Partial discharges (according to IEC standard 60270) are harmful electrical discharges in electrical systems. The early detection of these anomalies, as well as the identification of the exact location of the emission, will help to carry out corrective actions on time and with high efficiency. These discharges release energy from time to time, creating a distinctive acoustic signature that can be captured by an ultrasound listening device (ULD).
According to current statistics, 85% of power failures occur in medium and high voltage transformers. Partial discharges such as the corona effect are the main causes of power loss and damage to electrical equipment insulation, hence the need to have an inspection tool for these discharges at hand. This is more so the case in acoustic inspection, where there is often no temperature increase in corona discharges.
Partial Discharge – Corona
Corona discharge is the luminescent or electrical discharge around conductors when the surrounding air is stressed beyond its point of ionization without developing disruptive discharges. Corona looks for a path to land through the air. These high energy discharges can cause mechanical, electrical and thermal damage.
Corona is a changing zone of ionized gases, identifiable to the inspector through airborne ultrasound. Corona discharge occurs when the voltage in electrical conductors, such as an antenna or a high-voltage transmission line, exceeds the threshold value. The air around the high transmission line begins to ionize to form a blue or purple glow, generally visible in total darkness and with high humidity in the environment.
Corona itself is not always a sign of impending failure, however it is an opportunity to investigate the consequences of the crown to determine if corrective action should be taken. Corona, however, can be catastrophic. It can lead to the loss of expensive power management components.
The ultrasonic signature of the Corona Effect is very characteristic: a buzz type background sound, with blows on it that are equivalent to the individual discharges. These blows, as seen in the waveform graph at the top of figure 1, are very constant in time series and are on the same frequency as electric power; 50 Hz or 60 Hz depending on the country where they are taken. By having broad redundancy of the discharges, they generate a spectrum full of harmonics.