Increased sensitivity of capacitive couplers for in-service PD measurement in rotating machines

Capacitive couplers are widely used as sensors for in-service/online partial discharge (PD) measurement in rotating machines. They are permanently installed in operating generators and motors to detect partial discharges. The capacitive couplers traditionally used for PD detection on rotating machines have been 80 pF. Recently it has been found that higher detection sensitivity can be obtained by increasing capacitance of 80 pF couplers. This paper presents theoretical analysis, laboratory tests and field test experience to compare traditional 80 pF couplers and higher-capacitance couplers for in-service PD measurement in rotating machines. The frequency bandwidth of various capacitive couplers is characterized and analyzed in relation to the PD detection sensitivity. This investigation concludes capacitive couplers with a higher capacitance than 80 pF are more sensitive for in-service PD measurement in rotating machines. More partial discharges and higher PD magnitudes can be acquired by using higher capacitance couplers than 80 pF couplers.