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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 (30 June 2020) | Viewed by 44146

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Special Issue Editor

Prof. Dr. Yuriy Serdyuk

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Guest Editor

Department of Electrical Engineering, Chalmers University of Technology, Chalmersplatsen 4, 412 96 Göteborg, Sweden
Interests: power system components; high voltage technologies; electrical insulation; HVDC insulation; electric fields; composite materials; nano-composites; charge transport; dielectric properties

Special Issue Information

Dear Colleagues,

You are invited to submit your contributions for consideration for the Special Issue “Advances in electric insulating materials for components of power systems”.

In modern society, energy demands are growing continuously and, therefore, electric energy generation and transportation are fundamental questions that need to be resolved to ensure further development. It is commonly accepted that future sustainable electric energy systems will be based on renewable energy sources (hydro, wind, solar), which need to be integrated into existing systems. The implementation of this concept is challenging and requires new solutions in many aspects. Thus, the remote locations of renewable energy sources from consumption sites suggest the utilization of high voltage direct current (HVDC) technology for electric energy transportation. There is also a trend towards utilizing higher voltages in AC systems to enhance their efficiency. Electrical insulation as an integral element of power components responsible for their reliability should be capable of withstanding the high electrical and thermal stresses appearing during normal operation as well as under specific circumstances (lightning overvoltages, switching operations, polarity reversals in HVDC systems, etc.). These requirements, together with the demands for the compactness (and thus higher energy density) of modern devices, push insulating materials in power components to their physical limits. Understanding the behavior of insulating materials under extreme electric and combined electric–thermal stresses is therefore crucial for developing future electric energy systems.

This Special Issue of Energies focuses on recent achievements in the research and development of electric insulating materials for power components. The topics include, but are not limited to:

High field effects in insulating materials
Electric conduction mechanisms
Space and surface charge dynamics
Effects of nano-fillers
Electric tree formation and development
Diagnostics of insulating materials
Ageing and lifetime prediction
Self-healing insulating materials

Review papers, contributions presenting experimental investigations, theoretical studies, and computer simulations are welcome.

Prof. Dr. Yuriy Serdyuk
Guest Editor

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Keywords

electrical insulation
HVDC insulation
composite materials
nano-composites
charge transport
multi-layered insulation
space charge
surface charge
electric conduction
dielectric response
insulation ageing
insulation diagnostics
partial discharges
electric treeing
self-healing insulation

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Published Papers (11 papers)

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Research

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12 pages, 2631 KiB

Open AccessArticle

Streaming Electrification Phenomenon of Electrical Insulating Oils for Power Transformers

by Maciej Zdanowski

Energies 2020, 13(12), 3225; https://doi.org/10.3390/en13123225 - 22 Jun 2020

Cited by 20 | Viewed by 2534

Abstract

The subject matter of this study was the problem of the ECT (electrostatic charging tendency) of mineral insulation oils during their flow. The electrostatic charges generated may lead to partial discharges, and as a consequence, to the breakdown of a power transformer insulation system. In this study, the results of the ECT of mineral oils used in transformers were compared. The method of streaming electrification of insulation liquids using a flow-through system was used. The influence of flow speed, temperature, and the pipe material on the values of the electrification current and volume charge density q_w were analyzed_. The results obtained in this study should be taken into account regarding the operation of power transformers. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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10 pages, 4076 KiB

Open AccessArticle

Temperature and Field Induced Variations of Electric Conductivities of HTV Silicone Rubbers Derived from Measured Currents and Surface Potential Decay Characteristics

by Shahid Alam, Yuriy V. Serdyuk and Stanislaw M. Gubanski

Energies 2020, 13(11), 2982; https://doi.org/10.3390/en13112982 - 10 Jun 2020

Cited by 8 | Viewed by 1927

Abstract

The temperature and field dependencies of electric conductivities of two types of silicone rubber-based polymers intended for use in high voltage direct current applications are presented and discussed. The conductivities obtained with the standard method by measuring a current through the material sample placed between metallic electrodes in response to the applied voltage are compared with those deduced from the measured potential decay on pre-charged material surface in an open circuit configuration. The measurements were conducted in the range of the applied electric field strength (0.5–5) kV/mm and temperatures ranging from 22 °C to 70 °C. It is shown that the values of the conductivities obtained by the two methods are in agreement and their temperature dependences obey Arrhenius law yielding similar activation energies. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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16 pages, 9388 KiB

