Laboratory Testing Results Of Vintage Medium Voltage Nuclear Power Plant Cables

Abstract

All nuclear power plants (NPPs), and other types of industrial facilities, over time suffer from faults and operating issues in cable circuits, connectors, shielding, and end devices while in service. These types of issues occur in both low voltage (LV) and medium voltage (MV) circuits, and a wide range condition monitoring (CM) technologies have been developed to identify, locate, and quantify the severity of age-related degradation that occurs in these systems and components. In general, the degradation and failure mechanisms that occur in MV cable circuits can be significantly different than those that occur in low voltage (LV) cables. One of the primary concerns of MV cables installed in NPPs is moisture related degradation that can cause a buildup of localized electrical stresses in the cable insulation polymer. These electrical stresses can lead to the formation and growth of water trees and/or electrical trees in the insulation and cause partial discharges to occur in the cable polymer. Over time, these moisture related issues can cause degradation of the insulation material and/or cable failure. As NPPs and other industrial facilities age, identifying, locating, and quantifying degradation in systems important to safety, operation and production along with their associated cables is becoming a higher priority for long term reliability. This paper presents the results and findings of a cable condition assessment that was performed using laboratory CM techniques to assess the aged condition of six (6) 5 kV MV cable samples removed from service at an operating nuclear power plant. During operation, these six (6) samples and other sections of this cable were submerged in water, and after nearly forty years of service had noticeable signs of moisture intrusion into the cable (e.g. low insulation resistance measurements and swelling and blistering of the cable jacket). Based on the results of the laboratory condition assessment, the jacket and insulation materials of these cable samples are susceptible to moisture related degradation, which can lead to water/electrical tree formation and growth in the cable insulation. These types of issues lead to degradation of the electrical properties of the cable (e.g. decreasing insulation resistance) and can eventually cause cable failure.