INSULATORS FOR TRANSMISSION LINES.
Line insulators, pins, and cross-arms all go to make up paths of more or
less conductivity between the wires of a transmission circuit. The
amount of current flowing along these paths from one conductor to
another in any case will depend on the combined resistance of the
insulators, pins, and cross-arm at each pole.
As a general rule, the wires of high-voltage transmission circuits are
used bare because continuous coverings would add materially to the cost
with only a trifling increase in effective insulation against high
voltages. In some instances the wires of high-pressure transmission
lines have individual coverings for short distances where they enter
cities, but often this is not the case. At Manchester, N. H., bare
conductors from water-power plants enter the sub-station, well within
the city limits, at 12,000 volts. From the water-power at Chambly the
bare 25,000-volt circuits, after crossing the St. Lawrence River over
the great Victoria bridge, pass overhead to a terminal-house near the
water-front in Montreal. In order to reach the General Electric Works,
the 30,000-volt circuits from Spier Falls enter the city limits of
Schenectady, N. Y., with bare overhead conductors.
Where transmission lines pass over a territory exposed to corrosive
gases, it is sometimes desirable to give each wire a weather-proof
covering. An instance of this sort occurs near Niagara Falls where the
aluminum conductors forming one of the circuits to Buffalo are covered
with a braid that is saturated with asphaltum for some distance.
Each path, formed by the surface of the insulators of a line and the
pins and cross-arm by which they are supported, not only wastes the
energy represented by the leakage current passing over it, but may lead
to the charring and burning of the pins and cross-arm by this current.
To prevent such burning, the main reliance is to be placed in the
surface resistance of the insulators rather than that of pins and
cross-arms. These insulators should be made of glass or porcelain, and
should be used dry--that is, without oil. In some of the early
transmission lines, insulators were used on which the lower edges were
turned inward and upward so that a circular trough was formed beneath
the body of the insulator, and this trough was filled with heavy
petroleum. It was found, however, that this trough of oil served to
collect dirt and thus tended to lower the insulation between wire and
cross-arm, so that the practice was soon abandoned. Glass and porcelain
insulators are rivals for use on high-tension lines, and each has
advantages of its own. Porcelain insulators are much stronger
mechanically than are those of glass, and are not liable to crack
because of unequal internal expansion, a result sometimes met with where
glass insulators are exposed to a hot morning sun. In favor of glass
insulators it may be said that their insulating properties are quite
uniform, and that, unlike porcelain, their internal defects are often
apparent on inspection. In order to avoid internal defects in large
porcelain insulators, it has been found necessary to manufacture some
designs in several parts and then cement the parts of each insulator
together.
Defective insulators may be divided into two classes--those that the
line voltage will puncture and break and those that permit an excessive
amount of current to pass over their surfaces to the pins and
cross-arms. Where an insulator is punctured and broken, the pin,
cross-arm, and pole to which it is attached are liable to be burned up.
If the leakage of current over the surface of an insulator is large, not
only may the loss of energy on the line where the insulator is used be
serious, but this energy follows the pins and cross-arm in its path from
wire to wire, and gradually chars the former, or both, so that they are
ultimately set on fire or break through lack of mechanical strength. The
discharge over the surface of an insulator may be so large in amount as
to have a disruptive character, and thus to be readily visible. More
frequently this surface leakage of current over insulators is of the
invisible and silent sort that nevertheless may be sufficient in amount
to char, weaken, and even ultimately set fire to pins and cross-arms.
All insulators, whether made of glass or porcelain, should be tested
electrically to determine their ability to resist puncture, and to hold
back the surface leakage of current, before they are put into practical
use on high-tension lines. Experience has shown that inspection alone
cannot be depended on to detect defective glass insulators. Electrical
testing of insulators serves well to determine the voltage to which they
may be subjected in practical service with little danger of puncture by
the disruptive passage of current through their substance. It is also
possible to determine the voltage that will cause a disruptive discharge
of current over the surface of an insulator, when the outer part of this
surface is either wet or dry. This is as far as electrical tests are
usually carried, but it seems desirable that such tests should also
determine the amount of silent, invisible leakage over the surface of
insulators both when they are wet and when they are dry, at the voltage
which their circuits are intended to carry. Such a test of silent
leakage is important because this sort of leakage chars and weakens
insulator pins, and sets fire to them and cross-arms, besides
representing a waste of energy.
