LIGHTNING PROTECTION DEVICES
Lightning protection devices, or lightning _arresters_, are devices
for providing a path by which lightning disturbances or other static
discharges may pass to the earth.
Lightning arresters, designed for the protection of transmission lines,
must perform this function with a minimum impairment of the insulation
of the lines.
In general the construction of lightning arresters comprise
1. Air gaps;
2. Resistances;
3. Inductances;
4. Arc suppressing devices.
~Ques. What are the causes of static charges?~
Ans. They may be caused by sandstorms in dry climates, or may be due to
grounds on the high pressure side of a system.
~Ques. What causes high frequency oscillations?~
Ans. They are usually due to lightning discharges in the vicinity of
the line.
~Ques. What are the requirements of lightning protection devices?~
Ans. They must prevent excessive pressure differences between line and
ground, line and line, and between conductor turns in the electrical
apparatus.
~Air Gap Arresters.~--method of relieving any abnormal pressure
condition is to connect a discharge air gap between some point on an
electric conductor and the ground. The resistance thus interposed
between the ground and the conductor is such that any voltage very
much in excess of the maximum normal will cause a discharge to ground,
whereas at other times the conductor is ungrounded because of the air
gap. This forms the principle of air gap arresters.
[Illustration: FIG. 2,349.--Non-arcing multi-gap arrester. Based on
the principle of employing for the terminals across which the arc is
formed, such metals as are least capable of maintaining an alternating
arc between them. This so called non-arcing property of certain metals
was discovered by Alexander Wurtz. The action is such that the "line
current" which follows the lightning discharge follows as an arc, but
is stopped at the end of one alternation because of the property of the
non-arcing metals to carry an arc in one direction, but requiring an
extremely high voltage to start a reverse arc. The non-arcing metals
ordinarily employed are alloys of zinc and copper. Plain multi-gap
arresters as here shown operate satisfactorily with the smaller
machines and on circuits of limited power, but for large machines
of close regulation, and therefore of very large momentary overload
capacity, especially when a number of such are operated in parallel,
such arresters were found insufficient, the line current following the
lightning discharge frequently was so enormous that the circuit did
not open at the end of the half wave, that is the arrester held the
arc and was destroyed. The introduction of synchronous motors made it
necessary that the arc should be extinguished immediately, otherwise
the synchronous motors and converters would drop out of step, and the
system would in this way be shut down. To insure the breaking of the
arc, resistance was introduced in the arrester, the modified device
being known as the low equivalent arrester as shown in fig. 2,350.]
The single gap while adequate for telegraph line protection, was found
insufficient for electric light and power circuits, because since the
current in such circuits is considerable and usually at high pressure
it would follow the lightning discharge across the gap. Thus the
problem arose to devise means for short circuiting the line current
resulting in various modifications of gap arrester.
~Multi-gap Arresters.~--The essential elements of an arrester of this
type are a number of cylinders spaced with a small air gap between them
and placed between the line to be protected and the ground, or between
line and line.
_In operation_, the multi-gap arrester discharges at a much lower
voltage than would a single gap having a length equal to the sum of the
small gaps. In explaining the action of multi-gaps, there are three
things to consider:
[Illustration: FIG. 2,350.--Low equivalent arrester. This is a
modification of the multi-gap arrester shown in fig. 2,349. About half
of the total number of gaps are shunted by a resistance, and another
resistance inserted between the cylinders and the earth. With this
arrangement the middle point is at ground pressure, and there are
between line and ground only one half of the total number of gaps.
This is sufficient to prevent a bridging of the gaps under normal
conditions.]
1. The transmission of the static stress along the line of the
cylinders; 2. The sparking at the gaps; 3. The action and duration of
the current which follows the spark, and the extinguishment of the arc.
~Ques. What is a spark?~
Ans. The conduction of electricity by air.
~Ques. What is an arc.~
Ans. The conduction of electricity by vapor of the electrode.
~Distribution of Static Stress.~--The cylinders of the
multi-gap arrester act like plates of condensers in series.
This condenser function is the essential feature of its
operation.
When a static stress is applied to a series of cylinders
between line and ground, the stress is immediately carried from
end to end.
If the top cylinder be positive it will attract a negative
charge on the face of the adjacent cylinder and repel an equal
positive charge to the opposite face and so on down the entire
row.
The second cylinder has a definite capacity relative to the
third cylinder and also to the ground; consequently the charge
induced on the third cylinder will be less than on the second
cylinder, due to the fact that only part of the positive
charge on the second cylinder induces negative electricity
on the third, while the rest of the charge induces negative
electricity to the ground. Each successive cylinder, counting
from the top of the arrester, will have a slightly smaller
charge of electricity than the preceding one.
[Illustration: FIG. 2,351.--General Electric 2,200 volt multi-gap
arrester for station installation. It consists of fourteen ⅝" knurled
cylinders and two shunt resistance rods mounted on a porcelain base.
One of these rods has a low resistance, and shunts nine gaps; the
other rod has a high resistance, and shunts eleven gaps. The effect
of the shunt resistance in extinguishing the line current arc is the
same, therefore, as that of an equal series resistance but is without
the objectionable features of the latter. Series resistance limits
the discharge current to such an extent that an arrester with series
resistance fails to protect against destructive rises of voltage when
the conditions are severest. Graded shunt resistance responds to all
frequencies and opens a discharge path for excessive voltage when the
frequencies are high as well as when they are low. Its further effect
in withholding the line current from the gaps after the relieving
discharge has occurred, is to aid the non-arcing quality of the metal
cylinders in quickly suppressing the arc that follows a discharge. The
arc is extinguished at the end of the half cycle of line current in
which the discharge takes place.]
~Sparking at the Gaps.~--The quantity of electricity induced
on the second cylinder is greater than on any lower cylinder
and its gap has a greater pressure strain across it as shown in
fig. 2,357. When the voltage across the first gap is sufficient
to spark, the second cylinder is charged to line voltage and
the second gap receives the static strain and breaks down. The
successive action is similar to overturning a row of ten-pins
by pushing the first pin against the second. This phenomenon
explains why a given length of air gap concentrated in one gap
requires more voltage to spark across it than the same total
length made up of a row of multi-gaps.
