Kun Zhao

The damped alternating current (DAC) technique has been widely applied for partial discharge (PD) testing of high‐voltage (HV) cross‐linked polyethylene (XLPE) cables. However, for transmission applications, the unipolar DC charging stage required in conventional DAC systems leads to space charge accumulation, which deteriorates the cable insulation condition. CIGRE TB 841, therefore, recommends the use of a bipolar charging manner instead of the unipolar manner. To address this requirement, a compact bipolar HVDC generator based on a modified voltage multiplier is proposed. In the proposed design, bidirectional rectifier switches are employed in place of unidirectional diode stacks to enable flexible polarity switching of the output voltage. Simulation results demonstrate an 80% reduction in space charge accumulation under bipolar DAC voltage, validating the effectiveness of the mitigation strategy. A three‐stage prototype was constructed, which is capable of delivering up to 180 kV. Laboratory tests on a lumped capacitor and field tests on a 110 kV XLPE cable validated the feasibility and effectiveness of the proposed method. The system maintains the compactness and portability of conventional DAC systems while extending its capability to provide bipolar DAC voltage, thus providing a practical solution for on‐site PD detection of XLPE cables.

Cable shields may develop holes when the sheath is damaged. Time–frequency domain reflectometry (TFDR) is an effective method for detecting cable defects, but it cannot directly evaluate hole sizes. To address this issue, we analyze the impact of shield hole sizes on TFDR signals. Building on this analysis, we propose an improved dual-frequency TFDR method to measure shield holes and evaluate their sizes. This method directly measures the characteristic impedances and damage ratios using dual-frequency TFDR, followed by a two-step evaluation process to determine the hole center angles and lengths based on these measurements. Simulations and experiments validate the proposed method. In laboratory-scale experiments using a scaled cable model, and considering measurement noise, the maximum relative errors for shield hole length and center angle are 11% and 5%, respectively.

Cable shield corrosion defects are a common threat to power cables, requiring precise detection and sizing for effective management. Such defect detection and location methods are widely researched. However, essentially none of them can accurately assess the length of shield corrosion defects. To solve this issue, we develop a model to represent the impact of shield corrosion. Building upon this model, we propose a double-frequency time-frequency domain reflectometry (TFDR) method. The innovation of double-frequency TFDR lies in conducting two successive measurements, with the center frequency of the second signal being twice that of the first. The length of the shield corrosion defects is determined by analyzing the measured signals using the spectrum ratio. The proposed method is validated through simulations on a 10-kV system and experiments with a SYV50-type cable. The results show that estimation errors for the defects’ equivalent inductance and length are below 2% and 0.3%, respectively. Hence, double-frequency TFDR emerges as a powerful tool for assessing cable shield corrosion.

Partial discharge (PD) diagnosis is a crucial technique for assessing the insulation condition of high-voltage (HV) cables. The double-sided travelling wave method is effective for online PD localization on HV cables. However, the PD localization accuracy is limited by three main factors: inaccurate time synchronization among sensors, imprecise pulse propagation velocity, and noise interference. To address these issues, a bidirectional pulse-injected time synchronization method is proposed to correct the timing bases of detectors and assess the pulse propagation velocity along cables independently of cable parameters. Furthermore, a denoising method combining the short-time Fourier transform (STFT) and variational mode decomposition (VMD) is designed to enhance the robustness of the system to high-intensity noise. Finally, an integrated PD detector with pulse injection and signal detection functions and switchable measurement ranges is designed to improve detection sensitivity. To validate the effectiveness of the proposed method, a portable system is developed, and experiments are conducted on both a 10 kV cable in the laboratory and a 110 kV cable in the field. The experimental results confirm that the proposed method enhances the accuracy of PD localization.

Identifying the location of partial discharges (PDs) and providing clear information on insulation vulnerabilities in power systems can help utility companies to improve their maintenance efficiency. This paper presents a PD monitoring and locating system for medium-voltage switchgears using a set of transient earth voltage (TEV) detectors. A time-of-flight methodology was used to identify the switchgear affected by PD among a row of switchgears. A time sharing access mechanism, which was implemented by multiple high-frequency surface mounting relays integrated into each TEV detector, was applied to automatically identify the site of PD and minimize the number of expensive high-speed acquisition cards. The operation principle and implementation of the modular system were formulated, and the efficiency of the proposed system in identifying the switchgear with PD among three 10 kV metal-clad switchgears was confirmed experimentally.

