POLE LINES FOR POWER TRANSMISSION.
Long transmission lines should follow the most direct routes between
generating and sub-stations as far as practicable. The number of poles,
cross-arms, and insulators increases directly with the length of line,
and the weight of conductors with the square of that length, other
factors remaining equal. Every material deviation from a straight line
must therefore be paid for at a rather high rate.
Distribution lines necessarily follow the public streets in order to
reach consumers, but the saving of the cost of a private right of way
and ease of access are the main considerations which tend to keep
transmission lines on streets and highways. Except in very rough or
swampy country, the difficulty of access to a pole line on a private
right of way is not a serious matter and should be given but little
weight. The cost of a private right of way may be more important, and
should be compared with the additional cost of the pole line and
conductors if erected on the public highway. In this additional cost
should be included any items for paving about the poles, extra pins,
insulators, and guys made necessary by frequent turns in the highway,
and the sums that may be required to secure the necessary franchises.
There is also the possible contingency of future legislation as to the
voltage that may be maintained on wires located over public streets.
These considerations taken together give a strong tendency to the
location of long transmission lines on private rights of way, especially
where the amount of power involved is great and the voltage very high.
A transmission line 80.3 miles in length recently erected between
Rochester and Pelham, N. H., by way of Portsmouth, where the generating
station is located, to feed an electric railway system, operates at
13,200 volts and is mainly located on private rights of way. Deeds
conveying the easements for this right of way provide that all trees or
branches within one rod on either side of the line may be cut away. The
transmission line between Niagara Falls and Buffalo, about twenty-three
miles long and operating at 22,000 volts, is largely on a private way
thirty feet wide.
[Illustration: FIG. 80.--Transmission Line of New Hampshire Traction
Company over Hampton River Bridge, 4,623 Feet Long.]
For the transmission between Cañon Ferry and Butte the line is mainly
located on a private way. Between Colgate and Oakland the transmission
line is mostly on private way, and this is also true of the greater part
of some other high-pressure lines in California. These private rights of
way range from fifty to several hundred feet wide, it being necessary in
forests to cut down all trees that are tall enough to fall onto the
wires.
In some cases of transmission at very high voltage two independent pole
lines are erected and one or more circuits are then run on each set of
poles. This construction has been followed on the transmission line
between Niagara Falls and Buffalo, Cañon Ferry and Butte, Welland Canal
and Hamilton, and between Colgate and Oakland. Such double pole lines
are more usually located on the same right of way, this being true of
the Cañon Ferry and Colgate systems, but this is not always the case. In
the Hamilton system the two lines of poles, one thirty-five miles and
the other thirty-seven miles in length, are located several miles apart.
The two sets of poles on a part of the Buffalo line are less than thirty
feet, on the Colgate line are twenty-five feet, and on the Cañon Ferry
line are forty feet apart.
The main reasons for the use of two pole lines instead of one are the
probability that an arc started on one circuit will be communicated to
another on the same poles, and the greater ease and safety of repairs
when each circuit is on a separate line of poles. On each pole line of
the Cañon Ferry transmission, and also on each pole line of the Colgate
transmission, there is only one three-wire circuit. On the Cañon Ferry
line each wire of the two circuits has a cross-section of only 106,500
circular mils, and on the Colgate line one circuit is of 133,225
circular mils wire and the other circuit is of 211,600 circular mils
cable. In contrast with these figures the line of the Standard Electric
Company between Electra and Mission San José, a distance of ninety-nine
miles, is made up of only three conductors, each being an aluminum cable
of 471,034 circular mils section. Inductance increases with the
frequency of the current in a conductor, and in each of the three
systems just considered the frequency is sixty cycles per second.
