The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]-[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption.

50 years in the development of insulating liquids

The present review article represents a comprehensive study on polymer micro/nanocomposites that are used in high-voltage applications. Particular focus is on the structure-property relationship of composite materials used in power engineering, by exploiting fundamental theory as well as numerical/analytical models and the influence of material design on electrical, mechanical and thermal properties. In addition to describing the scientific development of micro/nanocomposites electrical features desired in power engineering, the study is mainly focused on the electrical properties of insulating materials, particularly cross-linked polyethylene (XLPE) and epoxy resins, unfilled and filled with different types of filler. Polymer micro/nanocomposites based on XLPE and epoxy resins are usually used as insulating systems for high-voltage applications, such as: cables, generators, motors, cast resin dry-type transformers, etc. Furthermore, this paper includes ample discussions regarding the advantages and disadvantages resulting in the electrical, mechanical and thermal properties by the addition of micro- and nanofillers into the base polymer. The study goals are to determine the impact of filler size, type and distribution of the particles into the polymer matrix on the electrical, mechanical and thermal properties of the polymer micro/nanocomposites compared to the neat polymer and traditionally materials used as insulation systems in high-voltage engineering. Properties such as electrical conductivity, relative permittivity, dielectric losses, partial discharges, erosion resistance, space charge behavior, electric breakdown, tracking and electrical tree resistance, thermal conductivity, tensile strength and modulus, elongation at break of micro- and nanocomposites based on epoxy resin and XLPE are analyzed. Finally, it was concluded that the use of polymer micro/nanocomposites in electrical engineering is very promising and further research work must be accomplished in order to diversify the polymer composites matrices and to improve their properties.

Properties of Polymer Composites Used in High-Voltage Applications.

This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al2O3, with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material’s direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials.

http://publications.lib.chalmers.se/records/fulltext/233507/local_233507.pdf

Charge Transport in LDPE Nanocomposites Part I—Experimental Approach

The performance of electrical equipment and devices is determined to a great extent by the properties of their insulating materials. In power systems and electrical devices, insulating materials have to work in extreme circumstances that can include high temperature differences, intense radiation, and strong electric fields. Such conditions demand high-quality insulating materials with superior electrical, thermal, and mechanical properties as well as resilience to other environmental stresses. This study focuses on advances in insulating materials since the early 20th century and reviews the many developments in their properties and applications, including electric breakdown strength, thermal conductivity, temperature resistance, corona resistance, and specific energy storage in dielectrics. Some research progress on other properties is also covered, such as non-linearity and radiation resistance. Investigations into the properties of insulating materials can greatly assist in understanding their interface effects and composite structures, which in turn is helpful for discovering methods to improve the performance of electrical devices. Future directions for research are proposed to guide new investigations and support the development of even better insulating materials.

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Progress in and prospects for electrical insulating materials

Nanodielectrics, a nowadays popular term in the dielectrics community, has been the subject of intensive research over the past 10 years. In fact, the term "nanodielectrics" stems out of the field of polymer nanocomposites—an emerging field of nanotechnology. Whereas polymer nanocomposites concern polymers within which nanometer-sized fillers are homogeneously dispersed at just a few weight percentage (wt%), the term "nanometric dielectrics," [1] or "nanodielectrics" [2], [3], refers to nanocomposites of specific interest in connection with their dielectric characteristics. Nevertheless, for the scope of HV electrical insulation research, the terms "nanocomposites" and "nanodielectrics" are used interchangeably to refer to polymer/ nanoparticle mixtures of dielectric interest. The year 2015 marked the 21st anniversary of nanodielectrics from their first conceptual introduction in the 1990s [1]. Over this period, progress in nanodielectric research can be divided into two main phases [4], [5], i.e., the one before 2002 and the one thereafter. Initially, experimental studies on nanodielectrics were scarce and countable [6], [7]. Researchers only began to shift their attention to these materials when promising applications of nanodielectrics in electrical insulation were experimentally demonstrated by Nelson and his coworkers in 2002 [8]. Whereas conventional microfilled materials come with reduced dielectric strength due to bulk charge accumulation, Nelson et al. [8] discovered mitigated space charge accumulation and enhanced charge decay in nanofilled materials as compared with their microfilled counterparts, which could lead to improved dielectric strength of nanofilled materials over microfilled materials. Since then, research interest in nanodielectrics has increased tremendously, and various electrical insulation properties such as partial discharge resistance, treeing progression, space charge formation, and dielectric breakdown strength of nanodielectrics have been compared with their unfilled and microfilled counterparts. Promising improvements in these properties have been reported with nanofiller addition [9]–[17].

Nanodielectrics: opportunities and challenges

Advances in polymer nanodielectrics over the past 50 years, using colloid science, polymer composite technology, and nanotechnology, are reviewed. Despite rapid progress during the past 10 years, interdisciplinary investigation is needed to yield additional benefit to society.

T. Imai

Papers

Advances in nanodielectric materials over the past 50 years