Megger DELTA 4000, Reference Manual

Page: 37 / 48

ZM-AH02E DELTA 4000 37
3 TESTING POWER SYSTEM COMPONENTS
Liquids
Test procedure
To measure the dissipation factor of insulating liquids, a
special test cell such as the Megger Catalog No. 670511 Oil
Test Cell is required. It is constructed with electrodes which
form the plates of a capacitor and the liquid constitutes the
dielectric. The test cell is a three-terminal type with a guard
electrode to avoid measuring fringe effects and the insula-
tion for the electrode supports.
When samples of insulating liquid are tested, the specimen
capacitance may also be used for etermining the dielectric
constant (permittivity) of the insulating liquid. The ratio of
the test cell capacitance measured when filled (liquid dielec -
tric) to the test cell capacitance measured when empty (air
dielectric) is the value of dielectric constant of the liquid.
Miscellaneous assemblies and
components
When an apparatus is dismantled to locate internal trouble
and make repairs, dissipation factor measurements can be
valuable in detecting damaged areas of insulation to such
parts as wood or fiberglass lift-rods, guides or support
members. Sometimes existing metal parts can be used as
the electrodes between which measurements can be made.
Sometimes it will be necessary to provide electrodes. Con-
ductive collars, can be used; aluminum foil also works well.
Whenever conducting material is used, ensure that intimate
contact is made with the critical areas of the insulation. Pe-
troleum jelly or Dow Corning #4 insulating grease applied
at the interface surface often helps to obtain better physical
contact.
It may sometimes be necessary to separate volume losses
from surface losses by providing a third (guard) terminal
on or within the specimen insulation system. For example,
an insulating tube formed over a metal rod may be tested
for internal damage in the insulation. A conductive band
(or foil) is applied near the center of the insulating tube
with additional conductive (guard) bands on each side,
separated from the center band by enough clean insulation
to withstand the intended test voltage. With the metal rod
grounded, the test set will measure the capacitance and
dissipation factor of the volume of insulation between the
center conductive band (high-voltage) and the metal rod.
Figure 13 shows a typical test setup.
Comparisons between dissipation factors of suspected
areas and components against similar parts which can be
assumed to be in good condition are of prime importance
in analyzing insulation components. Dissipation factor volt-
age measurements can indicate the presence of ionization
in a component by a sudden tip-up of dissipation factor as
the test voltage is increased. Delaminations within a mate-
rial can also be detected in this way. Avoid overstressing
component insulation by indiscriminate use of the available
test voltage. Consider the voltage on the component under
normal operating conditions.
Figure 13: GST test with guarding on insulated tube cover-
ing metal rod