[Illustration: FIG. 2,352.--General Electric 2,200 volt arrester in the
act of discharging, and shunting the line current. The figure shows
an actual discharge taking place. It will be seen that the heavy line
current passes across only four of the gaps, and then goes through the
resistance rods; while the static discharge passes straight across
the entire series of thirteen gaps. When the gaps of an arrester are
shunted by even a low resistance, discharges of very high frequency
find it relatively difficult to pass through the resistance rods, owing
to the impedance of the rods, but comparatively easy to pass across
all the gaps, owing to the capacity effect in breaking down the gaps.
The higher the frequency, the more pronounced is this effect, hence
the discharges select different paths through gaps and resistances
depending upon the frequency. By frequency is meant, not the frequency
of the line current but the lightning frequency, which may run into
hundreds of thousands, or into millions of cycles. The equivalent
needle gap for this arrester is shown by tests to be nearly the same
for all frequencies and quantities of discharge; that is, the arrester
is equally responsive to all frequencies.]
[Illustration: FIGS. 2,353 to 2,355.--Oscillograph record of the
phenomena that take place in the different circuits or selective paths
of a multi-gap arrester during a discharge such as shown in fig.
2,352.]
As the spark crosses each successive gap, the voltage gradient
along the remainder readjusts itself.
~How the Arc is Extinguished.~--When the sparks extend across
all the gaps the line current will follow if, at that instant,
the line pressure be sufficient. On account of the relatively
greater line current, the distribution of pressure along the
gaps becomes equal, and has the value necessary to maintain the
line current arc on a gap.
The line current continues to flow until the voltage of the
generator passes through zero to the next half cycle, when the
arc extinguishing quality of the metal cylinders comes into
action.
[Illustration: FIGS. 2,356 and 2,357.--Diagram showing condenser action
of cylinders and pressure gradient for static stress.]
The alloy contains a metal of low boiling point which prevents
the reversal of the line current. It is a rectifying effect,
and before the pressure again reverses, the arc vapor in the
gaps has cooled to a non-conducting state.
~Effect of Frequency.~--The higher the frequency of the
lightning oscillation, the more readily will the multi-gap
respond to the pressure.
Briefly stated, the problem is to properly limit the line
current so that the arc may be extinguished; to arrange a shunt
circuit so that the series resistance will be automatically cut
out if safety demand it on account of a heavy lightning stroke
and, while retaining these properties, to make the arrester
sensitive to a wide range of frequency.
It should be noted that series resistance limits the rate of discharge
of the lightning as well as of the line current. The greater the
value of the line current, the greater the number of gaps required to
extinguish the arcs.
~Graded Shunt Resistances.~--Any arc is unstable and can be
extinguished by placing a properly proportioned resistance in parallel
with it. All the minor discharges then pass over the resistances and
the unshunted spark gaps, the resistance assisting in opening the line
current after the discharge.
Very heavy discharges pass over all the spark gaps, as a path without
resistance, but those spark gaps which are shunted by the resistance,
open after the discharge.
The line current, after the first discharge is accordingly deflected
over the resistances, and limited thereby, the circuit being finally
opened by the unshunted spark gaps. The arrangement of shunted
resistances is shown in fig. 2,358.
[Illustration: FIG. 2,358.--Arrangement of graded resistances on
multi-gap arrester.]
~The Cumulative or "Breaking Back" Effect.~--The graded shunt
resistance gives a valuable effect, where the arrester is considered as
four separate arresters. This is the "cumulative" or "breaking back"
action.
When a lightning strain between line and ground takes place, the
pressure is carried down the high resistance H (figs. 2,365 and 2,366),
to the series gaps GS, and the series gaps spark over.
Although it may require several thousand volts to spark across an
air gap, it requires relatively only a few volts to maintain the arc
which follows the spark. In consequence, when the gaps GS spark over,
the lower end of the high resistance is reduced practically to ground
pressure.
If the high resistance can carry the discharge current without giving
an ohmic drop sufficient to break down the shunted gaps GH, nothing
further occurs--the arc goes out.
If, on the contrary, the lightning stroke be too heavy for this, the
pressure strain is thrown across the shunted gaps, GH, equal in number
to the previous set. In other words, the same voltage breaks down
both of the groups of gaps, GS and GH, in succession. The lightning
discharge current is now limited only by the medium resistance M, and
the pressure is concentrated across the gaps, GM.
If the medium resistance cannot discharge the lightning, the gap GM
spark, and the discharge is limited only by the low resistance.
The low resistance should take care of most cases but with
extraordinarily heavy strokes and high frequencies, the discharge can
~break back~ far enough to cut out all resistance.
In the last steps, the resistance is relatively low in proportion to
the number of shunt gaps, GL, and is designed to cut out the line
current immediately from the gap, GL. This "breaking back" effect is
valuable in discharging lightning of low frequency.
[Illustration: FIGS. 2,359 to 2,364.--Westinghouse safety spark gaps.
Fig. 2,359, indoor type; figs. 2,360 to 2,364, outdoor type. It is
well known that with transformers, operating on high voltage lines and
having large ratios of transformation, there may occur, on the low
tension side, momentary voltages to ground greatly in excess of the
normal. These momentary increases in voltage between the low tension
circuits and ground are commonly called ~"static disturbances."~ In
general they are the result of a change in the static balance of
the high tension side and its connecting circuits. Unless certain
precautions are taken, such a static disturbance on the low tension
side may cause serious stresses in the secondary insulation of a
transformer with a high ratio of transformation. This induced static
voltage is independent of the ratio of transformation. The static
stresses are more serious in a high ratio transformer simply because
the insulation of its secondary is less able to withstand them. _A
method of relieving this disturbance_ is to connect a discharge spark
gap between some point of the low tension side of the transformer to
be protected (a middle or neutral point, if one be available) and the
ground. The spark gap opening is such that any voltage very much in
excess of the maximum normal will cause a discharge to ground, and
thus the low tension side is practically tied to ground during such
disturbance, while at other times it is ungrounded. The Underwriters
recommend the grounding of the neutral point of low tension circuits
when the conditions are such that the maximum normal voltage between
the point connected and ground will not exceed 250 volts. The rule
allows one side of a 250 volt circuit or the middle point of a 550 volt
circuit to be grounded. ~The spark gaps shown above~ are designed for
use on transformer secondary circuits and for protecting individual
series arc lamps. These spark gaps are single pole, and consist of
two cylinders of non-arcing metal with an air gap between. One of the
cylinders is connected to the ground, the other to the line.]
