Application of ultrasound to the NDT of solid insulation

The use of ultrasound is proposed as the most economic and practical nondestructive test method for the detection of electrical degradation in insulating materials. The method is non-invasive and easily applied to onsite, in-service conditions. A variety of insulating materials and inhomogeneities have been investigated using pulse-echo techniques in the A-scan mode, to establish the range of applicability of this technique. 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 structured fibres, paper, mica and polymer films to determine the ultrasonic visibility of such composites. Simulated electrical trees with a diameter of 10 mu m have been easily detected in polyester resin. In polyethylene the limiting diameter was 25 mu m at 10 MHz. Air bubbles, which simulate voids in the material, and delaminations in multilayered systems are both detected with ease using this technique. Flaws in practical systems, such as failures in SRBP bushings and delaminations in multilayer stator bar insulation, have been successfully investigated. The authors conclude that ultrasound has great potential for flaw detection in a variety of insulating systems, including voids, cracks, delaminations and electrical and water trees.<>