Given the different load conditions in electrical
transmission lines (load due to the weight of conductors, insulators, radial
ice on
conductors, wind on conductors) the cross arm requires ductility, i.e. the
ability of
the material to plastically deform while continuing to carry loads without
fracture,
even after micro cracking. Also, concrete is not easily usable with thin
sections of a
cross arm. A cross arm made of concrete would be large, bulky, heavy and would
require steel reinforcement for structural bending capacity and stirrups for
shear
reinforcement. For at least the above reasons concrete has not generally been
used
for cross arms across the transmission industry.
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Summary
[0012] In an aspect, there is provided herein a transmission class structural
cross
arm, including: a C-channel comprising a glass fibre reinforced polymer with
UV
inhibitors.
[0013] In another aspect, there is provided herein a support structure for a
conductor,
including: a) at least one utility pole; a) a first cross arm comprised of
glass fibre
reinforced polymer, coupled to the at least one utility pole; b) a second
cross arm
comprised of glass fibre reinforced polymer, coupled to the at least one
utility pole,
on the opposite side of the pole; wherein the first cross arm and the second
cross
arm are provided in a back-to-back arrangement and the conductor is supported
between the first cross arm and the second cross arm.
Drawings
[0014] The following figures set forth embodiments in which like reference
numerals
denote like parts. Embodiments are illustrated by way of example and not by
way of
limitation in the accompanying figures.
[0015] Figure 1 is an isometric view of a timber cross arm as known in the
prior art;
[0016] Figure 2 is a cross sectional view of the cross arm of Figure 1;
[0017] Figure 3 is an isometric view of a glass fibre reinforced polymer cross
arm
according to an embodiment;
[0018] Figure 4 is a cross sectional view of an embodiment of a cross arm;
[0019] Figure 5 is a cross sectional view of another embodiment of a cross
arm;
[0020] Figure 6 is a front view showing a cross arm installed between two
utility
poles;
[0021] Figure 7 is a side sectional view of a portion of Figure 6 showing a
pair of
cross arms coupled to a utility pole;
[0022] Figure 8 is a side sectional view showing cross arms supporting a
conductor;
[0023] Figure 9 is a side sectional view of single cross arm coupled to a
utility pole;
[0024] Figure 10 is a side sectional view of a single cross arm supporting a
conductor; and
[0025] Figure 11 is an isometric view of a portion of a cross arm according to
an
embodiment.
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Detailed Description
[0026]Referring to Figures 1 and 2, cross arms of the prior art are generally
shown.
In general, cross arms are coupled to utility poles in order to provide
support for
conductors, in an electrical transmission grid. As shown, prior art cross arms
may be
solid and include a rectangular cross section, or may include a solid
rectangular
exterior with a hollow interior.
[0027]Referring to Figures 3, 4 and 5, a cross arm 1 according to an
embodiment is
generally shown. The cross arm 1 is a transmission class structural cross arm
and
has a generally C-shaped cross-section. The cross arm 1 is made of glass fibre
reinforced polymer. The cross arm 1 includes a rectangular back member 10, a
top
extension 20 and a bottom extension 30. The cross arm 1 includes a first end
105, a
second end 115 and a middle 125. The top and bottom flanges 20, 30 are
provided
along the length of cross arm 1 to provide additional strength. The top flange
20
extends from a top edge of back member 10. Similarly, the bottom flange 30
extends from a bottom edge of back member 10.
[0028] In the embodiment of Figures 3 and 5, top and bottom flanges 20, 30
extend
from the back member 10. Both the top and bottom flanges 20, 30 are tapered
slightly and fillets 185 are provided where the top and bottom flanges 20, 30
meet
the back member 10.
[0029] In the embodiment of Figure 4, top and bottom flanges 20, 30 extend
generally perpendicularly from back member 10. Further, the top and bottom
flanges
20, 30 are untapered and the fillets are not present.
[0030]The function of the cross arm 1 is to support conductors within a frame,
such
as an "H-frame", for example, which is shown in Figure 6. In H-frames, the
cross
arm 1 extends between two utility poles 100 to support conductors 170, which
are
typically suspended from the cross arm 1 near ends 105 and 115 and at middle
125,
as shown. Cross braces 101, 102, which may be coupled to opposite sides of the
utility poles 100, may further be provided to support the H-frame.