[Illustration: FIGS. 2,365 and 2,366.--Graded shunt resistance arrester
connections. Fig. 2,365, connections for 33,000 volt ~Y~ system with
grounded neutral; fig. 2,366, connections for 33,000 volt delta or
ungrounded ~Y~ systems. The type of arrester shown above may be
considered as ~four arresters in one~. ~First,~ for small discharges
there are a few gaps in series with a high shunt resistance. This
part of the arrester will safely discharge accumulated static and
also all disruptive discharges of small ampere capacity. This path
is shown through H (resistance) and GS (gaps). ~Second,~ there are a
number of gaps in series with a medium shunt resistance which will
discharge disruptive strokes of medium ampere capacity. This path is
shown through M (resistance) and GH plus GS (gaps). ~Third,~ there are
a greater number of gaps in series with a low shunt resistance which
will discharge heavy disruptive strokes. This path is shown through L
(resistance) and GM plus GH plus GS (gaps). ~Fourth,~ the total number
of gaps has no series resistance, thus enabling the arrester to freely
discharge the heaviest induced strokes. This path is shown through zero
resistance and GH plus GM plus GH plus GS (gaps). In each of the above
circuits the number of gaps and the resistance are so proportioned as
to extinguish the line arc at the end of the half cycle in which the
lightning discharge takes place.]
[Illustration: FIG. 2,367.--Installation of a General Electric 12,500
volt, three phase, multi-gap lightning arrester in the Garfield Park
sub-station of the West Chicago park common. The "V" unit multi-gap
arrester, which is plainly seen in the illustration, is made up of
"V" units consisting of gaps between knurled cylinders and connected
together at their ends by short metal strips. The base is of porcelain,
which thoroughly insulates each cylinder, and insures the proper
functioning of the multi-gaps. The cylinders are made of an alloy
that contains metal of low boiling point which gives the rectifying
effect, and metals of high boiling point which cannot vaporize in the
presence of the one of low boiling point. The cylinders are heavily
knurled. As the arc plays on the point of a knurl it gradually burns
back and when the metal of low boiling temperature is used up, the gap
is increased at that point. The knurling, thus, insures longer life to
the cylinder by forcing successive arcs to shift to a new point. When
worn along the entire face, the cylinder should be slightly turned.
The low resistance section of the graded shunt is composed of rods of
a metallic alloy. These rods have large current carrying capacity, and
practically zero temperature coefficient up to red heat. The medium and
high resistance rods are of the same standard composition previously
used. The contacts are metal caps shrunk on the ends; the resistances
are permanent in value and the inductance is reduced to a minimum. The
rods are glazed to prevent absorption of moisture and surface arcing.]
After the spark passes, the arcs are extinguished in the reversed
order. The low resistance, L, is proportioned so as to draw the arcs
immediately from the gaps, GL. The line current continues in the next
group of gaps, GM, until the end of the half cycle of the generator
wave.
[Illustration: FIGS. 2,368 to 2,370.--Multi-gap or low equivalent
lightning arrester. It consists of: 1, a number of gap units in series;
2, a number of gap units in shunt with a resistance; and 3, a series
resistance. ~All resistances~ are wire wound and the series resistance
is non-inductive. The shunt resistance and gap units are mounted on
marble. ~When a discharge occurs,~ the series gaps are broken down,
and if the discharge be heavy enough, it will meet opposition in the
shunt resistance and pass over the shunted gaps, through the series
resistance to the ground. ~The arc~ which tends to follow the discharge
is then withdrawn from the shunted gaps by the shunt resistance, and
aided by both resistances is suppressed by the series gaps. ~The
pressure of discharge~ is determined by the number of series gaps as
sufficient number is used to withstand the normal voltage and yet give
a proper factor of safety for the severest service.]
At this instant the medium resistance, M, aids the rectifying quality
of the gaps, GM, by shunting out the low frequency current of the
alternator.
On account of this shunting effect the current dies out sooner in the
gaps, GM, than it otherwise would.
In the same manner, but to a less degree, the high resistance, H, draws
the line current from the gaps, GH.
This current now being limited by the high resistance, the arc is
easily extinguished at the end of the first one-half cycle of the
alternator wave.
~Ques. What is the difference between arrester for grounded Y and
non-grounded neutral systems?~
Ans. The connections are shown in figs. 2,365 and 2,366. The difference
in design lies in the use of a fourth arrester leg between the
multiplex connection and ground or ungrounded system.
~Ques. Why is the fourth leg introduced?~
Ans. The arrester is designed to have two legs between line and line.
If one line become accidentally grounded, the full line voltage would
be thrown across one leg if the fourth or ground leg were not present.
[Illustration: FIG. 2,371.--Westinghouse three pole or four pole
arrester in weather proof wooden case which protects the arrester units
from rain and snow when they are installed in exposed locations, as on
poles or buildings.]
On a ~Y~ system with a grounded neutral, the accidentally
grounded phase causes a short circuit of the phase and the
arrester is relieved of the strain by the tripping of the
circuit breaker. Briefly stated, the fourth or ground leg of
the arrester is used when, for any reason, the system could be
operated, even for a short time with one phase grounded.
~Ques. Describe the multiplex connection.~
Ans. It consists of a common connection between the phase legs of the
arrester above the earth connection and provides an arrester better
adapted to relieve high pressure surges between lines than would
otherwise be possible.
Its use also economizes in space and material for delta and
partially grounded or non-grounded ~Y~ systems.
[Illustration: FIGS. 2,372 and 2,373.--Westinghouse multi-gap
lightning arrester and views showing parts. ~In construction,~ a
series of gaps, between non-arcing metal cylinders arranged in a row,
is connected between line and ground in series with a composition
stick resistor having a resistance of something between 80 and 120
ohms. ~In operation,~ if an excessive pressure be developed on a
line, electric discharge arcs form between the metal cylinders, and
the charge of electricity flows to ground, relieving the excessive
stress. The resistance of the stick resistor limits the flow so that
an excessive power current cannot pass through the arrester. The
tendency for a destructive power arc to follow the discharge arc is
thus counteracted. ~The composition resistors~ and the gap cylinders
are mounted in pairs on a porcelain base, and complete units are
arranged within weather proof wooden boxes as indicated. ~For two pole
arresters,~ one unit is mounted on the back of the box. ~For three pole
and four pole arresters,~ two units are used; one is secured on each
side of the box.] [Illustration: FIGS. 2,374 to 2,376.--Connections
for Westinghouse multi-gap (type G) arresters. These arresters may be
installed outside on poles or buildings, or indoors on station walls.
