THE DEVELOPMENT OF EVALUATION METHOD OF THE HV AND EHV CABLE POLYMERIC INSULATION LIFE CURVE. FIRST RESULTS AND AREAS OF APPLICATION
The testing procedure and experimental unit are described which permit to derive the life curve i.e. electrical tree induction period vs. applied voltage dependence for the crosslinked low density polyethylene extruded insulation. The developed equipment permits to test samples of materials both made in laboratory and cut from commercially manufactured cables at room temperature as well as at long-term permissible and overload temperature. The fact of electrical tree inception is established by means of application of light microscope directly during simultaneous high voltage test of 10 samples. The optical system and micromanipulation technique used provide the spatial resolution equal to approx. 1,5 ^m and high precision of the microelectrode delivery to the area of interest inside the sample as well as permanent and reliable contact between the microelectrode and material and preservation of residual mechanical stress field. The results obtained may be used for the development of new HV cable construction. The derived life curve permits by using the corresponding mathematical model to calculate the insulation wall thickness, which for instance is equal to 19-20 mm for 220 kV cable.