Very Low Partial Discharge Values Found While Commissioning A New Wind Farm
During a recent wind farm commissioning project in the southern United States multiple defects were identified within the collector cable system. One particular cable exhibited defects in the form of very low levels of partial discharge at two times the operating voltage. The defects identified by the IMCORP Factory Grade® assessment had very small discharge values, measuring 7 and 8 pico-Coulombs (pC) respectively, indicating imperfections that are small in physical size. Since IMCORP’s Factory Grade® technology is the only available field assessment with better than 5 pC sensitivity, the contractor began to question the validity of the results and rumors arose doubting the existence of any defects at all. As such IMCORP deployed its location matching technology to determine the position of the PD sites and a rigorous recovery process was followed in order to extract the cable segment for laboratory analysis. No physical evidence of damage to the cable was observed during the extraction process. Upon extraction the cable segment was sent to IMCORP for full dissection analysis in the presence of the site owner, contractor and cable manufacturer.
During the laboratory analysis the cable was tested again revealing the same electronic PD signatures that had been identified on site. Upon removal of the jacket and concentric wires voids were observed on the outer semiconducting layer. Further removal of this layer revealed multiple points of delamination on the insulation, evenly spaced longitudinally, most likely attributed to a fault in the cable manufacturing process.
Defects in cable systems have a serious impact on renewable energy suppliers as downtime impacts both the top and bottom lines in terms of lost revenue, emergency fault location expenses, and excavation and repair costs. Recent surveys have concluded that the typical cost of failure can top $100k per incident. In addition, IMCORP compiles an annual report measuring the cable system quality rating (CSQR) for utility-scale renewable energy sites in the United States. This report has revealed that approximately 15% of cable collector systems have at least one substandard component that requires extensive repair actions. This cost of failure is avoidable through field testing cable systems to manufacturers’ standards, using IMCORP technology, during commissioning and planned maintenance outages for any new renewable site.
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