Comparison of new cables and aged cables out of the grid by the Thermal Step Method

In order to evaluate the "ageing" of a cable, we have to compare some parameters with those of the same non-aged cable or with a population of new cables comparable with the tested cable. During four years, in Laborelec a bank of cables (150 kV and 70 kV, new and out of the grid) has been constituted in order to characterize the insulation material. The Thermal Step Method (TSM) developed by the Laboratoire d\´Electrotechnique de Montpellier (LEM) has measured the cables in particular. Other chemical techniques (FourierTransform-InfraRed, Differential Scanning Calorimetry) have been also used. So, we have actually a good collection of the products installed in the Belgium grid since four years and considered as reference. A first ageing marker has been defined as the distance between the maximum charge density of a tested cable and the robust line calculated through the space defined by the maximum of charge density and the minimum of charge density (about 30 cables from the bank). If the distance of a cable is greater than three standard deviations of the distribution of the established parameters (charge density), the assessed cable can be considered on base of statistical approach, as modified and further measurements are needed to assess profoundly its state. A second ageing marker is the electrostatic pressure or even better: the sum of the absolute values of the electrostatic pressure. The greater the number of stress zones, the greater the risk of accelerated ageing.