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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "D1: Advanced Energy Materials".

Deadline for manuscript submissions: closed (31 January 2021) | Viewed by 43546

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Prof. Dr. Pawel Rozga

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Institute of Electrical Power Engineering, Lodz University of Technology, Stefanowskiego 20, 90-537 Lodz, Poland
Interests: high-voltage engineering; pre-breakdown and breakdown phenomena in dielectric liquids; alternative dielectric liquids for electrical purposes; statistics in electrical engineering; partial discharges; insulation of power transformers
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Prof. Dr. Abderrahmane Beroual

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Guest Editor

Département Electronique, Energie Electrique, Automatique, École Centrale de Lyon, 69134 Ecully, France
Interests: high voltage engineering; insulation coordination; outdoor insulation; dielectric materials (gases, liquids, nanofluids, interfaces); long air gaps discharge and lightning; modelling of discharges and composite materials
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The proper operation of high-voltage devices, especially transformers, is mainly determined by their insulation conditions. Solid, liquid, and gaseous dielectrics used as electrical insulation in high-voltage equipment must ensure correct, continuous, uninterrupted, and safe operation of the devices. In addition, ensuring an appropriate level of environmental safety is extremely important. Insulation materials work under conditions of high temperature, high electric fields, or mechanical stresses. Such conditions require high-quality materials, which should not change over time. Therefore, it is very important to study the properties of materials used in high-voltage equipment from various perspectives, such as electrical (AC, DC, LI, or combined exposure), thermal (e.g., accelerated aging), or chemical (impact of environmental factors or material compatibility). In all these areas, researchers are seeking to find optimal solutions for insualting materials that will work well in industrial applications. The development of the HVDC technology, which involves specific stress conditions and thus specific physical phenomena associated with stress, has become extremely important in this respect.

Since insulating materials are constantly developing and new materials keep appearing in the market (e.g., biodegradable insulating liquids in the case of liquid dielectrics or nano-fluids), this Special Issue will be focused on all aspects of new solutions proposed for use in high-voltage applications.

Therefore, papers that deal with the study of the dielectric and thermal properties of insulation materials for electrical applications and those focused on other properties of dielectrics are warmly welcomed. Studies on the chemical structure of materials and on the impact of this structure (admixtures, nanoparticles) on the properties of single dielectrics and mixtures are also of interest. Papers may as well report research methods to study insulating materials in the laboratory testing phase as well as in operating conditions.

Prof. Dr. Pawel Rozga
Prof. Dr. Abderrahmane Beroual
Guest Editors

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Keywords

New solutions within solid, liquid, and gaseous dielectrics
Testing of dielectric materials under AC, DC, and LI voltages
HV equipment isolation systems (power transformers, transmission lines, substations, switchgears etc.)
Pre-breakdown and breakdown phenomena in dielectrics
Aging tests
Measurement techniques
Characterization methods and techniques

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Published Papers (14 papers)

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Editorial

Jump to: Research, Review

4 pages, 168 KiB

Open AccessEditorial

High Voltage Insulating Materials—Current State and Prospects

by Pawel Rozga and Abderahhmane Beroual

Energies 2021, 14(13), 3799; https://doi.org/10.3390/en14133799 - 25 Jun 2021

Cited by 2 | Viewed by 1975

Abstract

Progression in the field of insulating materials for power transformers and other high voltage devices is visible regardless of the type of insulation: solid, liquid, or gas [...] Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

Research

Jump to: Editorial, Review

25 pages, 10421 KiB

Open AccessArticle

Investigation of Survival/Hazard Rate of Natural Ester Treated with Al₂O₃ Nanoparticle for Power Transformer Liquid Dielectric

