Analysis and verification of signal propagation as method for temperature monitoring of underground power cables

Underground power cables are increasingly becoming a bottleneck in the medium-voltage grid due to congestion. Limitations arise from the lack of knowledge on their thermal state. This paper investigates for two commonly applied cable types, PILC and XLPE, how signal propagation velocity can be exploited as means for thermal monitoring. To this end, the transit time of signals sent along a cable is interpreted in terms of the temperature dependency of the cable insulation permittivity. The sensitivity is verified by laboratory and field tests. Shifts in waveforms can be distinguished with a resolution down to 1 °C, but uncertainties in cable parameters hamper a direct conversion of the propagation characteristics to an absolute temperature scale. Instead, changes in waveforms need to be calibrated with respect to a reference signal at a known temperature. Parameter uncertainties then only arise in the relative shift and an accuracy below 5 °C is obtained. The method is demonstrated for a multi-conductor power cable exhibiting distinct signal propagation modes.