The fundamental principles of water drop corona as an aging mechanism for nonceramic insulators are presented. It is demonstrated that water drops in the sheath regions enhance the electric field and may cause corona which can play an important role in long-term performance. Electric field enhancement caused by water drops in different locations on the shed and sheath are demonstrated by means of electric field calculations and small scale experiments. The threshold magnitude of the surface electric field for corona from water drops is presented for two silicone rubber surfaces having different hydrophobic properties. The effect of water drop corona activity on the properties of the surface material is shown using small scale aging experiments.

Aging of nonceramic insulators due to corona from water drops

Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided.

Diagnostic Methods for Outdoor Polymeric Insulators

This paper provides an overview of the electric field (E-field) distribution on transmission line composite insulators applied in alternating current applications. Factors that affect the E-field distribution are discussed as well as the influence of the E-field distribution on the short and long term performance. Modeling and measurement methods are reported and examples of calculated E-field magnitudes determined are presented together with corona ring application information. This paper was developed by the IEEE Task force on electric fields and composite insulators.

Electric Fields on AC Composite Transmission Line Insulators

This paper surveys the properties which give silicone rubber its distinctive and highly desirable performance characteristics for use as outdoor insulation. The methods of assessing these properties arediscussed and this is followed byadetailedsurveyof whatis knownof agingaud recovery mechanisms. Corona is identified as the principle electrical aging agent and evidence is presented for irreversible loss of polymer as a result of aging. However, the surface recov. ery mechanisms are robust and, provided there is an adequate resting period, the hydrophobic properties recover, This supports the view, from service experience, that there is no substantial evidence to show that normal aging, unless it results in the exposure of the glass fiber core, detracts from the performance of insulators.

Review of aging and recovery of silicone rubber insulation for outdoor use

Currently, there is a strong tendency to replace rigid electronic assemblies by mechanically flexible and stretchable equivalents. This emerging technology can be applied for biomedical electronics, such as implantable devices and electronics on skin. In the first step of the production process of stretchable electronics, electronic interconnections and components are encapsulated into a thin layer of polydimethylsiloxane (PDMS). Afterwards, the electronic structures are completely embedded by placing another PDMS layer on top. It is very important that the metals inside the electronic circuit do not leak out in order to obtain a highly biocompatible system. Therefore, an excellent adhesion between the 2 PDMS layers is of great importance. However, PDMS has a very low surface energy, resulting in poor adhesion properties. Therefore, in this paper, PDMS films are plasma treated with a dielectric barrier discharge (DBD) operating in air at medium pressure (5.0 kPa). Contact angle and XPS measurements reveal that plasma treatment increases the hydrophilicity of the PDMS films due to the incorporation of silanol groups at the expense of methyl groups. T-peel tests show that plasma treatment rapidly imparts adhesion enhancement, but only when both PDMS layers are plasma treated. Results also reveal that it is very important to bond the plasma-treated PDMS films immediately after treatment. In this case, an excellent adhesion is maintained several days after treatment. The ageing behaviour of the plasma-treated PDMS films is also studied in detail: contact angle measurements show that the contact angle increases during storage in air and angle-resolved XPS reveals that this hydrophobic recovery is due to the migration of low molar mass PDMS species to the surface.

https://iopscience.iop.org/article/10.1088/0022-3727/40/23/021/pdf

Adhesion enhancement by a dielectric barrier discharge of PDMS used for flexible and stretchable electronics

This paper identifies corona and local arcing as the major ageing influences on silicone rubber surfaces. The hydrophobic properties of silicone rubber lead to the formation of discrete water droplets on the surface and these can lead to local enhancement of the electric field and corona activity. This activity leads to surface ageing and a consequent loss of hydrophobicity. Once wetting takes place, surface currents flow and dry bands form. Arcing across dry bands also leads to ageing but this is not as deleterious as corona ageing. Experimental evidence on corona ageing and the stages in the ageing process, in service, are presented.

http://ieeexplore.ieee.org/iel5/6616/17709/00821219.pdf

Surface ageing mechanisms and their relationship to service performance of silcone rubber insulation

The paper identifies three different corona onset mechanisms for a water drop attached to the underside of a conductor as a function of air pressure. The three different corona onset mechanisms are a result of two different corona onset processes: water drop instability and ionisation of the surrounding air. Both processes are considered in the paper. A model which determines the corona onset potential as a function of air pressure for a water drop attached to a conductor is presented. The model identifies the three onset mechanisms and accounts for the previously unexplained transition between the ionisation and instability modes. The corona onset potential is determined by modelling both the stability of the drop and the ionisation of the surrounding air as a function of the potential applied to the conductor. This is done over a range of air pressures resulting in an ‘onset versus air pressure’ curve. The entire model is then verified by experimentation. The model accounts for changes in the drop shape with changes in applied potential and the influence of this on the E-field distribution and hence the ionisation processes. Drop volume, conductor surface conditions and applied E-field divergency are accounted for.

Consideration of corona onset from a water drop as a function of air pressure

The paper discusses the use of a ferromagnetic coating applied to the busbars of an SF6 busduct as a means of controlling disconnect transients. A software travelling wave model is described. The model is based on a frequency-domain characterisation of the busbars in the system. The model accounts for the skin effect in coated materials, and incorporates the effect of discontinuities in the system. The influence of the various coating attributes are noted. The model is shown to perform accurately as a travelling-wave analysis tool. Consideration is also given to the transient skin effect, which is shown to increase the attenuation of transients. Results recorded in a laboratory system with and without ferromagnetic busbars are presented. Ferromagnetics are shown to introduce marked risetime lengthening, as well as an envelope attentuation to disconnect transients.

J.P. Reynders

Papers

Review of aging and recovery of silicone rubber insulation for outdoor use

Surface ageing mechanisms and their relationship to service performance of silcone rubber insulation

Consideration of corona onset from a water drop as a function of air pressure

Consideration of skin effect in high-voltage coaxial systems under transient and steady-state conditions, and its impact on steep travelling waves