High Voltage最新文献

This paper presents an approach to accurately forecast the yearly occurrence of flashovers caused by lightning-induced voltage in overhead power lines in the presence of layered ground. The aim is to improve the accuracy of flashover rate estimation by using the insulator volt-time model. In particular, two-layer horizontal ground structures are considered. A 3D finite element method is used to calculate induced voltages and a Monte Carlo simulation is applied to determine the annual flashover rate. Volt-time insulator characteristics are used to identify the flashover condition. The results are compared with those obtained by using the standard 1.5 times the critical flashover (CFO) threshold criterion. The results indicate that the conventional 1.5 times the CFO criterion may underestimate the rate of flashovers, particularly in regions with horizontally stratified soil, as the volt-time method offers a more accurate presentation of the flashover process. Furthermore, the effect of upper soil depth, upper soil conductivity, pole spacing, and different flashover distance calculation techniques on flashover rates are analysed. This paper presents a new mathematical formula for estimating yearly flashovers based on the results obtained by the volt-time method in the presence of stratified ground. The derived analytical formula provides an insightful tool for power system engineers to evaluate the lightning performance of overhead lines and implement efficient mitigation strategies.

The high voltage Padova test facility (HVPTF) is an experiment set in Padova, Italy, operating in the framework of the Neutral Beam Test Facility project of the International Thermonuclear Experimental Reactor (ITER). One of the purposes of HVPTF is to study the phenomenology of discharge events occurring between electrodes at high voltage differences over long vacuum gaps, which is crucial in the development of the neutral beam injector foreseen for ITER. The facility hosts a cylindrical vacuum vessel with stable pressure control, where two electrodes of different possible geometries can be mounted. Two independent power supplies allow for total voltage differences up to 800 kV_DC with adjustable gap widths up to 250 mm. Among the diagnostics, a gas electron multiplier (GEM) detector is installed for acquisition of x-ray emission on a radial line of sight of the vessel. This paper presents a study of the experimental sessions featuring stainless-steel needle-plane electrodes. The analysis is based on the GEM data, in relation to the information on current and voltage of the two power supplies. The events are characterised in terms of both temporal and spatial evolution, providing sequential emission profiles with spatial resolution of tens of millimetres on timescales of the order of hundreds of nanoseconds.

Silicone rubber of composite insulators is extensively utilised because of its superior hydrophobicity and hydrophobicity transfer characteristics, especially in heavily polluted environments. This work investigates the hydrophobicity transfer process of silicone rubber materials with different insoluble substances and explores the impact of microscopic parameters on hydrophobicity transfer. Experiments employed diatomaceous earth and kaolin of various particle sizes and origins as inert substances to analyse their effects on hydrophobicity transfer characteristics. Results indicate that diatomaceous earth, with its porous structure, facilitates faster transfer compared to the dense structure of kaolin. Pore size and specific surface area are critical parameters influencing transfer rates. Larger outer pores accelerate the initial ‘fast process’ while smaller inner pores govern the subsequent ‘slow process’ Fractal characteristics of pores also affect transfer efficiency with higher fractal dimensions leading to more extensive transfer. The hydrophobicity transfer process in silicone rubber involves dynamic diffusion influenced by the complexity of internal channels and surface structures. Enhanced understanding of these mechanisms can improve the performance and reliability of composite insulators in polluted environments.

The effect of a metal shell on the launch efficiency of an asynchronous coil launcher (coil AC pulse linear motor) remains insufficiently understood in terms of the underlying mechanisms and principles. To address this gap, this study conducted extensive modelling simulations and calculations, varying the shell's conductivity, permeability, and dimensions. Through comparative analysis of these models, this paper identifies a unique ‘tick-shaped efficiency curve’ for the asynchronous coil launcher: the launch efficiency first decreases and then increases as the shell's electromagnetic parameters are enhanced. Enhancements that bolster the electromagnetic induction effect within the shell—such as increased conductivity, permeability and dimensions—are termed as the augmentation of electromagnetic parameters. This study delves into Lenz's law of electromagnetism to elucidate the observed phenomena, attributing them to the spatio-temporal force characteristics of the multi-peak and multi-valley armature of the transmitting device, and the resulting ‘tick-shaped efficiency curve’. A comprehensive summary of shell-related research in electromagnetic emission reveals that the driving current fundamentally dictates the shell's impact on launch efficiency. DC-driven launchers conform to the monotonic effect efficiency curve, whereas AC-driven launchers conform to the tick-shaped efficiency curve.

The air gap at the interface inside the cable terminations for high-speed trains and the partial discharge caused by it are two important factors affecting the insulating performance. The development of air gap and partial discharge will eventually lead to breakdown faults. To investigate the evolutionary characteristics of the air gap and partial discharge, the simulation models and samples of cable terminations containing defects are constructed in this paper. By analysing the variation law of the electric field and the multidimensional information of partial discharge, the evolution process of the air gap is divided into four well-characterised stages. Especially in the third stage, the partial discharge extinction voltage is 51.41% lower than that of the defect-free samples and even lower than the working voltage. The asymmetry of discharge is the most significant factor. The volume of discharge in the third quadrant is significantly higher than that in the first quadrant. This important feature can be applied to the inspection and evaluation of the insulating state of the cable terminations. The partial discharge characteristics of the air gap revealed in this paper are proposed to provide an important theoretical supplement to the study of interface discharges between heterogeneous dielectrics.

