On-site examination for predicting the remaining life-time of paper and synthetic insulated cables

On-site, nondestructive examination of long and medium voltage paper-, polyethylene-, and PVC insulated cables is rather difficult with 50 Hz frequency at nominal voltage due to the excess power consumption and network disturbances. But the benefit of these tests is that the parameters and limits which result have been fairly accurately and reliably determined after long experience acquired through laboratory tests of short cable sections. For solving this problem, DEDASZ Rt. has been studying its old paper-, polyethylene-, and PVC insulated cables for 5 years, after a cable diagnostic vehicle was developed. Analysis, especially as follow-up after a cable failure, is important in planning for cable replacement. This paper describes the special testing device and the on-site methods for measuring capacitance, loss factor, partial discharge, and the changing of parameters as a function of voltage. Once the rate of deterioration over time is known, with the help of a computer the remaining life of the cables can be determined with reasonable accuracy, based on the results of regular measurements. The life extension of two long cables by replacing the old cables in sections is described in this paper. Finally, background on the diagnosis of 4 complete cable replacements is presented