Model to estimate the trapping parameters of cross-linked polyethylene cable peelings of different service years and their relationships with dc breakdown strengths

In this study, an improved trapping/detrapping model was used to simulate the charge dynamics in cross-linked polyethylene peelings from different-year aged cables. Injection barrier of trapping parameters was estimated by the model fitted to experimental data for each type of sample. Moreover, dc breakdown tests were operated on those samples. It has been found that the dc breakdown strength of inner-layer samples is the lowest in cable sections with thicker insulation layer taken from high-voltage ac (HVAC) 220 kV service condition, whereas for the cable with thinner insulation from HVAC 110 kV, middle-layer samples have worst breakdown performance. This might be explained by the space charge issues under long-term HVAC condition. More importantly, a clear relationship between estimated model parameters, including injection barrier, trap depth and trap density, with the dc breakdown strength in each layer has been reported in this study

I run as fast as a rabbit, can you? A Multilingual Simile Dialogue Dataset

A simile is a figure of speech that compares two different things (called the tenor and the vehicle) via shared properties. The tenor and the vehicle are usually connected with comparator words such as "like" or "as". The simile phenomena are unique and complex in a real-life dialogue scene where the tenor and the vehicle can be verbal phrases or sentences, mentioned by different speakers, exist in different sentences, or occur in reversed order. However, the current simile research usually focuses on similes in a triplet tuple (tenor, property, vehicle) or a single sentence where the tenor and vehicle are usually entities or noun phrases, which could not reflect complex simile phenomena in real scenarios. In this paper, we propose a novel and high-quality multilingual simile dialogue (MSD) dataset to facilitate the study of complex simile phenomena. The MSD is the largest manually annotated simile data ($\sim$20K) and it contains both English and Chinese data. Meanwhile, the MSD data can also be used on dialogue tasks to test the ability of dialogue systems when using similes. We design 3 simile tasks (recognition, interpretation, and generation) and 2 dialogue tasks (retrieval and generation) with MSD. For each task, we provide experimental results from strong pre-trained or state-of-the-art models. The experiments demonstrate the challenge of MSD and we have released the data/code on GitHub.Comment: 13 Pages, 1 Figure, 12 Tables, ACL 2023 finding

The influence of frequency of AC component on space charge behaviours in polyethylene under combined AC and DC electric fields

Combined ac and dc voltages may occur in high voltage power transmission system, which bring challenges to the dielectric materials. Generally speaking, space charge is well recognised under high voltage dc conditions but has received little attention under high voltage ac conditions. Under ac electric fields, the formation space charge in the material depends strongly on the frequency. The knowledge of space charge behaviour in dielectric materials under combined voltages is desirable in terms of both experimental and simulation. In this paper, a numerical model based on the bipolar charge injection/transport model is used to analyse characteristics of space charge in polyethylene under combined electric fields at room temperature. The DC component ratio and the overall root mean square (RMS) are kept the same, while the frequency of the AC component is altered from O.05Hz to 50Hz. It has been found that the amount of the accumulated charges reduces with increasing frequency due to the more frequent polarity reversal. Moreover, the time to reach the steady states of charge amount increases with frequency as wel

Space charge behaviours in polyethylene under combined AC and DC electric fields

Polyethylene has been one of the widely studied polymeric insulation materials. One of the major issues related to polymeric materials is the easy formation of space charge which may cause electric field enhancement. In this paper, a numerical simulation based on a bipolar charge injection/transport model is used to obtain characteristics of space charge in polyethylene under the combined AC and DC high voltage at room temperature. The bipolar charge injection/transport model, which is widely used in HVDC space charge simulation, is applied in this combined condition. The overall applied voltage, consisted of root mean square (RMS) values of 50 Hz AC voltage and DC voltage, is kept the same, while the DC component has been varied from 0 to 1. The simulated charge distributions present notable differences when DC offset is added compared with pure AC conditions. Besides, these differences become more significant when the offset ratio is increased. The total positive and negative charge amounts are calculated respectively by integrating the charge in the material, and a curve of net charge amount changing along with time is obtaine

