The water tree is one of the key factors leading to the insulation property deterioration in cross-linked polyethylene (XLPE) cables. An accelerated ageing experiment has been performed to produce water trees with the purpose of ageing the full-size XLPE cable under the high-frequency electric field. Based on Polarization and Depolarization Current (PDC) method, the diagnostic tests are employed to evaluate the ageing extent of the cable subjected to water tree at different ageing time. Results show that the average length of water trees is proportional to the ageing time, and thus the experiment is effective to accelerate the growth of water trees. Furthermore, such various diagnostic indicators as the I*t~lnt curves, the loss factor at different frequency, have their rules to assess the ageing condition of the cable with the duration of ageing time. Therefore, information obtained from PDC current can be used for determining the ageing extent of the cables.

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This article discusses the main structure of HV cables as well as the characteristics of buffer layers and detection of ablation defects. To effectively detect ablation defects in the buffer layer, two approaches, terahertz time-domain measurements and commercial computed tomography (CT), were used in the laboratory to evaluate samples of cable with these defects and compare different preset conditions for detecting defects. The test results showed that the terahertz time domain measurement method was more sensitive than CT to moisture intrusion into the cable, but terahertz wave energy was reflected on the surface of the aluminum sheath of the cable and could not penetrate the sheath, thus, it was difficult to use this method to detect internal defects. The commercial CT method used X-rays to penetrate the aluminum sheath, and it could effectively obtain data for the three-dimensional reconstruction of each layer of material, other than the copper core. The structure of the cable was mapped through three-dimensional reconstruction using VGstudio Max software, and the three-dimensional map and gray value analysis were combined. The results confirmed that defects with different densities in the buffer layer could be detected by commercial CT. Thus, commercial CT detection methods provide an effective solution for identifying hidden defects in cables and have profound practical significance for the development of methods to detect defects when cables are in use.

With the continuous development of the power system, 500kV transmission lines have become the backbone of power grid transmission lines. The construction of a large number of transmission lines is intertwined with the activities of birds. Line failures caused by birds have become the main cause of power system trip accidents. First, it seriously threatens the safe operation of the power system, and its degree of damage is second only to lightning strikes and external damage. Through related research and analysis, the bird damage of 500kV transmission line is mainly caused by three aspects: bird droppings, bird body and bird pecking. This paper analyzes the formation mechanism of bird damage on 500kV transmission lines, and provides some guidance for the corresponding prevention measures for the protection of transmission lines from bird damage.