Open AccessArticle

Space Charge Accumulation at Material Interfaces in HVDC Cable Insulation Part I—Experimental Study and Charge Injection Hypothesis

by Espen Doedens, E. Markus Jarvid, Raphaël Guffond and Yuriy V. Serdyuk

Energies 2020, 13(8), 2005; https://doi.org/10.3390/en13082005 - 17 Apr 2020

Cited by 14 | Viewed by 3784

Abstract

On-site installation of accessories on extruded polymeric high voltage cables in a common practice. The procedure requires the shaping of the physical interface between the cable insulation surface and the pre-molded accessory body. On such interfaces, rough surfaces should be avoided in order to limit space charge accumulation in the insulation, which affects the cable performance by reducing insulation life-time, creating conditions for local field enhancement, and, respectively, the formation of possible breakdown path e.g. by electrical treeing. Space charge measurements on cable insulation peelings were undertaken to assess the space charge injection and accumulation on interfaces with varying degrees of surface roughness in order to improve understanding on this subject. The results of the measurements confirm the hypothesis regarding the enhancement of charge injection from rough surfaces when electric field strength exceeds a certain level. The accumulated charge density in the material is shown to strongly depend on the field strength and temperature in both polarization and subsequent depolarization measurements. These results emphasize that a bipolar charge transport model that incorporates field and temperature dependencies of charge injection, trapping, detrapping, and recombination processes needs to be adopted to accurately describe the observed electric conduction phenomena. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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24 pages, 14658 KiB

Open AccessArticle

Space Charge Accumulation at Material Interfaces in HVDC Cable Insulation Part II—Simulations of Charge Transport

by Espen Doedens, E. Markus Jarvid, Raphaël Guffond and Yuriy V. Serdyuk

Energies 2020, 13(7), 1750; https://doi.org/10.3390/en13071750 - 6 Apr 2020

Cited by 20 | Viewed by 4016

Abstract

Extruded high voltage direct current (HVDC) cable systems contain interfaces with poorly understood microscopic properties, particularly surface roughness. Modelling the effect of roughness on conduction in cable insulation is challenging, as the available results of macroscopic measurements give little information about microscopic charge distributions at material interfaces. In this work, macroscopic charge injection from interfaces is assessed by using a bipolar charge transport model, which is validated against a series of space charge measurements on cable peelings with different degrees of surface roughness. The electric field-dependent conduction and charge trapping effects stimulated by the injection current originating from rough surfaces are assessed. It is shown that by accounting for roughness enhanced charge injection with the parameters derived in part I of the paper, reasonable agreement between computed and measured results can be achieved at medium field strengths (10–40 kV/mm). Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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17 pages, 4706 KiB

Open AccessArticle

Charging and Discharge Currents in Low-Density Polyethylene and its Nanocomposite

by Anh T. Hoang, Yuriy V. Serdyuk and Stanislaw M. Gubanski

Energies 2020, 13(6), 1518; https://doi.org/10.3390/en13061518 - 23 Mar 2020

Cited by 5 | Viewed by 2891

Abstract

Charging and discharge currents measured in low-density polyethylene (LDPE) and LDPE/Al₂O₃ nanocomposite are analyzed. The experiments were conducted at temperatures of 40–80 °C utilizing a consecutive charging–discharging procedure, with the charging step at electric fields varying between 20 and 60 kV/mm. A quasi-steady state of the charging currents was earlier observed for the nanofilled specimens and it was attributed to the enhanced trapping process at polymer–nanofiller interfaces. An anomalous behavior of the discharge currents was found at elevated temperatures for both the studied materials and its occurrence at lower temperatures in the nanofilled LDPE was due to the presence of deeply trapped charges at polymer–nanofiller interfaces. The field dependence of the quasi-steady charging currents is examined by testing for different conduction mechanisms. It is shown that the space-charge-limited process is dominant and the average trap site separation is estimated at less than 2 nm for the pristine LDPE and it is at about 5–7 nm for the LDPE/Al₂O₃ nanocomposite. Also, location of the trapping sites in the band gap structure of the nanofilled material is altered, which substantially weakens electrical transport as compared to the unfilled counterpart. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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15 pages, 6979 KiB

