The ultrasonic location of discharges in transformers-a new approach

Abstract

The location of discharges within large transformers and the identification of the nature of the discharge source is a problem. The conventional approach involves the use of a multiplicity of ultrasonic sensors attached to points on the transformer tank chosen to be close to the supposed location of the discharge activity. The output of the ultrasonic sensors is recorded in synchronism with the onset of discharge activity, detected by a sensor coil wound around the transformer's neutral lead. The location of the discharge is estimated according to the differences in time taken for the sound associated with the discharge to reach each transducer.The established approach does not involve analysing the ultrasonic signals, but revolves around the Occurrence of an output from the ultrasonic transducers. The new approach to be described utilises information contained within the signal, to determine the position of the discharge. It relies on the fact that for a given discharge, the ultrasonic signal received by the transducer contains two components, the relative extent of which is determined by the angle between the tank wall and the line joining the transducer to the discharge source. Using this fact, a discharge can be located without having to detect associated electrical activity in the neutral. This makes the location of discharging points much easier, particularly in the electrically noisy environment of a large high voltage substation. Conversely, if the electrical discharge signals are available to synchronise readings from the transducers, then the number of transducer readings required is much reduced for a given accuracy of location. ACOUSTIC EMISSIONAcoustic emission is the process by which sound is produced by rapid energy release within a material. It is possible to listen to this release of energy as the sound wave propagates through the surrounding media. The received waveform depends on the nature of both the source and the materials through which the wave passes. It is a common technique in the field of NonDestructive Testing (NDT) and has found applications in a number of industries [l]. The application of acoustic emission to the location of discharges within a transformer is not new. Commercial systems are available which make use of the acoustic signal in conjunction with electrical signals picked up in the neutral. We have developed a technique which analyses the acoustic signal alone. Tests have been carried out using a laboratory based system with discharges located to a high degree of accuracy.