The low temperature flow properties of synthetic esters were investigated based on molecular dynamics simulations, and the influence of the molecular structure of pentaerythritol esters on the pour point was explored. When the carbon number and parity of the ester chain are the same, the pour point of the ester with a branched chain is relatively low. The pour point is related to the position of the branched chain, which near the ester group or methyl group is slightly less effective in reducing the pour point than in the β-C position. When the number of carbons in the ester chain increases from an even to an odd number (from 6 to 7, or from 8 to 9), the pour point rises by approximately 14.5°C (from −47.65°C to −33.15°C) or 9.5°C (from −9.65°C to −0.15°C). When the number of carbons increases from odd to even numbers (from 7 to 8, or from 9 to 10), the pour point rises by approximately 23.5°C (from −33.15°C to −9.65°C) or 15.5°C (−0.15°C to 15.35°C). The branched-chain modification method for reducing the pour point of pentaerythritol ester has been proposed. The pour point of the pentaerythritol ester was −40°C when the percentage of branched chain acid in the raw material was 10% and 15%.

Biaxially oriented polypropylene (BOPP) thin film is the predominant dielectric material used in film capacitive energy storage for pulsed power engineering and power conversions due to its remarkable high dielectric strength and low conduction loss. However, the design rating of BOPP film capacitors in high power density conversion systems operated also under high temperature is still based on the empirical criteria due to the lack of systematic mechanism studies at elevated temperature. In this work, the temperature-dependent electrical conduction in tenter and bubble BOPP films up to their breakdown strength was systematically studied using a specialised circuitry featuring dynamic gain-controlled capacitive current cancellation. Both tenter and bubble BOPP films exhibit an extended trap-limited conduction region at the high electric field, followed subsequently with a trap-filled limited conduction until breakdown. This trap-filled-limited conduction presents characteristics of carriers transport with detrimental high mobility and soaring conduction loss. Overall, the shallow localised states revealed by the Arrhenius analysis, the large bandgap, and high barrier height of BOPP film together render its exceptional electrical integrity. In comparison, the enhanced crystallinity and larger crystallite sizes in tenter BOPP produced by the sequential stretching result in a higher upper operational temperature and slightly higher breakdown strength than bubble BOPP, suggesting the important role of processing induced enhancements to intrinsic properties of molecular origin. This study provides insights into the high-field characteristics of BOPP films at elevated temperature with promising learning outcomes useful to the expedited designs of the next generation polymer films for capacitive energy storages.

This paper focuses on the space charge and breakdown characteristics of polypropylene (PP)-based insulation interface in extrusion moulded joint (EMJ) for high-voltage direct current (HVDC) submarine cables. The double-layered flat samples and cylindrical samples are prepared to imitate the interface in the PP-insulated EMJ. The DC conductivity, space charge, and breakdown strength are tested. The results demonstrate that in the EMJ manufacturing process, the lower wielding temperature leads to microdefects at the insulation interface. As shallow traps, the microdefects exacerbate hetero charge accumulation, thereby intensifying the electric field distortion and increasing the conductivity. Meanwhile, the interfacial microdefects lead to a reduction in the insulation breakdown strength. At 90°C, the normal and tangential breakdown strengths decrease by a maximum of 20.6% and 54.5%, respectively. Notably, the space charges and microdefects lead to a rapid decline in the breakdown strength after hetero polarity pre-stressing. Especially for the tangential breakdown strength, the maximum decrease rate reaches 22.9%. Therefore, the interfacial microdefects caused by the drop in the welding temperature are the primary factors leading to a serious decrease in the electrical properties of EMJ insulation, making the EMJ insulation weaker than PP cable insulation.

Quickly and accurately obtaining the internal temperature distribution of a transformer plays a key role in predicting its operating conditions and simplifying the maintenance process. A reasonable equivalent thermal circuit model is a relatively reliable method of obtaining the internal temperature distribution. However, thermal circuit models without targeted consideration of operating conditions and parameter corrections usually limit the accuracy of the results. This paper proposed a five-node transient thermal circuit model with the introduction of nonlinear thermal resistance, which considered the internal structure and winding layout of the core-type high-frequency transformer. The Nusselt number, a crucial variable in heat convection calculations and directly related to the accuracy of thermal resistance parameters, was calibrated on the basis of the distribution of external cooling air. After parameter calibration, the maximum computational error of the hotspot temperature is reduced by 5.48% compared with that of the uncalibrated model. Finally, an experimental platform for temperature monitoring was established to validate the five-node model and its ability to track the temperature change at each reference point after calibrating the Nusselt number.

This study focuses on the distribution of high-resistance media (pores and spinels) within $��\text{ZnO}�$ varistors and explores the mechanical and electrical failure mechanisms of varistors under different pulse actions. Micro-CT technology revealed that the proportion of high-resistance media in the edge area is much higher than in the internal area. Simulation results indicated that a high porosity significantly increased temperature rise and thermal stress concentration, while a high spinel proportion exacerbated current concentration but had a relatively minor impact on the distribution of temperature rise and thermal stress. Under an electric field of 1000–1250 V/mm, pores transition from an insulating state to a conductive state, especially in the edge area, leading to concentrated temperature rise and thermal stress. Once the thermal stress exceeded the critical value of the mechanical strength of the pores, cracking failure occurred. The high spinel proportion in the edge area further intensified current concentration under high electric fields, working together with the conductivity of the pores to produce a significant local temperature rise, melting grain structure, and ultimately leading to puncture failure. This study provides a new perspective for understanding the failure mechanism of $��\text{ZnO}�$ varistors and lays a theoretical foundation for the development of varistor materials with high energy absorption capacity.