Investigation of Photovoltaic DC Cable Insulation Integrity under Thermal Stress

DC cables account for only 1-2 % of the overall cost of the photovoltaic system but have a significant impact on the output of the power system. During service, the DC cables have to bear harsh conditions as they have to endure high temperatures, mechanical and atmospheric stresses. Out of these stresses, the thermal stress is a constant stress in which the cables are exposed to causing degradation in the insulation and jacket of the cable. This could reduce the service life of the cable insulation and may result in embrittlement, cracking and eventual failure of the insulation, and risking a possible short circuit. This paper has been aimed to study the overall degradation of the low voltage photovoltaic unshielded DC cables under thermal stress by adopting dielectric spectroscopy and hardness as non-destructive condition monitoring techniques. The cables were accelerated thermally aged under 120°C temperature for seven cycles. The insulation and jacket were kept intact and the overall state of the cable was examined. The significant variation in the real and imaginary part of permittivity was observed at low frequency, 0.01 Hz. While with aging the cable became harder, showing the sign of degradation in the cable. The results show the potential capability of the techniques to be used as diagnostic techniques for the low voltage unshielded cables.