The weather proof wooden case (as shown in fig. 2,371) protects the
arrester units from rain and snow when installed in exposed places.
Fig. 2,374 shows single phase installation, fig. 2,375, two phase
installation, and fig. 2,376, three phase installation. ~On a two
pole circuit~ one line wire is connected to the top of each of the
composition resistors of each arrester unit, as shown in fig. 2,374,
and the ground wire is connected to the middle point of the gap series.
~On four pole circuits,~ fig. 2,375, the same scheme of connections is
used, but two arrester units are necessary and the connections of both
are the same. ~On three pole circuits,~ two arrester units are used,
with the same connections as for four pole circuits, except that there
are but three line connections instead of four as in fig. 2,376.]
~Horn Gap Arresters.~--A horn gap arrester consists essentially of two
horn shaped terminals forming an air gap of variable length, one horn
being connected to the line to be protected and the other to the ground
usually through series resistance as shown in fig. 2,378.
~Ques. How does the horn gap arrester operate?~
Ans. The arc due to the line current which follows a discharge, rises
between the diverging horn and becoming more and more attenuated is
finally extinguished.
[Illustration: FIG. 2,377.--Horn gap arrester, diagram showing arrester
and connections between line and ground. The horn type arrester
was invented by Oelschlaeger for the Allgemeine Electricitaets
Gesellschaft, and like the Thomson arc circuit arrester, ~its
operation~ is based on the fact that _a short circuit once started at
the base, the heat generated by the arc will cause it to travel upward
until it becomes so attenuated that it is ruptured_. On circuits of
high voltage this rupture sometimes takes a second or two, but seems
to act with little disturbance of the line. ~Sometimes~ _a water
resistance_ is used, a _choke coil being_ inserted in the circuit in
series. ~In one installation~ for a 40,000 volt line, the horns were
made of No. 0,000 copper wire with gap knees 2¼ to 3 or 3¼ inches. The
capacity of the water resistance receptacle was 15 gallons. ~Users
differ~ as to whether the water should contain salt. ~The choke coil~
can be made of about 18 turns of iron wire wound on a 6 inch cylinder.]
~Ques. What is the objection to the horn gap on alternating current
circuits?~
Ans. The arc lasts too long for synchronous apparatus to remain in step.
~Ques. What provision was made to shorten the duration of the arc?~
Ans. A series resistance was inserted in the arrester circuit as shown
in fig. 2,377.
~Ques. What difficulty was caused by the series resistance?~
Ans. With sufficient series resistance to prevent loss of synchronism,
the arrester failed to protect the system under severe conditions.
~Ques. With these objections what use was found for the horn gap
arrester?~
Ans. It is used as an emergency arrester on some overhead lines, to
operate only when a shut down is unavoidable, also for series lighting
circuits.
[Illustration: FIG. 2,378.--General Electric horn gap with charging
resistance for cable system. Arresters for cable systems differ from
arresters for overhead circuits only in the construction of the horn
gaps. The necessity for this difference is due to the fact that a
cable system has a very much higher electrostatic capacity and much
less inductance than an overhead system. In consequence, the currents
which flow into the arrester during charging are somewhat higher. It
is desirable to avoid these heavier currents, especially during the
time of breaking the arc at the horn gap. This is accomplished by
using a special horn gap and resistance. This consists of an auxiliary
horn mounted above and insulated from the regular horn in such a
manner as to intercept the arc if it rise on the regular horns. Enough
resistance is connected in series with this auxiliary horn so that the
current flow and arc across this gap are always limited to a moderate
value. Such a device has several advantages. Since the mechanism is so
arranged that the charging is always done through the auxiliary horn
the current rush is limited during the charging and thus troubles from
carelessness or ignorance are avoided. It also gives a nearer uniform
charging current. In the use of this auxiliary horn gap and resistance
there are three successive stages, as follows: 1, light discharges
will pass across the smaller gaps to the auxiliary horn and through
the series resistance to the cells; 2, if the discharge be heavy, the
resistance offers sufficient impedance to cause the spark to pass to
the main horn. This is accomplished with only a slight increase in
pressure because the gap is already ionized. If the cells be in normal
condition, the spark at the gap is immediately extinguished, without
any flow of line current; 3, if the cells be in poor form, the line
current may follow the discharge across the main gap and the arc will
rise to the safety horn and be extinguished through a resistance. For
mixed overhead and cable systems the choice of arrester will be a
matter of judgment. If there be a comparatively short length of cable,
the usual practice for overhead systems may be adopted. For direct
connection to busbars, arresters with charging resistance should be
used.]
[Illustration: FIGS. 2,379 and 2,380.--Diagram showing connections of
horn type lightning arresters on series circuits.]
The necessity of service requires that series lightning systems
be fully equipped against damage by lightning and similar
trouble. The most common disturbances occurring on series
circuits are the surges set up by the sudden opening of the
loaded circuit. These disturbances are especially severe where
circuits are accidentally grounded, due to contact of the wires
where they pass through other circuits.
~Ques. How are the spark gaps adjusted?~
Ans. They are set to give a low spark pressure relative to the voltage
of the line.
[Illustration: FIG. 2,381.--General Electric horn type arrester,
mounted for 15 light series arc circuit. The horn type arrester
consists of a horn gap with series resistance between each line and
ground. The resistances and horn gaps are mounted on porcelain bases
and the latter on insulating wooden supports. The supports have
asbestos barriers (except for lowest voltages), and backs to eliminate
liability of damage from the arc which forms in the horn gap at the
time of the discharge. The spark gaps are adjusted to give a low spark
pressure relative to the voltage of the circuit. The number and ohmic
value of the resistance rods used in the various arresters depend upon
the voltage and current of the circuit.]
~Ques. Why are horn arresters well suited to protect series lighting
circuits against surges?~
Ans. Because the surges are damped out before the arc which forms
across the horn gaps is interrupted.
These arcs last for several cycles, since the length of the
time of action of the arrester depends upon the lengthening of
the arc between the horn gaps, limited by the series resistance.
Since practically all disturbances on lighting circuits are
of low frequency, the series resistance can be used with good
results; it aids the horn in extinguishing the arc, limits the
size of the arc and prevents short circuits occurring during
the period of discharge.
