Initial Experience with Acoustic Imaging of PD on High Voltage Equipment

Directional ultrasonic microphones have been used for decades to locate surface partial discharge and corona sites in high voltage equipment. However, there was always some uncertainty of exactly where the discharge sites were due to reflections and the field of view of the detector. In addition, scanning a complete winding took some time. Recently, a significant advance in this technology was achieved with the commercial development of an acoustic “camera” that can show where the sound is occurring with respect to a normal visible-light image of the test object. That is, the device produces a sound image of the PD on the test object, much like a UV camera locates the ultraviolet light from PD on an image of the test object. The acoustic camera takes advantage of a large array of wideband microphones and is able to display the acoustic signal in selectable frequency ranges in the sonic and ultrasonic ranges. The effectiveness of this new tool was evaluated on a point to plane corona test object, as well as stator coils and stators with known PD. The optimum surface discharge detection band seems to be from 30–50 kHz: Intense PD in internal voids may also be detected, albeit at a lower frequency than for surface PD. Precise locations of multiple discharge sites are rapidly identified in different test objects.