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Partial discharge is an incident where two conductors create a small electrical spark that does not link the two conductors completely, hence the term “partial.” Voids inside insulation systems are places where discharges can usually occur. A partial discharge that occurs frequently can make the void bigger, wear away the insulation, and ultimately damage the system.
Occurrence of a partial discharge usually indicates a potentially serious problem with the machine itself. Some of these problems include incorrect assembling and installation and damaged parts. Even new machines can suffer if poor-quality materials are used. Contaminants gathered over time could also be a cause for discharges. Old machines can also experience this kind of problem. Partial discharges can also be a common issue with machines that produce high voltages of electricity.
Solid insulation systems that use fabrics and paper can suffer from partial discharge when there are irregular voids. Gas bubbles, however, inside gas and liquid insulation are more prone to having discharges. High levels of humidity can also increase the appearance of discharges, as water is a very good conductor as well.
If the current becomes too high, the partial discharge would struggle through the void and can form into an “electrical tree,” with its branches spreading out. Solid insulation like papers and plastics would usually display these branches as dark, twisted lines. The problem with partial discharges is that their currents can be so erratic, appearing and disappearing so quickly they cannot be detected, unless they recur continually. One electronic device useful for detection is the oscilloscope, which records and displays fluctuating voltages.
Electronic waves from a partial discharge usually spread from a single location. The oscilloscope’s probe attached to the insulation system will pick the current up and show it on the screen as peaks and valleys. Another method of finding partial discharges is through ultrasonic detectors. Aside from electric currents, discharges also emit ultrasounds that human ears cannot hear, but can be picked up by sensitive machines. If an ultrasonic activity is discovered, then there is probably a partial discharge occurring in the system.
Another way of detecting discharges is through “transient earth voltages" (TEV) measurement. Discharges often result in a another byproduct of radio waves, which can be easier to detect than ultrasound. The location where TEV is strongest can be the prime site of the partial discharges. Many modern sensors and detectors employ both the TEV and ultrasonic measurement and other technologies to accurately and instantly discover any partial discharge.