Open AccessArticle

Electrical Characterization of a New Crosslinked Copolymer Blend for DC Cable Insulation

by Sarath Kumara, Xiangdong Xu, Thomas Hammarström, Yingwei Ouyang, Amir Masoud Pourrahimi, Christian Müller and Yuriy V. Serdyuk

Energies 2020, 13(6), 1434; https://doi.org/10.3390/en13061434 - 19 Mar 2020

Cited by 24 | Viewed by 3318

Abstract

To design reliable high voltage cables, clean materials with superior insulating properties capable of operating at high electric field levels at elevated temperatures are required. This study aims at the electrical characterization of a byproduct-free crosslinked copolymer blend, which is seen as a promising alternative to conventional peroxide crosslinked polyethylene currently used for high voltage direct current cable insulation. The characterization entails direct current (DC) conductivity, dielectric response and surface potential decay measurements at different temperatures and electric field levels. In order to quantify the insulating performance of the new material, the electrical properties of the copolymer blend are compared with those of two reference materials; i.e., low-density polyethylene (LDPE) and peroxide crosslinked polyethylene (XLPE). It is found that, for electric fields of 10–50 kV/mm and temperatures varying from 30 °C to 70 °C, the DC conductivity of the copolymer blend is in the range of 10⁻¹⁷–10⁻¹³ S/m, which is close to the conductivity of crosslinked polyethylene. Furthermore, the loss tangent of the copolymer blend is about three to four times lower than that of crosslinked polyethylene and its magnitude is on the level of 0.01 at 50 °C and 0.12 at 70 °C (measured at 0.1 mHz and 6.66 kV/mm). The apparent conductivity and trap density distributions deduced from surface potential decay measurements also confirmed that the new material has electrical properties at least as good as currently used insulation materials based on XLPE (not byproduct-free). Thus, the proposed byproduct-free crosslinked copolymer blend has a high potential as a prospective insulation medium for extruded high voltage DC cables. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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14 pages, 3442 KiB

Open AccessArticle

Solubility and Diffusivity of Polar and Non-Polar Molecules in Polyethylene-Aluminum Oxide Nanocomposites for HVDC Applications

by Shima L. Holder, Mattias E. Karlsson, Richard T. Olsson, Mikael S. Hedenqvist and Fritjof Nilsson

Energies 2020, 13(3), 722; https://doi.org/10.3390/en13030722 - 7 Feb 2020

Cited by 1 | Viewed by 3501

Abstract

The best commercial high-voltage insulation material of today is (crosslinked) ultra-pure low-density polyethylene (LDPE). A 100-fold decrease in electrical conductivity can be achieved by adding 1–3 wt.% of well-dispersed inorganic nanoparticles to the LDPE. One hypothesis is that the nanoparticle surfaces attract ions and polar molecules, thereby cleaning the surrounding polymer, and thus reducing the conductivity. LDPE-based nanocomposites with 1–12 wt.% octyl-coated aluminum oxide nanoparticles were prepared and the sorption and desorption of one polar compound (acetophenone, a crosslinking by-product) and one non-polar compound of a similar size (limonene) were examined. Since the uptake of acetophenone increased linearly with increasing filler content, whereas the uptake of limonene decreased, the surface attraction hypothesis was strengthened. The analytical functions for predicting composite solubility as a function of particle size and filler fraction were derived using experimental solubility measurements and Monte Carlo simulations. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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13 pages, 2592 KiB

Open AccessArticle

Degradation Assessment of Polyethylene-Based Material Through Electrical and Chemical-Physical Analyses

by Simone Vincenzo Suraci, Davide Fabiani, Laura Mazzocchetti and Loris Giorgini

Energies 2020, 13(3), 650; https://doi.org/10.3390/en13030650 - 3 Feb 2020

Cited by 29 | Viewed by 2923

Abstract

The usability of any material hinges upon its stability over time. One of the major concerns, focusing on polymeric materials, is the degradation they face during their service life. The degradation mechanisms are deeply influenced by the aging temperature to which the material is subjected. In this paper, low-density polyethylene (LDPE) flat specimens were thermally aged under two different temperatures (90 °C and 110 °C) and analyzed. Specimens were characterized through both the most common mechanical and chemical measurements techniques (e.g., tensile stress, thermal analyses, oxidation induction time) and electrical measurements (dielectric spectroscopy, in particular), which are examples of non-destructive techniques. As a result, a very spread characterization of the polyethylene-based materials was obtained and a very good correlation was found to exist between these different techniques, highlighting the possibility of following the aging degradation development of polymers through electrical non-destructive techniques. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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17 pages, 6719 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Endurance of Polymeric Insulation Foil Exposed to DC-Biased Medium-Frequency Rectangular Pulse Voltage Stress

by Raphael Färber, Thomas Guillod, Florian Krismer, Johann W. Kolar and Christian M. Franck

