Combining synchronous UHF and IEC-compliant PD measurements for an effective noise suppression

On-side PD measurements are very often interfered by external disturbances as corona from neighboring systems and switching activity from the grid, respectively. Therefore no clear assignment of PD sources to the object is possible. However, a substantiated risk assessment requires such assignment and, even more, an identification and location of the PD fault. As the classical PD measurement methods, according to the IEC 60270, are very sensitive to external interferences, UHF detection methods are very common, allowing a high level of noise suppression. Due to the propagation characteristics of PD pulses in the UHF-range, a special UHF sensor, mounted close to the object to be tested, permits a sensitive focusing on partial discharges, as external noise and disturbances are highly attenuated. Hence, UHF signal detection ensures a high level of sensitivity and selectivity, respectively [1]. As a major disadvantage, UHF measurements can not be calibrated in terms of PD charge magnitudes. With international standards implying limit values to be specified in pico-Coulombs, the PD measurement needs a valid calibration to deliver comparable results. If possible two consecutive PD measurements can be done in order to A) detect PD in the UHF-range and B) evaluate them in the IEC-range. With a highly disturbed IEC-range, this may not be possible at all. With a modern synchronous PD measuring system it is possible to perform these two measurements simultaneously. Additionally, and as a main benefit, it is possible to use the UHF PD data as trigger source for the I EC PD measurement. This allows detecting PD with a high sensitivity and selectivity in the UHF-range, but allocating the appropriate PD level in terms of pico-Coulombs with the calibrated IEC-compliant PD detection.