The polarization and depolarization current (PDC) measurement has been widely used to assess the aging degree of cross-linked polyethylene (XLPE) cables. However, the aging type identification of XLPE cables has not been fully considered. In this study we conduct PDC measurement on thermal-aged and water tree-aged XLPE cable samples respectively, upon which a graphical approach to assess the insulation condition and distinguish the aging type of XLPE cables is proposed. First, to designate the evaluation indexes of the insulation condition, multiple PDC measurement based dielectric parameters are considered. Then, to reflect the sensitivity of each evaluation index to the condition of XLPE cable, a combined weighting method based on the group fuzzy analytic hierarchy process (GFAHP) and entropy method (EM) is introduced. Furthermore, the improved radar chart method (RCM) based graphical approach is proposed to comprehensively and intuitively characterize the evaluation indexes. Finally, the aging degree of the XLPE cable samples is quantitatively assessed by calculating the area of the polygon in the chart. Simultaneously, utilizing the position of the areal coordinate and the shape characteristic parameter of the polygon, the thermal-aged and water tree-aged samples are effectively distinguished.

To extract partial discharge (PD) signals from white noise efficiently, this paper proposes a denoising method for PD signals, named adaptive short-time singular value decomposition (ASTSVD). First, a sliding window was moved along the time axis of a PD signal to cut a whole signal into segments with overlaps. The singular value decomposition (SVD) method was then applied to each segment to obtain its singular value sequence. The minimum description length (MDL) criterion was used to determine the number of effective singular values automatically. Then, the selected singular values of each signal segment were used to reconstruct the noise-free signal segment, from which the denoised PD signal was obtained. To evaluate ASTSVD, we applied ASTSVD and two other methods on simulated, laboratory-measured, and field-detected noisy PD signals, respectively. Compared to the other two methods, the denoised PD signals of ASTSVD contain less residual noise and exhibit smaller waveform distortion.

To improve the reliability of power systems, online cable condition monitoring must be implemented to replace traditional regular power outage maintenance. Existing online monitoring methods have certain limitations. In this article, a novel method based on low-frequency signal injection is proposed to monitor power cable insulation conditions online. For this method, a low-frequency signal is injected into the power system via the potential transformer (PT) open delta configuration. The cable conductor voltage and leakage current are detected. The interpolating windowed fast Fourier transform (FFT) algorithm is applied to calculate the dielectric loss angle. Then, the cable tangent delta ( $\tan \delta $ ) can be deduced, and the cable condition can be assessed. A distribution network system is built based on the power system computer-aided design (PSCAD) software. The frequency and amplitude of the injected signal are simulated and discussed. An online monitoring platform is established in the laboratory. Thermal-aged cable samples and water tree-aged cable samples are prepared and measured. The experimental results confirm that the proposed method can effectively assess power cable insulation.

Condition assessment is significant to the maintenance management of high-voltage cross-linked polyethylene (XLPE) power cables. This paper presents a condition assessment method for corrugated aluminum sheathed cables based on evolved gas analysis. First, simultaneous thermogravimetric and mass spectrometric analyses of the pyrolysis of each cable material were conducted to characterize the gas generation caused by thermal faults. Simulated experiments were designed to investigate gas generation caused by electrical faults. The results show that different cable faults (low and high-temperature pyrolysis of semiconductors, insulation, and buffer materials; low and high energy discharge activity across insulation materials and electrochemical corrosion of the buffer material) show distinct gas generation characteristics. Based on the evolved gases, a method of identifying faults from the key gas concentration is proposed in this study. Moreover, gas transport within the corrugated aluminum sheathed cable with a buffer layer was analyzed, based on which a gas sampling method for cables with a buffer layer is proposed. Finally, the developed method was validated by practical applications. The proposed method is comparable to conducting a blood test on a human body in that it can help discover various incipient cable faults. The fundamentals and data interpretation methods are adaptable to all XLPE cables.

Moisture at the insulation interface is the main cause for electrical breakdown and insulation failure of cable accessories. However, it is still unclear how the presence of moisture affects the discharge development and interfacial breakdown process. This paper clarifies the reasons for the breakdown failure of cable accessory insulation interfaces under the effect of moisture. First, the moisture and breakdown characteristics of faulty cable accessories in the field are presented. Second, the discharge evolution characteristics during the breakdown process of dry and wet interfaces are experimentally described. Then, the effect mechanism of moisture on the development of interfacial breakdown is explained by combining product analysis and electric field analysis. Finally, the proposed breakdown mechanism of insulation interface under the effect of moisture are verified through a field case. The experiment results show that the discharge develops in stages and generates gases during the breakdown of the insulation interface, and there exist multiple discrete arc discharges with random paths during the breakdown of the wet interface. The severe electric-field distortion and its dynamic movement caused by the gas bubble areas formed in the wet interface are the main reasons for the increase in randomness of interface discharge and the decrease in breakdown strength. Through current impulse detection and generated gas analysis, it is expected to provide new solutions for early fault detection for cable accessories.