Dependencies of the PD- and tan(δ)-characteristics on the temperature and ageing status of MV PILC cables

Diagnostic tools and methods for the state evaluation of MV (medium voltage) and HV (high voltage) power cables are mostly based on single key parameters like the partial discharge (PD) levels (inception voltage and intensity) or the values of the dissipation factor tan(δ). The main goals are the determination of the functionality and actual condition of the equipment. Additionally, the prediction of the rest life-time of power components could help to improve the investment planning and the maintenance strategy. In the field of cables and especially in case of PILC (Paper Insulated Lead Covered) cables, the requirements on the measurement technology and diagnostic systems are rather complex. In many cases the attention is directed to the methodology and the accuracy of the measurement equipment itself or to the value or spectrum of the measurement frequencies. However, the reliability of these methods is also very dependent on the experience of the test engineers and the knowledge and interpretation of all measured parameters. Therefore, besides the electrical parameters, in particular the temperature and humidity during the measurement, the equipment's dimensions, cable lengths and noise levels besides others must be taken into account. In this article, the principal dependencies of two major electrical criteria, which are the tan(δ)- and the PD-characteristics on the cable temperature and the cable age or the operational history of the cables, are presented. The strong correlation on e.g. the temperature makes clear, that the interpretation of a single criteria and a subsequent prognoses of the cable status are nearly impossible without further information. The derived results would be a subject of an immense scattering depending on seasonal influences, the load status during the disconnection of the analyzed lines, time constants, cable length, etc.