Temperature Dependence of FDR Response for Thermally Aged Low-Voltage Cables

Abstract

Frequency Domain Reflectometry (FDR) has attracted significant attention for use in nuclear power plants (NPPs) for non-destructive cable health monitoring. However, cable aging detection using FDR faces challenges due to its high sensitivity to environmental factors such as temperature, vibrations, proximity to other cables, and so on. This work aims to evaluate the influence of measurement temperature on the FDR reflected signal in a low voltage cable undergoing accelerated thermal aging. A 100 ft long multi-core low-voltage unshielded power cable insulated with flame retardant ethylene propylene rubber (FR-EPR) and covered by a chloro-polyethylene (CPE) jacket was selected for this study. The cable was energized during aging by a 480 ${\mathrm {V}}_{\mathbf{AC}}$ 3-phase motor, and a 30 ft mid-section of this cable was routed through an air circulating oven held at $140 ^{\circ}\text{C}$ for up to an effective aging time of 62 days. FDR measurements were taken periodically with the oven on (at $140 ^{\circ}\text{C})$ and with the oven off (at $22 ^{\circ}\text{C})$. A comparative analysis of data collected at both temperatures showed that the FDR response was strongly dependent on measurement temperature. FDR measurements at ambient temperature showed large peaks corresponding to impedance changes in the aged section of the cable after 3 days of aging. These peaks continued to rise steadily with increasing aging time. However, for measurements taken at $140 ^{\circ}\text{C}$, slowly rising peaks in the oven region (aged section) were observed only after a lead time of 23 days of aging. This work highlights the importance of measurement temperature on the performance of FDR as a condition monitoring tool for aging cables.