The voltage employed to test insulators should vary in amount according
to the purpose for which any particular test is made. Glass and
porcelain, like many other solid insulators, will withstand a voltage
during a few minutes that will cause a puncture if continued
indefinitely. In this respect these insulators are unlike air, which
allows a disruptive discharge at once when the voltage to which it is
exposed reaches an amount that the air cannot permanently withstand.
Because of this property of glass and porcelain insulators, it is
necessary in making a puncture test to employ a voltage much higher than
that to which they are to be permanently exposed. In good practice it is
thought desirable to test insulators for puncture with at least twice
the voltage of the circuits which they will be required to permanently
support on transmission lines.
For the first transmission line from Niagara Falls to Buffalo, which was
designed to operate at 11,000 volts, the porcelain insulators were
tested for puncture with a voltage of 40,000, or nearly four times that
of the circuits they were to support.
Porcelain insulators for the second line between Niagara Falls and
Buffalo, after the voltage of transmission had been raised to 22,000,
were given a puncture test at 60,000 volts. Of these insulators tested
at 60,000 volts only about three per cent proved to be defective. These
puncture tests were carried out by placing each insulator upside down in
an open pan containing salt water to a depth of two inches, partly
filling the pin hole of the insulator with salt water, and then
connecting one terminal of the testing circuit with a rod of metal in
the pin hole, and the other terminal with the pan. Alternating current
was employed in these tests, as is usually the case (Volume xviii.,
Transactions A. I. E. E., pp. 514 to 520). For the transmission lines
between Spier Falls, Schenectady, Albany, and Troy, where the voltage is
30,000, the insulators were required to withstand a puncturing test with
75,000 volts for a period of five minutes after they had been soaked in
water for twenty-four hours.
There is some difference of opinion as to the proper duration of a
puncturing test, the practice in some cases being to continue the test
for only one minute on each insulator, while in other cases the time
runs up to five minutes or more. As a rule, the higher the testing
voltage compared with that under which the insulators will be regularly
used, the shorter should be the period of test. Instead of being tested
in salt water as above described, an insulator may be screwed onto an
iron pin of a size that fits its threads, and then one side of the
testing circuit put in contact with the pin and the other side connected
with the wire groove of the insulator. Care should be taken where an
iron pin is used either in testing or for regular line work, that the
pin is not screwed hard up against the top of the insulator, as this
tends to crack off the top, especially when the pin and insulator are
raised in temperature. Iron expands at a much higher rate than glass or
porcelain, and it is desirable to cement iron pins into insulators
rather than to screw them in. There seems to be some reason to think
that an insulator will puncture more readily when it is exposed to
severe mechanical stress by the expansion of the iron pin on which it is
mounted.
Tests of insulators are usually made with alternating current, and the
form of the voltage curve is important, especially where the test is
made to determine what voltage will arc over the surface of the
insulator from the line wire to the pin. The square root of the mean
square for two curves of alternating voltage or mean effective voltage,
as read by a voltmeter, may be the same though the maximum voltages of
the two curves differ widely. In tests for the puncture of insulators,
the average alternating voltage applied is more important than the
maximum voltage shown by the highest points of the pressure curve,
because of the influence of the time element with glass and porcelain.
On the other hand, when the test is to determine the voltage at which
current will arc over the insulator surface from the line wire to the
pin, the maximum value of the pressure curve should be taken into
consideration because air has no time element, but permits a disruptive
discharge under a merely instantaneous voltage.
Alternators used in transmission systems usually conform approximately
to a sine curve in the instantaneous values of the pressures they
develop, and it is therefore desirable that tests on line insulators be
made with voltages whose values follow the sine curve. Either a single
transformer or several transformers in series may be employed to step up
to the required voltage, but a single transformer will usually give
better regulation and greater accuracy. An air-gap between needle points
is not a very satisfactory means by which to determine the average
voltage on a testing circuit, because, as already pointed out, the
sparking distance between the needle points depends mainly on the
maximum instantaneous values of the voltage, which may vary with the
load on the generator, and the saturation of its magnets. For accurate
results a step-down voltmeter transformer should be used on the testing
circuit.
