Composite insulators for high-voltage overhead lines have better performances and are lighter than traditional designs, especially in heavily polluted areas. However, since it is a relatively recent technology, reliable methods to perform live-line diagnostics are still under development, especially with regard to internal defects, which provide few external symptoms. Thermal cameras can be employed, but their use is not always straightforward as the sun radiation can hide the thermal footprint of internal degenerative effects. In this work, an optical E-field sensor has been used to diagnose the internal defects of a set of composite insulators (bandwidth 200 mHz–50 MHz, min. detectable E-field 100 V/m). Moreover, a modelling activity using finite elements has been carried out to identify the possible nature of the defects by comparing experimental E-field profiles with those simulated assuming a specific defect geometry. The results show that the sensor can detect the presence of an internal defect, since its presence distorts the E-field profile when compared to the profile of a sound insulator. Moreover, the measured E-field profiles are compatible with the corresponding simulated ones when a conductive defect is considered. However, it was observed that a defect whose conductivity is not at least two orders of magnitude greater than the conductivity of the surroundings remains undetected.

Polymeric insulation employed in electrical power industry undergo irreversible and unpredictable ageing due to partial discharges (PDs). In particular, the dielectric properties of polyethylene (PE) used in high voltage cables are often compromised by the formation of electrical trees. In this work we assume that the propagation of treeing channels involves the injection of carbonic material into the gas interacting with the surface of the defect. Experimental characterizations proved that, in certain conditions, disordered graphitic carbon can form in some areas of the electrical trees, thus increasing surface conductivity and inhibiting PDs. The chemical mechanism involved in this process is yet to be clarified. Here we propose a model for this process. By means of a series of molecular dynamics simulations, we show how the chemisorption of gaseous molecules on a PE surface can lead to a bidimensional carbonic structure. The characterization of the density of states of such systems suggests that the presence of pure carbon adsorbed on the polymer causes an increase in surface conductivity.

Plasma simulation is getting increasingly important to reproduce technically relevant configurations in electrical engineering. For instance, simulation tools are used to represent the evolution of partial discharges in internal defects such as voids and electrical treeing. In these cases the simulation includes both the gaseous defect and the polymeric bulk where the defect is embedded. A major role is played by the physical characteristics of the surface separating the defect from the bulk and, among these parameters, the surface conductivity is one of the most important aspects. The latter may take very high values due to the cumulative interaction of dielectric materials with a large number of partial discharges. The simulation of configurations involving high surface conductivities is particularly tough since, as we will show, this may reduce the maximum time step increase of simulation algorithms. In this work, we introduce a proper numerical filter that can circumvent this problem and we show that, using this technique, we can significantly enhance the performances of simulation algorithms without compromising their accuracy. The performances of this approach will be shown in a set of numerical examples.

We report a characterization of the chemical conditions that might cause an electron emissionElectron Emission from a polyethylenePolyethylene surface and trigger a partial discharge in an isolated void. In the framework of the electrical power industry, polyethylene is, commonly, the most used material to form the insulating layer of electrical cables. Unfortunately, under AC, it is known that this polymer suffers deterioration, which is usually associated with the treeing process. The latter phenomenon starts within a gaseous defect encased in the polymeric matrixMatrices, inside which the electrical strength undergoes a significant decrease and an electron is emitted from the polymer into the void. This creates the conditions for the triggering of a series of partial discharges that degrade the material from within and creates a tree of cavities in continuous, and self-sustaining, expansion. The mechanism by which the electron emissionElectron Emission occurs, causing the discharge to be triggered, is, most likely, the Schottky effect. It is, therefore, very important to define the chemical conditions that favor the initial surface electron ejection. In the present study, we performed a series of density functional theoryDensity Functional Theory (DFT) calculations for the characterization of the electronic structure of several defected polyethylenePolyethylene systems. Our purpose was to find a combination of chemical defects that could significantly reduce the surface work functionWork Function, and potentially give a Schottky emission consistent with our experimental reference. The work function of each system has been the key parameter we followed for assessing its Schottky emission properties. According to the several tests we conducted, we stress that is really unlikely to have a Schottky emission from polyethylenePolyethylene without any residual electron charge on the surface, which, in turn, needs to be localized thanks to the additive electronic states given by chemical defects. In particular, we found that an oxidized, and negatively charged, polyethylenePolyethylene surface returned a work functionWork Function in line with the experiment.