The use of one circuit of larger wire instead of two circuits of smaller
wire has the obvious advantage of greater mechanical strength in each
conductor, saves the cost of one pole line and of the erection of the
second circuit. With voltages above 40,000 to 50,000 on long
transmission lines there is a large loss of energy by leakage directly
through the air from wire to wire. To keep this loss within desirable
limits it may be necessary to give each wire of a circuit a greater
distance from the others of the same circuit than can readily be had if
all the wires of each circuit are mounted on one line of poles. If there
is only one three-wire circuit to be provided for, three lines of poles
or two lines with a long crosspiece between them may be set with any
desired distance between the lines so that the leakage through the air
with one wire on each pole will be reduced to a small quantity. On a
line built in this way it would be practically impossible for an arc to
start between the wires by any of the usual means.
Distances from pole to pole in the same line vary somewhat with the
number, size, and material of the conductors to be carried. On ordinary
construction in a straight line poles are often spaced from 100 to 110
feet apart--that is, about fifty poles per mile. On curves and near
corners the spacing of poles should be shorter. Poles for the 80.3
miles, mentioned in New Hampshire, are regularly located 100 feet apart.
Of the two pole lines between Niagara Falls and Buffalo, the older was
designed to carry twelve copper cables of 350,000 circular mils each,
and its poles were spaced only 70 feet apart. The newer line is designed
to carry six aluminum cables of 500,000 circular mils each and its poles
are 140 feet apart. Poles in each of the lines between Cañon Ferry and
Butte are regularly spaced 110 feet apart and each pole carries three
copper cables of 106,500 circular mils.
[Illustration: FIG. 81.--Chambly-Montreal Line Crossing the Chambly
Canal.]
[Illustration: FIG. 82.--Special Wooden Structures on Line Between Spier
Falls and Schenectady.]
The two 142-mile lines between Colgate and Oakland are each made up of
poles 132 feet apart, and one line of poles carries the three copper
conductors and the other line of poles the aluminum conductors already
named. As aluminum wire has only one-half the weight of copper wire of
equal conductivity, the length of span between poles carrying aluminum
wire may be greater than that where copper is used. Only a part of the
strain on poles is due to the weight of wires carried, however. Where a
body of water must be crossed, a very long span, with special supports
for the wires at each side, may be necessary. A case of this sort was
met where the Colgate and Oakland line crosses the Carquinez Straits at
a point where the waterway is 3,200 feet wide. It was necessary to have
the lowest part of the cables across these straits at least 200 feet
above the surface of the water so that vessels with the tallest masts
could pass underneath. To secure the necessary elevation for the cables
a steel tower was built on each bank of the straits at such a point that
the distance between the points for cable support on the two towers is
4,427 feet apart. As the banks rise rapidly from the water level, one
steel tower was given a height of only 65 feet, while the height of the
other was made 225 feet. Between these two towers four steel cables were
suspended, each cable being made up of nineteen strands of galvanized
steel wire, having an outside diameter of seven-eighths inch and
weighing 7,080 pounds for the span. The breaking strain of each cable is
98,000 pounds, and it has the electrical conductivity of a No. 2 copper
wire. The cables are simply supported on the towers by steel rollers,
and the pull of each cable, amounting to twelve tons, is taken by an
anchorage some distance behind each tower, where the cable terminates.
Each anchorage consists of a large block of cement deeply embedded in
the ground, and with anchor bolts running through it. Each cable is
secured to its anchorage through a series of strain insulators, and the
regular line cables of copper and aluminum are connected with the steel
cables just outside of the shelter built over the strain insulators of
each anchor. Steel cables were used for the long span across the straits
because of the great tensile strength that could be had in that metal.
This span is, no doubt, the longest and highest that has ever been
erected for electrical transmission at high voltage.
[Illustration: FIG. 83.--Special Structure on Line Between Spier Falls
and Schenectady.]
It has been suggested in one instance that steel towers ninety feet high
and 1,000 feet apart be substituted for pole lines and the wires strung
from tower to tower. Such construction would increase the difficulty of
insulation and would cost more at the start than a line of wooden poles.
The question is whether a lower maintenance and depreciation rate for
the steel towers would offset their disadvantages compared with poles.
