Contd.
GENERAL CONSIDERATIONS
THE PD SOURCE

A partial discharge (PD) results in localized, nearly instantaneous release of energy. The energy released in a PD produces a number of effects such as chemical and structural changes in the material, electromagnetic effects etc. The Acoustic discharge detection is based on detection of the mechanical signals emitted from the discharge.

A fraction of the released energy heats the material adjacent to the PD and can evaporate some of it, creating a small explosion. The discharge acts as a point source of acoustic waves that propagates throughout the insulation. As the discharge duration is very short, the acoustic spectrum of the emitted wave is very broad (several MHz). The intensity of the emitted acoustic wave is proportional to the energy released in the discharge. Thus the amplitude of the wave is proportional to the square root of the energy in the discharge. As the energy released is often proportional to the charge squared, a linear relationship between the amplitude of the acoustic wave and the discharge magnitude (in pico-Coulombs) is common.

The Acoustic wave can be detected by a suitable sensor kept on the apparatus tank, the output of which can be analyzed using a conventional data acquisition system. The shape of the detected signal depends on the source, the detection apparatus and the sensor.

Sound propagates through a medium by means of wave motion, i.e., the propagation of a local disturbance through the medium. When a wave propagates through a structure, the intensity of the wave decreases as a function of distance from the source. This results from several mechanisms including geometrical spreading of the acoustic wave, acoustic absorption (conservation of acoustic energy to heat) and scattering of the wave front. These phenomena result in a reduction of the intensity of the wave as it moves away from the source. When an acoustic wave propagates from one medium to another, which has a different density and/or elasticity (i.e. different acoustic impedance), reflection and refraction will take place. This results in a reduction of the energy in the transmitted wave.

Hence, the acoustic propagation path from the discharge to the sensor is specific to the apparatus under test. It ranges from the simple case of discharges on overhead lines, where the sound wave propagates through only air, to the complicated case of sound propagating through the complex structure of a power transformer. An assessment of the probable acoustic PD signals characteristics must consider both changes in signal amplitude and signal shape as the signal propagates away from the source.

THE SENSOR

The choice of an acoustic sensor from the wide range of available sensors depends on a number of fundamental considerations. The optimization between bandwidth and sensitivity is always important.