[Illustration: FIG. 2,382.--General Electric horn arrester for pole
installation. Quite frequently series circuits are run underground
in cables for some distance from the generating station. In order to
protect the cables it is advisable to place horn arresters at the
points where the cable joins the overhead wires. The resistance units
are mounted in the wooden box. This design is used to economize space,
since if the horn gaps be placed in the box the latter would have to
be made very large to accommodate the asbestos barriers and backs. In
installing this type of arrester it is advisable to place it as near as
possible to the top of the pole so that the arc may rise unobstructed
and thus avoid the likelihood of live wires coming in contact with the
horns which, during the operation of the series current, are alive.]
~Electrolytic Arresters.~--Arresters of this class are sometimes
called aluminum arresters because of the property of aluminum on which
their action depends; that is, _it depends on the phenomenon that a
non-conducting film is formed on the surface of aluminum when immersed
in certain electrolytes_.
If however, the film be exposed to a higher pressure, it may be
punctured by many minute holes, thus so reducing its resistance that a
large current may pass. When the pressure is again reduced the holes
become resealed and the film again effective.
[Illustration: FIGS. 2,383 and 2,384.--Elevation and plan of General
Electric horn gaps and operating stand for high voltage arresters.]
In construction, the aluminum arrester consists essentially of a system
of nested aluminum cup shaped trays, supported on porcelain and secured
in frames of heated wood, arranged in a steel tank.
The system of trays is connected between the line and ground, and
between line and line, a horn gap being inserted in the arrester
circuit which prevents the arrester being subjected to the line voltage
except when in action.
The electrolyte is poured into the cones and partly fills the space
between the adjacent ones. The stack of cones with the electrolyte
between them is then immersed in a tank of oil. The electrolyte between
adjacent cones forms an insulation. The oil improves this insulation
and prevents the evaporation of the solution.
[Illustration: FIG. 2,385.--Cross section of General Electric aluminum
(electrolytic) lightning arrester.]
A cylinder of insulating material concentric with the cone stack is
placed between the latter and the steel tank, the object being to
improve the circulation of the oil and increase the insulation between
the tank and the cone stack. The arrester, as just described consists
of a number of cells connected in series.
~Ques. Of what does a single cell consist and what are its
characteristics?~
Ans. It consists of two of the cone shaped aluminium trays or plates
and an electrolyte, which forms a condenser that will stand about 350
volts before breaking down. When this voltage is exceeded the cell
becomes a fairly good conductor of electricity, but as soon as the
voltage drops its resistance again resumes a very high value.
~Ques. What is the critical voltage?~
Ans. The voltage at which the current begins to flow freely.
[Illustration: FIG. 2,386 to 2,390.--Parts of General Electric 15,000
volt aluminum lightning arrester, not including horn gaps, etc.]
Up to a certain voltage the cell allows an exceedingly low
current to flow, but at a higher voltage the current flow is
limited only by the internal resistance of the cell, which is
very low. A close analogy to this action is found in the well
known safety valve of the steam boiler, by which the steam is
confined until the pressure rises above a given value, when
it is released. On the aluminum plates there are myriads of
minute safety valves, so that, if the electric pressure rise
above the critical voltage, the discharge takes place equally
over the entire surface. It is important to distinguish between
the valve action of this hydroxide film and the failure of any
dielectric substance.
~Ques. When a cell is connected permanently to the circuit what two
conditions are involved?~
Ans. The _temporary_ critical voltage and the permanent _critical
voltage_.
For instance, if the cell have 300 volts applied to it constantly,
and the pressure be suddenly increased to, say 325 volts, there will
be a considerable rush of current until the film thickness has been
increased to withstand the extra 25 volts; this usually requires
several seconds. In this case 325 volts is _the_ ~temporary~ _critical
voltage of the cell_.
Similar action will occur at any pressure up to about the ~permanent~
critical voltage, or _the voltage at which the film cannot further
thicken_, and therefore allows a free flow of current.
If the voltage be again reduced to 300 the excess thickness of film
will be gradually dissolved, and if it vary periodically between two
values, each of which is less than the permanent critical value, the
temporary critical voltage will be the higher value. This feature is of
great importance as it provides a means of discharging abnormal surges,
the instant the pressure rises above the impressed value.
[Illustration: FIG. 2,391.--Volt ampere characteristic curve of a
General Electric aluminum (electrolytic) cell on alternating current.
The permanent critical voltage is between 335 and 360 volts. With
alternating current, the cell acts as a fairly good condenser, and
there is not only the leakage through the film, but also a capacity
current flowing into the cell. The phase of this current, then, is
nearly 90 degrees ahead of the pressure and represents a very low
energy factor.]
~Ques. How is the number of cells required for a given circuit
determined?~
Ans. The number required for a given operating voltage is determined by
allowing about 250 to 300 volts per cell.
~Ques. In putting cells in commission how is the electrolyte
introduced?~
Ans. It is poured into the aluminum trays and the overflow drawn off at
the bottom of the tank.
~Ques. Describe the further operations in putting cells in commission.~
Ans. After putting in the electrolyte it is allowed to stand for a few
days until part has evaporated, then the oil is poured over the surface
to prevent further evaporation.
[Illustration: FIG. 2,392.--Westinghouse electrolytic lightning
arrester, for three phase ungrounded neutral service, 25,000 maximum
voltage. These arresters are designed for the protection of alternating
current circuits from all kinds of static disturbances. They have been
standardized for installation on three phase circuits of voltages of
2,200 to 110,000. They cannot be used for voltages of less than 13,500.
For voltages below this the horn gaps cannot, with safety, be set
close enough together, out of doors, to take advantage of the freedom
of discharge of the electrolytic element. If the horn gaps be set too
close together they may be short circuited by rain. A shelter should be
built for arresters of 13,500 volts and below for their protection when
installed outside.]
~Ques. What action takes place when the trays stand in the electrolyte
and cell is disconnected from the circuit?~
Ans. Part of the film deteriorates.
~Ques. What is the nature of the film?~
Ans. The film is composed of two parts, one of which is hard and
insoluble, and apparently acts as a skeleton to hold the more soluble
part. The action of the cell seems to indicate that the soluble part of
the film is composed of gases in a liquid form.
~Ques. What action takes place when a cell which has stood for some
time disconnected, is reconnected to the circuit?~
[Illustration: FIGS. 2,393 and 2,394.--Aluminum trays for Westinghouse
electrolytic lightning arresters.]