Pole lines should be staked out with a transit, and the same instrument
can be used to give a perpendicular position to each pole and bring it
into line. Wooden poles are used in most cases of high-voltage
transmission lines. Iron poles would make it unsafe to work on any
circuit carried by them when it was transmitting current at high
voltage. With iron poles a defective insulator might lead to the
destruction of the conductors at that point through continuous arcing on
to the iron.
[Illustration: FIG. 84.--Crossing of Delaware and Hudson Railway Tracks
by 30,000-volt Line at Saratoga, N. Y.]
The kinds of wood used for poles vary in different sections of the
country. In New England, chestnut poles are a favorite and were used on
the 80.3 miles of transmission line mentioned in New Hampshire. Cedar
poles are used to some extent in nearly all parts of the country,
including Canada. Spruce and pine poles are employed to some extent,
especially in lengths of more than fifty feet. In the Rocky Mountain
region and in California round cedar poles from the forests of Oregon,
Washington, and Idaho are much used. Sawed redwood poles from the trunks
of large trees were erected on the 147-mile line between Electra
power-house and San Francisco. For the Colgate and Oakland line Oregon
cedar poles were selected, and the transmission between Cañon Ferry and
Butte was carried out with cedar poles from Idaho. For transmission
circuits it is far more important at most points to have poles very
strong rather than very long. Where wires or obstructions must be
crossed by the high-voltage circuits the poles should be long enough to
carry these circuits well above everything else. In the open country,
where no obstructions are to be avoided, it does not pay to use poles
with a length greater than thirty-five feet.
[Illustration: FIG. 85.--Pole Line from Spier Falls over Mount
McGregor.]
Short poles offer less surface to the wind, the length of the lever
through which wind pressure acts to break the pole at the ground
decreases with the length of pole, and the shorter the poles the smaller
is the strain on struts and guy wires. If poles are only thirty or
thirty-five feet long, they may be large in diameter without excessive
cost. As a rule, no pole should be used with a top less than seven
inches in diameter, and a pole with this top should not be required to
carry more than three wires. A pole with seven- or eight-inch top and
thirty feet long should measure not less than twelve inches in diameter
at the butt. For longer poles the diameters at the butt should increase
up to at least eighteen inches for a round pole sixty feet long.
In the New Hampshire transmission above named the standard length of
poles is thirty-five feet. On the line between Cañon Ferry and Butte the
poles range from thirty-five to ninety feet in length. The round cedar
poles used in the Colgate and Oakland line range from twenty-five to
sixty feet in length, from eight to twelve inches diameter at the top,
and from twelve to eighteen inches diameter at the butt. On the line
between Electra and San Francisco the square-sawed redwood poles are
reported to have the following dimensions, in a paper read at the annual
convention of Edison Illuminating Companies in 1902.
+-------+-------+---------------+----------+
|Height,| Top, | Butt, | Depth |
| Feet. |Inches.| Inches. |in Ground.|
+-------+-------+---------------+----------+
| | | | |
| 35 | 7 × 7 |12 × 12 | 5.5 |
| 40 | 8 × 8 |13-1/2 × 13-1/2| 6 |
| 45 | 9 × 9 |15 × 15 | 6.5 |
| 50 |10 × 10|16 × 16 | 7 |
| 60 |11 × 11|17 × 17 | 8 |
+-------+-------+---------------+----------+
The relative dimensions of these poles are of interest because, being
sawed from the trunks of large trees, they could have any desired
measurements at the tops and butts. These poles, over the greater part
of the line, carried the three aluminum cables of 471,034 circular mils
each, previously mentioned. Depth to which poles are set in the ground
ranges from about five feet for twenty-five- or thirty-foot poles to
eight feet for poles sixty feet long. In locations where the soil is
very soft or where poles must resist heavy strains the stability of each
pole may be much increased by digging the hole two feet or more larger
in diameter than the butt of the pole, and then filling in cement
concrete--one part, by measure, of Portland cement, three of sand and
five of broken stone--all around the butt of the pole after it is in the
hole. The butts of poles up to a point one foot or more above the ground
line are frequently treated with hot tar, pitch, asphalt, or carbolineum
before the poles are erected, and in Salt Lake City salt is said to be
used around pole butts after they are in the hole.