by Raymon Antony Raj, Ravi Samikannu, Abid Yahya and Modisa Mosalaosi

Energies 2021, 14(5), 1510; https://doi.org/10.3390/en14051510 - 9 Mar 2021

Cited by 33 | Viewed by 2458

Abstract

Increasing usage of petroleum-based insulating oils in electrical apparatus has led to increase in pollution and, at the same time, the oils adversely affect the life of electrical apparatus. This increases the demand of Mineral Oil (MO), which is on the verge of extinction and leads to conducting tests on natural esters. This work discusses dielectric endurance of Marula Oil (MRO), a natural ester modified using Conductive Nano Particle (CNP) to replace petroleum-based dielectric oils for power transformer applications. The Al₂O₃ is a CNP that has a melting point of 2072 °C and a low charge relaxation time that allows time to quench free electrons during electrical discharge. Al₂O₃ is blended with the MRO and Mineral Oil (MO) in different concentrations. The measured dielectric properties are transformed into mathematical equations using the Lagrange interpolation polynomial functions and compared with the predicted values either using Gaussian or Fourier distribution functions. Addition of Al₂O₃ indicates that 0.75 g/L in MRO has an 80% survival rate and 20% hazard rate compared to MO which has 50% survival rate and 50% hazard rate. Considering the measured or interpolated values and the predicted values, they are used to identify the MRO and MO’s optimum concentration produces better results. The test result confirms the enhancement of the breakdown voltage up to 64%, kinematic viscosity is lowered by up to 40% at 110 °C, and flash/fire points of MRO after Al₂O₃ treatment enhanced to 14% and 23%. Hence the endurance of Al₂O₃ in MRO proves to be effective against electrical, physical and thermal stress. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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18 pages, 8014 KiB

Open AccessArticle

The Influence of the Window Width on FRA Assessment with Numerical Indices

by Szymon Banaszak, Eugeniusz Kornatowski and Wojciech Szoka

Energies 2021, 14(2), 362; https://doi.org/10.3390/en14020362 - 11 Jan 2021

Cited by 7 | Viewed by 1682

Abstract

Frequency response analysis is a method used in transformer diagnostics for the detection of mechanical faults or short-circuits in windings. The interpretation of test results is often performed with the application of numerical indices. However, usually these indices are used for the whole frequency range of the recorded data, returning a single number. Such an approach is inaccurate and may lead to mistakes in the interpretation. An alternative quality assessment is based on the estimation of the local values of the quality index with the moving window method. In this paper, the authors analyse the influence of the width of the input data window for four numerical indices. The analysis is based on the data measured on the transformer with deformations introduced into the winding and also for a 10 MVA transformer measured under industrial conditions. For the first unit the analysis is performed for various window widths and for various extents of the deformation, while in the case of the second the real differences between the frequency response curves are being analysed. On the basis of the results it was found that the choice of the data window width significantly influences the quality of the analysis results and the rules for elements number selection differ for various numerical indices. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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11 pages, 1799 KiB

Open AccessArticle

Statistical Analysis of AC Dielectric Strength of Natural Ester-Based ZnO Nanofluids

by Hidir Duzkaya and Abderrahmane Beroual

Energies 2021, 14(1), 99; https://doi.org/10.3390/en14010099 - 27 Dec 2020

Cited by 46 | Viewed by 2590

Abstract

Due to environmental concerns and increased energy demand, natural esters are among the alternatives to mineral oils in transformers. This study examines the electrical behavior of natural ester-based ZnO nanofluids at different concentrations in the range of 0.05–0.4 g/L. AC breakdown voltages are measured in a horizontally positioned sphere–sphere electrode system according to IEC 60156 specifications. The measurement data are analyzed using Weibull and normal distribution functions. Breakdown voltages with 1%, 10% and 50% probability are also estimated, these probabilities being of great interest for the design of power electrical components. Experimental results show that AC breakdown voltage increases with the concentration of ZnO nanoparticles, except for the concentration of 0.05 and 0.4 g/L of ZnO. Moreover, breakdown voltages at 1% and 10% probability increase by 22.7% and 13.2% when adding 0.1 g/L ZnO to natural ester, respectively. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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13 pages, 5301 KiB

Open AccessArticle

Ageing Tests of Samples of Glass-Epoxy Core Rods in Composite Insulators Subjected to High Direct Current (DC) Voltage in a Thermal Chamber

by Krzysztof Wieczorek, Przemysław Ranachowski, Zbigniew Ranachowski and Piotr Papliński

Energies 2020, 13(24), 6724; https://doi.org/10.3390/en13246724 - 20 Dec 2020

Cited by 4 | Viewed by 2354

Abstract

In this article, we presented the results of the tests performed on three sets of samples of glass-reinforced epoxy (GRE) core rods used in alternating current (AC) composite insulators with silicone rubber housing. The objective of this examination was to test the aging resistance of the rod material when exposed to direct current (DC) high voltage. We hypothesized that the long-term effects of the electrostatic field on the GRE core rod material would lead to a gradual degradation of its mechanical properties caused by ionic current flow. Further, we hypothesized that reducing the mechanical strength of the GRE core rod would lead to the breakage of the insulator. The first group of samples was used for reference. The samples from the second group were subjected to a temperature of about 50 °C for 6000 h. The third group of samples were aged by temperature and DC high voltage for the same time. The samples were examined using the 3-point bending test, micro-hardness measurement and microscopic analysis. No recordable degradation effects were found. Long-term temperature impact and, above all, the combined action of temperature and DC high voltage did not reduce the mechanical parameters or change the microstructure of the GRE material. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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14 pages, 3674 KiB