Ans. There is a momentary rush of current which reforms the part of the
film which has dissolved.
This current rush will have increasing values as the intervals
of rest of the cell are made greater.
Many electrolytes have been studied, but none has been found
which does not show this dissolution effect to a greater or
lesser extent.
If the cell has stood disconnected from the circuit for some
time, especially in a warm climate, there is a possibility that
the initial current rush will be sufficient to open the circuit
breakers or oil switches. This current rush also raises the
temperature of the cell, and if the temperature rise be great,
it is objectionable.
When the cells do not stand for more than a day, however, the
film dissolution and initial current rush are negligible.
~Ques. What is the object of using horn gaps on electrolytic arresters?~
Ans. The use is threefold: 1, it prevents the arrester being subjected
continually to the line voltage; 2, acts as a disconnecting switch to
disconnect the arrester from the line for repairs, etc., and 3, acts as
a connecting switch for charging.
[Illustration: FIG. 2,395.--Horn gaps and transfer device of General
Electric aluminium lightning arrester for 12,500 volt non-grounded
neutral circuit. The object of the transfer device is to provide a
means for interchanging the ground stacks with one of the line stacks
of cones during the charging operation so that the films of all the
cells will be formed to the same value. The transfer device consists of
a rotating switch which may be turned 180 degrees, thus interchanging
the connections of the ground stack and one of the line stacks.
For arresters up to 27,000 volts the device is mounted with three
insulators on the pipe frame work, and is operated by a hand wheel; for
arresters of higher voltage, the transfer device is mounted directly
over the tanks and is operated by bevel gears and hand wheel.]
~Charging of Electrolytic Arresters.~--In electrolytic arresters all
electrolytes dissolve the film when the arrester is on open circuit,
the extent of the dissolution depending upon the length of time the
film is in the electrolyte, and upon its temperature. It is therefore
necessary to _charge_ the cells from time to time and thus prevent the
dissolution and consequent rush of current which would otherwise occur
when the arrester discharges.
~Ques. Describe the charging operation for arresters with grounded
circuits.~
Ans. It consists in simply closing simultaneously the three horn gaps
so that the full pressure across the cells causes a small charging
current to flow and form the films to their normal condition.
[Illustration: FIG. 2,396.--Sectional view of General Electric ~vacuum
tube arrester~ for railway signal circuits. ~The arrester~ _is
essentially a gap in a vacuum_. ~In construction~, the gap is formed
between the inner wall of a drawn metal shell and a disc electrode
mounted concentric with it. ~The electrode~ is supported on a brass rod
which serves as the lead in connection, and has ample current carrying
capacity. The electrode system is insulated from the tube and rigidly
supported in position by a bushing made of vitreous material. The
bushing does not form the vacuum seal, that being made by a special
compound. ~The open end of the tube~ is finally closed by a porcelain
bushing. The tube is exhausted in a special machine which solders
a small hole in the end after the vacuum has been established. The
possibility of solder entering the active part of the vacuum space
is prevented by a diaphragm punching, and both the electrode and the
lining of the tube are of non-arcing metal. ~The arrester has a spark
pressure of~ from 350 to 600 volts direct current, and an equivalent
needle gap of about .005 inch. The arrester will not stand a continuous
flow of current due to excessive heating, hence if there be a
possibility of this due to high pressure crosses, fuses should be used.
R.R.S.A. standard terminals are used.]
~Ques. Describe the charging operation for arresters for non-grounded
circuits. ~ Ans. First, the horn gaps are closed for five seconds and
opened again to normal position, thus charging the cells of the three
line stacks. Second, with the horn gaps still in normal position, the
position of the transfer device is reversed and the horn gaps are again
closed for five seconds and returned to the normal position.
The complete charging operation takes but a few moments and
should be performed daily. The operation is valuable, not only
in keeping the films in good condition, but also in giving the
operator some idea of the condition of the arrester by enabling
him to observe the size and color of the charging spark.
[Illustration: FIG. 2,397.--Highland Park sub-station, Charlotte, N.C.,
showing old lightning arrester tower on the left and General Electric
aluminum (electrolytic) cell lightning arrester and horn gaps in
foreground.]
~Grounded and Non-grounded Neutral Circuits.~--It is important to avoid
the mistake of choosing an arrester for a thoroughly grounded neutral
when the neutral is only partially grounded, that is, grounded through
an appreciable resistance. Careful consideration of this condition will
make the above statement clear.
In an arrester for a grounded neutral circuit, each stack of cones
normally receives the neutral pressure when the arrester discharges,
but if a phase become accidentally grounded, the line voltage is thrown
across each of the other stacks of cones until the circuit breaker
opens the circuit. The line voltage is 173 per cent. of the neutral or
normal operating voltage of the cells and therefore about 150 per cent.
of the permanent critical voltage of each cell. This means that when
a grounded phase occurs, this 50 per cent. excess pressure is short
circuited through the cells until the circuit breaker opens.
[Illustration: FIG. 2,398.--Westinghouse ~electrolytic station
lightning arrester~ for direct current up to 1,500 volts consists of a
tank of oil in which are placed, on properly insulated supports, a nest
of cup shaped aluminum trays. The spaces between the trays are filled
with electrolyte, a sufficient quantity for one charge being furnished
with each arrester. The top tray is connected with the line through a
60 ampere fuse, and the bottom tray is connected to the tank which is
thoroughly grounded by means of a lug. The fuse is of the enclosed type
and mounted on the cover of the arrester. ~A small charging current~
flows through the trays continuously and keeps the films on the trays
built up, so that no charging is required. This charging current is
not, however, of sufficient value to raise the temperature appreciably.
The immersed area of each tray is 100 square inches. The shape and
the arrangement of the trays is such that any gases generated by the
discharge can pass out readily without disturbing the electrolyte
between the trays.]
The amount of energy to be dissipated in the arrester depends upon the
kilowatt capacity of the generator, the internal resistance of the
cells, and the time required to operate the circuit breakers. It is
evident that the greater the amount of resistance in the neutral, the
longer will be the time required for the circuit breakers to operate.
Therefore, in cases where the earthing resistance in the neutral
is great enough to prevent the automatic circuit breakers opening
practically instantaneously, an arrester for a non-grounded neutral
system should be installed.
[Illustration: FIGS. 2,399 to 2,401.--Westinghouse ground fittings.
