Diagnosis of power cable insulation using isothermal relaxation current technique

Isothermal Relaxation current (IRC) is an indication of the energy distribution in the various traps distributed in power cable insulation. A correlation has been established between aging of the cable insulation and IRC. IRC is one of the nondestructive methods to determine the remaining life of a working power cable. Determination of the characteristic aging factor involves the determination of the various relaxation time constants associated with IRC. A computerised measurement system to do IRC measurement is developed by which the cable is energized up to 1 kV dc and a current up to 10 pA can be measured. The signal processing algorithms are developed to filter the high frequency noise in the measurement, to fit the data to the desired number of relaxation factors and to determine the aging factors. The developed system is tested with the samples supplied by Shanghai Electrical Cable Research Institute and Singapore Power Grid. The results are compared with the aged and non aged samples. A close indication of aging is found by the application of extended Debye method and Universal relaxation law