Pd approach for on-line pd monitoring of cable systems using the minimum number of pd sensors

Summary

The new TS IEC 62478 introduces different PD approaches for unconventional PD measurements in high voltage cable systems. However several questions arise when a continuous on-line PD monitoring is required: What is the most appropriate frequency measuring range in order to minimize the number of PD sensors? Should all accessories be monitored? What is maximum distance between two consecutive sensors to be sure a good monitoring is achieved? What kind of PD monitoring (PD detection or PD location) should be chosen? What is the relationship between PD sensitivity of the measuring instrumentation under background noise and the number of PD sensors required? PD measurement in the HF range is the most efficient approach to reduce the number of PD sensors when large cable system must be monitored using the minimum number of PD sensors. However, noise rejection in the HF range is the most critical problem for the PD instruments operating in this frequency range. When only a narrowband measuring frequency higher than few megahertz is used to perform on-line PD measurements special attention must be paid to be sure the selected narrowband measuring frequency is not affected by noise influence for time period where the PD monitoring will be performed. Higher measuring frequency values are usually less affected by electrical noise but for frequency values higher than 8 MHz signal attenuation along cable system could be too high. Consequently, cable length to be monitored could be very short and the number of PD sensors too much. PD approaches in frequency range between several hundred of kilohertz and few tens of MHz using powerful signal processing tools for noise suppression are appropriate enough for on line continuous PD measurements, because they are able to reconstruct the original PD pulse after filtering process maintaining significant amplitude values in any frequency range where the PD pulse was measured. Automatic noise rejection approach without any expertise intervention and with a PD sensitivity at least of 10 pC is convenient for on-line PD monitoring to be able to detect internal defects with time enough to make the corresponding maintenance decisions.

Additional informations

Authors

LANDINGER, ICHIKAWA, KAZMI

Keywords

Cable systems, sensor, partial discarges, signal procesing, measurements