Study Of High Voltage Frequency Domain Dielectric Properties For Thermal Aged XLPE Cable

XLPE cables are affected by the service life and operating environment,and will produce different degrees of overall or partial thermal aging,which will lead to a decline in insulation performance.These two kinds of thermal aging have different harm to cables and different maintenance methods.At present,the cable thermal aging diagnosis method can diagnose the overall thermal aging of the cable,but it can not effectively distinguish and diagnose the local thermal aging of the cable.Therefore,based on the high-voltage frequency domain dielectric spectroscopy,the detection and diagnosis of XLPE cable insulation thermal aging are carried out.In this paper,the whole thermal accelerated aging(internal heating method)test and local thermal accelerated aging(external heating method)test are designed,and the aging samples are connected in series with the intact cable to simulate the local thermal aging phenomenon of the cable.The mechanical properties,FTIR and XRD of the samples with different aging cycles were tested.The physical and chemical properties such as elongation at break,carbonyl index and crystallinity were obtained.High-voltage frequency sedomain dielectric spectroscopy tests were carried out on the whole aging sample and local thermal aging sample,the dielectric characteristic parameters,such as delamination S and nonlinearity η,are extracted and compared with 0.1 Hz very-low frequency dielectric loss test.The results show that the elongation at break,carbonyl index and crystallinity of the samples do not change significantly in the early stage of aging,but in the middle and late stage of aging,with the increase of aging time,the elongation at break and crystallinity of the samples decrease,while the carbonyl index increases.Although the 0.1 Hz very-low frequency dielectric loss test can reflect the aging degree of the overall thermal aging cable,it can not effectively diagnose the local thermal aging of the cable.However,the delamination S and nonlinearity η of high-voltage frequency domain dielectric spectroscopy are less affected by the intact cable.The results show that the delamination S is greater than 1 and the nonlinearity η increases with the increase of aging time in both the whole thermal aging and the local thermal aging(0.4m aging sample is connected with 4m and 10m intact cable in series).The whole thermal aging and high proportion local thermal aging can be distinguished and diagnosed by delamination S and nonlinearity η.And the delamination S and nonlinearity η improve the sensitivity of cable thermal aging diagnosis.