Determination of Trap Distribution Based on Voltage Response Measurement for Condition Monitoring of Nuclear Power Plant I&C Cables

The voltage response (VR) measurement was developed decades ago as a condition monitoring technique for oil-paper insulation of electrical equipment. Later, the improved version of the method, namely, the extended voltage response (EVR), was introduced as an efficient tool for determining the electrical model and the dielectric response of insulations. This article presents a new interpretation of the EVR measurement results, namely, the deduction of how the trap distribution could be determined from the measurement results and how the trap distribution provided by the method can be used as a potential condition indicator for nuclear cables. The proposed method was tested on gamma-irradiated and thermally aged ethylene-propylene rubber (EPR)-insulated instrumentation and control (I&C) cable samples. The shallow and deep traps around 0.76- and 0.86-eV energy depth were identified, and the trap densities showed an increasing trend with aging time for irradiation and thermal aging. The results were compared with elongation at break (EaB) data, the most generally accepted condition indicator for polymeric cable components in the nuclear industry. Since a strong correlation between the EaB and deep trap density was shown, regression curves were calculated. In addition, based on the regression curves, threshold trap density values were determined for 25% of the remaining lifetime. The results revealed that the deep trap density can be a reliable condition indicator for nuclear power plant (NPP) cables.