Energies 2020, 13(1), 13; https://doi.org/10.3390/en13010013 - 18 Dec 2019

Cited by 22 | Viewed by 4579

Abstract

The endurance of polymeric insulation foil is investigated under a mixed medium-voltage stress (DC + medium-frequency rectangular pulse) by means of accelerated lifetime testing. A dedicated setup is used that allows us to selectively eliminate the known risk factors for premature insulation failure under medium-frequency pulse voltage stress: partial discharges (PDs) during pulse transitions, excessive dielectric heating, and systemic overvoltages. The obtained results on polyethylenterephtalat (PET) insulation foil suggest that the adequate consideration of these factors is sufficient for eliminating the adverse effects of the pulse modulation under the investigated conditions. Indeed, if all mentioned risk factors are eliminated, the time to failure observed under a pure DC stress is shorter than with a superimposed pulse (keeping the same peak voltage). There is then no indication of an additional detrimental “per pulse” degradation process (i.e., the time to failure is not dependent on pulse frequency). In contrast, when repetitive PDs are present, the lifetime under combined DC + rectangular pulse stress strongly decreases with increasing pulse switching frequency. PD erosion of the foil is quantified by means of confocal microscopy, and the applicability of the streamer criterion for predicting PD inception is discussed. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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Review

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24 pages, 2623 KiB

Open AccessReview

Review of Fiber Optic Diagnostic Techniques for Power Transformers

by Janvier Sylvestre N’cho and Issouf Fofana

Energies 2020, 13(7), 1789; https://doi.org/10.3390/en13071789 - 8 Apr 2020

Cited by 49 | Viewed by 5893

Abstract

Diagnostic and condition monitoring of power transformers are key actions to guarantee their safe operation. The subsequent benefits include reduced service interruptions and economic losses associated with their unavailability. Conventional test methods developed for the condition assessment of power transformers have certain limitations. To overcome such problems, fiber optic-based sensors for monitoring the condition of transformers have been developed. Flawlessly built-up fiber optic-based sensors provide online and offline assessment of various parameters like temperature, moisture, partial discharges, gas analyses, vibration, winding deformation, and oil levels, which are based on different sensing principles. In this paper a variety and assessment of different fiber optic-based diagnostic techniques for monitoring power transformers are discussed. It includes significant tutorial elements as well as some analyses. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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23 pages, 6855 KiB

Open AccessReview

Review of the PEA Method for Space Charge Measurements on HVDC Cables and Mini-Cables

by Giuseppe Rizzo, Pietro Romano, Antonino Imburgia and Guido Ala

Energies 2019, 12(18), 3512; https://doi.org/10.3390/en12183512 - 12 Sep 2019

Cited by 37 | Viewed by 7251

Abstract

This review takes into account articles and standards published in recent years concerning the application of the Pulsed Electro Acoustic (PEA) method for space charge measurement on High Voltage Direct Current (HVDC) cables and mini-cables. Since the 80s, the PEA method has been implemented for space charge measurements on flat specimens in order to investigate space charge phenomena and to evaluate the ageing of dielectrics. In recent years, this technique has been adapted to cylindrical geometry. Several studies and experiments have been carried out on the use of the PEA method for full size cables and HVDC cable models. The experiments have been conducted using different arrangements of the measurement setup and focusing attention on different aspects of space charge phenomena. In this work, the importance of space charge measurement is highlighted and the state-of-the-art PEA method application to full size cables and mini-cables is described. The main aim of this paper is to offer a complete and current review of this technique. In addition, limits on the use of PEA method are examined and main possible directions of research are proposed in order to improve the applicability, reliability, and replicability of this method. Full article

(This article belongs to the Special Issue Advances in Electric Insulating Materials for Components of Power System)

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