Condition Monitoring of Photovoltaic Cables Based Cross-Linked Polyolefin Insulation Under Combined Accelerated Aging Stresses: Electrical and Mechanical Assessment

Abstract

The degradation of insulating materials has been recognized as a prime cause of the precocious failure of cable systems. In this research, the role of simultaneous thermal and mechanical stresses on the insulation integrity of cross-linked polyolefin insulation based low voltage photovoltaic cables was studied. The cable samples were subjected to combined thermal–mechanical aging at 120 °C for 120, 189, 258, 396, 636, 876, 1000, and 1120 h aging cycles. The effect of the accelerated aging tests on the insulation integrity of the cross-linked polyolefin was explored by electrical and mechanical techniques. The electrical assessment was based on the measurement of the complex permittivity with frequency variation between 100 mHz to 500 kHz and the measurement of the decay and return voltage slopes. The Shore D hardness as a mechanical testing technique has been carried out. Surprisingly, after 1120 aging hours, the real part of permittivity was almost the same as the pristine case. Also, the imaginary part of permittivity retarded below the pristine case. That was more dominant at frequencies higher than 10 Hz. A good agreement was found between the decay voltage slope and the imaginary permittivity at 1 Hz. Also, between the return voltage slope and the real permittivity. The cable hardness showed anomalous behavior with aging. This shows the ability of the cross-linked polyolefin to recover its mechanical properties when subjected to elevated temperature.