Application of ultrasonic pulse-echo method to insulation condition diagnosis for large generators

It is a challenge to diagnose insulation condition of stator bars using an ultrasonic technique, due to the complex structures of the stator bars and the high attenuation of the stator insulation. In this paper, five low frequency transducers were used in a pulse-echo configuration to try to test a standby stator bar of a generator (300 MW, 18 kV). With an available transducer, four types of insulation structures were tested on the stator bar. The short side insulation was compared with the large one of the stator bar in the slot. The stator bar near the exit from the slot and the end stator bar, were compared with the stator bar in the slot. The semiconductive coating covered stator insulation was compared with the uncoated one. The stator insulation having an insulation-copper interface was compared with that having an insulation-gas interface. In addition, insulation diagnosis tests in the laboratory were performed on two sets of stator bars, which had been in service for 18 and 23 years, respectively in two generators (300 MW, 18 kV). Field tests were performed on the stator bars near the exit from the slot and the end stator bars of a generator (300 MW, 18 kV). The experimental results demonstrated an availability of low frequency ultrasonic pulse-echo method for insulation condition diagnosis of large generators.