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.

In this paper, a fractured 500 kV ac composite insulator, whose fracture characteristics differ significantly from those of brittle fracture, is analyzed in order to identify the aging mechanisms involved and their causes. Several sheath punctures occurred along the insulator which apparently originated from inside the insulator. Removal of the sheath revealed weak adhesion as well as tracking along the sheath-core interface. Weak microscopic as well as macroscopic interfaces were indicated by the insulator's weak performance in the water diffusion and dye penetration test according to IEC 62217. The core material was analyzed by means of FTIR identifying glass corrosion, ion exchange and hydrolysis as major degradation mechanisms. Based on the results of analysis, a water induced aging process is proposed and recommendations for online monitoring of composite insulators with respect to interfacial aging are given.

Analysis of a fractured 500 kV composite insulator - identification of aging mechanisms and their causes

We studied the dielectric properties (dielectric constant and loss factor) of epoxy molding compounds used for electronic packaging, as a function of frequency (100 Hz–100 kHz) and temperature (25–100 °C). Studies were performed for samples with different formulations (various silica and carbon black contents). At room temperature a loss peak is found at 50 kHz, whose intensity is enhanced by carbon black addition. Additional loss is detected below 1 kHz when the temperature is increased up to 100 °C. We also studied the influence of post-mold curing time (0–12 h at 165 °C) on the dielectric properties. The dielectric constant monotonically decreases with post-mold cure to level off to a minimum value for long post-cure durations. The loss factor first increases for short post-curing times, and then decreases as post-cure is continued. The origin of loss is discussed with reference to common relaxation processes observed in epoxy polymers.

Dielectric properties of epoxy/silica composites used for microlectronic packaging, and their dependence on post-curing

We report on the effects of hydrothermal ageing on the dielectric properties of epoxy-silica composites used for microelectronic packaging. The study was carried out for samples with various degrees of curing. Epoxy compounds were subjected to moisture exposure (standard JEDEC procedures) and consecutive thermal stress (240°C). Changes in the dielectric constant and in the loss factor were measured in the 100 Hz–100 kHz frequency range. It is found that water absorption increases with curing. After ageing the dielectric constant is decreased and the loss factor increased. Cured materials have a higher resistance against hydrothermal ageing.

Combined effects of humidity and thermal stress on the dielectric properties of epoxy-silica composites

https://iopscience.iop.org/article/10.1088/2053-1591/ab3f0d/pdf

Aging of silicone rubber-based composite insulators under multi-stressed conditions: an overview

This paper deals with two different types of composite insulating materials for HV outdoor insulation technology. For outdoor applications fiberglass reinforced polymer (FRP) are used mainly as transmission line insulators (composite long rods),and as power apparatus housings (composite hollow core insulators). During the last decades mineral filled polymer (MFP) are found to be very suitable for outdoor insulation in the medium and HV ranges. To meet the outdoor demands, long-term stability and durability against environmental stresses are necessary, Composite insulation consists of more than one dielectric component, and linking at least two different kinds of materials leads to interface problems. These regions always appear as weak material structures that can be attacked by various aging mechanisms. Especially the long-term performance and aging resistance are determined by the interface quality. One can distinguish four principal kinds of interface: microscopic, e.g. those between fillers and matrix components; macroscopic, e.g. those between glass fiber rod and polymeric shielding material; internal, in the insulation bulk; and external, solid surface against a liquid or gaseous phase. Today the use of composite insulating materials in HV technology is state of the art. They offer a wide range of superior properties for indoor as well as for outdoor applications. Further improvements should focus on the hydrophobicity and on the long term resistance of the external interface and the stability of the internal interface.

J. M. Seifert

Papers

Interfacial phenomena in composite high voltage insulation