LIGHTNING:
THE MOST COMMON
SOURCE OF OVERVOLTAGE
®
®
POWER SYSTEMS, INC.
Hi-Tension News
Reprinted from
9001-H
Fall/Wint. 1990
ISO 9001-94
Cert. No. 002196
The Ohio Brass Co.
Wadsworth, OH USA
®
POWER
SYSTEMS, INC.
573-682-5521
ANDERSON
Fax 573-682-8714
®
http://www.hubbellpowersystems.com
®
®
®
UNITED STATES • 210 N. Allen • Centralia, MO 65240 • Phone: 573-682-5521 • Fax: 573-682-8714 • e-mail: [email protected]
CANADA • 870 Brock Road South • Pickering, Ontario L1W 1Z8 • Phone: 905-839-1138 • Fax: 905-831-6353 • e-mail: [email protected]
MEXICO • Av. Coyoacan No. 1051 • Col. Del Valle • 03100 Mexico, D.F. • Phone: 525-575-2022 • Fax: 525-559-8626 • e-mail: [email protected]
NOTE: Because Ohio Brass has a policy of continuous product improvement, we reserve the right to change design and specifications without notice.
©
Copyright 2001 • Hubbell/ Ohio Brass
Bulletin EU1422-H
Printed in U.S.A.
Lightning: The Most Common
Source of Overvoltage
At any given moment, nearly
2,000 thunderstorms occur over the
earth‘s surface.
Lightning strikes the earth 100
times each second, often devastating property and lives.
Development of Lightning
Although lightning is still the
most common source of overvoltage surges and is the most destructive of all elements associated with
thunderstorms, the exact mechanism that produces lightning
strokes is not completely known.
Lightning is an effect of electrification within a thunderstorm.
Thunderstorms begin with moist,
heated air rising and saturating to
form visible cumulonimbus clouds.
As clouds form and grow, the
moisture condenses to rain, snow,
and ice within the cloud mass.
Wind updraft and downdraft
drives the moisture up and down in
freezing and thawing cycles until
Figure 1
the moisture droplets are heavy
enough to overcome the updraft
and precipitation begins. The
process of air and moisture movement results in the accumulation of
electrical charges in portions of the
cloud. The frozen upper layers
generally acquire a large positive
charge while the lower layers have
a small positive charge above a
much larger negatively charged
area nearest the earth. Most lightning occurs between the positive
and negative charges in the cloud,
but some is between the cloud and
the earth.
Touchdown to Earth
The maximum current measured
in lightning strokes is over 200,000
amperes. Based on a number of
records of lightning strokes to lines,
10 percent of the stroke currents
will exceed 75 kA crest and 50
percent will exceed 45 kA. Fifty-five
percent have at least two stroke
components while 17 percent have
at least six components with an
average of about 2-1/2 components
per stroke. The total flash duration,
including all components, averages
200 milliseconds or 12 power
frequency cycles. Almost 10 percent
exceed 500 milliseconds.
About 83 percent of the lightning
strokes to earth are of negative
polarity. The normal fair weather
voltage gradient at the earth's
surface ranges from 67-317 volts/
meter over land and 128 volts/
meter over oceans. It varies with
altitude and is usually assumed to
average 100 volts/meter. The field
gets weaker at high altitudes and is
very small at an altitude of 50
kilometers. The total potential
difference between the earth and
the atmosphere at that altitude is
about 400 kV.
The air is not a perfect insulator,
so there is a small current flowing
between the earth and the atmosphere of about 10 picoamperes per
square meter. The current density
varies substantially over the surface
of the earth, but the total current
between the earth and the upper
atmosphere is very nearly constant
at 1800 amperes. With the 400 kV
potential, the result is a power flow
of about 700 megawatts.
With such a large power flow, it
would appear that the negative
charge on the earth would soon be
discharged. It seems it should take
only about a half hour to discharge
the entire earth, but the charge is
maintained. The reverse current
flow that maintains this charge is
lightning.
There are about 40,000 thunderstorms per day all over the earth.
The peak lightning activity occurs
at 7 p.m. Greenwich mean time.
The best estimates on the number of
strokes are about 100 lightning
flashes per second worldwide.
The earth is normally negatively
charged with respect to the atmosphere. But as the thunderstorm
passes over the ground, the negative charge in the base of the cloud
induces a positive charge on the
ground below and for several miles
around the storm. The ground
charge follows the storm like an
electrical shadow, growing stronger
as the negative cloud charge
increases. The attraction between
positive and negative charges
makes positive ground current flow
up buildings, trees, and other
elevated objects in an effort to
establish a flow of current. But air,
which is a poor conductor of
electricity, insulates the cloud and
ground charges, preventing a flow
of current until huge electrical
charges are built up. Lightning
occurs when the difference between
the positive and negative charges—
the electrical potential—becomes
great enough to overcome the
resistance of the insulating air and
to force a conductive path for
current to flow between the two
charges. Electrical potential in these
cases can be as high as 100 million
volts. Lightning strokes proceed
from cloud to cloud, cloud to
ground, or where high structures
are involved from ground to cloud.'
