PMDT Airborne Ultrasonic
Airborne Ultrasonic Parabolic Dish Concentrator, Internal, and Ultrasonic Extension Wand, 20kHz-80kHz, center frequency 40kHz. Each time a PD event occurs, an ultrasonic emission is transmitted from the site of the defect. The emissions are typically in the ultrasonic range from 20kHz - 300kHz and may also extend lower into the audible range. If an insulation defect is close to the surface, then the ultrasonic signal will spread out and become airborne. The airborne ultrasonic signal may be detected by one or more of the 3 types of airborne ultrasonic sensors which are commonly used.
One type is built into the main handheld PD test unit. Also the extension wand can be used to listen in hard-to-reach seams of Switchgear cabinets or other enclosures. Another type is the Parabolic Dish Concentrator, which is used to extend the distance range of the ultrasonic microphone. The parabolic dish is ideal for checking bushing and insulators for surface tracking and harmful corona. When airborne ultrasonic PD signals are detected on exposed insulator surface, the Parabolic Dish enables very quick and precise pinpointing of the PD location. Airborne ultrasonic is best for detecting corona PD and surface tracking PD. Floating electrode PD will also produce a weak airborne ultrasonic signal. But when airborne ultrasonic signals are detected within enclosures, it is not immediately possible to determine the exact location of the PD. Sometimes you cannot see the insulator which has the PD. By using multi-channel PD Diagnostics, 3 or 4 ultrasonic contact sensors can be used to triangulate the signal origin.