[0031]The back member 10 of the cross arm 1 abuts the utility pole 100 when
installed. Referring to the embodiment of Figure 7, pair of cross arms 1 are
provided back-to-back on opposite sides of the utility pole 100. The cross
arms 1
are coupled to utility poles 100 by rods 120, which may be made of galvanized
steel
and include threads. As shown, the rod 120 extends through aperture 155 of a
first
cross arm 1, through the utility pole 100 and through aperture 155 of a second
cross
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arm 1. Rods 120 are secured in place by a washer 130 and a nut 140, which is
threaded onto the rod 120, however, an other suitable securing arrangement may
alternatively be used. A bracket 110 may be provided between the cross arm 1
and
the utility pole 100 to provide additional support to the cross arm 1.
[0032] As shown in Figure 8, a hardware component 150 is provided to support
conductors 170. The hardware component 150 includes flanges, which extend
outwardly at a top end thereof. The flanges are sized to abut both cross arms
1 so
that the hardware component 150 is supported between the cross arms 1. An
insulator 160 hangs from the hardware component 150 and the conductor 170 is
coupled to the insulator 160 by a clamp 175.
[0033] In another embodiment, which is shown in Figures 9 and 10, a single
cross
arm 1 is provided between utility poles 100. The single cross arm 1 is secured
to the
utility poles 100 in a similar manner as has been described with respect to
the back-
to-back arrangement of Figure 7. In this embodiment, a plate 200, which is
secured
to cross arm 1 using two bolt 215 and nut 220 combinations, is used to support
the
insulator 160. A U-ring 230 is coupled to the bottom of the plate 200 and is
suspended therefrom. The U-ring 230 is coupled to the plate 200 by bolt 241
and
nut 237.
[0034] It will be appreciated by a person skilled in the art that in addition
to the H-
frame configuration, the cross arm 1 may be used in many other common
transmission line configurations like wishbone, single pole, multipole or Y-
frame
construction.
[0035] The cross arm 1 is made of Glass Fibre Reinforced Polymer (GFRP), which
is
a polyurethane-based composite that is manufactured using a pultrusion
process.
The resin of the GFRP is a two component polyurethane resin and the glass is E-
glass (Electrical grade glass). E-glass is commonly used in electrical
applications in
which high insulation, dielectric, fire retardant and high modulus properties
are
desirable. In one embodiment, the wo component polyurethane resin is Rimline
TM ,
which is manufactured by Huntsman TM .
[0036] In one embodiment, the volume fraction ratio of E-glass (Electrical
grade
glass) to resin of the GFRP of the cross arm 1 is approximately 53:47. Other
ratios
are also possible, including volume fraction ratios of E-glass to resin of
between 50
and 58: between 50 and 42.
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[0037] GFRP is a stronger material than timber, which has been used to
manufacture
cross arms of the prior art, to allow the cross arm 1 provides sufficient
strength and
durability. GFRP further has an extremely high strength-to-weight ratio so
that
weight issues relating to field construction are less significant.
[0038]As shown in Figure 11, the cross arm 1 includes different fibreglass
layers
including: a continuous filament mat 250, unidirectional roving 260 and triax
fabric
270. The continuous filament mat 250 includes 1.5 oz/ft2 internal layers and
2.0
oz/ft2 surface layers with polyester veil stitched. The unidirectional roving
260 is 56-
yield with approximately 608 total ends. The triax fabric 270 is E-TTXM4008-10
triax
fabric including 12 oz/yd2 in each 45 , 16 oz/yd2 in 90 and 6.75oz/yd2
chopped
mat. The resin may be pigmented to a specified color.
[0039]The cross arm 1 further includes a polyester veil that coats the cross
arm 1 to
provide protection against aging and weathering of the cross arm 1. The GFRP
may
further be coated with an aliphatic urethane gel coat, a solar reflective
clear coat or
another suitable coating to provide further protection against aging and
weathering.
In one embodiment, a single aliphatic polyurethane gel coating is provided for
protection against aging and weathering.
[0040] In one embodiment, the rovings and mattings of layers 250, 260, 270 may
be
provided in a longitudinal direction along the length of the cross arm 1. In
another
embodiment, the strength of the cross arm 1 is customized by altering the mat
orientations of one or all of the layers 250, 260, 270. In this embodiment,
the rovings
and mattings may be provided in a transverse direction relative to the length
of the
cross arm 1, for example. Alternatively, the strands of the mattings may be
provided
with an angle of approximately 45 degrees to one another.
[0041] In another embodiment, the strength properties of the cross arm 1 may
be
increased by including an aliphatic polyurethane gel coating. In another
embodiment, strength may be increased by deleting some or all of the glass
roving
or mats and instead including carbon fibres, rovings or mats or titanium
fibres,
rovings or mats.