Electrical Treeing is one of the main phenomena that influence the aging of insulating components in the power grid. There are plenty of measurement systems that can detect partial discharges associated with the treeing. However, due to the complexity of this phenomenon, it is usually difficult to estimate the extension and progression rate of the treeing. In this work we take a step forward towards the development of a model, based on first principles, that can represent the evolution of a plasma in treeing channels. This can be considered a first step towards a better estimation of the residual life of insulating components.

The present work is devoted to the modelling of the deterioration process of polyethylene exposed to reactive plasma. In particular, we present the comparison between two different models. The first one, already presented in the literature, considers the H-abstractions due to the OH. and O. radicals, and some successive reactions, involving mainly other gaseous species. The second model, in addition to the first model reactions, is enriched by including the effects (both H-abstractions and chemisorptions) due to the collisions of a series of plasma ions with the surface. These collisions have been reproduced by ab initio molecular dynamics simulations. The performance of both models has been tested by implementing them in a code for simulation of surface chemical reactions induced by a plasma discharge. The related results are then compared with some experimental data. The first model is able to well reproduce the experimentally detected oxidation of the polyethylene surface exposed to plasma discharges. The second model, including also the decisive effects of the NO. radical on an already oxidized surface, is able to explain the presence of nitrogen on the sample surface.

In this work, we treat the discretization of Poisson's equation in two domains where interface conditions cannot be expressed by simple algebraic equations since they also contain differential terms. In particular, the more general and physical framework regards the solution of electrodynamic problems where surface conduction phenomena are taken into account. This kind of problems include, just to name a few, the interaction of plasma with solid dielectrics, or semi-conductive surfaces, the motion of charged ions in electro-active polymers and the interaction of charged fluids with porous matrices. In this work, we will identify a representative Poisson's problem with non-algebraic interface conditions and we will study several discrete approaches to solve it. For each approach, we study the resolvability of the associated algebraic problem and we test its performances using some numerical tests. (C) 2021 Elsevier Inc. All rights reserved.

To enhance grid reliability, weak points must be monitored. One of the weaknesses is the cable joints, which are prone to failure and can cause great losses from both a technical and economical point of view. Joints failures are usually caused by impurities unintentionally added during installation that cause partial discharges (PDs). In time, these discharges erode the insulation and generate treeing up to a destructive discharge between the conductor and the ground shield. For this reason, a method for the early detection of defects in joint installation and their online monitoring is required. A previously developed sensor was improved by adapting it for this purpose. It is based on the measurement of the induced current on a conductor due to a charge variation. It was experimentally tested on an actual joint in which defects were artificially introduced. Results show that the sensor is able to detect partial discharges. Moreover, a method for PD localization was developed. The first results show a coherency between the possible defect location, numerical simulations and historical background.

The study of cold plasma represents a very active field of applied physics with technical applications ranging from medical treatments to estimation of aging of electrical components due to internal partial discharges and treeing. In particular, the simulation of this category of plasma plays an increasingly important role since more and more complex, and technically relevant, configurations can be represented. Various kinds of models have been considered, one possible classification is relative to the way the electronic energy is computed. In the local electric field approximation a simple algebraic relationship is used which directly links the electric field strength to the electron energy. On the contrary, in the local mean energy approximation a proper differential equation is solved. In most cases this equation is coupled with a conservation equation which predicts the electron concentration. We will tackle this latter case and we will introduce a formulation capable of decoupling the electron density equation from the electron energy one. We will study the properties of the new formulation and we will build a proper numerical scheme capable of preserving, at a discrete level, these properties. Moreover, we will also discuss the existence of the discrete solution and test the performances of the scheme both in simple test cases, where an exact solution is known, and in a technically relevant configuration such as the formation of a treeing structure. (C) 2021 Elsevier Inc. All rights reserved.

