Abstrad -Capacitive couplers are widely used as sensors for in-semieelon-line partial discharge (PD) measurement in rotating machines. They am permanently installed in operating generators and motors to detect partial discharges. The capacitive couplers traditionally used for PD detection on rotating machines have heen 80 pF. Recently it has been found that higher detection sensitivity can he obtained by increasing capacitance of 80 pF couplers. This paper presents theoretical analysis, laboratory test and 6eld 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 hlgher 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.
Monitoring the health of large electrical machines, especially power station generators, is now an integral part of their operation to maintain and extend life. This work studies the use of electromagnetic sensors to detect inter-lamination insulation faults in the stator cores of large generators before they propagate to a level that can lead to catastrophic failure. The work develops a deeper understanding of the electromagnetic behaviour of core faults so that condition-monitoring sensors can be more specific about the location and severity of the fault. The study develops two new three-dimensional (3D) analytical models, one for predicting the fault current distribution in a stator core fault and the second for predicting the varying detection of such current by air-cored sensors. This further analysed the 3D detection efficiency of typical short fault lengths to compare with the two dimensional (2D) default of infinite-length faults. Different fault positions were modelled so that a clearer understanding of the location and severity of the fault is possible. These were validated on a specially constructed experimental test core that can impose controlled fault currents. The study also demonstrates how small core faults can escalate then self-limit radially, but may propagate axially into longer more damaging faults.
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