Life assessment of cable paper using slow thermal ramp methods

Rapid thermal ramp techniques are widely used to assess the properties of p olymers (e.g. DTA, TGA, etc.). For certain types of processes, slow thermal ramp methods can assist the process of assessing the longer term ageing an d life prediction of a polymeric material in complex environments. We will illustrate this in relation to recent work on the ageing of Kraft electrica l insulation paper in insulating oil in relation to the longer term ageing of oil-filled power cables. In this case the key indicators of degradation were the degree of polymerisation (DP) and the production of gases capable of dissolving in the oil and measured by dissolved gas in oil analysis (DCA ). Kinetic modelling of the change in the DP value was used to generate Arrhen ius parameters and cable life prediction calculations were made for a range of potential cable operating temperatures. The DP decreases with age and i ncreasing temperature in a complex chain scission reaction. In parallel, ab ove 150 degrees C there is a rapid exponential increase in the evolution of CO, CO2, CH4, C2H4 and C2H6 with increasing temperature. We found that the rate of degradation of the paper/oil system is dependent on the degree of containment of the ageing experiments. These results indicate that containm ent effects and resulting pressure effects exist in this type of accelerate d non-isothermal ageing experiment and should be accounted for in practical ageing environments. The estimated life of the paper can be calculated if we assume that the Arr henius parameters are representative of the entire degradation process and if suitable end-of-life criteria can be defined. It is also possible to cal culate the average operating temperature of an electrical cable over a fixe d time period, knowing the starting and finishing DP. Some of the attractio ns, potential weaknesses and use of the slow thermal ramp method in complex environments are also discussed. (C) 1999 Elsevier Science B.V. All rights reserved.