When voltage is applied to the insulation, a current is
established consisting of a charging current (IC) and an in-phase component
current (IR). The charging current leads the in-phase component current by 90°.
The vector sum of the charging current and the in-phase
component current is the total current (IT) drawn by the insulation specimen.
The in-phase component current is also referred to as the resistive current,
loss current, or conduction current. The ideal insulation (ideal capacitor)
behaves somewhat differently under the application of DC versus AC voltages
which are discussed below.
DC Voltage Tests
When a DC voltage is applied to the insulation, a large
current is drawn at the beginning to provide the charging energy, however, this
current decreases to a minimum level over time. The minimum current is due to
continuous leakage or watt loss through the insulation.
The energy required to charge an insulation is known as the
dielectric absorption phenomenon. In actual practice, the losses from
dielectric absorption (i.e., the absorption current) are much higher than the
continuous leakage losses.
In the case of DC voltage testing, the effect of dielectric
absorption becomes minimum over time and therefore measurements of continuous
leakage current can be made. Dielectric absorption losses are very sensitive to
changes in moisture content of an insulation, as well as the presence of other
contaminants.
Small increases in moisture content of an insulation cause a
large increase in dielectric absorption. The fact that dielectric losses are
due to dielectric absorption makes the dielectric loss, PF, or DF test a very
sensitive test for detecting moisture in the insulation.
When a DC voltage is applied to an insulation, the total
current drawn by the insulation is comprised of capacitance charging current,
dielectric absorption current, and continuous leakage currents.
AC Voltage Tests
In the case of AC voltage application to an insulation, a
large current is drawn which remains constant as the AC current alternately
charges and discharges the insulation. The effect of dielectric absorption currents
remains high because the dielectric field never becomes fully established due
to the polarity of the current reversing each half cycle.
When an AC voltage is applied to an insulation, the currents
drawn by the insulation are due to capacitance charging, dielectric absorption,
continuous leakage current, and corona which are discussed below:
Capacitance Charging Current: In the case of AC
voltage, this current is constant and is a function of voltage, the dielectric
constant of the insulating material, and the geometry of the insulation.
Dielectric Absorption Current: When an electric fi
eld is set up across an insulation, the dipole molecules try to align with the
field. Since the molecules in an AC field are continually reversing and never
fully align, the energy required is a function of material, contamination,
(such as water), and electrical frequency. It is not a function of time.
Leakage Current (conductivity): All insulation
materials will conduct some current. If voltage is increased beyond a certain
level, electrons will be knocked off of molecules causing current to pass
through the insulation.
This is a function of the material, contamination
(especially water), and temperature. Excessive conductivity will generate heat
causing the insulation to cascade into failure.
Corona (ionization current): Corona is the process by
which neutral molecules of air disassociate to form positively and negatively
charged ions. This occurs due to over stressing of an air void in the
insulation.
Air voids in oil or solid insulations may be due to
deterioration from heat or physical stress, poor manufacture, faulty
installation, or improper operation. Corona breaks down the air into ozone
which, in combination with water, forms nitrous acid.
The ionized air bombards the surrounding insulation and
causes heat. The combination of these conditions will result in deterioration
of the insulation and carbon tracking. Corona losses increase exponentially as
voltage increases.
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