In electrical engineering, a partial discharge is a localized dielectric breakdown of a section of dielectrically stressed insulation path that occurs generally in voids, cracks or interfaces within that insulation system. By contrast, corona discharge is usually revealed by a glow or brush discharge. Partial discharge in insulation systems may or may not exhibit this.

A partial discharge is a localised discharge inside the dielectric of an electrical insulation system which is limited to a part of the insulation and so only partially bridge the electrodes. It occurs in cavities or voids within liquid or solid dielectrics. Also surface charges appear between different insulating materials.

Partial discharges in insulating material are caused by voids in the dielectric. Because the capacitance of the void is generally much less than the capacitance of the surrounding dielectric, higher voltages will appear across the void than across an equivalent distance of dielectric. If the applied voltage is increased sufficiently upto the inception voltage for the sample, then PD activity will start at the void.

The equivalent circuit of a dielectric incorporating a cavity includes a capacitive potential divider in parallel with another capacitor. The upper capacitor of the divider represents the parallel combination of the capacitanes in series with the void and the lower capacitor represents the capacitance of the void. The parallel capacitor, represents the remaining unvoided capacitance of the sample..

PD currents are difficult to measure becuas of their small magnitude and short duration. 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.

q= Cb delta(Vc) is not equal to the actual amount of charge at the discharge site, but is more realistic tha deltaVa

A number of discharge detection schemes have been invented since the importance of PD was realised early in the last century. Partial discharge currents tend to be of short duration and have rise times in the nanosecond region. On an oscilloscope, the discharges look like randomly occuring 'spikes'. The ususal way of quantifying partial discharge magnitude is in picocoulombs.

This is measured by calibrating the voltage of the spikes against the voltages obtained from a calibration unit discharged into the measuring instrument. The calibration unit is quite simple in operation and merely comprises a square wave generator in series with a capacitor connected across the sample. Usually these are triggered optically to enable calibration without entering a dangerous high voltage area. THe calibrators are usually left connected during the discharge testing.

In wideband detection, the coupling impedance eusually comprises a low Q parallel resonant L,R,C circuit . This cct tends to attenuate the exciting votage usually between 50 and 60 Hz and to amplify the voltage generated due to the discharges

Clearly the field testing of plant and equipment in service by the above method, although it has been done, is not very convenient. Therefore field testing for PD activity has taken on a slightly different role where accuracy of measurement is less important than basic indication of discharge activity. To this end, a number of specialised instruments have been developed many companies around the world.

The effects of PD within high voltage cables with solid insulation can be very serious and lead to complete failure of the cable. (an explosion) The continuous effect of discharge in solid dielectrics results in formation of discharge channels. This is called treeing. Discharge events deteriorate the insulating material by the energy dissipated by high energy electrons or ions and can cause chemical transformation of the dielectric that can lead to further deterioration of the dielectric.(ref K&S)

In paper insulated HV cables, the effects of partial discharge are initially shown as small pinholes penetrating the paper windings. As activity progresses, the discharge energy starts to cause chemical changes in the papers and its impregnation. Carbonisation of the papers is common. Once this occurs, a conducting path is formed in that part of the insulation. This generates heat and more charring. Eventually a complete breakdown path exists across the insulation and, unless the current is limited elsewhere, the cable suffers catastrophic damage.(ref LC)

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• High voltage engineering fundamentals, E.Kuffel,W.S. Zaengl, pub. Pergamon pPress. First edition, 1992 ISBN 0-08-024213-8

• Introduction to Partial Discharge</math>-->