[0042] The specifications of the GFRP of the cross arm 1 are determined based
on
the design loadings and structural requirements on the transmission lines in
addition
to the voltage level. GFRP products may be customized to suit different
environmental conditions, such as aging and weathering, UV inhibition and
resistance to chemicals, contaminants or corrosives. The customization depends
on
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the user requirements and resins and reinforcement orientation developed
accordingly. The suitability of the properties developed ensures that the
desired
mechanical and physical properties are maintained over a long period. The key
factor in developing the crossarm is a knowledge of the specific user
requirements,
structural finite element modeling to meet those requirements, electrical,
mechanical
and aging testing to confirm the product performance and field trials to
refine the
product application.
[0043] In one embodiment, a 230 kV H-frame includes a pair of utility poles
that are
spaced 5.5m. In this embodiment, the cross arm 1 has dimensions of 14"x 6"x
1/2"x
38 feet. Suitable strength and durability properties of the cross arm 1 are
achieved
by customizing the layers 230, 250, 270 and adjusting the two component
polyurethane resin to E-glass ratio. The cross arm 1 may be used in many
different
voltage environments including up to 287kV, for example.
[0044] Cross arm 1 may be manufactured industrially in a controlled
environment so
that weather conditions do not influence the continuous manufacture of the
cross
arm 1. Cross arm 1 is easily shipped and can be manufactured in large volumes,
with minimal environmental impact, particularly when compared to timber.
Installation holes, such as apertures 155, for example, may be pre drilled
before
delivery.
[0045] The cross arm 1 has many advantages over cross arms of the prior art.
Fire
retardant additives and E-glass are incorporated into the matrix of the glass
fibre
reinforced polymers, which allow the cross arm 1 to be non-combustible.
Further,
the cross arm 1 is environmentally benign, meaning that it has no negative
impact on
the local environment. Cross arm 1 generally has a long life expectancy, of at
least
75 years, and therefore a lower life cycle cost when compared to timber or
steel,
given the cost of replacement. Cross arm 1 can be installed using installation
equipment and methods commonly used with prior art cross arms.
[0046] Cross arm 1 is electrically non conductive, and can resist harsh
weather
conditions, for example, cross arm 1 is freeze and thaw resistant, ultra
violet light
resistant (because of either UV inhibitors incorporated into the resin, or the
use of a
Polyphatic veil and/or a specific UV resistant clear coat), corrosion
resistant, and
does not rot or decompose. Also the chemical properties of the resin/glass
matrix of
cross arm 1 become mechanically and physically stronger as temperatures become
colder.
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[0047] Cross arm 1 also provides several advantages when compared to concrete.
GFRPs do not have capillarity porosity. The fibres of the GFRPs provide
ductility to
cross arm 1 to allow deflection without fracture. In particular, cross arm 1
has
superior stress-rupture characteristics and is inherently creep resistant. In
addition,
GFRP has elastic deformation/ memory properties, meaning that once load has
been
removed from the cross Arm 1, the cross Arm 1 returns to its original
"straight line"
shape.
[0048] A further advantage of the cross arm 1 is that the cross arm 1 fits
into existing
electrical grids. In addition, the cross arm 1 may be stored outside, which
allows
cross arms 1 to be purchased in bulk. Cross arm 1 may be colored by adding
selected pigmentation to the resin. In one embodiment, a wood color may be
selected in order to match a color of the utility poles to which the cross
arms 1 are
attached. In another embodiment, the cross arms 1 are colored for marketing
and/or
public acceptance purposes.
[0049] Because dirt and other contaminants on cross arms 1 can be a source of
electrical conductivity, the cross arm 1 includes smooth and homogeneous
surfaces
to minimize dirt and contaminant accumulation over time. Wind and rain act as
surface cleansers for the cross arm 1. Installation, repair or replacement of
cross
arm 1 can be done on an energized electrical line because cross arm 1 is not
conductive. Cross arm 1 will not risk electrical interference causing partial
discharges. Further, based on continuous discharge withstand tests up to 800
kV,
cross arm 1 has been shown to withstand lightning strikes. Also as the glass
fibre
reinforced polymer is not combustible; it will not propagate fire in forest
fires. For this
reason, the risk from vandalism and fire is minimal. Cross arm 1 will have a
weight
less than a similarly sized timber cross arms.
[0050] Specific embodiments have been shown and described herein. However,
modifications and variations may occur to those skilled in the art. All such
modifications and variations are believed to be within the scope and sphere of
the
present embodiments.
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