An insulator that resists a puncture test may fail badly when subjected
to a test as to the voltage that will arc over its surface from line
wire to pin. This arc-over test should be made with the outer surface of
the insulator both wet and dry. For the purpose of this test the
insulator should be screwed onto an iron pin, or onto a wooden pin that
has been covered with tinfoil. One wire of the testing circuit should
then be secured in the groove of the insulator, and the other wire
should be connected to the iron or tin foil of the pin. The voltage that
will arc over the surface of an insulator from the line wire to the pin
depends on the conditions of that surface and of the air. In light air,
such as is found at great elevations, an arc will jump a greater
distance than in dry air near the sea-level. A fog increases the
distance that a given voltage will jump between a line wire and its
insulator pin, and a heavy rain lengthens the distance still further.
The heavier the downpour of rain the greater is the distance over the
outside surface of an insulator that a given voltage will arc over. The
angle at which the falling water strikes the insulator surface also has
an influence on the voltage required to arc over that surface, a
deviation from a downpour perpendicular to the plane of the lower edge
of the petticoat of the insulator seeming to increase the arcing
distance for a given voltage.
An insulator should be given an arc-over test under conditions that are
approximately the most severe to be met in practice. These conditions
can perhaps be fairly represented by a downpour of water that amounts to
a depth of one inch in five minutes for each square inch of the plane
included by the edge of the largest petticoat of the insulator, when the
direction of the falling water makes an angle of forty-five degrees with
that plane. A precipitation of one inch in depth on a horizontal plane
during five minutes seems to be a little greater than any recorded by
the United States Weather Bureau. Under the severe conditions just
named, the voltage required to arc over the insulator surface from line
wire to pin should be somewhat greater at least than the normal voltage
of the circuit where the insulator is to be used. For the transmission
line between Spier Falls and Schenectady, on which the maximum voltage
is 30,000, the insulators were required to stand a test of 42,000 volts
when wet, without arcing over from line wire to pin. In these wet tests
the water should be sprayed evenly onto the insulator surface like rain,
and the quantity of water that strikes the insulator in a given time
should be measured.
When the outside of an insulator is wet with rain, it is evident that
most of the resistance between the line wire and the insulator pin must
be offered by the inside surface of the petticoat of the insulator. For
this reason an insulator that is to withstand a very high voltage so
that no arc will be formed over its wet outside surface must have a
wide, dry surface under its petticoat. In some tests of line insulators
reported in Volume xxi., Transactions A. I. E. E., p. 314, the results
show that the voltage required to arc over from line wire to pin depends
on the shortest distance between them, rather than on the distance over
the insulator surface. Three insulators, numbered 4, 5, and 7 in the
trial, were in each case tested by a gradual increase of voltage until a
discharge took place between the wire and pin. The pins were coated with
tinfoil, and the testing voltage was applied to the tie wire on each
insulator and to the tinfoil of its pin. Insulators 4, 5, and 7
permitted arcs from wire to pin when exposed to 73,800, 74,700, and
74,700 volts respectively, the surfaces of all being dry and clean. The
shortest distances between wires and pins over insulator surface and
through air were 6-5/8, 6-1/4, and 7-7/8 inches respectively for the
three insulators, so that the arcing voltages amounted to 11,140,
11,952, and 9,479 per inch of these distances. Measured along their
surfaces, the distances between wires and pins on these three insulators
were 8, 11-1/4, and 15-1/2 inches respectively, so that the three arcing
voltages, which were nearly equal, amounted to 9,225, 6,640, and 4,819
per inch of these distances. These figures make it plain that the arcing
voltage for each insulator depends on the shortest distance over its
surface and through the air, from wire to pin. It might be expected that
the voltage in any case would arc equal distances over clean, dry
insulator surface or through the air, and the experiments just named
indicate that this view is approximately correct. The sparking distance
through air between needle points, which is greater than that between
smooth surfaces, is 5.85 inches with 70,000 volts, and 7.1 inches with
80,000 volts according to the report in Volume xix., A. I. E. E., p.
