Full resolution (JPEG) - On this page / på denna sida - 1958, H. 7 - Disturbances in the Swedish Electric Power System in Association with Magnetic Storms, by Per Olof Persson
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from the network with a consequent reactive power
transport as the result. The increased demand for excitation
current in a power transformer with direct current
excitation may be seen from the figure and provides an
explanation of the voltage variations and changes in
reactive power, as well as the zero sequence currents which
are set up by the harmonics of the excitation current. In
view of the fact that an excitation current is set up which
is not of sinusoidal form, harmonics will be obtained with
direct current excitation, and the third harmonic in
particular will predominate. The latter, in addition to
circulating in the transformer’s delta windings where such arc
available, and also circulating in the power network along
the lines, will give rise to a zero sequence current in
a summation-connected current transformer, which may
actuate the earth fault protection and cause
disconnection. This lias actually happened in the reported
disturbance that occurred in February this year.
The differential relay of transformer protection may also
be actuated by the excitation current. With a strong direct
current excitation of the transformer and the consequent
heavy demand for its excitation, the excitation currents
from outside the network may be of such an order of
magnitude that the functioning value of the differential
relays is exceeded and disconnection takes place.
As mentioned under the heading Theory, it may be
assumed that the earth current I induced in the earth’s
crust by atmospheric currents will cause a drop of
potential in the ground which is of such duration with respect
to size and direction that the currents set up may be
regarded as direct currents. With regard to the direct
current excitation of the transformer, this is dependent
not only upon the magnitude of the circulating direct
current but also upon the direction in which this current
flows through the windings. Both the direction and
magnitude are determined by the earthed stations’ potential in
the earth’s crust and Ity the resistances of the lines.
Fig. 2. Excitation current of power transformers during
normal service and with a superimposed direct
current. En = impressed voltage, i0 = excitation
current, a) Normal service without direct current
excitation, b) Normal service with direct current
excitation.
Depending upon the distribution and magnitude of these
direct currents, which can probably be verified
theoretically by model measurements, the conditions may be
such that the fluxes produced by the direct current in
the transformer windings may reinforce or weaken each
other at different points in the network. Apart from the
case in which the direct current is weak or entirely absent,
this may explain why disconnection only took place
through the transformers’ differential protection in a few
instances.
The fact that the disconnections mainly occurred in the
380 and 200 kV network may be a mere coincidence, but
the low line resistances in the 380 and 200 kV lines without
series capacitors certainly contributed to the production
of large direct currents and thus increased the risk of
unwarranted disconnections.
Summary
Disturbances in t lie Swedish heavy current system due
to magnetic storms have not been observed previously.
Since the disturbing effect of these storms is dependent
upon their intensity it may be readily assumed that the
risk of the repetition of a disturbance recently caused by
a magnetic storm is greatest during periods of high solar
activity, as it is generally believed that the magnetic storms
are the result of eruptions on the sun which usually occur
in connection with the sunspot maximum3,4. These
eruptions take place approximately every eleventh year and
will have produced their maximum disturbing effect during
the past year and also in the immediate future.
In view of the fact that the curve for the sunspot
maximum is based on purely statistical material it must be
assumed, however, that appreciable disturbances may also
lake place in the intervening periods, and this actually
occurred in the U.S.A. in 1940, namely, directly between two
sunspot maxima. It should likewise be borne in mind that
in addition to the part of the sun’s surface at which the
eruptions occur (maximum effect from the sun’s central
and western parts), the disturbing effect may also be
produced by other phenomena which are not periodic
(corona, etc.).
In the case of the present disturbance the sun was highly
active, but, according to the information available, no
specially violent outburst on the sun’s surface had been
observed, which might explain the magnetic disturbance.
More violent solar eruptions have been observed on other
occasions without any disturbance being noted
(approximately one day later) of the same magnitude as was the case
during the night between the 10th and 11th of February.
If this disturbance had taken place during the daytime
it would have resulted in a total breakdown of the
Swedish heavy current system. Since magnetic storms are
equally likely to occur during the daytime as at night,
disturbances of this nature might have serious consequences
for the operation of the Swedish power transmission
system.
No steps can be taken at the present time to prevent a
recurrence of the disturbances in the form of the
disconnection of the transformer- and line-protection due
to the effect of magnetic storms.
It may be desirable, however, to investigate the
possibilities for recording the direct currents set up in the power
system during magnetic disturbances, by the installation
of recording instruments at one or more points in the
network.
References
1. Chapman and Bartfj.s: Geomagnetism. Clarendon Press, Oxford
1040.
2. Davtdson W E: Sun-Spot Disturbances of Terrestrial Magne
tism. Electrical Engineering, Febr. 1911 p. 72—75.
3. Öhman Y: Aktuella frågor inom solforskningen. Stockholm 1048
4. Bi.ock L: Model Experiments on Au ro råe and Magnetic Storms
Tellus vol. 7, Stockholm 1955.
1 94 ELTEKN I K 1958
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