The burgeoning demand for enhanced power-system safety means that there is increased attention on technologies for the detection and location of cable faults. The double-terminal fault location method, which senses a fault current using a current transformer (CT), is the most effective and widely applied method in industrial practice. However, CT sensors may suffer from magnetic saturation when exposed to a high fault current, resulting in significant location errors, and their sensitivity cannot be adjusted. To address this problem, this paper describes an improved method for the acquisition of fault signal and the identification of fault-wave arrival time in power cables. First, a novel noncontact sensor (NCS) is developed for the detection of electric fields, and its sensitivity-adjustment curves are obtained by theoretical calculation and simulation. Then, based on feedback variational mode decomposition (FVMD) and the Wigner-Ville distribution (WVD), an improved method (the FVMD + WVD method) for the identification of fault-wave arrival time is devised, and its efficacy and accuracy are verified by simulations in Power Systems Computer-Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) software. Finally, the fault-wave arrival-time detection performance of the NCS is examined in a series of on-site DC and AC experiments based on damped AC technology. The results show that the NCS exhibits feasible and effective performance, and that its response can be appropriately tuned based on its sensitivity-adjustment curves. The FVMD + WVD method is more accurate than several other current methods, as its error is only 0.48%, and thus this method enables the practical location of cable faults. In sum, these findings demonstrate that the NCS and the FVMD + WVD method comprise an improved system for the detection and location of cable faults, which will enhance cable safety in power systems.

Broadband impedance spectroscopy (BIS) has become a new method for detecting cable insulation defects, but the sensitivity of this method to local defects in cables has not been fully studied. Taking long submarine cable as an example, firstly, the intrinsic correlations between the impedance spectroscopy characteristics and the cable parameters are studied, and the influence of local defects on the peak of impedance spectrum is studied by establishing a simplified model of long cables with insulation defects. Secondly, the formula for calculating the sensitivity of input impedance to the insulation defects is proposed and the characteristics of sensitivity are studied. Finally, the sensitivity of the input impedance to different positions and degrees of defects at the first resonant frequency is analyzed, which can be verified by simulation and experimental. The Results show that, for long cable with length l, under the condition of open-circuit, the sensitivity trends to zero when the defects occur at 25% or 75% of the cable length l, and under the condition of short-circuit, the sensitivity trends to zero when the defects occur to the end of the cable. Therefore, in the application of BIS, the above positions should be paid attention to, and the cable length can be changed and measured again to confirm its insulation status.

Partial discharge (PD) is an early manifestation of multiple cable faults. The traditional PD monitoring method has a weak monitoring effect and low positioning accuracy when monitoring the PD of cable. This paper proposes a distributed on-line monitoring method for cable PD based on phase-sensitive fiber-optic time domain reflection (φ-OTDR) principle. Firstly, the production and propagation model of the partial discharge ultrasonic wave (PDUW) of the cable was established and simulated, and the distribution of PDUW inside the cable with different conditions and different structures was obtained. Then we established a model of cable PDUW, and the backscattered Rayleigh light intensity in fiber and theoretically analyzed and experimentally verified the relationship between the cable PDUW and backscattered Rayleigh light intensity in fiber. The experimental results are consistent with the theoretical analysis. When the cable has PD, the backscattered Rayleigh light intensity change of the PD position is higher than the intact position. We analyzed the experimental data and obtained the judgment conditions of the cable PD. Finally, we calculate and obtain the sensitivity of the monitoring method to different cables and the warning value when cable PD is abnormal.

This paper presents a calculation method of short-circuit current (SCC) for DFIG-based wind farm considering the voltage distribution characteristics. When a short-circuit fault occur in a wind farm, a large SCC will be contributed by the dozens or even hundreds of doubly-fed induction generators (DFIGs) in the wind farm. And the security and stability of the wind farm will be affected by the SCC. The SCC characteristics of the DFIG are closely related to its post-fault terminal voltage (PFTV). Therefore, a practical calculation model of SCC in single DFIG is proposed in this paper, based on electromagnetic transient (EMT) analysis. And according to a correction method of topological matrix for post-fault wind farm proposed in this paper, the relationship model between voltages and currents under short-circuit faults is established. Then the PFTVs and SCCs of the DFIGs in the wind farm can be obtained by solving the model equation. Finally, the proposed method is validated through MATLAB/Simulink-based simulation results, and a comparison with the national standard calculation method is analyzed. The results show that this method has better accuracy, which provides a more accurate basis for the electrical safety design of wind farms.