Corona ring improvement to surface electric field stress mitigation of 400 kV composite insulator

This paper aims to enhance the performance of a 400 kV composite insulator by minimizing electric field stress along its surface using COMSOL Multiphysics. The study employs optimization techniques for the corona ring, initially targeting key parameters such as the ring tube radius, corona ring radius, and corona ring height. Recently used optimization algorithms for engineering applications, namely Constrained Optimization by Linear Approximation (COBYLA), Coordinate Search (CS), and Nelder-Mead (NM), are utilized to determine the most effective one for the optimization purpose. The results indicate that the three algorithms converge around similar values, with COBYLA exhibiting a lower iteration number. Furthermore, our research confirms that increasing the corona tube radius results in a reduction of electric field when considering the other optimized parameters; however, this leads to an increase in the corona ring weight. To resolve this concern, we conduct a re-optimization of the ring shape by targeting eight specific points that form a polynomial interface replacing the original ring. The obtained results demonstrate that the re-optimized shape effectively reduces surface electric field, although it leads to an increase in the ring weight. Nonetheless, this approach aids in identifying critical regions of surface deformation that interact with the electric field. In light of this, we explore an alternative approach that involves replacing the new shape with multiple corona rings positioned in the most deformed regions and then optimizing their positions. This approach helps to achieve better results of surface electric field and overall weight, under clean and polluted surfaces

Correlations between physico-chemical properties and dielectric behavior of thermally aged XLPE cables insulation

The carried out investigations aim at clarifying the correlations between physico-chemical properties and the variation of dielectric losses of Cross-Linked Polyethylene (XLPE) as Medium Voltage cable insulation during thermal aging. The used cables were manufactured by ELSEWEDY CABLES. They have been installed in Algeria where they must resist high temperatures that could exceed 50 degrees in the south of the country. For this aim, thermal aging measurements were performed on XLPE cables samples at 80, 100 and 140 °C for aging time of 5000 hours. The chemical properties were evaluated through Fourier Transform Infrared Spectrum (FTIR). In addition, the effect of aging on the crystallinity ratio, the lamellar thickness and the melting peak temperature was assessed using Differential Scanning Calorimetry (DSC). Frequency domain dielectric spectroscopy has been conducted on the aged XLPE cables in order to study the evolution of the dielectric losses. The obtained results showed that the principal aging mechanism is thermal oxidation which heads to deteriorate the morphology and increase both the polarization and losses, principally at low frequencies. It has been found that the color changes caused by the chains scission indicate the increase of interfaces between the cable insulating and semi-conductive layers, which could head to increase the dielectric losses due to the interfacial polarization

Correlations between structural changes and dielectric behavior of thermally aged XLPE

International audienceThis study aims at elucidating the correlations between structural changes and the variation of dielectric properties of cross-linked polyethylene (XLPE) during thermal aging. For this purpose, thermal aging experiments were carried out on XLPE samples at 80, 100, 120 and 140 °C for different aging times for a maximum total duration of 5040 hours. Chemical changes were assessed by Fourier Transform Infrared Spectrum (FTIR). Besides, the crystallinity ratio and the melting temperature were evaluated using Differential Scanning Calorimetry (DSC). Frequency domain dielectric spectroscopy was used to investigate the evolution of the dielectric constant and losses of XLPE during aging. The results show that aging at 80 and 100 °C could help to improve the crystalline state of XLPE which leads to the decrease of the dielectric constant. However, for higher thermal stress, the main aging mechanism of XLPE is thermal oxidation which leads to deteriorate the morphology and increase both the polarization and losses, mainly at low frequencies. It has been found also that in some cases, the non-homogeneity of the color of XLPE seems to indicate the increase of interfaces in the material which could lead to increase the dielectric losses due to the interfacial polarization

A New Analytical Equation for the Linear Resistance of Discharge During Flashover of Polluted Insulators Under AC or DC Voltages

International audienceThis paper presents a new equation for the linear resistance of discharge channel during the flashover of polluted insulators basing on an electro-energetic balance and by using some mathematical formulations. This equation shows the dependency on the applied voltage, the temperature, thermal conductivity and length of the discharge channel, the resistance and capacitance of pollution layer; the radius of discharge channel is assumed to be variable. It is valid during the whole discharge process (i.e.; during the different steps of the discharge development) in AC and DC voltages and in both static and dynamic regimes. The proposed equation has the particularity to be an analytical equation, in opposition with the main equations used till today to determine the linear resistance of discharge propagating over polluted insulators up to flashover and which are empirical equations

Optimizing corona ring design for 400 kV cap-pin string insulator to minimize surface electric field under various surface conditions

The objective of this paper is to reduce the electric field stress along the surface of the 400 kV cap-pin string insulator by optimizing its corona ring design. The cap-pin string insulator, composed of glass material and consisting of 1512 L units, is designed and investigated within a 2D-axisymmetric environment using COMSOL software. Firstly, we optimize the corona ring’s radius, tube radius, and height. Secondly, the corona ring shape is optimized to more effectively decrease the surface electric field, despite the resulting increase in its weight. Finally, the optimized shape is replaced by multiple corona rings, each with a small radius, positioned in the most deformed area, to enhance the insulator’s performance in terms of both surface electric field and corona ring weight. The COBYLA algorithm is utilized as the optimization method throughout the study. This research examines clean, uniformly polluted, and Room Temperature Vulcanizing (RTV) silicon rubber-coated surface conditions. The obtained results are presented and discussed

Power Transformer Fault Prediction using Naive Bayes and Decision tree based on Dissolved Gas Analysis

Power transformers are the basic elements of the power grid, which is directly related to the reliability of the electrical system. Many techniques were used to prevent power transformer failures, but the Dissolved Gas Analysis (DGA) remains the most effective one. Based on the DGA technique, this paper describes the use of two of the most effective machine learning algorithms: Naive Bayes and Decision Tree for the identification of power transformer’s faults. In our investigation, 9 different input vectors have been developed from widely known DGA techniques. 481 samples have been used and 6 types of faults have been considered. The evaluation result of the implementation of the proposed methods shows an effectiveness of 86.25% in power transformer’s fault recognition

A New Analytical Equation for the Linear Resistance of Discharge During Flashover of Polluted Insulators Under AC or DC Voltages

International audienceThis paper presents a new equation for the linear resistance of discharge channel during the flashover of polluted insulators basing on an electro-energetic balance and by using some mathematical formulations. This equation shows the dependency on the applied voltage, the temperature, thermal conductivity and length of the discharge channel, the resistance and capacitance of pollution layer; the radius of discharge channel is assumed to be variable. It is valid during the whole discharge process (i.e.; during the different steps of the discharge development) in AC and DC voltages and in both static and dynamic regimes. The proposed equation has the particularity to be an analytical equation, in opposition with the main equations used till today to determine the linear resistance of discharge propagating over polluted insulators up to flashover and which are empirical equations

Cap-pin glass insulator simulation and leakage current waveform extraction

This paper presents a simulation of a HV glass insulator and a methodology to extract leakage current signal. Using 3D profiles, COMSOL-based simulations are conducted at a 50 Hz voltage waveform of different magnitudes. The 1512L cap and pin glass insulator is employed since it is widely used in Algerian desert regions. Potential and electric field distributions, along with the temperature rising, are presented and discussed. The current density is extracted and used to compute the leakage current signal. The obtained results are presented and discussed