Open AccessArticle

Streaming Electrification of Nycodiel 1255 Synthetic Ester and Trafo EN Mineral Oil Mixtures by Using Rotating Disc Method

by Maciej Zdanowski

Energies 2020, 13(23), 6159; https://doi.org/10.3390/en13236159 - 24 Nov 2020

Cited by 11 | Viewed by 2151

Abstract

Power transformers are the main element of an electric power system. The service life of these devices depends to a large extent on the technical condition of their insulation system. Replacing mineral oils with natural or synthetic ester (retrofilling process) may increase the efficiency and operational safety of transformers, and also limit their adverse environmental impact. It is technically unfeasible to completely remove mineral oil from a transformer. Its small residues form a mixture with fluid ester, with different physicochemical and electric properties. Streaming electrification is one of the phenomena which, under unfavorable conditions, may damage the insulation system of a forced oil cooled transformer. It is necessary to run prophylactic tests for the ECT (electrostatic charging tendency) of insulating liquid mixtures from the point of view of transformer retrofilling, which is being used more often than before. The article presents the results of studies on selected physicochemical, and electrical properties, and the ECT of mixtures of fresh and aged Trafo EN mineral oil with Nycodiel 1255 synthetic ester. In this regard, the density, the kinematic viscosity, the conductivity, and the relative dielectric constant were measured. The molecular diffusion coefficient was determined using Adamczewski’s empirical dependency. The streaming electrification was tested in a rotating disc system. The impact of the rotation time, the diameter, and the disc’s rotation speed on the amount of the electrification current generated were analyzed. In addition, the co-relation between the electrification current and the composition of the mixture was determined using fresh and aged mineral oil. On the basis of the electrification model, the volume density of the q_w charge was calculated, which is a parameter defining the ECT of insulating liquids. Based on the results, it was concluded that the synthetic ester is characterized by a higher susceptibility to electrification than the mineral oil. However, combining synthetic ester with a small amount (up to 20%) of fresh or aged mineral oil significantly reduces its ECT, which is beneficial from the point of view of retrofilling power transformers. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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18 pages, 4319 KiB

Open AccessArticle

Analysis of Polarization and Depolarization Currents of Samples of NOMEX^®910 Cellulose–Aramid Insulation Impregnated with Mineral Oil

by Stefan Wolny and Adam Krotowski

Energies 2020, 13(22), 6075; https://doi.org/10.3390/en13226075 - 20 Nov 2020

Cited by 13 | Viewed by 2717

Abstract

The article presents results of laboratory tests performed on samples of NOMEX^®910 cellulose–aramid insulation impregnated with Nynas Nytro 10× inhibited insulating mineral oil using the polarization and depolarization current analysis method (PDC Method). In the course of the tests, the insulation samples were subjected to a process of accelerated thermal degradation of cellulose macromolecules, as well as weight-controlled dampening, thereby simulating the ageing processes occurring when using the insulation in power transformers. The effects of temperature in the ranges typical of normal transformer operation were also taken into account. On the basis of the obtained data, the activation energy was then fixed together with dominant time constants of cellulose–aramid insulation relaxation processes with respect to the temperature and degree of moisture, as well as thermal degradation of cellulose macromolecules. It was found that the greatest and predictable changes in the activation energy value were caused by the temperature and the degree of moisture in the samples. A similar conclusion applies to the dominant time constant of the relaxation process of cellulose fibers. Degree of thermal degradation samples was of marginal importance for the described parameters. The final outcome of the test results and analyses presented in the article are regression functions for the activation energy and the dominant time constants depending on the earlier listed parameters of the experiment, which may be used in the future diagnostics of the degree of technical wear of cellulose–aramid insulation performed using the PDC method. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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10 pages, 2036 KiB