Space charge and AC electric breakdown strength in polyethylene

The effect of space charge on dielectric performances is generally neglected under high voltage alternate current (HVAC) electric fields, due to the limited charge amount accumulated. This paper intends to expound the characteristics of space charge in polymeric under AC stresses and its importance to the breakdown strength of polyethylene. A numerical model based on space charge dynamics under high voltage electric fields has been used to explain the differences of the breakdown strength in polyethylene under AC and DC stresses. Bipolar charge transport theory is adopted to analyse the charge transportation processes. Space charge profiles at the time of breakdown occurrence under different electric fields (including DC voltage, 0.5, 5 and 50 Hz AC voltages) and in the samples with different thicknesses (from 50 to 200 µm) are simulated. The relationship between space charge and the breakdown strength is discussed. The simulated results agree well with the trend of experimental breakdown results: generally, breakdown occurs easier under AC stress than DC stress; the breakdown strength is increased with the decrease of the sample thickness. The results suggest the presence of space charge under AC stresses has a great impact on electric breakdown strength of the material

Space charge dynamics in polyethylene under periodical high voltage electric fields

During the last two decades, space charge has been recognised to be a major factor influencing the electrical performance of cable insulation. A significant amount of work has been carried out to investigate space charge dynamics within polymeric insulation under high voltage direct current (HVDC) fields. Modern charge mapping techniques are adopted to obtain the necessary information about space charge within the insulation. However, the underlying physics of charge transport and the charge trapping characteristics of space charge are not well understood. Employing theoretical modelling based on numerical simulation to analyse the space charge features can provide an insight view into the charge distribution in dielectrics under realistic conditions. This thesis focuses on the analysis of space charge phenomenon within polyethylene insulation under common operating electric fields through both experimental and numerical investigations.An improved pulsed electro‐acoustic system along with a data processing procedure has been developed to investigate space charge in polyethylene under AC and superposed AC and DC voltages. Raman spectra and Attenuated Total Reflectance Fourier‐transform Infrared (ATR‐FTIR) spectra are collected to confirm the influences of the magnitudes and frequency of AC fields on the physical characteristics of polyethylene. Evaluation of pure AC and DC voltage tests as specified in the international standard, BS EN 61378‐2:2001, has also been done by comparing the space charge profiles under the real superposed AC and DC voltage and the deduced testing voltages.A numerical simulation model based on bipolar charge transport theory has been developed to analyse space charge phenomenon in polyethylene under periodical complex electric fields. The build‐up of space charge in polyethylene under DC electric fields has been modelled, and the simulation setting has been optimised based on the measured results. The model is also introduced to simulate the charge dynamics under AC and superposed AC and DC fields. The simulation results exhibited good agreement with the measured profiles.Besides, the effects of applied field characteristics (frequency, field magnitude, and field composition) on the charge formation and transportation have also been investigated and using both the experimental and numerical approaches. Furthermore, the numerical model has been further applied to analyse the relationship between space charge phenomenon and electrical breakdown in insulation. It has been found that the different region where breakdown happens caused by different charge dynamics is a significant reason leading the various breakdown strengths of the same material different, under AC and DC voltages.The outcome of this dissertation can aid the fundamental understanding of charge dynamics in the insulating materials under general operating high voltage electric fields

Determination of threshold electric field for charge injection in polymeric materials

Accurately determining the threshold electric field at which charge injection from the electrodes starts is important for reliable operation of dielectric materials as the presence of charge in the material can lead to electric field enhancement, resulting in degradation and early failures of the material. The two methods in charge measurements that have been commonly used to find out the threshold field have been compared to the proposed method, which overcomes the drawbacks of the two methods. Such method offers (i) high sensitivity as the effect of capacitive charge has been eliminated and (ii) contributions from both mobile and slow charges; hence, providing a more accurate value for the threshold electric field. Based on the proposed method, it has been found that the threshold field for low density polyethylene is around 8 kV/mm, which is lower than thereported value obtained from the other method

Numerical analysis on space charge and AC-DC combined breakdown strength in polyethylene

In high voltage direct current (HVDC) power transmission systems, electrical equipment at the valve side usually withstands complex stresses like AC-DC combined voltages. Understanding the charge migration and accumulation characteristics under complex stresses and their influences on the breakdown strength are of great significance. This paper presents an in-depth simulation study on the AC-DC combined breakdown strength using a modified bipolar charge transport model, which takes the deterioration of materials into consideration. Firstly, the simulation results of charge profiles under AC voltages agree well with experimental results, which partly illustrates the appropriate selection of parameters. Then the AC breakdown strengths with different ramping rates and frequencies are studied. Charges accumulated relatively close to the surface of samples can cause severe distortion of the electric field after the change of voltage polarity. After that, the breakdown voltages with different ratios of the AC to DC component are calculated and analyzed, especially on how AC and DC component influence the charge migration and accumulation during the evolution of the aging process eventually leading to breakdown. The simulation results of AC-DC combined breakdown voltages show good agreement with previous experimental results