Partial Discharge Test (PD)

Partial Discharge (PD), as its name would suggest, is an electrical discharge that occurs across a portion of the insulation between two conducting electrodes, without completely bridging the gap. Partial Discharges can occur in voids in solid insulation (paper, polymer), gas bubbles in liquid insulation or around an electrode in a gas (corona).Partial Discharge activity can initiate under normal working conditions in high voltage equipment where the insulation condition has deteriorated with age and/or has been aged prematurely by thermal over-stressing. PD can also be observed, on occasion, with the commissioning of new equipment due to improper installation, poor design and/or workmanship (this is seen particularly in cable joints and terminations which are made-up on site). After initiation, the PD can propagate and develop into electrical trees until the insulation is so weakened that it fails completely with breakdown to earth or between the phases of a 3-phase system.

Once begun, PD causes progressive deterioration of insulating materials, ultimately leading to electrical breakdown. The cumulative effect of partial discharges within solid dielectrics is the formation of numerous, branching partially conducting discharge channels, a process called treeing. Repetitive discharge events cause irreversible mechanical and chemical deterioration of the insulating material. In paper-insulated high-voltage cables, partial discharges begin as small pinholes penetrating the paper windings that are adjacent to the electrical conductor or outer sheath. As PD activity progresses, the repetitive discharges eventually cause permanent chemical changes within the affected paper layers and impregnating dielectric fluid. Over time, partially conducting carbonized trees are formed. This places greater stress on the remaining insulation, leading to further growth of the damaged region, resistive heating along the tree, and further charring (sometimes called tracking). This eventually culminates in the complete dielectric failure of the cable and, typically, an electrical explosion. It is known that whilst some discharges can be extremely dangerous to the health of the insulation system (e.g. discharges within polymeric cables and accessories) other types of discharge can be relatively benign (e.g. corona into air from outdoor cable sealing ends). It is necessary therefore, when testing for PD, that the Test Engineer is able to ascertain the type of discharge present and its origin.

PD testing is particularly important where HV plant has a high criticality. This may be due to its age, historical failures or the consequences of its failure (position in the network).

Application includes: Transformers, Current and Voltage Transformers, Bushings, Generators, Motors, Switchgear (SF6).