Full resolution (JPEG) - On this page / på denna sida - 1958, H. 4 - An Insulated Cable for Heavy Power Transmission, by Bror Hansson
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
Tyessu.-e
yo/ref
Fig. 1. Diagram uf oil flow in new cable.
voltage. Before this additional heat is dissipated, the
temperature may have increased to such an extent
that lower fractions of the oil evaporate or the heat
may help the dielectric stress to split the oil
molecules. Then hydrogen is generated. Ionization starts
in the gas bubbles and may, even when the
over-voltage has disappeared, be maintained by the rated
voltage. More heat, more gas and final destruction
(breakdown) is the result.
It is recognized that a high ionization point of a
cable is a good criterion of its quality. For perfectly
impregnated "oil filled" designs the classical opinion
that the ionization point is a threshold value on a
tg<5-voltage curve is, however, not quite true. The
ionization voltage is dependant on the time of
application of the test (measuring) voltage. It indicates
that the test voltage is so high and has been applied
during a time long enough to produce so much gas
in faulty spots that our testing instrument, which
measures the bulk losses of the dielectric, may detect
the losses in the ionized gas bubbles.
These opinions have been proved by many years’
successful service of Liljeholmen’s (Asea’s)
foil-cooled pressure capacitors and cables for 380 and
425 kV, the design of which is founded on the same
working hypothesis.
The three principles of design of the new cable are:
1. An oil pump forces the insulating oil axially
through the inner channel of the conductor at an
elevated pressure, thus cooling the oil and giving
to it a high insulating strength, which is
dependant upon low temperature and high oil pressure.
2. The pump also forces the oil radially through
the insulation which is cooled by the oil. At the
same time gas which may have been liberated at
a time of overvoltage or overtemperature is
removed by the oil.
3. On its return to the pressure pump the oil is
sprayed into a vacuum chamber and thus relieved
from the gases. This vacuum chamber also serves
as a cheap expansion vessel for the oil.
It is believed that degasifying is the only
regeneration needed for overstressed oil, but treatment
with fullers earth, active carbon or the like may
readily be arranged in the return oil pipe line.
The means for carrying these principles into
practice are as follows:
1. An oil pump which creates an overpressure, and
2. forces the oil through the cable, axially through
the channel in the conductor and radially through
the insulation,
3. throttle valves by means of which the oil pressure
in different parts of the cable may be regulated,
and
4. the vacuum vessel in which the oil is degasified
and stored.
5. Alternatively, the oil may also be passed through
regenerative filters before the vacuum treatment.
The advantages of this cable are good cooling,
high oil pressure and therefore high dielectric
strength and also low dielectric losses, or in other
words, this cable may carry high current at high
voltage.
The cooling, the oil pressure and the oil
regeneration all increase the dielectric strength of the
impregnated paper. This permits the use of more porous
paper, which has a lower dielectric strength (short
time) but which also has a lower s and lower tg<5.
Both help to decrease the losses and the working
temperature of the cable and a low f also decreases
the charging current.
The disadvantages of the cable are moving parts,
the oil pumps and a vacuum pump, and the power
they consume. Economically, the costs for the pumps
and their energy are compensated by the cheap
expansion vessel and reduced dielectric losses, but
moving parts in a cable installation are only
acceptable for cables of great importance e.g. where the
current or the voltage or the length of the cable are
large.
It should perhaps already be mentioned here that
when once the cable installation is degasified, as is
always done before a cable is taken into service, the
vacuum pump should only stand by for an emer-
ELTEKNIK 1958 1 50
<< prev. page << föreg. sida << >> nästa sida >> next page >>
