Experiences with diagnostics on complete in-service MV cable feeders

By using an on-line continuous condition monitoring system, many problems in cable connections can be detected before leading to an outage. The system used for the experiments in this paper is capable of detecting and locating weak spots in complete cable feeders, continuously and while the cable remains on-line, by using only two sensors for the complete cable connection. The system uses partial discharges (PDs) that originate from weak spots in the cable as its diagnostic principle. For MV cables, joints en terminations, detection of partial discharges is proven to be a reliable method. However, this statement is only valid if the PDs are measured correctly and interpreted reliably. The paper summarizes the measuring principles that guarantee the correct detection of partial discharges. The main part of the paper discusses field experiences and results. The system was designed for application on existing, sometimes old, cable connections. However, some interesting phenomena have been observed in new cable feeders too. This, together with the basic principles behind the technology, makes the system extremely applicable in the majority of practical field situations. From the first field experiences important lessons have been learned to increase the reliability of the interpretation. Partial discharge signals are being measured, but risks on failures are produced.