An Introduction to Partial Discharge (PD)

What is PD and why should we test for it?

PD is an electrical discharge that occurs across a localised area of the insulation between two conducting electrodes, without completely bridging the gap. It can be caused by discontinuities or imperfections in the insulation system. PD testing thus gives an indication of deterioration of the insulation and is an indicator of incipient faults. In general PD will occur in systems operating at voltages of 3000 V and above; it should be noted though that in some cases PD can also occur at lower voltages.

Where does PD occur?

PD can occur at various points in the insulation system, for example: voids in the insulation medium, at the interface between insulation layers or in gas bubbles in liquid insulation.
PD can be observed at the commissioning of new equipment, caused by improper installation or poor design and/or workmanship – particularly in cable joints and terminations which are made up on-site. Poor workmanship at the manufacturing stage of an asset can lead to premature failure, with a disproportionately high percentage of insulation failures being observed within the first 1–3 years of service compared to the rest of the working life of the asset.
PD activity can start under normal working conditions in high‑voltage equipment where the insulation condition has deteriorated with age, or has been aged prematurely by thermal or electrical over-stressing, or due to improper installation.

How does PD develop?

Once started, PD can develop into electrical trees and surface tracking, eventually leading to a breakdown between phase and earth or between phases of a 3-phase system. Depending on the discontinuity and/or imperfection type and location in the insulation system, a failure can take anything from a few hours up to several years to occur.
While some discharges can be extremely dangerous to the health of the insulation system (e.g. discharges within polymeric cables and cable accessories), other types of discharge can be relatively benign (e.g. such as corona into air from sharp, exposed points on HV overhead networks). The key to on-line PD (OLPD) diagnostic testing is to be able to differentiate between the different types of PD which can occur.

PD monitoring and asset management

OLPD monitoring allows for trends in PD activity to be observed over time. While PD is incepted by the high-voltage stresses, it can be influenced by the other operating stresses. Correlation with environmental (temperature, humidity, etc.) or service conditions (e.g. changes in load) can sometimes be observed. As PD activity is often present well in advance of insulation failure, it is possible by observing PD trends to make strategic decisions about refurbishing and renewal programmes. Often it may not be necessary to replace an older asset which is perfectly serviceable, and thereby the capital replacement cost can be deferred by using condition-based maintenance.
PD testing is particularly important when the MV/HV asset is critical to the operation of a network; this may be due to the asset's age, past failures or the financial consequences of its failure. Qualification of PD criticality within the HV network can be achieved quickly and easily using HVPD's 4-Phase Asset Management Solution to provide an early warning for any incipient insulation faults. Examples of MV and HV plant that can be tested:
  • Cables and cable accessories (terminations and joints)
  • Switchgear (AIS, SIS and GIS)
  • Power transformers and bushings
  • Motors and generators
  • Instrument transformers (voltage and current)
  • Capacitors


On-line partial discharge (OLPD) testing

OLPD testing of MV and HV plant gives an advance warning of pending insulation failure, allowing the plant owner to take remedial maintenance action during planned outages. Our past projects have shown that in general the earlier the advance warning can be made, the cheaper the maintenance or intervention costs will be.
OLPD testing and monitoring gives an accurate picture of the HV plant's health and performance under normal service conditions, including the effect of load, temperature and humidity.

Benefits of OLPD field measurements

  • Trending analysis is predictive and indicates insulation degradation in advance of the failure
  • Testing is non-intrusive, causing no interruption of service, and is performed under normal operating voltage, load and environmental conditions
  • Testing is non-destructive as it does not test to failure or put any additional stresses on the equipment under test
  • Overvoltage testing is not used so the tested equipment is not exposed to higher voltage stresses than those encountered under normal operating conditions
  • Trending is accomplished by storing baseline measurement results to enable comparison with future test results
  • In many instances the occurrence of the PD can be located within the asset under test so the fault can be accurately and more economically repaired
  • Low cost of performing an OLPD survey, compared with off-line testing, allows for annual surveys to be performed economically at most facilities