Full resolution (JPEG) - On this page / på denna sida - 1958, H. 6 - Up-to-date Criteria in the Construction of Equipment for High Voltage Power Systems According to the Experience of the 400 kV System in Sweden, by Gunnar Jancke - Induced Lightning Over-voltages, by SG
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Fig. 10. Specific transmission costs as a function of the
power transmitted per line. Optimum series
capacitor compensation is indicated.
1. WO kV line with 2X593 mm2 ACSR conductors.
2. 400 kV line with 3 X 593 mm1 ACSR conductors.
3. 500 kV line with 3 X 593 mm2 ACSR conductors.
4. 650 kV line with 4X593 mm2 ACSR conductors.
circuit-breaker tests. Otherwise the apparatus has
functioned faultlessly.
The Network of the Future
Notwithstanding the fact that we are entirely
satisfied with the 400 kV network, we have recently
undertaken exhaustive studies regarding the choice
of voltage for future installations. The reason for
this is that a revised assessment of Sweden’s water
power resources has disclosed appreciably higher
values than those with which we reckoned in 1946.
By about 1980 it will be necessary for us to transmit
45 X 109 kWh/year or 8 000 MW over an average
distance of 800 km. High voltage technique has made
considerable advances since 1946.
We investigated the alternatives for the continued
extension of the 400 kV network, the reconstruction
of the existing network for 500 kV and continuing
to build for this voltage, the superposing of a new
650 kV network, and finally, high voltage direct
current.
Direct current was unable to compete on account
of the costs for interconnection with the existing
network. The results shown in fig. 10 were obtained
for the alternating current alternatives. They apply
to a fully extended network, and the conditions will
be less favourable for the higher voltages during a
part of the building period. The gain resulting from
raising the voltage to 650 kV is so small therefore,
that it does not justify the associated complication
of the network and the risks. We have decided to
proceed with 400 kV and employ three 593 mm2
conductors per phase for all trunk lines. Since such
lines can be operated at 500 kV from the point of
view of radio interference and the insulation
requirements fall in step with the increase in the short
circuit power, it is possible that certain lines will
be operated at 500 kV in the future. They will then
be equipped with auto-connected transformers which
will be connected up as a part of the lines.
Although our studies have not led to any decisive
change in constructional policy they have
nevertheless proved very valuable inasmuch as they have
materially increased our knowledge of the subject.
Whereas we were formerly working at the margin
of the latter, we are now of the opinion that we
possess a relatively comprehensive knowledge of
the problems involved and the effect of different
parameters. As a consequence of this, we are now
in a position to carry out further extensions of the
system more cheaply and with greater operating
reliability and, especially, to come closer to the
zeal, which is to produce an optimum overall
solution of the transmission problem, where each
integrating part is matched for the best total economy
and reliability.
Induced Lightning Over-voltages
In order to gain Doctor’s degree of technology
civilingenjör Sune Rusck defended on March, 27th, 1958 at Kungl.
Tekniska Högskolan in Stockholm a dissertation entitled
"Induced lightning over-voltages on power-transmission
lines with special reference to the over-voltage protection
of low-voltage networks".
Earlier methods of calculating induced over-voltages are
according to the author not satisfactorily solved as the
vector potential of the inducing field has been neglected.
The author has therefore developed a new theory of
induced voltages on a long insidated multi-phase
power-transmission line. This new theory can be applied with an
arbitrarily varying field and shows that the induced
voltage is dependent on the magnitude of the time derivative
of the inducing field and proportional to the height of the
conductors, but otherwise independent of the design of
the line if all the conductors are insulated from the earth.
If one conductor is earthed, the voltage of the other ones
will be reduced with 25—50 per cent.
As it is only the lightning strokes to the ground which have
a rapidly varying field, it is only this kind of lightning
discharge which can cause high induced voltages. With
the present knowledge of the mechanism of the lightning
stroke as a starting point, the inducing field has been
calculated. These expressions have then been substituted in the
previously deduced equations. It is found that the induced
voltage obtains its maximum value at that point in the
line which is nearest to the lightning stroke. The front
of the induced voltage may be represented by a voltage
linearly rising to the maximum value during a time a
little longer than the front-time of the lightning current.
The front-time has no great influence on the magnitude
of the induced voltage on a long line except in the case
of lightning strokes very close to the line. The attenuation,
which is fairly heavy, as well as the influence of
discontinuities of the line have been studied. The curve showing
the frequency of occurrence ol the induced voltages has
been calculated. The induced voltages are of no importance
in the case of lines insulated for a system voltage higher
than 70 kV. On the other hand, the majority of the
lightning faults on lines for 20 kV and lower are caused
by the induced voltages.
With the induced voltage on a long insulated line as a
starting point, the conditions in a low-voltage network
have been studied taking into account the protective effect
of lightning arresters. In a network without lightning
arresters about ten spark-overs per installation and year
can be expected. If lightning arresters are installed at a
distance of 300—(500 in from each other, the sparkovers
will be reduced to a tenth of the value without arresters.
Where it has been possible, the theoretical results have
been compared with measuring results and operating
experiences. The agreement is satisfactory. SG
1 92 ELTEKN I K 1958
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