What’s a partial discharge?

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Partial discharge is an accident in which two conductors create a small electric spark that does not fully connect the two conductors, hence the term “partial”. Voids within insulation systems are places where flashovers can usually occur. A partial discharge that occurs frequently can make the gap larger, wear down the insulation, and ultimately damage the system.

The occurrence of a partial discharge usually indicates a potentially serious problem with the machine itself. Some of these problems include incorrect assembly and installation and damaged parts. Even new cars can suffer if poor quality materials are used. Contaminants built up over time could also cause runoff. Even older machines can experience this kind of problem. Partial discharges can also be a common problem with machines that produce high voltages of electricity.

Solid insulation systems using fabrics and paper can undergo partial discharge in the presence of irregular voids. However, gas bubbles within gas and liquid insulation are more prone to discharge. High humidity levels can also enhance the appearance of drains, as water is also a very good conductor.

If the current gets too high, the partial discharge would struggle through the vacuum and could turn into an “electric tree,” with its branches spreading out. Solid insulations such as paper and plastic usually show these branches as dark, twisted lines. The problem with partial discharges is that their currents can be so erratic, appearing and disappearing so quickly that they can’t be detected unless they repeat themselves over and over again. A useful electronic device for sensing is the oscilloscope, which records and displays fluctuating voltages.

The electronic waves from a partial discharge usually spread from a single location. The oscilloscope probe connected to the isolation system will sense the current and show it on the screen as peaks and valleys. Another method of finding partial discharges is through ultrasonic detectors. In addition to electric currents, the discharges also emit ultrasounds that human ears cannot hear, but can be picked up by sensitive machines. If ultrasonic activity is detected, a partial discharge is likely to occur in the system.

Another way to detect flashovers is by measuring “transient earth voltages” (TEV). The discharges often give rise to another by-product of radio waves, which can be easier to detect than ultrasound. The location where the VTE is strongest may be the primary site of partial discharges. Many modern sensors and detectors use both TEV and ultrasonic measurement and other technologies to accurately and instantaneously detect any partial discharge.

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