Why is the Direct-current (DC) testing important?

(1) Capacitance-charging current. Capacitance-charging current is the current resulting from the charge absorbed by the capacitor formed by the capacitance of the bushing. This current is a function of time which also depends on applied voltage, insulation resistance,  and the time constant of the circuit.

(2) Dielectric-absorption current. Dielectric-absorption current is the current that is absorbed and trapped in the dielectric during the charging of the piece of equipment and released after the equipment is discharged. This current can be calculated from test voltage, capacitance, and time. In air-cooled and hydrogen-cooled AC rotating machines, dielectric absorption varies with temperature.

(3) Surface leakage current. The passage of current over the surface of insulation rather than through its volume is surface leakage current. In measuring the insulation resistance of armature and field windings in rotating machines (rated 1 hp, 750 W or greater), a high surface leakage current is usually caused by moisture or another type of partly conductive contamination present in the rotating machine.

(4) Partial discharge (corona current). This is a type of localized discharge resulting from transient gaseous ionization in an insulation system when the voltage stress exceeds a critical value. Partial discharge is not desirable and should be eliminated. It is caused by the overstressing of air at sharp corners of the conductor due to high test voltage. In shielded power cable systems (rated > 5 kV), humidity or wind can increase the corona current.

(5) Volumetric leakage current. The current that flows through the volume insulation itself is a volumetric leakage current. It is the current that is of primary interest in the evaluation of the condition of the insulation. In shielded power cable systems (rated > 5 kV), humidity, condensation, and precipitation on the surface of termination can increase surface leakage current and volumetric leakage current. Surface leakage current and volumetric current added together is the quotient of the applied voltage and insulation resistance. In large AC rotating machines rated 10,000 kVA or greater and rated 6000V or higher, moisture, on the end windings, increases surface leakage current and volumetric leakage current especially when dirt is also on the winding. Since the insulation may weaken over time, age may cause an increase in the various currents during testing.