Research On Diagnositic Method Of XLPE Cables Degradation And Measurement Of Space Charge

XLPE cables are widely used in urban distribution system due to their advantages compares with the traditional oil-filled cables and oil impregnated paper cables, such as light quality, easy installation and maintenance, excellent mechanical and electrical properties. The dielectric property of XLPE cables will degrade with time due to the combined effects of electrical field, temperature and water in the environment, which may lead to break down of the cables and affect the reliability of power transmission. Therefore, it has great theory and application value to investigate the diagnosis technique of XLPE cables.In this paper a new diagnostic method called Thermal Pulse Current Method (TPCM) is proposed based on the correlation between degradation of XLPE cable and space charge accumulation. The TPC involves applying a temperature step to a short-circuited insulating sample and measuring the capacitive current response, which is then used as an index to evaluate the ageing degree of the XLPE cables. Two variants of the TPM can be used: the inner heating technique (IHT) and outer cooling technique (OCT).10 kV XLPE cables are employed as specimens in the experiment, which were aged by Accelerated Water-Tree Test (AWTT) in a curved water-filled tube. The cables were subjected to an AC voltage of 26.2 kV and a daily thermal load cycles (8 hours per day) through the use of inductive current. Three groups of cable were marked A, B and C with ageing time of 360 days, 180 days and 120 days respectively. Then the specimens are tested by the IHT and OCT and the correlation between the thermal step current and the ageing degree is studied. Obtained results show that the amplitude of the thermal step current increase with increasing the ageing degree; the TPC on the same specimen is dependant on the applied DC voltage; The IHT and OCT are The IHT and OCT are applied to the evaluation of the mean insulating state and the analysis of local defect respectively, but the IHT is more sensitive to a slightly aged cable. The validation of the TPC system on the evaluation of the cable condition is verified. Obtained data and experiences in the experiment are useful for the diagnosis of the field aged cable.