In some cases the poles of transmission lines are painted over their
entire length. Pole tops should always be pointed or wedge-shaped, to
shed water, and paint or tar should be applied to these tops. In some
cases poles are filled with crude petroleum or other preservative
compound in iron retorts, where moisture is withdrawn from the pole by
exhausting the air, and then, after treatment with dry steam, the poles
have the compound forced into them by hydraulic pressure.
In favorable soils cedar poles may remain fairly sound for twenty years,
chestnut poles more than one-half of that time, and spruce and pine
about five years. Poles up to forty feet in length may be readily set
with pike poles, but when they are much longer than this a derrick will
save time and labor. A derrick should have a little more than one-half
the length of the poles to be set.
[Illustration: FIG. 86.--Chambly-Montreal Line Crossing the Richelieu
River.]
Poles should be guyed or braced at all points where there are material
changes in the direction of the line, and on long straight stretches
about every fifth pole should be guyed or braced in both directions to
prevent the poles setting back when the line wire is cut or broken at
any point. Where there is room for wooden struts, as on a private right
of way, they should be used instead of guys because of their more
substantial character and the higher security of insulation thus
obtained. Ordinary strain insulators cannot be relied on with lines that
operate at very high voltages, and where guys must be used a timber four
by six inches and ten to twenty feet long may have the guy twisted
about each end of it and be used as a strain insulator. A guy or strut
should come well up under the lower cross-arm on a pole to avoid
breaking of the pole at the point of attachment.
Where poles have heavy circuits and several cross-arms each it is
sometimes desirable to attach a guy or strut beneath the lowest arm and
also a guy close to the pole top. Galvanized iron or steel wire is the
material best suited for guys, and the cable form has greater strength
and is more flexible than solid wire.
[Illustration: FIG. 87.--Cross-arms and Insulators on the Line Between
the Chambly Plant and Montreal.]
On the transmission line between Electra and San Francisco, which is
intended to operate at 60,000 volts, the use of guys has been mostly
avoided and struts employed instead. Where a guy had to be used, a
strain insulator of wood six by six inches and twenty feet long was
inserted in it.
The number and spacing of cross-arms on the poles of transmission lines
are regulated by the number of circuits that each pole must carry and by
the desired distance apart of the wires. Formerly it was common to carry
two or more circuits on a single line of poles, but now a frequent
practice is to give each pole line only one circuit and each pole only
one cross-arm, except that a small cross-arm for a telephone circuit is
placed some feet below the power wires. With only one transmission
circuit per pole line, one wire is usually placed at the top of the pole
and the other two wires at opposite ends of the single cross-arm. The
older pole line for the transmission between Niagara Falls and Buffalo
carried two cross-arms per pole for the power wires, these cross-arms
being two feet apart. Each cross-arm was of yellow pine, twelve feet
long, four by six inches in section, and intended to carry four
three-wire circuits, but only two circuits have been erected on these
two cross-arms. On the later pole line for this same transmission each
pole carries two cross-arms, the upper intended for four and the lower
cross-arm for two wires, so that one three-wire circuit may be strung on
each side of the poles, two wires on the upper and one on the lower arm
in the form of an equilateral triangle. The pole lines between Cañon
Ferry and Butte, Colgate and Oakland, and Electra and San Francisco all
have only one cross-arm for power wires per pole, and the third wire of
the circuit in each case is mounted at the top of the pole so that the
three conductors are at the corners of an equilateral triangle.
This relative position of the conductors makes it easy to transpose them
as often as desired. On the line from Cañon Ferry to Butte the
cross-arms are each eight feet long with two holes for pins
seventy-eight inches apart, and are attached to the pole five feet ten
and one-half inches from the top. Gains for cross-arms should be cut
from one to two inches deep in poles before they are raised, and one
hole for three-quarters or seven-eighths-inch bolt should be bored
through the centre of the cross-arm and of the pole at the gain. Each
cross-arm should be attached to the pole by a single bolt passing
entirely through the pole and cross-arm with a washer about three inches
in diameter next to the cross-arm. One large through bolt weakens the
pole and arm less than two smaller bolts or lag-screws, and the arm can
be more easily replaced if there is only one bolt to remove. Alternate
poles in a line should have their cross-arms bolted on opposite sides,
and at corners double arms should be used.
