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Partial Discharge
Treeing and paper degradation

Introduction to Partial Discharge

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. PD's are caused when there is a discontinuity in the insulation system and as a general 'rule-of-thumb' PD will occur in systems operating at voltages of 3000V and above (although it should be noted that PD can occur at lower voltages than this).

Partial discharges can occur in voids in solid insulation (paper, polymer etc), along the interfaces of multi-layer solid insulation systems, in gas bubbles in liquid insulation or around an electrode in a gas (corona discharge).

Partial Discharge activity can initiate under normal working conditions in high voltage equipment where the insulation condition has deteriorated with age, has been aged prematurely by thermal or electrical over-stressing or due to improper installation (this leads to 'infant mortality').

Partial Discharge
Tracking in paper insulation
Partial Discharge

PD can often be observed 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). It is known that poor workmanship can lead to 'infant mortality' of MV/HV networks with a disproportionate percentage of insulation failures being observed within the first 1-3 years of service compared to the rest of the service life of the cables/plant.

After initiation, the PD can propagate and develop into electrical trees and interfacial tracking until the insulation is so weakened that it fails completely with breakdown to earth or between the phases of a 3-phase system. Depending on the discontinuity in the insulation system and where it is positioned, a failure can take anything from a few hours up to several years to track through to produce a complete earth or phase-phase fault.

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 cable accessories) whilst other types of discharge can be relatively benign (e.g. such as corona into air from sharp, exposed points on HV overhead networks or on the outside surfaces of outdoor cable sealing ends). The key to on-line, diagnostic PD testing is to be able to differentiate between the dangerous and the benign. This becomes more difficult as the voltage of the system increases.

Partial Discharge
Start of tracking on VMX spouts

It is necessary therefore, when testing for PD, that the Test Engineer is able to ascertain the type of discharge present and its origin. HVPD provide our customers with the highest level of diagnostic support and test services presently available in the marketplace to make these decisions correctly.

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.

On-line PD testing of MV and HV plant gives an advance warning of pending insulation failure, thereby allowing the plant owner to take remedial maintenance action during planned outages. Past projects by HVPD have shown that, in general, the earlier the advance warning can be made, the cheaper the maintenance or intervention costs will be.

Unlike off-line testing, on-line PD 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.

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

HV Plant Asset and Risk Management

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

Qualification of PD 'criticality' within the plant owner's HV network can be achieved quickly and easily using HVPD's on-line, screening and diagnostic PD test technology to provide an 'early warning system' for these incipient insulation faults.

PD Monitoring and Trend Analysis

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.

Partial Discharge
Tracks on SOHI epoxy resin busbar

The Benefits of On-Line Partial Discharge Field Measurements

  • 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, load and environmental conditions.
  • It is a nondestructive test; it does not test to failure or adversely affect the equipment under test in any way.
  • It does not use any overvoltage, thereby not exposing the tested equipment to higher voltage stresses than those encountered under normal operating conditions.
  • Trending can be accomplished by storing baseline measurement results to allow comparison with future tests.
  • In many instances the site of the partial discharge occurrence can be located within the cable/plant under test, so the localised problem can be repaired.
  • The cost to perform a PD survey is relatively inexpensive compared with off-line testing, allowing annual surveys to be performed economically at most facilities.

Examples of HV & MV Plant That Can Be Tested

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

For further information and advice on partial discharge testing please contact us.

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