Polyethylene is one of the most used solid state insulators in electrical power industry. It is particularly used to electrically insulate high-voltage cables. Under the stresses associated with AC power supplies, this material undergoes ageing, which is often associated with treeing. It is thought that this phenomenon starts from gaseous defects embedded in the insulator bulk, leading to the formation of a cluster of cavities. Treeing is able to dig the matrix until complete breakdown of the insulating components. Cavities are generated by a sequence of partial discharges. Each discharge is triggered by an electron emission from the surface at the interface with gas. The Schottky effect is believed to be the most likely mechanism able to cause this electron emission. Our DFT modelling has suggested that electron emission is highly unlikely to occur if the surface is neutral. DOS analysis has revealed that the Schottky effect is also related to chemical defects. The latter must exhibit electronic states slightly under the conduction band. Furthermore, these sites must be able to act as a trap for negative charge excess. A polyethylene system with an excess electron, combined with specific oxidative groups, has proved to be consistent with experimental data.

In this paper we aim to make possible the simulation of plasma -solid interaction in real insulating components using a set of first-principle partial differential equations. The high ratio between the conductivities of the many materials present in electrical components causes the associated numerical problem to become very stiff and this curtails the maxi-mum allowed time step increase. To simulate some realistic components using sufficiently large time steps a filtering method is applied. A novel theoretical analysis has been developed showing that the fil-ter allows the use of larger time steps without affecting the accuracy of the method. This analysis is backed by the results of a significant numerical experiment. (c) 2021 Elsevier Inc. All rights reserved.

The development of electro active polymers represents a very active branch of research with many applications principally devoted to biological systems. In particular the simulation of these materials is a very important topic in order to predict their behavior. In this work we propose and analyse, from a mathematical point of view, an algorithm for the simulation of active polymers arranged in possibly complex geometries. This algorithm has been designed to be implemented with three dimensional unstructured grids coupled with shell elements to take into account, in a computationally efficient manner, boundary effects and boundary layers. In particular we focus on the existence of a discrete and positive solution of the algorithm. A comparison between the results obtained and the experimental data is included: we aim to reproduce the results obtained by two classical tests in this field i.e. the electric impedance spectroscopy and the displacement of polymers when subjected to an external electric field.

The purpose of this work is to develop an experimental system capable of precisely locating the partial discharge (PD) events inside the insulating part of axisymmetric electrical components and of extracting the real dipole moment generated by each discharge. Moreover, using some novel simulation tools based on first principles and the solution of a set of partial differential equations (PDEs), we aim to shed a light on the internal evolution of PDs.

A predictive diagnosis of the health of insulating electrical components subjected to partial discharges seems still hard to achieve. Especially the determination of the residual life is, in many cases, very uncertain. In highly valuable components, such as cables and joints, the precise localization of partial discharge events may provide a valuable tool to estimate the progression of degeneration phenomena such as the electrical treeing. In this work we introduce a novel technique used to locate the position of moving charges that exploits the induced current on a set of properly placed electrodes. This technique has been previously developed in the framework of radiation detectors. In particular, we develop and analyze the performances of a localization method based on the measurement and analysis of induced currents and especially tailored to locate the internal discharges in axis symmetric components. The method is also capable to determine the real dipole moment produced by a discharge. This provides a measurement of the discharge strength that is more physically relevant than the commonly used apparent charge mentioned by international standards. The evaluation of the performances of the method is performed by building a proper test bed with a known defect.

