Partial discharge (PD) measurement of turbine generators under operating condition is disturbed by noise originating from arcing, sparking and corona outside generators. Characteristics of both the external noise and PD in stator windings are investigated by using simultaneous measurement with multiple sensors located inside and outside an operating turbine generator. Pulses of the external noise detected with capacitive couplers inside the generator do not have high frequency components as the PD pulses have. The pulses occurring at a grounding brush of the rotor shaft propagate into the stator windings and are detected with the capacitive couplers. The radiation from unknown noise sources outside the generator induces pulses in PD detection circuits. The external noise from isolated phase bus and exciter slip rings was not found with the capacitive couplers in this experiment.
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