APPLICATION OF ULTRASOUND TO THE NDT OF SOLID INSULATION

The use of ultrasound is proposed as the most economic and practical n ondestructive test method for the detection of electrical degradation in insulating materials. Unlike current investigative methods for the assessment of degradation, it is non-invasive and easily applied to on site, in-service conditions. A variety of insulating materials and inh omogeneities have been investigated using pulse-echo techniques in the A-scan mode, to establish the range of applicability of this techniqu e. Polyester resin, in its pure state and filled with aluminium oxide, and crosslinked polyethylene have been used as base materials. These materials have been compounded by the inclusion of layers of structure d fibres, paper, mica and polymer films to determine the ultrasonic vi sibility of such composites. Simulated electrical trees with a diamete r of 10 mu m have been easily detected in polyester resin. In polyethy lene the limiting diameter was 25 mu m at 10 MHz. Air bubbles, which s imulate voids in the material, and delaminations in multilayered syste ms are both detected with ease using this technique. Flaws in practica l systems, such as failures in SRBP bushings and delaminations in mult ilayer stator bar insulation, have been successfully investigated. The authors conclude that ultrasound has great potential for flaw detecti on in a variety of insulating systems, including voids, cracks, delami nations and electrical and water trees.