Development of high sensitivity capacitors for PD measurement in operating motors and generators

80 pF high voltage capacitors have been widely used as a sensor for PD measurement in rotating machines for more than ten years. However, its detection sensitivity with respect to various types of rotating machines has not been fully investigated. As we know, PD activity in lower-voltage (e.g. 3.3 kV-6.9 kV) rotating machines is usually lower than that in higher-voltage rotating machines. Therefore, PD sensors with a higher sensitivity are required to detect relatively small PD pulses in lower-voltage rotating machines. This paper presents theoretical analysis, laboratory tests and field test experience to compare the sensitivity of 80 pF capacitors and 500 pF capacitors for on-line PD measurement in lower-voltage rotating machines. The PD detection sensitivity is studied by characterizing the frequency bandwidth of various capacitors and the frequency spectrum of a PD pulse. The field test on a 6.9 kV operating motor verifies that the 500 pF capacitor can detect the PD activity which the 80 pF capacitor was not able to. 500 pF capacitors have been installed on more than forty rotating machines in operating motors and generators. A comparison of statistics analysis of a number of motors installed with 500 pF capacitors and 80 pF capacitors shows that 500 pF capacitors detect more PD activity than 80 pF capacitors