2002 - 2003 - Fachgebiet Hochspannungstechnik

- 8 -
3.2 Berichte
Dipl.-Ing. B. Dolata
Tests on ester fluid and mineral oil under impulse- and AC voltage stress
Homogeneous field
To investigate the behaviour of both fluids in
homogeneous electrical fields at gap spacings
larger than 2,5 mm some breakdown tests have
been performed with a sphere - sphere electrode
configuration. Two sphere shaped electrodes of 100
mm diameter have been positioned above each
other in a sealed vessel, where one of the spheres
was grounded and not adjustable positioned at the
bottom and the other sphere, to which high voltage
was applied, could be fixed in a distance between 0
mm and 80 mm. Figure 1 shows a photograph of
the test vessel. According to the aim to keep the
fluids dry the vessel could not be opened during the
whole test, thus there was the demand to make the
surface of the electrodes very resistant against
abrasion by arcing. As a result of this, on both
spheres a segment of 60 mm diameter was replaced
by an alloy of tungsten (80 %) and copper (20 %).
In the arrangement of the spheres the alloy parts
were positioned on both sides of the gap between
the electrodes to force the arc of the breakdown to
Figure 1
Test vessel used for the breakdown
experiments at homogeneous
electrical field
occur between the mechanically strong
sections of the electrodes. A schematic view
of the electrodes is depicted in Figure 2.
Due to the avoidance of vessel opening the
water content of the examined mineral oil
only raised during the whole time of the
tests from 8,1 ppm to 12,6 ppm while the
water content of the ester liquid has been
32 ppm at the end of the investigations.
Impulse voltage tests
Figure 2 Schematic view of the electrodes
After the vessel was filled by evacuating the
air out and sucking dried liquid in, the
arrangement was exposed to impulse and
AC voltages. To reduce the influence of
energy to the fluid, the impulse voltage tests
have been performed in advance to the
breakdown tests with rising 50 Hz AC
voltage at a rise speed of 2 kV per second.