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

Capacitive couplers are widely used as sensors for in-service/on-line parti al discharge (PD) measurement in rotating machines. They are permanently in stalled in operating generators and motors to detect partial discharges. Th e capacitive couplers traditionally used for PD detection on rotating machi nes have been 80 pF. Recently it has been found that higher detection sensi tivity can be obtained by increasing capacitance of 80 pF couplers. This paper presents theoretical analysis, laboratory test and field test ex perience to compare traditional 80 pF couplers and higher-capacitance coupl ers for in-service PD measurement in rotating machines. The frequency bandw idth of various capacitive couplers is characterized and analyzed in relati on to the PD detection sensitivity. This investigation concludes capacitive couplers with a higher capacitance than 80 pF are more sensitive for in-se rvice PD measurement in rotating machines. More partial discharges and high er PD magnitudes can be acquired by using higher capacitance couplers than 80 pF couplers.