The influence of corona near to the bushing of a transformer on partial discharge measurement with an acoustic emission sensor

Partial discharge (PD) detection is one of the most important issues in order to determine the insulation condition of power transformers; however the procedures for electrical partial discharge detecting are sometimes not so beneficial for onsite and on-line measurements due to excessive interferences. Acoustic emission (AE) PD detecting method may have some advantages in comparing electrical as well as optical and chemical methods. With this method it is possible to identify the type of PD from the collected information and with a set of sensors to locate the PD source and to evaluate the severity of PD activity. Using multiple sensors, the time delay between the sensors can be used in order to locate the PD source within the power transformer. Protection against electromagnetic interference (EMI) is another advantage of the acoustic method over the electrical method. However the acoustic emission signal from e.g. corona on a transformer bushing could cause some influence on acoustic emission signals due to the short distance between acoustic source and sensor. In this paper, the effect of this kind of acoustic noise has been measured with two different types of acoustic sensors (75 kHz and 150 kHz) at different PD activities and amount of apparent charges, in order to compare the sensitivity of the two sensors on the acoustic noise superimposed on the PD signal. The results are that the two sensors show different sensitivity and the sensor with the maximum frequency resonant at 150 kHz is more reliable for acoustic signals of PD inside the power transformer.