The influence of ageing on the characteristics of oil-filled cable dielectric

An investigation has been made of the electrical and mechanical characteristics of the insulation of samples of cables in the range of 60-230 kV after many years of operation in order to evaluate the influence of ageing on the dielectric of oil-filled cables. To determine the effect of thermal ageing alone and in combination with electrical stress, a long series of laboratory tests has been carried out on components of cable insulation, on cable models and on actual cables. A comparison has been made between the laboratory investigations and the state of the insulation of cables after many years of operation. The following conclusions have been drawn: (a) Paper is the component most affected by temperature. (b) The electric strength is not influenced, within the limits of the tests, by the combined effects of temperature and electric stress. (c) Cables examined after more than 20 years of operation are still in perfect condition and will go on operating satisfactorily for many more years if present loading conditions are maintained. It is suggested that the mechanical deterioration of paper should be taken as a criterion of the state of used cables and to fix, for both new and used cables, the temperature limits for normal and emergency loading in relation to the desired life of the cable.