Dielectric test ASTM D-1816 (VDE electrode): The present ASTM D-877 gap
consists of 1 in. diameter disk, square-edged electrodes spaced at 0.1 in.
The use of this test gap results in a uniform electrostatic fi eld at the center line
of the test disks and a highly nonuniform fi eld at the edges of the disk.
To attain uniform fi eld strength at all points, spherical electrodes would have
to be used. Between these extremes of a highly distorted fi eld and an ideal uniform
fi eld, a third gap confi guration, designated as VDE, has been used. The
VDE gap specifi cations call for a sector diameter of 36 mm and a 25 mm radius
of curvature for the spherically capped electrodes. A gap of about 0.08 in.
between electrodes has been found to give about the same breakdown voltage
relationships in the 25–30 kV range as the ASTM D-877 confi guration.
Tests have shown the following:
VDE confi guration depicts more accurately the average electric strength
and scatter of the oil as the transformer sees it
VDE gap is relatively sensitive to oil quality
ASTM D-877 is less sensitive
Point electrodes are almost completely insensitive to oil quality
The VDE cell, in which a quart of oil is tested between VDE electrodes, while
being mildly circulated, realistically measures changes in oil strength, which
determine the electrical strength of typical transformer construction. This
test method (ASTM D-1816) is similar to ASTM D-877. The procedure for the
VDE (ASTM D-1816) test is the same as for the disk electrodes (ASTM D-877).
4.2.2.2 Acidity Test
New transformer liquids contain practically no acids if properly refi ned.
The acidity test measures the content of acids formed by oxidation. The acids
are directly responsible for sludge formation. These acids precipitate out, as
their concentration increases, and become sludge. They also react with metals
to form another form of sludge in the transformer. The ASTM D974 and D664
are laboratory tests whereas D1534 is a fi eld test which determines the
approximate total acid value of the oil.
The acid number of the neutralization number is the milligrams (mg) of
potassium hydroxide (KOH) required to neutralize the acid contained in 1 g
of transformer liquid. Test data indicate that the acidity is proportional to
the amount of oxygen absorbed by the liquid. Therefore, different transformers
would take different periods of time before sledge would begin to
appear. Transformers