PNNL

Abstract

Research conducted by the Electric Power Research Institute (EPRI) and other research institutions have concluded that water-trees are one of the leading degradation mechanisms that contribute to the loss of dielectric insulation strength in medium-voltage cable insulating materials in wet or submerged environments. The electrochemical reactions are caused by the combined effect of water presence and relatively high electrical stress. Records of cable failures provided by the licensees in response to Generic Letter (GL) 2007-01 (NRC 2007) have called into question the reliability of medium voltage cables in wetted or submerged environments. EPRI’s dissipation factor or Tan Delta testing guidelines and acceptance criteria have been adopted by most nuclear power plant operators as the primary tool for condition monitoring of medium voltage cables in wet or submerged environments. EPRI has been collecting member data since late 2009 to analyze and provide feedback to members, validate the EPRI-developed acceptance criteria guidelines, support analysis of test results, recommend appropriate actions for the "action required" category, and gather candidate cables for EPRI-sponsored forensic research on causes for insulation degradation. EPRI has collected data from 37 nuclear sites, which represent 44 operating units. The test results have been organized by insulation type such as cross-linked polyethylene (XLPE), butyl rubber, black, pink, and brown ethylene-propylene rubber (EPR; and mixed insulation (hybrid insulations). The data have been analyzed, and follow-up information was obtained from members for "action required" test results. EPRI has also performed correlations between Tan Delta tests and the information gathered under the EPRI forensic research on medium voltage cables. In addition, EPRI has developed guidance on how to systematically analyze Tan Delta test results. In order for the NRC to perform the evaluation of the EPRI criteria issued in reports 3002000557 and 3002005321, EPRI has agreed to provide the NRC with the Tan Delta data collected from the licensees. The objective of this project is to perform a statistical analysis to determine whether the Tan Delta test data collected by EPRI supports the EPRI criteria issued in EPRI Reports 3002000557 and 3002005321 to manage the aging of cables in submerged environments. The specific questions to be addressed are: • Are the EPRI recommended testing intervals and thresholds for cables that test green (good) and yellow (further study) suitable to manage the aging of cables in submerged environments? • Based on the data, are the thresholds and interval guidelines statistically significant? • Is the data provided by licensees to EPRI aligned with the test guidelines? • After binning the data in ranges from 0 -10 years, 10 – 20 years, 20 – 30 years, 30 – 40 years, 40 – 50 years, and 50 – 60 years, consider failure rates and test data to assess correlations. This analysis reviewed the two primary reports (EPRI 3002000557 and EPRI 3002005321) plus two derivative reports that were cited in the primary reports (EPRI 1028262 and EPRI 1021070). The principal results, conclusions, and recommendations from this project’s analyses were as follows: • Cable insulation degradation failures are relatively few during the first 10 years following installation. Thereafter, however, there is no strong correlation between cable age and failure rate. The correlations between the tan d test data and service year are at most 0.1 (low on scale from -1 to +1) when considered at the cable level and at the phase level. • The thresholds set under (EPRI 2013) have resulted in relatively few false positive and false negative indications. Cables identified as “good” have rarely failed before their subsequent planned inspection and forensic investigations of cables identified as “repair or replace” have almost always identified cable segments with problems. The false positive rate (the