The relationship between electro-chemical-tree ageing and charge relaxations behaviour of XLPE power cables

During the degradation of organic insulation, deterioration products are formed by oxidation, resulting in a change of the space-charge behavior. The initiation and growth of electrochemical treeing (ECT) implies a more complex decomposition by the simultaneous presence of water, electrical field, and a disturbed polymer. The chain of disintegration forms a greater amount of polymer radicals, hydroxyls, and many molecules with different lifetimes, causing a characteristic difference in charge relaxation behavior. This phenomenon was evaluated, separating the relaxation current out of the transient discharge current of aged and unaged XLPE (cross-linked polyethylene) power cables ready for service with a modified charge measurement technique. The change of the dielectric relaxation phenomenon which accompanies ECT generation is investigated by a computer evaluation of the relaxation current using the idea of approximating the measured current by a superposition of exponential functions. The result is a diagnostic tool with a high power of distinction on ECT-free and ECT-contaminated XLPE power cables