Temperature dependent signal propagation velocity : possible indicator for MV cable dynamic rating

Insulation temperature is the critical parameter for dynamic rating of underground power cables. Power cables on medium voltage (MV) level are too numerous and widespread to economically justify solutions like optical fibers as temperature sensors, often employed for high voltage power cables. This paper proposes to utilize the propagation velocity of high frequency signals as indicator for MV cable dynamic rating. Laboratory scale tests are performed for both PILC and XLPE cables. Test results show that the high frequency signal propagation velocity for XLPE insulation increases with the temperature rise while PILC cable shows opposite behavior. The variation of propagation velocity with temperature is consistent with the variation of permittivity measured on test samples of both materials. The variation of propagation velocity of XLPE is confirmed by data of a power cable in service subjected to strong load cycling monitored over a week. Field data on propagation velocity also matches load variation observed for a PILC cable for load cycling recorded over one week. From laboratory tests and field measurements, it seems feasible to extract thermal information from MV cables for dynamic rating by means of high frequency propagation velocity.