What is Partial Discharge?
Partial Discharge (PD) is a localized electrical discharge that only partially bridges the insulation between areas of different electrical potential, such as a High Voltage conductor and the ground plane. PD in general is a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. It can be initiated by voids, cracks, or inclusions within a solid dielectric, at interfaces within solid or liquid dielectrics, in bubbles within liquid dielectrics, or along the boundary of different insulation materials. PD can cause progressive and irreversible damage to liquid and solid insulation systems. With time, PD activity becomes more intense and dangerous. The process of deterioration can propagate and develop until the insulation is unable to withstand the electrical stress, leading to a flashover. The term “corona” is often used interchangeably with PD, however corona is better known as a type of PD, where discharges occur and dissipate in air.
Why Measure PD?
PD measurement is a reliable and non-intrusive method that can be used anytime to
diagnose the insulation condition of an electrical asset. Compared with other dielectric diagnostic methods, PD measurement provides you with very sensitive information to help you effectively detect, localize and assess weak points in the insulation system.
PD activity can pose different risk levels to an electrical asset. For instance, internal PD in the dielectric material of XLPE cable systems can lead to rapid failure. On the other hand, PD in self-healing insulations such as air, tends to have a much more gradual effect on
surrounding insulating surfaces. The fundamental properties of discharge and insulation types, coupled with accurate PD measurement and analysis can help engineers,
operators and asset managers assess their unique systems and make strategic decisions regarding the timely repair or replacement of the equipment before an unexpected
failure occurs.
Fundamentals of PD
PULSE GENERATION– A partial discharge is generated when a defect present in the insulating systems is sufficiently large and is exposed to a suitably high electrical field.
SIGNAL PROPOGATION– The process by which an electrical signal generated by a PD event travels away from the PD source both galvanically and radiatively.
SIGNAL DETECTION – The use of sensor(s) and measurement system(s) with sufficient sensitivity to detect the signal produced by a PD source within the insulation system.
DATA INTERPRETATION – Knowledge, rules and tools used to locate and identify the PD source as well as a means of assessing the severity or criticality of the defect.
DATA REPORTING – Conveying the results of a PD test to the owner or stake holder with enough information to allow for a decision on the remediation that will be pursued.
Partial Discharge testing and assessment requires a great deal of judgment and experience. The physics of this high frequency phenomenon must be well understood, followed by an understanding of signal propagation within the specific high voltage system. Further in depth knowledge of available acquisition hardware and software is needed in order to select the appropriate measurement methodology.