Full resolution (JPEG) - On this page / på denna sida - 1958, H. 4 - An Insulated Cable for Heavy Power Transmission, by Bror Hansson
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Fig. 5. Cable model for test.
be replaced by new oil and now this rejuvenated
insulation will withstand not only the lower voltage,
but it is also found that for the overvoltage V volts,
the same time, t seconds, will again elapse before
gassing starts.
Oil overpressure thus increases the dielectric
strength as long as it is applied but oil flow through
the insulation will rejuvenate an already damaged
insulation.
Thus, we can summarize three good ways of
improving the dielectric strength of oil-impregnated
paper insulation.
1. Good cooling.
2. High oil pressure.
3. Continuous exchange of the oil for new oil.
The oil pressure and the oil flow, moreover,
increase the dielectric strength so that paper with
lower losses and lower s may be used.
Losses and cooling
As cables are designed, the losses occur just where
we should prefer not to have them.
The copper losses are located in the conductor —
in the centre of the cable surrounded by
impregnated paper — which is not only a good electrical but
also a good thermal insulator.
The dielectric losses also culminate near the
conductor, because due to the geometrical dimensions
we there have the highest dielectric stresses.
If by using so-called graded insulation we try to
lower the stresses near the conductor, we
unfortunately make things worse. Grading of the stresses is
accomplished by placing dense paper near the
conductor and more porous paper outwards, but both
the high e and the high tg <5 of dense paper increase
the losses, as illustrated by fig. 6, according to which
dense 70 jx paper has 30 % higher e and 70 % higher
tg<5 than 130 [.i paper and thus causes approximately
120 % higher watts/dm3.
If we want to make cables which can carry more
load (amps and volts) than present cables, it seems
that we must do everything possible not only to
remove the losses but also to reduce them. In the
Swedish 425 kV 900 mm2 cables (fig. 7 b) the
dielectric losses — taken as the bulk losses in the total
insulation — are half the copper losses. But the
specific dielectric losses seem extremely high near
the conductor. If this 425 kV cable was used for
500 kV, which is quite permissible, and on the occa-
Layer nr 1 2 3
Density ............ .... 1.15 1.0 0.7
tg<5 ................ . ... 0.33 0.28 0.19
. .. . 4.2 3.8 3.2
Fig. 6. Dielectric stress (a1} at) in kV/mm and specific
losses (b„ bt) in W/dm’ in a 425 kV cable. au bt
refer to a cable with homo>genous insulation, at, bt
to a cable with graded insulation. See table.
ELTEKNIK 1958 1 53
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