Introduction to Partial Discharge

Tracking in paper insulation

What Is Partial Discharge and Why Should We Test For It?

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 etc), 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.

Treeing and paper degradation below
dry termination

Electrical tree

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.

Start of tracking on VMX spouts

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.

Failure of High Voltage insulation is the No. 1 cause of HV system failures with IEEE statistics indicating that electrical insulation deterioration causes up to 90% of electrical failures of certain high voltage equipment.

Failure site in XLPE insulation, with bow
tie trees accompanied by electrical trees

On-line PD testing of HV plant gives advance warning of pending insulation failure thus allowing the plant owner to take remedial action during planned outages.

Unlike off-line testing, on-line PD testing and monitoring gives an accurate picture of the plants health and performance under service conditions.

Tracks on SOHI epoxy resin busbar

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).

Identification of the 'critical plant' within the plant owner's HV network can be achieved quickly and easily using IPEC's on-line PD Testing technology to provide an 'early warning system' for incipient faults.

On-line PD monitoring allows for analysis trends in PD activity to be observed over time. This may reveal correlation with environmental (temperature, humidity etc) or service conditions (changes in load etc). As PD activity is often present well in advance of insulation failure it is possible by observing its development that strategic decisions can be made about refurbishing and renewal programmes.

• It is truly a predictive test, indicating insulation degradation in advance of the failure.
• It is a nonintrusive test, requiring no interruption of service and is performed under normal operating voltage and load.
• It is a nondestructive test; it does not test to failure or adversely affect the equipment under test.
• It need not use any overvoltages, thereby not exposing the tested equipment to higher voltage stresses than those encountered under normal operating conditions.
• Trending can be accomplished by storing results to allow comparison with future tests.
• In many instances the site of the partial discharge occurrence can be located within the test object, so the localised problem can be repaired.
• The cost to perform a p.d. survey is relatively inexpensive compared with off-line testing, allowing annual surveys to be performed economically at most facilities.

• Cables and Cable Accessories (terminations and joints)
• Switchgear (AIS and GIS)
• Instrument transformers (voltage and current)
• Power transformers and bushings
• Motors and generators
• Surge arrestors
• Capacitors