Frequency domain reflectometry (FDR) technologies could be applied to locate defects in cables effectively. Owing to the large space span and complex operating environment of the actual XLPE distribution cables, segmented thermal aging phenomenon occurs easily, causing the nonuniform distribution of electrical parameters along cables, which might affect the defect location accuracy ultimately. In this paper, the segmented thermal aging cable models were established and the effect of segmented-aging in XLPE distribution cables on defect location accuracy was explored based on simulation. It was found the floating range of defect location deviation in segmented thermal aging cable models could reach 10% of the whole cable length, which could not be ignored. Further research and analysis revealed that the defect location deviation in the segmented-aging cable mainly attributes to the differences of distributed wave velocities in the cable. On this basis, a correction coefficient of defect location $M$ was put forward for the segmented-aging cable models established, which has achieved remarkable results. Finally, a segmented-aging cable model with a length of 119 m was built in the laboratory, and the validity for improving defect location accuracy by considering the differences of wave velocities was verified through experiments. Results show the defect location deviation can be reduced from 2.19% to 0.39% by introducing $M$ .

The broadband impedance spectroscopy (BIS) method, a commonly used method for locating cable defects, is a method based on measuring the amplitude of reflected signals at different frequencies. However, noise may seriously interfere with the amplitude measurement and cause errors. Therefore, this paper presents a novel method based on the frequency modulated continuous wave (FMCW) to precisely locate defects in a cable. This method measures the frequency difference between incident and reflected electromagnetic waves in a coaxial cable within a bandwidth of tens of MHz. The anti-noise interference performance of frequency difference measurement is better than that of amplitude measurement. The noise power that the FMCW method can withstand is 7 dB greater than that of the BIS method. The experimental results show that FMCW method is effective for locating and detecting defects. For the minor defects prepared in this experiment, the effective location range of FMCW is greater than 800 meters, while BIS is less than 700 meters. The defect location deviation of the FMCW method is better than 0.5 % and the locating accuracy of the FMCW method is not inferior to that of the BIS method. FMCW method can be a very sensitive defect locating tool usable for real cable systems.

Off-line PD measurement is important for GIS. Interference signal generated by frequency-tuned resonant test set has a huge impact on PD measurement. Damped alternating current (DAC) voltage has been widely used in field testing of power cable and has no power interference. Whether the DAC can be used for PD testing of GIS still needs to be studied. The particle on insulator can greatly reduce the dielectric strength compared to clean condition, which is a very destructive defect. In this paper, the PDIV of AC and DAC were measured, the number of discharges per cycle was counted, and the PRPD pattern was obtained. For the defect model we set up, test results show that PDIV is independent of waveform and frequency; PD distribution area and the number of PDs per cycle increase with frequency. This is because SF ₆ gap needs to recover its dielectric strength after discharge. When the recovery time and discharge interval are on the same order, it's possible to excite PD at a lower voltage, thereby increasing the number of PDs in each cycle. DAC is effective for detecting the defect of particle on insulator in GIS, and the PD activity of DAC is more obvious.

To research the effect of temperature and humidity on corona performance of UHV DC transmission line, a mini corona cage has been constructed and put in a manual environment room, which temperature and humidity can be controlled. At the same time, a corona performance measuring system by using vary methods such as current impule method, acoustic method have been set up too. The corona inception corona voltage gradient of a copper line have been measured in a mini corona cage not only at different absolute humidity from 3 g/m³ to 12 g/m³ but also at different temperature from 0degC to 35degC . The relationship between inception corona voltage gradient and temperature and humidity have been achieved.

Crosslinked polyethylene (XLPE) distribution cables are prone to segmented thermal aging after long-term operation owing to the large spatial spans and complex operating environments, and accurate residual life prediction of each aging cable segment could provide a theoretical basis and reference for performance monitoring, maintenance and the replacement of cables. Existing studies mainly focus on the residual life prediction methods for uniform aging cables, which are not suitable for segmented-aging cables. In this paper, a residual life prediction method for segmented-aging XLPE distribution cables based on the time-temperature superposition principle (TTSP) by non-destructive BIS measuring on site was proposed. Firstly, the applicability of the TTSP in the transformation of the changing process of elongation at break (EAB) of XLPE at different thermal aging temperatures was verified based on the Arrhenius equation. Secondly, to better simulate the thermal aging process under working conditions, XLPE cables were subjected to accelerated external stress aging at 140 °C for different aging times, and the corresponding changing process of EAB along with aging time was further measured. The relationship between the EAB of XLPE cables and aging time was well fitted by an equation, which could be used as a reference curve to predict the thermal aging trends and residual life of service-aged XLPE cables. After that, a calculation method for the transformation of the changing process of EAB of XLPE at different thermal aging temperatures was proposed, in which the corresponding multiplicative shift factor could be obtained based on the TTSP instead of the Arrhenius equation extrapolation. Moreover, the availability of the above calculation method was further proved by accelerated thermal aging experiments at 154 °C; the results show that the prediction error for the cable’s EAB is no more than 3.15% and the prediction error for residual life is within 10% in this case. Finally, the realization of non-destructive residual life prediction combined with BIS measuring on site was explained briefly.

To research the effect of wind on corona current generated from DC transmission lines, a mini corona cage and a wind generated system have been constructed. The corona current generated from a copper line in corona cage have been measured with wind and no wind. The relationship between corona current and voltage with wind and no wind have been observed and compared. There were about 3% ~ 7% increased for corona current generated from DC conductor caused by wind after corona onset.