Fig. 2,399, ground plate; fig. 2,400, ground point; fig. 2,401,
cap. The ground plate consists of a circular piece of cast iron,
12 inches in diameter, 1⅜ inches thick with a ¾ inch pipe tap in
center to connection to arrester. The surface is increased by means
of corrugations, as shown in the accompanying illustrations, to 461
square inches, affording ample contact with the earth and enabling it
to take care of all discharges through the arrester. The plate should
preferably be buried at the foot of the pole so that the ground wire
runs to it in a straight line from the arrester. Care should, of
course, be taken to see that the earth in which the plate is buried
is damp. If the ground wire be placed within the pipe leading to the
ground plate it should be soldered to a cap at the top of the pipe
to eliminate the inductive effect due to the wire being surrounded
by iron. A simple and effective method of securing a good ground is
by means of an iron pipe with a malleable iron point having a dipped
galvanized finish, and a brass cap with a lug for soldering the ground
wire. The pipe may be driven into the earth, or if it be too hard to
permit driving, a hole may be dug and the pipe placed therein. It
should extend from eight to ten feet above and below the earth to
secure, respectively, a good ground and prevent any tampering with the
ground wire. Should it be desired to make use of a longer pipe which
would be inconvenient to drive into the earth, two pieces can be used
and connected together by a coupling. The brass cap and malleable iron
point are tapped for use with ¾ inch pipe.]
~Ground Connections.~--In all lightning arrester installations it is
of the utmost importance to make proper ground connections, as many
lightning arrester troubles can be traced to bad grounds. It has been
customary to ground a lightning arrester by means of a large metal
plate buried in a bed of charcoal at a depth of six or eight feet in
the earth.
A more satisfactory method of making a ground is to drive a number of
one inch iron pipes six or eight feet into the earth surrounding the
station, connecting all these pipes together by means of a copper wire
or, preferably, by a thin copper strip. A quantity of salt should be
placed around each pipe at the surface of the ground and the ground
should be thoroughly moistened with water. It is advisable to connect
these pipes to the iron framework of the station, and also to any water
mains, metal flumes, or trolley rails which are available.
[Illustration: FIGS. 2,402 to 2,404.--General Electric ~magnetic blow
out arrester~ for use on railways. ~It consists of~ _an adjustable
spark gap in series with a resistance_. ~Part of the resistance~ is in
shunt with a blow out coil, between the poles of which is the spark
gap. The parts are mounted in a strong, porcelain box, which, for car
and pole use, is in turn mounted in a substantial asbestos lined,
wooden box. ~In operation~, when the lighting pressure comes on the
line, it causes the spark gap to break down and a discharge occurs
through the gap and the resistance rod to ground. Part of the current
shunts through the blow out coil producing a strong magnetic field
across the spark gap. The magnetic field blows out the discharge arc
and restores normal conditions. The resistance is only 60 ohms (for 500
volt rating work), and the spark gap only one-fortieth of an inch (.025
in.).]
The following suggestions are made for the usual size station.
1. Place three pipes equally spaced near each outside wall,
making twelve altogether, and place three extra pipes spaced
about six feet apart at a point nearest the arrester.
2. Where plates are placed in streams of running water, they
should be buried in the mud along the bank in preference to
being laid in the stream. Streams with rocky bottoms are to be
avoided.
3. Whenever plates are placed at any distance from the
arrester, it is necessary also to drive a pipe into the
earth directly beneath the arrester, thus making the ground
connection as short as possible. Earth plates at a distance
cannot be depended upon. Long ground wires in a station cannot
be depended upon unless a lead is carried to the parallel
grounding pipes installed as described above.
4. As it is advisable occasionally to examine the underground
connections to see that they are in proper condition, it is
well to keep on file exact plans of the location of ground
plates, ground wires and pipes, with a brief description, so
that the data can be readily referred to.
[Illustration: FIG. 2,405.--General Electric ~magnetic blow out
arrester~ for line use. ~It consists~ essentially of _a small spark
gap which is in series with a resistance, and between the poles of a
magnet_. ~The operation~ is similar to that of the arrester shown in
figs. 2,402 to 2,404, but the magnet is a permanent magnet instead
of an electromagnet. The spark gap and the magnet are mounted within
porcelain blocks in such a way that the discharge arc is blown by the
magnet through an arc chute and a cooling grid which is also held by
the porcelain. ~The cooling grid~ in the arc chute materially assists
the magnet in extinguishing the discharge arc, giving the arrester a
high arc rupturing quality. ~The series rod~ is carborundum and is
connected externally to the other portion of the arrester. The arrester
is self-contained.]
5. From time to time the resistance of these ground connections
should be measured to determine their condition. The resistance
of a single pipe ground in good condition has an average value
of about 15 ohms. A simple and satisfactory method of keeping
account of the condition of the earth connections is to divide
the grounding pipes into two groups and connect each group to
the 110 volt lighting circuit with an ammeter in series.
~Choke Coils.~--A lightning discharge is of an oscillatory character
and possesses the property of self-induction, accordingly it passes
with difficulty through coils of wire. Moreover, the frequency of
oscillation of a lightning discharge being much greater than that of
commercial alternating currents, a coil can readily be constructed
which will offer a relatively high resistance to the passage of
lightning and at the same time allow free passage to all ordinary
electric currents.
Opinions on the design of choke coils for use with lightning
arresters vary considerably. Some engineers recommend the
use of very large choke coils, but while large choke coils
of high inductance do choke back the high frequency currents
better than smaller coils of less inductance, they cost more,
and under many conditions they are a menace to the insulation
unless the lightning arresters be installed on both sides of
them.
[Illustration: FIG. 2,406.--Westinghouse line suspension choke coil.
It is so designed that it can be inserted directly in the transmission
line wire or in the station wiring and held in position therein by
the tension of the line or station wires. Because of the fact that no
insulators are required, solely to support this choke coil, and that
it can be installed in either a vertical or a horizontal position it
can often be utilized effectively in power and sub-station layouts.
Terminals, each having a ½ inch round hole, to accommodate the
conductors are provided at each end of the coil. Three square headed
binding screws are supplied which clamp the conductors in position.