721. Comparing these distances with the shortest distances between wires
and pins in the tests of insulators numbered 4, 5, and 7, which broke
down at 73,800 to 74,700 volts when dry, it seems that a given voltage
will arc somewhat further over clean, dry insulator surface than it will
through air. This view finds support from the fact that only a part of
each of the shortest distances between wire and pin was over insulator
surface, the remainder being through air alone.
The fact that the dry part of the surface of an insulator and the air
between its lower wet edge and the pin or cross-arm offer most of the
resistance between the line wire and the pin and cross-arm is plainly
brought out by the results of the tests above mentioned, in the cases of
insulators numbered 4 and 7. While 73,800 volts were required to arc
from line-wire to pin when the entire insulator was dry and clean, the
arc was formed at only 53,400 volts during a moderate rain-storm, in the
case of No. 4 insulator. With insulator No. 7 the arcing voltage was
74,700 when the entire surface was clean and dry, but the arc from wire
to pin was started at 52,800 volts during a moderate rain. No. 5
insulator seems to present an erratic result, for when dry and clean the
arc jumped from wire to pin at 74,700 volts, and yet during a moderate
rain no arc was formed until a voltage of 70,400 was reached. For each
of the seven insulators on which tests are reported as above, the
voltage required to arc from line wire to pin was nearly or quite as
great during a dry snow-storm as when the insulator surface was clean
and dry. When the insulators were covered with wet snow their surface
insulation broke down at voltages that were within ten per cent above or
below the arcing voltages during a moderate rain in five cases. With two
insulators the arcing voltages, when they were covered with wet snow,
were only about sixty per cent of the voltages necessary to break down
the surface insulation between wire and pin during a moderate rain.
When the outside surface of an insulator is wet, as during a moderate
rain, it seems that the under surface of the insulator, and the distance
through air from the lower wet edge of the insulator to the pin or
cross-arm, make up most of the insulation that prevents arcing over from
the wire to the pin or cross-arm. It further appears that it is useless
to extend the distance across the dry under surface of the insulator
indefinitely without a corresponding increase of the direct distance
through air from the lower wet edge of the insulator to the wood of
cross-arm or pin. Insulator No. 7 in the tests under consideration had a
diameter at the lower edge of its outer petticoat of seven inches, and
was mounted on a standard wooden pin. The diameter of this pin in the
plane of the lower edge of the insulator was probably about 1-1/4
inches, so that the radial distance through air from this edge to the
pin must have been 2-7/8 inches approximately. During a moderate rain
the surface insulation of this insulator broke down and an arc was
formed from wire to pin with 52,800 volts. The sparking distance between
needle points at 50,000 volts is 3.55 inches, according to Volume xix.,
A. I. E. E., p. 721, and must be shorter between smooth surfaces, such
as the wire and pin in question, so that nearly all of the 52,800 volts
in this case must have been required to jump the 2-7/8 inches of air,
leaving very little to overcome the slight resistance of the wet outside
surface of the insulator. On this insulator the surface distance from
wire to pin was 15-1/2 inches, while the shortest breaking distance was
only 7-7/8 inches, so that the distance across the dry under surface of
the insulator must have been 15-1/2 - (7-7/8 - 2-7/8) = 10-1/2 inches
approximately. It is evidently futile to put a path 10-1/2 inches long
across dry insulator surface in parallel with a path only 2-7/8 inches
long in air, as an arc will certainly jump this shorter path long before
one will be formed over the longer. The same line of reasoning applies
to No. 3 insulator in this test, which had a diameter of 6-3/4 inches, a
surface distance from wire to pin of 13 inches, and a minimum distance
of 7-1/4 inches, and whose surface insulation broke down at 48,600 volts
during a moderate rain. The necessity of increasing the distance between
the lower wet edges of insulators and the pins and cross-arm, as well as
the distance across the dry under surfaces of insulators, led to the
adoption of the so-called umbrella type for some high-voltage lines. In
this type of insulator the main or outer petticoat is given a relatively
great diameter, and instead of being bell-shaped is only moderately
concave on its under side. With an insulator of this type mounted on a
large, long pin, the lower edge of the umbrella-like petticoat may be
far removed from the pin and cross-arm. Beneath the large petticoat of
such insulators for high voltages there are usually one or more smaller
petticoats or sleeves that run down the pin, and increase the distance
between it and the lower edge of the largest petticoat.
INSULATORS ON TRANSMISSION LINES.
