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In electrical engineering, partial discharge (PD) is a localised dielectric breakdown of a small portion of a solid or fluid  electrical insulation system under high voltage stress, which does not bridge the space between two conductors.

PD can cause damage to the surrounding insulation material by erosion of the insulation. In addition corrosive gases emitted by a PD source can cause further damage to surrounding insulation and metalwork often setting up further sites of PD activity. Ultimately the insulation medium can fail resulting in flashover and consequential damage to the electrical asset, power interruptions, fires, and explosions.

Types of Partial Discharge
Partial discharges are defined in IEC 60270 as a “localized electrical discharges that only partially bridges the insulation between conductors". This broad definition includes types of PD that are relatively harmless to types that are hard to detect in the field and can be very destructive. It is useful therefore to divide Partial Discharge into three categories.

Corona discharge is the discharge that occurs in the air or gas surrounding a conductor. It it is caused when the localised electric field exceeds the the breakdown strength of the surrounding air or gas. Typically this occurs at points or sharp edges on conductors. It is very common particularly on outdoor equipment.

Corona Discharge can be considered relatively harmless on outdoor equipment as the corrosive gases produced are blown or washed away by the weather. If however the corona discharge is enclosed in any way then the corrosive gases have nowhere to escape and can cause further damage. Corona discharge on outdoor equipment is often difficult to eliminate and some designs of equipment will inherently discharge. However it is good practice to eliminate sources of corona discharge whenever possible during routine maintenance as they can mask more serious problems. Surface Discharge is discharge that occurs on the surface of an insulator typically causing tracking damage to the insulator surface and reducing its efficacy. It is highly associated with contamination and humidity and is relatively common form of partial discharge.

Surface Discharge is particularly damaging to cast resin and polymeric insulation. If left unchecked, the discharge sites will grow and ultimately cause flashover. It can also crack the glaze on porcelain insulators and damage the ceramic beneath. If the cause of the surface problem is contamination, and it is caught early, then it is sometimes possible to clean glass and porcelain insulators before any long term damage occurs.

Internal Discharge is a type of discharge that occurs inside insulating material or fluid and is associated with voids, often microscopic iniitally, within that fluid or solid insulator. It is a relatively rare form of partial discharge.

Internal Discharge is the most difficult to diagnose in the field as there is no visible or audible indication that there is a problem. However if it is left and flashover does occur then there is nowhere for the resulting rapid heat energy to go and the result can be that the insulator will explode.

Note, however, that some text books and web sites will use different definitions of Corona Discharge. Sometimes Corona Discharge is used to cover both Corona Discharge (as defined above) and Surface Discharge. Sometimes Corona Discharge is used interchangeably with Partial Discharge to cover all three of the above sub definitions. Internal discharges are often overlooked.

Measurement and detection of partial discharge
When partial discharge is initiated, high frequency transient current pulses will appear and persist for nano-seconds to a micro-second, then disappear and reappear repeatedly. PD currents are difficult to measure because of their small magnitude and short duration. The event may be detected as a very small change in the current drawn by the sample under test. One method of measuring these currents is to put a small current-measuring resistor in series with the sample and then view the generated voltage on an oscilloscope via a matched coaxial cable. The ouput of such tests is generally summed up as a change in charge and is expressed in pC. This is the basis of the methodology described in IEC 60270 sometimes described as Apparent Charge measurement.

IEC measurements are ideal for laboratory measurement when the asset under test can be energised with a clean laboratory supply and test set up and the asset under test placed in a Faraday cage. Field measurements preclude the use of a Faraday cage and the energising supply can also be a compromise from the ideal. Field measurements are therefore prone to noise and consequently less sensitive.

Other methods have therefore been developed for field measurement which, while not as sensitive as an IEC measurement, are substantially more convenient. By necessity field measurements have to be quick, safe and simple if they are to widely applied by owners and operators of MV and HV assets.

Transient Earth Voltages(TEVs) are induced voltage spikes on the surface of the surrounding metalwork. These occurs because the partial discharge creates current spikes in the conductor and hence also in the earthed metal surrounding the conductor. TEV pulses are full of high frequency components and hence the earthed metalwork presents a considerable impedance to ground. Therefore voltage spikes are generated. These will stay on the inner surface of surrounding metalwork (to a depth of approximately 0.5 microns in mild steel at 100 MHz) and loop around to the outer surface wherever there is an electrical discontinuity in the metalwork. There is a secondary effect whereby electromagnetic waves generated by the partial discharge also generate TEVs on the surrounding metalwork - the surrounding metalwork acting like an antenna. TEVs are a very convenient phenomenon for measuring and detecting Partial Discharges as they can be detected without making an electrical connection or removing any panels.

Ultrasonic measurement relies on fact that the partial discharge will emit sound waves. The frequency bandwidth for emissions tends to be centred on 40kHz but will stretch into the audible area for extremely bad discharges. Ultrasound will not be emitted by an Internal Discharge. The usefulness of Ultrasonic detection is therefore restricted to Surface Discharge and Corona Discharge.

Electro Magnetic Field detection picks up the radio waves generated by the partial discharge. As noted before the radio waves can generate TEVs on the surrounding metalwork. More sensitive measurement, particularly at higher voltages, can be achieved using in built UHF antennas or external antenna mounted on insulating spacers in the surrounding metalwork.

Practical application
In practice, the detection of of partial discharge activity is a useful indication that insulation in MV/HV assets is beginning to degrade. For example, the UK Health and Safety Executive has published guidelines advocating regular testing for partial discharge activity on switchgear.

International Standards

 * IEC 60270:2000/BS EN 60270:2001 "High-Voltage Test Techniques - Partial Discharge Measurements"