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Disturbances in the Swedish Electric Power System in
Association with Magnetic Storms

Per Olof Persson, Divisional Chief,
Swedish State Power Board, Stockholm

621.3.051.025.088 : 551.594.5

General Survey

In I lie course of llie night between the 10th and 11th of
February 1958 a number of disconnections of the 380/200
kV transformers and 200 kV transmission lines occurred
1’or which there was no apparent justification. None of
these disconnections could be traced to direct faults in the
installations or the lines. Since violent magnetic storms
were reported to have occurred at the same time, however,
there was reason to assume that the disturbances in the
power system were due to these storms, and a
confirmation of this was found in the exact coincidence in time
between the magnetic disturbances measured and the
disconnections.

Theory

The manner in which the eartli fault currents are set
up on the occurrence of magnetic disturbances cannot
be explained with any certainty. The observations and
measurements that have been made tend to show,
however, that induction in the ordinary meaning of the term
does not take place in the lines, but that a potential
gradient is formed in the earth’s crust, whereupon a current
is set up in the manner indicated in fig. 1. It seems most
plausible that this current is not induced in the loop
power-line-earth, since it has been observed that an
increase in the resistivity of the earth’s crust is accompanied
by heavier currents in the line.

The current I is probably an induced current set up in
the earth’s crust caused by change in the space charges
(ionic currents due to eruptions on the sun, changes in
the earth’s magnetic field). It is not known how the space
currents are located, but according to some theories1
amongst others, it may be supposed that there are current
paths in the space of the outer atmosphere surrounding
the earth, which are symmetrical around the earth’s axis.
But it is also possible that there are unsymmetrical current
paths over the earth’s poles with a maximum intensity in
the southern and northern auroral zones, for which some
evidence is provided in the latter case by the intense
ionization of the air of the inner atmosphere on the
occasion of an aurora. Measurements carried out by the
State Railway Board have shown in every instance that
the potential gradients formed during magnetic
disturbances in Sweden are greater further to the north, which
might be due to the higher resistivity of the ground, but
probably is associated here also with the vicinity of I lie
magnetic pole. The results of these measurements have
shown that both in an cast-west and a north-south
direction, or vice versa, a potential gradient is present during

Fig. 1. Principle for current set-up in power line by
potential gradient in the earth’s crust.

t lie magnetic storms. During the most recent disturbance
it was confirmed that the north-south gradient was
somewhat greater than the east-west one.

From the measurements made during the most recent
disturbance it may be seen that the disturbing voltage
changes its direction with a varying periodicity for some
minutes. The measurements were undertaken on a
Södertälje—Stockholm cable connection earthed at one end and
with a recording instrument installed at the other end.
The maximum amplitude was nearly 100 V and it occurred
at about 3.00 a. m. This corresponds to a voltage
gradient of about 3 V per km. Higher values were observed
however. Values exceeding 7 V per km were measured
during the last magnetic storm. Similar values of the same
order of magnitude (10 V per mile) have been measured
in U.S.A. This would correspond to a current in a 200 kV
power line of about 50 to 200 A/phase at the maximum
disturbing effect, which appears to be of the correct order
of magnitude, since according to the American
measurements on one occasion during magnetic storms, direct
currents of the order of 50—100 A were measured at the
neutral points of earthed transformers. With respect to
the direction and magnitude of the gradient, the variation
in the latter may be explained by the fact that the current
spreads out in the earth’s crust, and owing to changes in
the earth’s resistance the gradient may occur locally in
different directions at separate places.

Summing up, therefore, it may be said that during
terrestrial magnetic disturbances potential gradients may be
formed in the earth’s crust which are of such an order of
magnitude that, when certain conditions are present
(power line with earthed terminals), slowly varying direct
currents may be set up in the power lines — but not in
power lines with series capacitors — and in the earth
conductors at transformer neutral points. Below the effect
is discussed which these direct currents are exercising on
I lie transmission system.

The Effect of a Superimposed Direct Current

on the 50-cycle Alternating Current System

By way of comparison it may be mentioned that during
the magnetic disturbances which occured in 1940 in the
American power transmission system2 the following
observations were made:

1. Large reactive power transports and voltage changes
in the power network.

2. The current which was registered on one occasion in
a transformer neutral conductor had a wave form which
is obtained when a power transformer operates with a
superimposed direct current.

Any changes occurring in the properties of the instrument
transformers are of secondary importance (verified by
measurements) since they operate with a low saturation of
the iron, which is not the case, however, in power
transformers. The occurrence of unbalance in the transformers’
circuits and the unwarranted zero sequence currents must
therefore probably be due to the flux saturation in the
power transformers by direct current excitation. As may
be seen from fig. 2 the impressed voltage will give rise
to an excitation current with a large harmonic content.
Greater excitation power is required and this is drawn

ELTEKNIK 1958 1 1 9

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