+----------------------------------+-------+---------+--------+-------+
| | |Material | Inches |Inches |
| |Voltage| of |Diameter|Height |
| Location of Line. | of | Insula- | of | of |
| | Line. | tor. |Insula- |Insula-|
| | | | tor. | tor. |
+----------------------------------+-------+---------+--------+-------+
|Electra to San Francisco | 60,000|Porcelain| 11 | 11-1/4|
|Colgate to Oakland | 60,000|Porcelain| 11 | 11-1/4|
|Cañon Ferry to Butte | 50,000|Glass | 9 | 12 |
|Shawinigan Falls to Montreal | 50,000|Porcelain| 10 | 13-1/2|
|Provo around Utah Lake | 40,000|Glass | 7 | 5-3/4|
|Santa Ana River to Los Angeles | 33,000|Porcelain| 6-3/4 | 4-7/8|
|Spier Falls to Schenectady | 30,000|Porcelain| 8-1/2 | 6-3/4|
|Apple River Falls to St Paul | 25,000|Glass | 7 | 5-3/4|
|Chambly to Montreal | 25,000|Porcelain| 5-1/2 | 6-1/2|
|Niagara Falls to Buffalo | 22,000|Porcelain| 7-1/2 | 7 |
|Portsmouth to Pelham, N. H. | 13,000|Porcelain| 5-1/4 | 3-3/4|
|Garvins Falls to Manchester, N. H.| 12,000|Glass | 5 | 4-3/4|
+----------------------------------+-------+---------+--------+-------+
The inner petticoat or sleeve that runs down over the pin and sometimes
reaches nearly to the cross-arm, of course becomes wet on its outside
surface and at its lower edge during a rain; but between this lower wet
part of the inner petticoat, or sleeve, and the lower wet edge of the
larger outside petticoat, there is a wide, dry strip of insulator
surface. A result is that an arc over the surface of the outside
petticoat can reach the wet edge of the sleeve only by crossing the
strip of dry under surface or jumping through the air.
The same type of insulator is used on the 60,000-volt lines between
Electra and San Francisco and between Colgate and Oakland, each
insulator having an outer petticoat 11 inches in diameter and one inner
petticoat or sleeve 6-1/2 inches in diameter. This inner petticoat runs
down the pin for a distance of 7-1/2 inches below the outer petticoat.
Slightly different pins are used for mounting the insulators on the two
transmission lines just named, so that on the former the distance
through air from the lower edge of the outer petticoat to the cross-arm
is 11 inches, and on the latter the corresponding distance is 11-1/2
inches. On the Electra line the lower edge of the inner petticoat of
each insulator is about 3-1/2 inches, and on the Colgate line about 4
inches above the cross-arm.
INSULATORS ON TRANSMISSION LINES.
+------------------------------+---------+---------+---------+----------+
| | Inches | Inches | Inches | Inches |
| | from | from | from | from |
| | Top of | Outside | Lowest | Edge of |
| Location of Line. |Insulator|Petticoat|Petticoat| Outside |
| | to | to | to | to Edge |
| | Cross- | Cross- | Cross- | of Lowest|
| | arm. | arm. | arm. |Petticoat.|
+------------------------------+---------+---------+---------+----------+
|Electra to San Francisco | 14-1/2 | 11 | 3-1/2 | 7-1/2 |
|Colgate to Oakland | 15 | 11-1/2 | 4 | 7-1/2 |
|Cañon Ferry to Butte | 13-1/2 | 7-3/4 | 1-1/2 | 6-1/4 |
|Shawinigan Falls to Montreal | 16-1/4 | 11-3/4 | 3-1/4 | 8-1/2 |
|Santa Ana River to Los Angeles| 8-5/8 | 3-3/4 | 3-3/4 | 0 |
|Spier Falls to Schenectady | 10-3/4 | 7-3/8 | 4-1/4 | 3-3/8 |
|Niagara Falls to Buffalo | 10 | 5-1/2 | 3 | 2-1/2 |
|Chambly to Montreal | 8-1/2 | 4-1/2 | 2 | 2-1/2 |
+------------------------------+---------+---------+---------+----------+
On each of the lines named in this table the wires are strung on the
tops of their insulators.