We have performed first-principles calculations (DFT) to estimate the secondary emission yield (SEY) through Auger neutralization mechanism (gamma(N)) related to the impact of a series of ions on a polyethylene surface. We have considered many relevant ionic species, such as Ar+, which is often used as a benchmark. Our main goal is to study dielectric surfaces but, to check the validity of our approach, we have also considered some metallic surfaces (Al, Cu, Cu:N, Cu:O and CuO) on which some more experimental data can be found. On the contrary, very few references are available regarding the Auger neutralization on insulating materials and, in particular, on polyethylene. The SEY outcomes for metals have basically reproduced the experimental references. In particular, the same decrease in gamma(N), which has been associated to a 'dirty' (gas-exposed) metal surface, was confirmed by calculations. Thus, the applicability of the method to plastic material was considered to give realistic results. The computed gamma(N) values associated to polyethylene are in the order of 10(-1) for most of the ionic species here considered. Moreover, we have observed that a few percentage points of variations of the surface energy levels predicted by DFT calculations, may cause, depending on the ion type, a substantial change of gamma(N). Therefore a detailed sensitivity analysis has been included to address this problem. The results associated to metals have shown that gamma(N) variations are, for some types of ions, very sharp, while this variability is milder for a polyethylene surface. Our calculations are fully compatible with the previous relevant literature and suggest that plastic materials are characterized by gamma(N)coefficients similar to those of metals only slightly smaller.

The on line diagnostic of high voltage insulators, used in overhead lines, has proved a powerful tool to detect and prevent the failure of such components. Among the many diagnostic techniques, E-field sensors have been extensively used to assess the presence of surface or internal defects. Electric field sensors are mostly based on capacitive metallic gauges, which induce a significant electric field distortion and may pose a threat to live line workers who perform the measurement. In this work, we introduce a new class of electric field sensors that are fully dielectric. The sensor is capable of measuring the modulus and the direction of the electric field in a plane with a high rejection of the components orthogonal to that plane in a very broad frequency interval.

Partial discharges represent one of the main mechanisms of ageing of dielectrics especially when an alternated current is used. This problem is particularly severe when partial discharges are associated with degenerative phenomena such as the electrical treeing. In this work, we present the latest development of our simulation codes, which are capable of simulating the evolution of discharges in complex three-dimensional geometries through parallel high-performance-computing technologies. The code is capable of both predicting the evolution of some macroscopic quantities, that can be measured directly, and estimating the progression of the internal ageing. For instance, it is possible to simulate the creation of chemically active species in the gas and their interactions with the surfaces of the branches. Some examples of the results obtained in a set of test cases will be discussed here.

Internal partial discharges and electrical treeing are some of the main aging mechanisms of all the polymeric insulating materials present in the power net. A better understanding of this process calls for the development of more sophisticated simulation tools, based on the numerical solution of first principles equations, i.e., a set of partial differential equations. The accuracy of the predictions of these models heavily relies on the quality of the set of physical parameters used as, for example, the swarm parameters. A key aspect is the correct representation of the first electron availability which triggers the discharge. These electrons are mainly injected into the gas from the interface between the gas itself and the solid polymeric matrix by means of the Schottky effect. The work function is a very relevant parameter in defining this effect, and so the entire quality of the simulation process depends on it. In this work, we estimate the value of the work function, using electronic structure calculations, and we perform some parametric analyses to show how the discharge simulations are affected by the variation of this quantity. In particular, we assume that the polymeric material is polyethylene, which is used extensively in the electric power transmission industry. The discharge simulations produce some relevant data that can be compared against experimental data for validation purposes.

Electrical applications require the development of electric field sensors that can reproduce vector electric field waveforms with a very large spectral width ranging from 50 Hz to at least 70 MHz. This makes it possible to measure both the normal operation modes of electrical components and abnormal behaviors such as the corona emission and partial discharges. In this work, we aim to develop a fully dielectric sensor capable of measuring two components of the electric field using a wide class of optical crystals including anisotropic ones, whereas most of the efforts in this field have been devoted to isotropic crystals. We report the results of the measurements performed at 50 Hz and with a lightning impulse, to validate the sensor.

Facial masks are suitable devices to limit the exposure to the electric field of a highly delicate part of the human body, particularly in Live Line Working. A mapping of the electric field in the face region is carried out by means of a dielectric optical sensing device. A couple of technologies used for the production of transparent shields have been analyzed and compared: metallic meshes and thin conductive layers. Their shielding are reported and the latter have been chosen as the leading application. A description of the production process of the transparent conductive films is included.