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Fig. 4. Summary of records
a Percentage distribution curve for front times
valid for currents > 5 kA
b Percentage distribution curve for times above
half values valid for currents > 5 kA
c Risk curve for current amplitude
d "Correlation" between current amplitude and
front time
rotated with 50 r/sec. The flux was read with a
head of soft iron having two coils connected to a
vacuum tube voltmeter. The remagnetisation was
made by separate coils on the same head which,
supplied by AC of the proper phase shift, gave a
remagnetization equivalent to the one by DC and a
stationary link5. Fig. 3 c and d give some examples
of records. From the figures it can also be seen that
the magnetization curves of different magnetic bars
fall within a rather narrow band.
Results
The number of measuring points per season is
shown in table 1 while table 2 and fig. 4 gives a
summary of the results. The records obtained are
of course not too extensive but may anyhow give
some idea about the average and variation range
of the interesting parameters of the strokes.
The measurements have involved 1 125 station
years, which as shown in table 2, has altogether
given 68 records, but the largest part of these is
below some few kiloamperes. It is a reasonable
assumption that the low current values are not
direct strokes but induced instead. This is supported
by the fact that all current values of 3 kA and above
are negative, while the lower current values often
are positive and oscillating. As the main interest of
this investigation relates to direct strokes all current
values below 5 kA have therefore been deleted.
A different choice of the lower measuring limit —
which of course can be made rather arbitrarily —
would considerably have effected such statistical
parameters as the current average and its dispersion.
Therefore all statements about "the average
lightning current" etc. are quite meaningless without a
clear statement about the lower measuring range of
the measuring device. Results from lightning
research are instead preferably presented in the form
of risk curves, i.e. curves showing the risk for a
current exceeding a certain value to a certain
structure. An example of such a risk curve valid for the
present investigation is shown in fig. 4 c.
In fig. 4 a is shown the distribution curve for front
times valid for currents of 5 kA and above. The curve
is a reasonably normal logarithmic distribution with
a logarithmic average of 2.0 |xs and a standard
deviation of 2.9. As the front time is defined as the
steepness divided by the current amplitude it, of
course, leads to somewhat shorter fronts than a
curve based on the conventional definition. It is
interesting to note that the curve trends towards
longer front times if currents below 5 kA are
included. The times above half values are shown in
curve 4 b. The logarithmic average is about 60 |xs
which is somewhat longer than the 40—50 ^is
generally obtained. The difference can be explained by
the fact that the link is no precision device and
especially that the 100 [i,s coil, on which most
evaluations were actually based has a tendency to magnify
the duration.
Correlation factors
When discussing the severety of lightning currents
the correlation between different parameters of the
lightning current is of course a factor of vital
importance. From a practical point of view it would
be considerably more serous if a surge with high
current amplitude generally also had high current
steepness or long duration. To illustrate a possible
correlation fig. 4 d is drawn showing current
amplitude on x-axis and the front time on the y-axes. This
figure indicates that these factors may be regarded
as non-correlated and consequently the risk for a
current having certain front time may be calculated
directly from the fig. 4 a and 4 c without
application of a correlation factor.
References
1. Stromberg å: Experimental check of steepness indicators. TM
9231, 1959 (Internal Asea Report).
2. Schlomann R H, Johnson I B, Price W S, Anderson J G: 1956
Lightning field investigation on the ovcc 345 AV system.
Transactions Paper No. 57—1037.
3. Blomberg H: Måtstavar för bestämning av varaktigheten hos
stötströmmar i kraftledningar (Magnetic links for recording of the
duration of surges in power lines). TM 5454, 1947 (Internal Asea
Report).
4. Blomberg H: Måtstavar för bestämning av varaktigheten hos
stötströmmar i kraftledningar (Magnetic links for recording of the
duration of surges in power lines). TM 5473, 1947 (Internal Asea
Report).
5. Stromberg å: Mätslavar för bestämning av varaktigheten hos
stötströmmar i kraftledningar (Magnetic links for recording of the
duration of surges in power lines). TM 5676, 1950 (Internal Asea
Report).
ELTEKNIK 1959 1 117
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