Open AccessArticle

Comparative Analysis of Optical Radiation Emitted by Electric Arc Generated at AC and DC Voltage

by Łukasz Nagi, Michał Kozioł and Jarosław Zygarlicki

Energies 2020, 13(19), 5137; https://doi.org/10.3390/en13195137 - 2 Oct 2020

Cited by 7 | Viewed by 2609

Abstract

The article presents a comparison of the spectra of electromagnetic radiation emitted by an electric arc. The spectrum ranges from ultraviolet through visible light to near infrared. Spectra from electric arcs were compared for different frequencies of generating current and for direct current. Characteristic peaks for each measurement were described, and the percentage of individual components of light emitted through the arc was presented. An electric arc is an undesirable phenomenon in many areas, and its detection and control depends largely on its source. There are also areas where an electric arc is used. A better understanding of the physical phenomena involved in different arcs can help optimize the use of the electric arc. Safety and economy through the elimination of parasitic energy shares i.e., in the welding arc can be based on the control of the arc by controlling its optical spectrum. The optical method used in this study is one of the methods of electrical discharge detection in electrical devices and systems. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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18 pages, 5201 KiB

Open AccessArticle

Using the Method of Harmonic Distortion Analysis in Partial Discharge Assessment in Mineral Oil in a Non-Uniform Electric Field

by Alper Aydogan, Fatih Atalar, Aysel Ersoy Yilmaz and Pawel Rozga

Energies 2020, 13(18), 4830; https://doi.org/10.3390/en13184830 - 15 Sep 2020

Cited by 14 | Viewed by 2771

Abstract

In high-voltage equipment, it is vital to detect any failure in advance. To do this, a determination of the partial discharges occurring at different voltage types as well as at different electrode configurations is essential for observing the oil condition. In this study, an experimental setup consisting of a needle–semi-sphere electrode configuration immersed in mineral oil is prepared for laboratory experiment. In such a way, a non-uniform electric field is created and the leakage currents are monitored from the grounded electrode. A total of six different electrode configurations are analyzed during the tests by the use of hemispheres of different diameters as grounded electrodes and copper and steel pointed (medical) needle high-voltage electrodes. In the experiments, the partial discharges occurring at four different voltage levels between 5.4 and 10.8 kV are measured and recorded. The effect of the different electrode configurations and voltage levels on the harmonic distortion are noted and discussed. It is experimentally confirmed that it is possible to measure the leakage current caused by the partial discharges of the corona type in oil at the different metal points, creating high-voltage electrodes and different electric field distributions based on the proposed non-invasive measurement technique. The studies showed that there is a significant rise of even harmonic components in the leakage current during the increase in the partial discharge intensity with the 5th harmonic as dominant. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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17 pages, 1213 KiB

Open AccessArticle

Effect of Moisture on the Thermal Conductivity of Cellulose and Aramid Paper Impregnated with Various Dielectric Liquids

by Grzegorz Dombek, Zbigniew Nadolny, Piotr Przybylek, Radoslaw Lopatkiewicz, Agnieszka Marcinkowska, Lukasz Druzynski, Tomasz Boczar and Andrzej Tomczewski

Energies 2020, 13(17), 4433; https://doi.org/10.3390/en13174433 - 27 Aug 2020

Cited by 18 | Viewed by 3117

Abstract

This paper presents the effect of the impact of moisture in paper insulation used as insulation of transformer windings on its thermal conductivity. Various types of paper (cellulose and aramid) and impregnated (mineral oil, synthetic ester, and natural ester) were tested. The impact of paper and impregnated types on the changes in thermal conductivity of paper insulation caused by an increase in moisture were analyzed. A linear equation, describing the changes in thermal conductivity due to moisture, for various types of paper and impregnated, was developed. The results of measuring the thermal conductivity of paper insulation depending on the temperature are presented. The aim of the study is to develop an experimental database to better understand the heat transport inside transformers to assess aging and optimize their performance. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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11 pages, 2515 KiB

Open AccessArticle

Electrostatic Charging Tendency Analysis Concerning Retrofilling Power Transformers with Envirotemp FR3 Natural Ester

by Maciej Zdanowski

Energies 2020, 13(17), 4420; https://doi.org/10.3390/en13174420 - 27 Aug 2020

Cited by 16 | Viewed by 2464

Abstract

Natural and synthetic esters are liquids characterized by insulating properties, high flash point, and biodegradability. For this reason, they are more and more often used as an alternative to conventional mineral oils. Esters are used to fill new or operating transformers previously filled with mineral oil (retrofilling). It is technically unfeasible to completely remove mineral oil from a transformer. Its small residues create with esters a mixture with features significantly different from those of the base liquids. This article presents electrostatic charging tendency (ECT) tests for mixtures of fresh and aged Trafo EN mineral oil with Envirotemp FR3 natural ester from the retrofilling point of view. Under unfavorable conditions, the flow electrification phenomenon can damage the solid insulation in transformers with forced oil circulation. The ECT of the insulating liquids has been specified using the volume density of the q_w charge. This parameter has been determined using the Abedian–Sonin model on the basis of the electrification current measured in the flow system, as well as selected physicochemical properties of the liquids. It was shown that ECT is strongly dependent on the type of insulating liquid and pipe material, as well as the composition of the mixtures. The most important finding from the research is that a small amount (up to 10%) of fresh and aged mineral oil is effective in reducing the ECT of Envirotemp FR3 natural ester. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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11 pages, 2941 KiB

