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.

Related post

No comments: