Sensitivity improvement of capacitive couplers for on-line partial discharge testing in rotating machines

Capacitive couplers are widely used as sensors for on-line partial discharge (PD) monitoring of rotating machines. Capacitive couplers in conjunction with Partial Discharge Analyzers (PDA) can detect PD pulses from operating generators and motors without an interruption of machine operation. The capacitance of the couplers traditionally installed for PD detection on rotating machines has been 80 ρF. Recent research by Adwel International Ltd. has improved the PD detection obtained with capacitive couplers for some types of rotating machines. This paper begins with an introduction to on-line, or in-service, PD testing of large rotating machines. It includes some calculations and discussions on the frequency bandwidth to be used for in-service PD testing. In-service PD field test results taken from the same machine using both the traditional 80 ρF capacitive couplers and 500 ρF capacitive couplers are presented to show the increased effectiveness of higher capacitance couplers. PD field testing indicates that 500 ρF capacitive couplers have a higher sensitivity than 80 ρF capacitive couplers without introducing significant noise. They can be more suitable than 80 ρF capacitive couplers for PD measurements in smaller or lower-voltage rotating machines