Localisation of partial discharge sites on power generator bars by means of ultrasonic measurements

The dissipation factor (tg δ) measurement of electrical power generator insulated bars allows one to monitor the quality of the production process. In case of anomalous tg δ values detection a problem arises: fast and reliable procedures to identify the defects and the “causes of non-conformity” are needed to drive a prompt correcting action on the production line. This work presents an innovative measuring procedure to solve such problems: the localisation sources of partial discharges (PD) by means of the simultaneous detection (through an electric method) of the PD signals and of the related sound waves (through an acoustic method). While a “new” acoustic measuring method employing two ultrasonic sensors was devised, a classic PD digital detection was adopted. The delay time between the two signals was the base to measure, under given sensitivity conditions and through a zeroing technique, the location of PD sites (defects), with an error of 2-3 cm on 2400×18×54 mm insulated bars. The longitudinal and the transversal velocities of the sound waves were measured for each defect in each tested bar. Such a measuring procedure was then successfully validated testing three insulated bars (one of them contained an artificial void)