Open AccessArticle

Laboratory Model Studies on the Drying Efficiency of Transformer Cellulose Insulation Using Synthetic Ester

by Piotr Przybylek, Hubert Moranda, Hanna Moscicka-Grzesiak and Mateusz Cybulski

Energies 2020, 13(13), 3467; https://doi.org/10.3390/en13133467 - 4 Jul 2020

Cited by 8 | Viewed by 2200

Abstract

This paper presents the results of laboratory tests of cellulose insulation drying with the use of synthetic ester. The effectiveness of the drying process was investigated depending on the initial moisture of cellulose samples (2%, 3%, and 4%), ester temperature (55, 70, and 85 °C), initial moisture of the ester (70, 140, and 220 ppm), drying time (48, 96, and 168 h), and the weight ratio of cellulosic materials to ester (0.067 and 0.033). A large influence of temperature and time of drying on the efficiency of the drying process was found. This is important information due to the application of the results in the transformers drying procedure. The heating and drying ester unit should provide the highest possible temperature. For the assumed experiment conditions the initial moisture of the ester had little effect on the drying efficiency. An ester with a moisture content below 140 ppm can still be considered as meeting the requirements for drying cellulose with significant moisture. The weight ratio of cellulose products to ester has no major effect on drying efficiency during cellulose drying by circulating dry ester. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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9 pages, 2481 KiB

Open AccessArticle

Energy Distribution of Optical Radiation Emitted by Electrical Discharges in Insulating Liquids

by Michał Kozioł

Energies 2020, 13(9), 2172; https://doi.org/10.3390/en13092172 - 1 May 2020

Cited by 5 | Viewed by 1978

Abstract

This article presents the results of the analysis of energy distribution of optical radiation emitted by electrical discharges in insulating liquids, such as synthetic ester, natural ester, and mineral oil. The measurements of optical radiation were carried out on a system of needle–needle type electrodes and on a system for surface discharges, which were immersed in brand new insulating liquids. Optical radiation was recorded using optical spectrophotometry method. On the basis of the obtained results, potential possibilities of using the analysis of the energy distribution of optical radiation as an additional descriptor for the recognition of individual sources of electric discharges were indicated. The results can also be used in the design of various types of detectors, as well as high-voltage diagnostic systems and arc protection systems. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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Review

Jump to: Editorial, Research

33 pages, 7481 KiB

Open AccessReview

A Review on Synthetic Ester Liquids for Transformer Applications

by Pawel Rozga, Abderrahmane Beroual, Piotr Przybylek, Maciej Jaroszewski and Konrad Strzelecki

Energies 2020, 13(23), 6429; https://doi.org/10.3390/en13236429 - 4 Dec 2020

Cited by 96 | Viewed by 9548

Abstract

Synthetic esters have become more and more popular in last few decades, explaining the increasing number of units filled with this liquid year by year. They have been investigated under different aspects, both from the fundamental point of view and breakdown mechanisms, well as from the application point of view. However, their use in high voltage equipment is always a challenge and deeper knowledge of the various aspects that can be encountered in their exploitation is needed. The intent of this review paper is to present the recent research progress on synthetic ester liquid in relation to the selected issues, most important for ester development in the authors’ opinion. The described issues are the breakdown performance of synthetic esters, lightning impulse strength and pre-breakdown phenomena of synthetic esters, synthetic esters-based nanofluids, combined paper-synthetic ester based insulating systems, application of synthetic ester for retro-filling and drying of mineral oil-immersed transformers, DGA(dissolved gas analysis)-based diagnosis of synthetic esters filled transformers as well as static electrification of synthetic esters. The different sections are based both on the data available in the literature, but above all on the authors’ own experience from their research work on synthetic ester liquids for electrical application purposes. Full article

(This article belongs to the Special Issue High Voltage Insulating Materials-Current State and Prospects)

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