Parametric Evaluation of Water Treeing in EPR-Insulated Medium Voltage Cables using Finite Element Analysis

Abstract

Medium voltage (MV) electric cables are used extensively in industrial settings, including nuclear power plants (NPPs). In NPPs, these cables provide supplementary power for safety systems to continue operating during emergency events. Despite efforts to maintain these cables, premature failure is known to occur, with the predominant causal factor being water tree-induced degradation of the cable’s insulation component. To better understand the effects of this degradation source, this thesis presents a parametric evaluation of various water tree and cable parameters using finite-element analysis (FEA). The parameters being evaluated for a MV cable insulated with ethylene propylene rubber (EPR) are water tree depth, composition, and geometry, as defined by aspect ratio (AR), and cable operating frequency and temperature. Evaluation is performed in five separate but interrelated areas pertaining to the measurement of degradation: global capacitance, global resistance, voltage and electric field distribution, localized specific energy absorption rate, and localized temperature rise. Results show that the rate of water tree-induced degradation is affected by each parameter. In general, rate of degradation was found to be directly related with water tree depth and AR, and cable temperature, but inversely related with cable operating frequency. Although values differed, these trends were largely maintained regardless of water tree composition. The results and findings of this parametric evaluation have provided an advanced understanding of water tree degradation in MV EPR-insulated cables. In addition, an argument for further use of FEA in conjunction with physical cable testing was presented, with the conclusion being that there exists a strong motivation to pair the two together.