Yellow pine is a favorite wood for cross-arms, though other varieties
are also used. The large, long pins necessary on high voltage lines tend
to increase the sectional area of cross-arms, and a section less than
five and one-half by four and one-half inches is seldom desirable. On
the line between Electra and San Francisco, which carries the three
aluminum cables of 471,034 circular mils each, the cross-arms of Oregon
pine have a section of six by six inches each. Standard dimensions of
some smaller cross-arms are four and three-quarters by three and
three-quarters inches, but it may be doubted whether these arms are
strong enough for long transmission work. Cross-arms should be surfaced
all over and crowned one-quarter to one-half inch on top so as to shed
water. After being kiln dried, cross-arms should be boiled in asphaltum
or linseed oil to preserve the wood and give it higher insulating
properties. Cross-arms longer than five feet should be secured by braces
starting at the pole some distance below each arm and extending to
points on the arm about half-way between the pole and each end of the
arm.
[Illustration: FIG. 88.--Tail Race and Pole Line at Chambly, Quebec
Power-station.]
Each brace may be of flat bar iron about one and one-half by one-quarter
inch in section, or the brace for both ends of an arm may be made of a
single piece of angle-iron bent into the proper shape. For high-voltage
lines it is undesirable to employ iron braces of any sort, since these
braces form a path of low resistance that comes much too close to the
pins on which the insulators and wires are mounted. Braces formed of
hard wood are much better as to insulation, and such braces of maple are
in use on the line between Butte and Cañon Ferry where the voltage is
50,000. Each brace on that line is thirty-six inches long and three
inches wide, with one end bolted to the centre of its pole and the other
end to the cross-arm twenty-three inches from the pole centre.
The line from Electra has hard-wood braces secured with wood pins.
Wood is the most common material for pins on which to mount the
insulators of high-voltage transmission circuits. Iron has been used for
pins to some extent, and its use is on the increase. Oak and locust pins
are generally used, the latter being stronger and more lasting. In
California, pins of eucalyptus wood are much used and are said to be
stronger than locust. All wooden pins should be boiled several hours in
linseed oil after being well dried. This increases the insulating and
lasting properties of the pins.
High-voltage lines require long pins to hold the lower edges of
insulators well above the cross-arms, and these pins must be much
stronger than those used on ordinary lines, because of the increased
leverage of each wire.
A pin twelve inches long over all and having a diameter of one and
one-half inches in the part that enters the cross-arm has been much used
for transmission circuits, but is much too short and weak for high
voltages. On the 50,000-volt line between Cañon Ferry and Butte the pins
are seasoned oak boiled in paraffin. Each of these pins is seventeen and
one-half inches long, two and one-half inches in diameter for a length
of four and one-half inches in the middle part, two inches in diameter
for a length of five and one-half inches that fits into the cross-arm or
pole top, and one and one-half inches in diameter at the top of the
thread inside of the insulator. These pins hold the outside edges of the
insulators nine inches above the tops of cross-arms. Each of these pins
is held in its socket by a three-eighths-inch bolt that passes entirely
through the pin and the cross-arm or pole top.
On the line between Electra and San Francisco the pins are each sixteen
and seven-eighths inches long, two and three-quarters inches in diameter
at the largest central part, and two and one-quarter inches in diameter
in the lower part, five inches long, that fits into the cross-arm or
pole top. One of these pins broke at the shoulder with a pull of 2,200
pounds at the threaded part. Carriage bolts one-half inch in diameter
pass through the cross-arm and pin two inches from the top of the arm,
and one bolt three inches from the pin on each side. Without these bolts
the arms split on test with a pull of 1,200 pounds on the pin, but with
the bolts the pin broke as above.