Super-hydrophobic coatings have recently drawn considerable attention in research and applications towards self-cleaning materials. This paper presents experiments and analysis of water droplet behaviors on glass, silicone rubber, and super-hydrophobic surface in an ac electric field applied parallel to the surface. Experimental results show that a water droplet tends to move on a super-hydrophobic surface while it tends to stretch or deform on glass and silicone rubber surfaces. A physical model of the water droplet deformation and motion mechanism is presented. An electrostatic force acting as the domain driving force plays a key role in water droplet motion on super-hydrophobic surface. The electrostatic stress acting along the external profile of a water droplet cross section was simulated using a surface integral of the Maxwell stress tensor. The simulation for surfaces with different wetting properties are in good agreement with the experimental results. Super-hydrophobic coating demonstrates a prospective electric field enhanced self-cleaning property.

Investigation of the electric field driven self-propelled motion of water droplets on a super-hydrophobic surface

Although the cost of investment in power lines insulators is 3 - 5% of the total cost of the installation, the impact of their performance on reliability, failure costs, maintenance routines, etc in power systems is tens of times higher. Composite insulators were introduced 50 years ago and have been used around the world with consistently good experience. Low weight, easy handling, good performance under high pollution, low maintenance costs, and resistance to vandalism are some of their advantages. Nevertheless, acid rain, salty dust deposition, corona discharges, ozone, UV radiation, and humidity among other factors, deteriorate the quality of the polymeric housing reducing their hydrophobicity. The synergistic action of ageing factors is extremely complex and the whole degradation process may change when any one variable is slightly modified. Many studies have been carried out to increase understanding of the physicochemical processes which control the electrical and mechanical stability of polymers during in-service ageing with the objective of predicting remaining life-times. Vital areas of knowledge about polymer insulators are still incomplete and lacking; three of them are: (1) early stages of degradation in service under different environmental conditions, (2) monitoring and diagnosis techniques suitable for distribution installations and (3) steps to establish an insulators management plan based on condition and risk of failure.In this research these three topics are covered. A full review of literature about management of electrical distribution assets is included, followed by a specific plan developed for monitoring, diagnosis and ranking of insulators mainly supported by visual inspections. Diagnosis of medium voltages EPDM insulators recovered from service aged under different conditions is done using both traditional techniques and, uniquely, dielectric impedance. The relationship between surface roughness and static contact angle is also used to characterize insulators? surfaces. Early stages of degradation are studied focusing the experimental work to evaluate the electrohydrodynamic processes which occur on new samples under different conditions, giving special attention to leakage current pulse analysis, electric field enhancement, and resistance/capacitive behaviour including phase of leakage current. Results from each specific topic offer additional understanding of polymer insulators degradation providing insight to monitoring, diagnosis and management. Additionally, results open new topics in which new investigations are proposed.

Improved Condition Monitoring of Composite Insulators

In the present work, epoxy nano composite samples of different clay content are subjected to water droplet initiated discharge tests using controlled harmonic content high voltage supply. The effect of a clay additive up to 1% by weight is shown to increase droplet contact angle by 8% with a subsequent reduction beyond this content. Using a UHF technique to identify the corona inception voltage, it is difficult to identify correlation with harmonic content. However, it was found that the corona inception factor (CIF) and the peak factor of the applied AC/harmonic AC voltages showed an inverse relationship. The rise time of the current pulse due to water droplet initiated corona discharge under AC/harmonic AC voltage was found to be about 700 ps and from FFT analysis, the corresponding UHF generated signal has a dominant frequency of about 1 GHz. The characteristics of the captured UHF signals are similar for all investigated clay contents and harmonic AC voltages. It was found that the discharges caused damage near the high voltage electrode and that the surface carbonization is related to the accumulated dissipated energy during the discharge process. Analysis of the voltage, current signals and phase resolved partial discharge (PRPD) studies indicate that discharges occur around the voltage peak and also when the rate of rise of voltage is high. The number of discharges is higher in the positive half cycle compared with the negative half cycle.

Understanding water droplet initiated discharges on epoxy nanocomposites under harmonic AC voltages adopting uhf technique

The ageing processes of composite insulators are heavily dependent on their surface hydrophobicity level. Corona discharges and dry-band arcing are the main phenomena associated with the degradation of polymer surfaces when insulators are exposed to pollution, humidity and electric fields. Additional environmental factors such as UV radiation, temperature and mechanical stresses act as accelerating factors for the ageing processes. Understanding of corona and dry band arc processes is crucial to evaluate, diagnose and monitor the insulators' surface condition in advanced ageing stages. At the same time, different experiments to study the effect of electric field distribution and its influence on water droplet behaviour, leakage current patterns, insipient corona discharges and water droplets behaviour are needed to increase the knowledge about early degradation stages. The study summarized in this paper was focused on metal electrodes shapes and their influence on electric field distribution, water droplet behaviour and leakage current characteristics. Keeping the gap distance constant, five different shapes of elongated electrodes (flat, three different circle-sections, and rectangular bars) were simulated using Finite Element Analysis (FEM) and four of them were experimentally tested. Contact angles of water droplets were monitored using a high speed camera. Images were synchronized with leakage current and voltage signals. Significant differences were found between the cases under study, and the influence of electrode shape on processes seen on the material surfaces has been established.

The dependency of water droplet behaviour and leakage current pattern on electrode configuration

The verification of electric field performance is one of the main challenges in the electric design of the fully composite pylon. Electric field computation based on numerical methods such as Finite Element Model (FEM) is an useful method to predict its electric field performance. However, verification of these theoretical computation is necessary. In order to evaluate the electric field distribution on the composite cross-arm surface in the fully composite pylon and assess its electrical design in wet conditions, water induced corona discharge tests on the full-scale cross-arm has been performed. According to test results, the corona inception electric field magnitude on the composite cross-arm surface in wet conditions is lower than the maximum electric field magnitude on the composite cross-arm surface with nominal operation voltage. Thus, the configuration design of the cross-arm needs further consideration, aiming to restrain the maximum electric field magnitude on its surface. Additionally, effects of the cross-arm inclined angle \({\varvec{\theta }}_{cross-arm}\) on the water induced corona discharge activities have been investigated, which gives valuable advice for the decision of optimal inclined angle \({\varvec{\theta }}_{cross-arm}\).

Electric Field Verification by High Voltage Experiments on the Composite Cross-Arm

To clarify the influence of a water droplet on discharge characteristics, water droplet was put on the polymer and the current and voltage characteristics of corona discharge was investigated under the application of AC voltage, using two different types of electrode systems (plane with needle to plane, needle to plane). The behavior of the water droplet was also investigated by using a high speed camera. In the case when AC high voltage was applied to the needle electrode, corona of the same polarity with the voltage applied to the needle was observed. When the water droplet tilted towards the positive electrode every half cycle, it was considered to possess a negative charge at the tilted tip of the water droplet.

M. Otsubo

Papers

Behavior of water droplet on polymer material and the influence on discharge characteristics