What Is Keraunic Level?
Keraunic level is defined as the
average annual number of thunderstorm days or hours for a given
locality. A daily keraunic level is called a thunderstorm-day and is
the average number of days per year on which thunder will be heard
during a 24-h period.
By this definition, it makes no
difference how many times thunder is heard during a 24-h period. In
other words, if thunder is heard on any one day more than one time,
the day is still classified as one thunder-day (or thunderstorm day).
The average annual keraunic level for
locations in the U.S. can be determined by referring to isokeraunic
maps on which lines of equal keraunic level are plotted on a map of
the country.
What Is Ground Flash Density?
Ground flash density (GFD) is defined
as the average number of strokes per unit area per unit time at a
particular location. It is usually assumed that the GFD to earth, a
substation, or a transmission or distribution line is roughly
proportional to the keraunic level at the locality. If thunderstorm
days are to be used as a basis, it is suggested that the following
equation be used (Anderson, 1987):
Nk = 0.12Td
or
Nm = 0.31Td
where
Nk is the number of flashes to earth
per square kilometer per year
Nm is the number of flashes to earth
per square mile per year
Td is the average annual keraunic
level, thunderstorm days
Lightning Detection Networks
A new technology is now being deployed
in Canada and the U.S. that promises to provide more accurate
information about ground flash density and lightning stroke
characteristics. Mapping of lightning flashes to the earth has been
in progress for over a decade in Europe, Africa, Australia, and Asia.
Now a network of direction-finding
receiving stations has been installed across Canada and the U.S. By
means of triangulation among the stations, and with computer
processing of signals, it is possible to pinpoint the location of
each lightning discharge.
Hundreds of millions of strokes have
been detected and plotted to date. Ground flash density maps have
already been prepared from this data, but with the variability in
frequency and paths taken by thunderstorms from year to year, it will
take a number of years to develop data that is statistically
significant. Some electric utilities are, however, taking advantage
of this technology to detect the approach of thunderstorms and to
plot the location of strikes on their system. This information is
very useful for dispatching crews to trouble spots and can result in
shorter outages that result from lightning strikes.
how does this parameter affect the design of earthing system???
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