Assessment of insulation condition of generator stator bars based on velocity of ultrasonic waves

The velocity of ultrasonic waves, which were launched using the pulse-echo method and propagated in a stator insulation, was measured to assess the insulation condition of the stator bars. A statistical minimum velocity (V/sub min/) and a statistical average velocity (V/sub av/) were computed for a new and three sets of stator bars which were in operation for 16, 18 and 23 years. After velocity measurements, several conventional electrical parameters such as dissipation factor, capacitance, and partial discharge were measured, and microscopic examinations of dissected insulation of tested bars were examined with a scanning electron microscope and an optical microscope. Experimental results showed that V/sub min/ and V/sub av/ decreased and more microscopic defects were formed in the insulation with increasing operating time of the stator bar. For the three sets of aged stator bars, the insulation conditions based on Y/sub min/ and V/sub av/ were consistent with those assessed by measuring electrical diagnostic parameters. Therefore, Y/sub min/ and V/sub av/ were used to assess the insulation condition of large generators. The measurement of ultrasonic waves may offer a broad prospect for stator insulation diagnosis.