Sonus PD Ultrasonic Detector7
Partial Discharge (PD) is an electrical discharge that does not completely bridge the space between two conducting
electrodes. The discharge may be in a gas lled void in a solid insulating material, in a gas bubble in a liquid insulator or
around an electrode in a gas. When partial discharge occurs in a gas, it is usually known as corona.
Partial discharge is generally accepted as the predominant cause of long term degradation and eventual failure of
electrical insulation. As a result, its measurement is standard as part of the factory testing of most types of high voltage
equipment. In-service monitoring of equipment for PD gives an advance warning of pending insulation failure. This allows
a plant operator to take remedial action during planned outages.
Partial discharge often occurs under normal working conditions, gradually deteriorating the dielectric until it can no longer
withstand the electrical stress and fails. Levels of PD activity will measure as insulating materials degrade. By detecting
this PD activity while the equipment is in operation, failure can be avoided.
A PD event will radiate energy in different forms and this energy can be picked up by the Sonus PD in order to detect the
source and locate it.
PD creates EM radiation that dissipates in all directions away from the source. Metal components, for instance the panels
around switchgear, will pick up this radiation and small voltages called Transient Earth Voltages (TEVs) are induced on
the surface. These very high frequency signals will be picked up by the Sonus PD and indicate that there is a PD source
nearby.
Partial Discharge also generates acoustic emissions energy across a wide band of frequencies. This acoustic energy can
be detected in the ultrasonic range when there is a ‘line of sight’ between the PD source and the detecting sensor. Sharp
points, for instance on air insulated cable terminations, are typical sources of corona that will produce acoustic emission.
Cast insulators are prone to Surface Tracking where electrical stress across the insulator’s surface causes discharge, and
deteriorates the insulator surface and creates carbon tracks. This can lead to ashover and failure of the equipment
2.1.2 Acoustic Emmissions (AE)
2 Partial Discharge
2.1 Radiated Energy
2.1.1 Electromagnetic Waves (EM)