INSULATION RESISTANCE AND DIELECTRIC ABSORPTION
TESTS
5. Definitions.
- The insulation resistance
of an insulation is defined as the resistance
(in megohms) offered by the insulation to an
impressed direct voltage. The resulting
current is called insulation current and
consists of two main components.
a. The current which flows within the
volume of the insulation, and is
composed of:
(1) Capacitance current
(2) Dielectric absorption current
(3) Irreversible conduction current
b. The current which flows in the leakage
paths over the surface of the insulation,
and is termed leakage current.
6. Theory of Insulation Resistance and
Dielectric Absorption Measurement.-
When a dc voltage from a high-voltage,
dc insulation test instrument is suddenly
applied to insulation, the insulation current
will start at a high value, gradually de-
crease with time, and finally level off to a
stable value. The low initial insulation
resistance is partly caused by the high initial
capacitance charging current. This
capacitance current rapidly decreases to a
negligible value (usually within 15 sec.) as
the insulation becomes charged. The low
initial insulation resistance is also partly
caused by the high initial dielectric absoption
current. This current also decreased with
time, but more gradually, requiring from 10
minutes to several hours to decay to a neg-
ligible value. However, for the pupose of
insulation resistance meter tests, the change
in dielectric absorpiton current after 10
minutes can be disregarded. The leakage
current does not change with time of voltage
application, and this current is the primary
factor on which insulation quality may be
judged. Insulation resistance varies directly
as the thickness and inversely as the area of
the insulation being tested. A curve plotted
between insulation current and time (or
insulation resistance and time) is known as a
dielectric absorption curve.
7. Skill Required in Making Insulation
Resistance and Dielectric Absorption
Measurements.- Unless made with a high
degree of skill, insulation resistance and
dielectric absorption measurements will show
major fluctuations due to variations of the
several factors discussed in the following
paragraphs, and will consequently be of little
value. Each factor causes large errors in the
measurement of the insulation resistance and
these errors may not be chargeable to
inaccuracy of the measuring instrument. This
discussion applies primarily to generator
stator winding insulation, but also applies
generally to the insulation of all rotating
electrical machinery, cables, transformers,
and other equipment except porcelain in-
sulators, lightning arresters, and bushings, on
which a favorable insulation resistance
reading cannot be accepted as indicating
good condition. The condition of bushings is
best determined by the power- factor method.
Field windings and other Iow-voltage
insulation should preferably be tested with a
voltage source not higher than 500 volts, to
avoid damage to the insulation. Before rotor
fields are tested, the brushes should be
raised and the slip-ring insulation carefully
cleaned with a good solvent. The rotor field
insulation resistance is as important as the
stator insulation resistance.
8. Effect of Previous Charge.- One
factor affecting insulation and dielectric
absorption measurements is the presence of
a previous charge on the insulation. The
charge may come from normal operation of a
generator with an ungrounded neutral, or from
a previous measurement of insulation
resistance. Time may be saved if the
generator winding is kept grounded until the
test on the winding is made.
9. Effect of Temperature.- Insulation re-
sistance varies inversely with temperature for
most insulting materials. To properly compare
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