Comparison of UHF antenna and VHF capacitor PD detection measurements from turbine generator stator windings

On-line partial discharge (PD) measurements are widely used to detect stator winding insulation problems. The most widely applied measurement method uses 80 pF capacitors to detect the PD in the VHF (30-300 MHz) frequency range. However, there are more than 1000 large turbine generators equipped with a UHF antenna called the stator slot coupler (SSC). The SSC method operates in the frequency range 10-1000 MHz. In several cases both PD measuring systems were installed on the same generator. This paper compares the results from 13 generators that have both the VHF and UHF PD measurement systems. In some cases the correlation between the systems is good. However, there are some situations where one system indicates high PD is present and the other does not. This is presumably caused by the differences in pulse attenuation that occur in the two frequency ranges and the influence of high level sparking noise at the machine terminals, to which the capacitive sensors are preferentially sensitive.