Partial discharge (PD) activity is often present well in advance of insulation failure. Therefore, partial discharge monitoring provides the most evident indication of defects and deterioration in transformers, turbines, motors, cable and switchgear.
Asset managers can evaluate partial discharge activity over time and make informed strategic decisions regarding the timely repair or replacement of the equipment before an unexpected outage occurs.
What is partial discharge?
Partial discharge occurs inside insulation in assets such as a transformer, turbine generator, motor, switchgear or cable. Partial discharge occurs when one part of the insulation cannot withstand electric stress, so it flashes over. At that time, the gases are ionized and the voltage drops quickly. Then the current pulse equalizes the remaining charge in the remaining insulation. It’s a rapid process that negatively affects the asset’s life.
Partial Discharge Activity
Where does partial discharge occur?
Partial discharge occurs in electrical assets that have insulation such as transformer bushings, switchgear, cables, turbine generators, motors, and more. It can occur at any voltage level.
Why does partial discharge occur in a transformer?
Whenever you have a change in the dielectric constant of a material, that can change the electrical distribution. Typically, there are three potential sources of PD generation in a power transformer – core and coils assembly, bushing, and load-tap changer. The following factors help determine the source of PD: operating voltage, voltage induced by main magnetic and stray flux, and impairment of the insulation properties due to moisture ingress and particle contaminants. Levels of insulation degradation in a power transformer can be reliably tracked using PD measurements.
What causes partial discharge?
There are multiple situations that can cause PD to occur. Many are due to human error: contamination, poor installation, or manufacturing defects. Some causes are environmental such as temperature and humidity. Over time, PD can also occur due to aging of the insulation.
Three Methods to Measuring Partial Discharge
Which method is best for partial discharge monitoring?
For decades, the electric method has been the most reliable form of PD measurement on transformers. Methods like the radio influence methods (RIV) calibrated in micro-volts are evaluated according to NEMA 107. The wide-band or narrow-band PD detectors calibrated in pico-coulombs are evaluated according to IEC publication 60270. There are no current accepted procedures or guidelines available for UHF or acoustic methods. Download this white paper that compares the three methods to learn more.