Partial Discharge Detection

Partial Discharge (PD) is an electrical phenomenon which causes insulation to deteriorate
and frequently is the reason for breakdown of an insulation system resulting in failure of
the equipment. Partial Discharge can be described as an electrical pulse or discharge in a gas-filled void
or on a dielectric surface of a solid or liquid insulation system. This pulse or discharge
would partially bridge phase to ground insulation or phase to phase insulation in an
electrical apparatus, thus causing the electrical equipment to fail


Ultrasound inspection may be performed on both open and enclosed electrical equipment at all voltages (low, medium, and high). When electrical apparatus such as switchgear, transformers, insulators, or disconnects and splice fail, the results can be catastrophic. This is just as true in industrial plants as it is in the power transmission and distribution side. Electrical discharges such as arcing, tracking or, in higher voltages, corona have the potential to create equipment failure and costly downtime.

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Transient Earth Voltage (TEV) detection technology has become one of the important detection methods for high voltage electrical equipment. TEVs occur 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 µm 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. While this method may be useful to detect some issues in switchgear and surface tracking on internal components, the sensitivity is not likely to be sufficient to detect issues within solid dielectric cable systems

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