The coil is provided with a strain insulator, so arranged within the
coil at its axis, that it assumes any mechanical tension transmitted
from the conductors. No mechanical tension reaches the turns of the
choke coil proper. In construction, the choke coil is made in but one
size having a current carrying capacity of 200 amperes and is suitable
for a voltage of 2,000 to 22,000. For higher voltages than 22,000,
several choke coils are connected in series. One coil is used for each
22,000 volts or fraction thereof, of the pressure between the wires of
the circuit. ~Application~: This type of choke coil may be used for
alternating current service for the entire range from 22,000 to 110,000
volts. It may be used on transformers, but is not recommended for the
protection of generators.]
Part of the functions of the choke coil are performed by the
end turns of a transformer and extra insulation is invariably
installed in all power transformers built in recent years.
The choice of choke coils must be influenced by the condition
of insulation in the transformers as well as by the cost,
pressure regulation, and nature of the lightning protection
required.
~Ques. What are the primary objects of a choke coil?~
Ans. To hold back the lighting disturbance from the circuit apparatus
during discharge, and to lower the frequency of the oscillation so that
whatever charge gets through the choke coil will be of a frequency too
low to cause serious pressure drop around the first turns of the end
coil in either alternator or transformer.
[Illustration: FIGS. 2,407 to 2,409.--General Electric choke coils.
Fig. 2,407, hour glass choke coil, 45,000 volts; fig. 2,408, low
voltage choke coil, 6,600 volts; fig. 2,409, low voltage choke coil,
4,600 volts.]
If there be no arrester, the choke coil cannot perform the
first function, accordingly a choke coil is best considered as
an auxiliary to an arrester.
~Ques. What is the principal electrical condition to be avoided with a
choke coil?~
Ans. Resonance. The coil should be so arranged that if continual
surges be set up in the circuit, a resonant voltage due to the presence
of the choke coil cannot build up at the transformer or generator
terminals. This factor is a menace to the insulation. Another way of
stating the condition is as follows: So arrange the choke coil as
not to prevent surges, originating in a transformer, passing to the
arrester.
~Ques. What is another electrical condition to be avoided and why?~
Ans. Internal static capacity between adjacent turns
[Illustration: FIG. 2,410.--Westinghouse choke coil for high pressure
transmission circuits, 2,200 to 25,000 volts. Choke coils of this type
are wound without iron cores on circular or elliptical center blocks.
They have a large number of layers and few turns per layer (except
those made for small currents, they usually have one turn per layer),
which give the best condition for insulating and cooling. They are air
cooled, heavily insulated and have a line lead at the top, as shown.
Choke coils are designed to prevent the short circuits sometimes caused
by the local concentration of pressure such as may be produced by a
lightning discharge. They limit, to some extent, an abnormal rise of
pressure on the apparatus by delaying the advance of a static wave from
the line and thus give the arrester more time to act. The disturbance
caused by a lightning flash passes along the line in the form of a
surge or "tidal wave." If this wave pass a choke coil, it is flattened
out, and if the coil be of sufficient power, becomes practically
harmless. It is evident, however, that the choke coil receives the
full force of the wave, and that, consequently, it must be heavily
insulated; moreover, the choke coil must not overheat under load, nor
introduce into the circuit excessive inductive resistance.] of the
choke coil, because this lowers the effectiveness of the coil.
~Ques. What is the object of making choke coils in the form of an hour
glass?~
Ans. To prevent sagging between the supports.
[Illustration: FIG. 2,411.--Westinghouse ~air cooled choke coil~
particularly suitable for outdoor use. ~The method of mounting~ is
such that insulation for any desired voltage is readily obtained with
the same type of porcelain, and mounting in any position is possible.
~The coil~ is a helix of aluminum rod, about 15 inches in diameter
and containing about 30 turns. Bracing clamps are provided to give
mechanical strength to the helix, and the rod used is of sufficient
diameter to carry 200 amperes. The coil is supported on two insulating
columns made up of porcelain insulators, which, except for the end
pieces, are interchangeable. The number of insulators used in the
columns depends on the voltage of the circuit in which the coil is to
be used. The apparatus can be mounted in any position convenient for
the wiring, on floor, wall, or ceiling. ~It is intended principally~
_for the protection of transformers_. Where greater reactance than is
afforded by a single coil is desired on the higher voltage circuits, it
is recommended that two or more coils be connected in series, one coil
being used for each 22,000 line voltage. This coil should not be used
for generators. The insulating columns are supported on substantial
cast iron blocks on wooden bases.]
~Ques. How are choke coils cooled?~
Ans. By air, or by oil.
~Ques. For what service are oil cooled choke coils used?~
Ans. On circuits of pressures above 25,000 volts, choke coils immersed
in oil, as are transformer coils, have advantages in that the coil is
amply insulated not only from the ground but against side flash, and
that copper of comparatively small section may be used without undue
heating.
[Illustration: FIG. 2,412.--Westinghouse ~air cooled choke coil,~
for voltages of from 2,200 to 110,000. ~In construction,~ the coils
are made of aluminum rod wound into a helix of about 15 inches in
diameter and having 20 turns. The helix is supported on two insulators.
For mechanical reasons it is necessary to have the aluminum rod of
sufficient size to secure rigidity, consequently every coil has a
capacity of 200 amperes and may be used on any circuit up to that
capacity. ~The coils~ are insulated according to the standard practice
for disconnecting switches, the insulators being mounted on wooden
pins supported by a wooden base. This apparatus can be mounted in
any position. The wiring of a station or sub-station is facilitated
because the protection may be placed so as to simply form part of the
wiring. The coils are symmetrical so that it is immaterial which end is
connected to the line or to the apparatus.]
~"Static" Interrupters.~--A static interrupter is a _combination of a
choke coil and a condenser_, the two being mounted together and placed
in a tank and oil insulated.
It is used on high pressure circuits and its function is to so
delay the erroneously called "static" wave in its entry into
the transformer coil, that a considerable portion of the latter
will become charged before the terminal will have reached full
pressure.
A choke coil alone sufficiently powerful to accomplish this would be
too large and costly on very high pressure and would interfere with the
operation of the system.
~Ques. How is the condenser and choke coil connected?~
Ans. The condenser is connected between the line and ground behind the
choke coil near the apparatus to be protected as shown in fig. 2,413.
[Illustration: FIG. 2,413.--Diagram showing connections of static
interrupter for protecting a transformer.]
~Ques. What is the effect of the condenser?~
Ans. The condenser, which has a very small electrostatic capacity, has
no appreciable effect upon the normal operation, but a very powerful
effect upon the static wave on account of its extremely high frequency.