The Cañon Ferry line is carried on insulators each of which has three
short petticoats and a long separate sleeve that runs down over the pin
to within 1-1/2 inches of the cross-arm. This sleeve makes contact with
its insulator near the pin hole. The outside petticoat of each
insulator on this line is 7-3/4 inches above the cross-arm and 6-1/4
inches above the lower end of the sleeve. Both the main insulator and
the sleeve, in this case, are of glass.
White porcelain insulators are used to support the 50,000-volt
Shawinigan line, and are of a recent design. Each of these insulators
has three petticoats ranged about a central stem so that their lower
edges are 4-1/2 inches, 9 inches, and 13 inches respectively, below the
top. The highest petticoat is 10 inches, the intermediate 9-3/4 inches,
and the lowest 4-1/4 inches in diameter. The height of this insulator is
13 inches, compared with 11-1/4 inches for those used on the Electra and
Colgate lines and 12 inches for the combined insulator and sleeve used
on the Cañon Ferry line. When mounted on its pin, this insulator on the
Shawinigan line holds its wire 16-1/4 inches above the cross-arm,
compared with a corresponding distance of 14-1/2 inches on the Electra,
15 inches on the Colgate, and 13-1/2 inches on the Cañon Ferry line. The
two upper petticoats on each of these insulators are much less concave
than the lowest one, and the edges of all three stand respectively
11-3/4, 7-1/4, and 3-1/4 inches above the cross-arm. From the edge of
the top to the edge of the bottom petticoat the direct distance is 8-1/2
inches.
Of the three transmission lines above named that operate at 50,000 to
60,000 volts, that between Shawinigan Falls and Montreal leads as to
distances between the line wire and insulator petticoats and the
cross-arm. On the Santa Ana line, where the voltage is 33,000, the
insulator is of a more ordinary type, being of porcelain, 6-3/4 inches
in diameter, 4-7/8 inches high, and having the lower edges of its three
petticoats in the same plane. Each of these insulators holds its wire
8-5/8 inches above the cross-arm, and has all of its petticoats 3-1/2
inches above the cross-arm. Unlike the three insulators just described,
which are mounted on wooden pins, this Santa Ana insulator has a pin
with an iron core, wooden threads, and porcelain base. This base extends
up from the cross-arm a distance of 3-1/8 inches, and the wooden sleeve,
in which the threads for the insulator are cut, runs down over the
central bolt of the pin to the top of the porcelain base, which is
5/8-inch below the petticoats.
The 30,000-volt lines from Spier Falls are carried 10-3/4 inches above
their cross-arms by triple petticoat porcelain insulators. Each of these
insulators is 8-1/2 inches in diameter, 6-3/4 inches high, and is built
up of three parts cemented together. A malleable-iron pin cemented into
each insulator with pure Portland cement carries the outside petticoat
7-1/2 inches and its lowest petticoat 4-1/4 inches above the cross-arm.
When the voltage on the Spier Falls lines was raised from about 13,000
to 30,000, the circuits being carried in part by one-piece porcelain
insulators, a number of these insulators were punctured at the higher
pressures, and some cross-arms and poles were burned as a result. No
failures resulted on those parts of these lines where the three-part
insulators were in use. The second pole line between Niagara Falls and
Buffalo was designed to carry circuits at 22,000 volts, or twice that
for which the first line was built. Porcelain insulators were employed
on both of these lines, but while the 11,000-volt line was carried on
three-petticoat insulators, each with a diameter of 7 inches and a
height of 5-1/2 inches, the 22,000-volt line was mounted on insulators
each 7-1/2 inches in diameter and 7 inches high, with only two
petticoats. The older insulator has its petticoats 2 inches above the
cross-arm, and the lower petticoat of the new insulator is 3 inches
above the arm. These two insulators illustrate the tendency to lengthen
out along the insulator axis as the voltage of the circuits to be
carried increases.
[Illustration: FIG. 93A.--The Old and New Insulators on the Niagara
Falls-Buffalo Line.]
For future work at still higher voltages, the advantage as to both first
cost and insulating qualities seems to lie with insulators that are very
long in an axial direction, and which have their petticoats arranged one
below the other and all of about the same diameter, rather than with
insulators of the umbrella type, like those on the Electra and Colgate
lines.