Dielectric behaviour of ac aged XLPE cables

Polyethylene has been used as electrical insulation material for coaxial cables used in underground distribution systems for electrical energy, for more than three decades. The widespread use of this insulating polymer may be attributed mainly to its outstanding physical and chemical properties. However, it has long been recognised that ageing effects during service, such as water treeing phenomena, may lead to a premature electrical breakdown of such cables. The search for a diagnostic technique, indicating the operating conditions of the insulation has attracted much attention and is regarded in both scientific and technological point of view as an important problem. Present paper reports results of a study of electrical ageing of XLPE cables field aged for 25 years, by Dielectric Spectroscopy over a wide range of frequency. Return Voltage technique and Fourier Transform Infrared Spectroscopy (FTIR) measurements. These results provide succinct evidence that the degradation of the cables arises from a simultaneous presence of both dipoles and injected space charges which create a nonlinear internal field.