Partial Discharge Test
Partial Discharge usually begins within:
• Inclusions within a solid dielectric
• At conductor-dielectric interfaces
• Or Bubbles within dielectrics
Since discharges are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes.
PD can also occur along the boundary between different insulating materials. Partial discharges within an insulating material are usually initiated within gas-filled voids within the dielectric.
When partial discharge is initiated:
• High frequency transient current pulses will appear and persist for nano-seconds to a micro-second, then disappear and reappear repeatedly.
• PD currents are difficult to measure because of their small magnitude and short duration.
• The event may be detected as a very small change in the current drawn by the sample under test.
• One method of measuring these currents is to put a small current-measuring resistor in series with the sample and then view the generated voltage on an oscilloscope via a matched coaxial cable.
• When PD occurs, electromagnetic waves propagate away from the discharge site in all directions. Detection of the high-frequency pulses can identify the existence and location of partial discharge.