A ground strike starts with a
pilot streamer accompanied by a
stepped leader moving toward
earth at a rate of 0.5 ft. per micro-
second. This stepped leader is only
a few hundred amps and is barely
visible. As the negatively charged
step leader nears earth, the positive
earth charge intensifies and sends
up a ground leader meeting the
step leader 20 to 30 yards above
ground. At this instant a return
stroke (what we see) is sent back
toward the cloud at approximately
100 ft./microsec. or 10 percent the
speed of light with a current of 5 to
200 thousand amperes creating a
channel between cloud and earth
with a temperature of 500,000
degrees Fahrenheit or more. This
rapid temperature increase causes
the air to expand very rapidly and
produces the thunder you hear.
Another stroke may follow this
channel to ground. Multiple strokes
are common, and they may appear
as one stroke to the human eye.
Where Lightning Strikes
Approximately 3 billion lightning strokes hit earth last year. The
industry maps lightning by thunderstorm days. The isokeraunic
map (Fig. 1) shows the thunderstorm days for the United States. A
generally accepted reference
benchmark is that an isokeraunic
level of 30 will result in one stroke
per mile of line per year. Lightning
stroke density quantifies the
number of strokes hitting the earth
per unit area per unit time.
Lightning invariably seeks the
easiest path between positive and
negative charge centers of the storm
area, even if such paths add substantial length to the strokes. When
lightning strikes a power line, a
zone extends to each side of the
actual stroke terminal where the
voltage may greatly exceed the
insulation level of the line, and
flashover to ground may occur
simultaneously. This results in
traveling waves generated in the
conductors on either side of the
flashover zone. The traveling wave
voltage is equal to the current
magnitude multiplied by the surge
impedance of the line (E = IZ = I
√L/C), and it is less than the
flashover voltage of the system
insulation. (See Fig. 2.) The surges
travel along the overhead line at
about 1,000 feet per microsecond
(the speed of light).
Surge Arresters
The lightning exposure of
distribution and station class surge
arresters is generally not the same.
Distribution class arresters are
typically exposed to higher average
currents, with about 50 percent over
1,200 amps and five percent over
9,000 amps. Station class arresters
are normally shielded from direct
strokes. Lightning overvoltages at
station entrances usually result
from transmission line traveling
waves. Thus, station arresters are
exposed to lower lightning curFigure 2
Insulation flashover and traveling
wave on a power line. Traveling wave
voltage is equal to the current magnitude multiplied by the surge impedance of the line.
rents, with 50 percent over 800 amps
and five percent over 4,000 amps.
(See Fig. 3.)
Lightning and Safety
When a thunderstorm threatens,
seek shelter inside a home, a large
building or an all-metal (not convertible) automobile. Do not use the
telephone except for emergencies. If
you are caught outside, do not stand
underneath a tall isolated tree or a
utility pole. Avoid projecting above
the surrounding landscape. For
example, don't stand on a hilltop. In
a forest, seek shelter in a low area
under a thick growth of small trees.
In open areas, go to a low place,
such as a ravine or valley.
Get off or away from open water,
tractors, and other metal farm
equipment or small metal vehicles,
such as motorcycles, bicycles, golf
carts, etc. Put down golf clubs and
take off golf shoes. Stay away from
wire fences, clotheslines, metal
pipes, and rails. If you are in a group
in the open, spread out, keeping
people several yards apart.
Remember, lightning may strike
some miles from the parent cloud.
Precautions should be taken even
Figure 3
Statistical data compare tower stroke
currents with station and distribution
arrester currents.
though the thunderstorm is not
directly overhead. If you are caught
in a level field or prairie far from
shelter and if you feel your hair
stand on end, lightning may be
about to strike you. Drop to your
knees and bend forward, putting
your hands on your knees. Do not lie
flat on the ground.
Over 200 people are killed each
year in the United States by lightning. One in five people struck by
lightning die. If struck, the lightning
generally moves around the body,
which acts as a lightning rod.
Persons struck by lightning receive a
severe electrical shock and may be
burned, but they carry no electrical
charge and can be handled safely.
Costly Interruptions
Hundreds of millions of dollars in
damage occur each year due to
lightning to utility equipment, with
additional damages or losses to end
users. A great concern to utilities
and their customers is interruptions.
Preventing these interruptions has
become a key issue to system
engineers.
One way to improve transmission
or distribution systems is with the
application of Protecta*Lite arresters.
Protecta*Lite arresters limit the
voltage which can occur across
insulators and thus reduce interruptions caused by lightning. They may
be applied as a supplement or as an
alternative to overhead shield wire.
Polymer-housed Protecta*Lite
arresters are selectively placed in
parallel with insulators on distribution or transmission lines.
Some of the many benefits
available through the use of
Protecta*Lite arresters are listed
below:
• Line compaction, line uprating,
improving outage performance and
improving line protection where
unbalanced insulation conditions
exist.
• Protecta*Lite systems are easy to
install on either existing or new
structures.
• Protecta*Lite eliminates or substantially reduces breaker operations
resulting in less maintenance and
improved service reliability.
• Protecta*Lite arresters can discharge thousands of strokes and
remain operative.
• Protecta*Lite arresters are available in combination with or without
an insulator for new and/or upgrade construction.
• Protecta*Lite incorporates a "no
lockout" feature in the unlikely event
of an arrester failure.
Lightning
Lightning is a constant threat to
utility quality and service. The way
to combat the severity of lightning is
to be as well prepared as possible.
Surge arresters, whether distribution, riser pole, station, intermediate
or Protecta*Lite, can help limit the
effects of lightning and improve
your reliability in delivering quality
power.
1Thunderstorms
and Lightning, U.S.
Department of Commerce, National
Oceanic and Atmospheric Administration, National Weather Service, June
1985.