Water droplets on hydrophobic surfaces of polymeric insulators oscillate under applied high voltage electric field stress. Deformation of the droplets increases local electric field stress in the triple zones, causing lower partial discharge inception voltage and consequently affecting the ageing performance of the insulator. For this contribution dominant modes of single water droplet oscillation on hydrophobic surfaces were investigated in a wide range of droplet volume and frequency of applied electric field using a high speed camera. 3D frame analysis showed for all volumes an increasing trend of mode number with frequency in the range of 20 to 100 Hz. Experimentally found resonance frequencies for water droplets on a hydrophobic surface approximately correlate with published frequencies of free water droplets. Conductivity showed no effect on the pattern of water droplet oscillation. Partial discharge inception voltages on two different hydrophobic surfaces (silicone rubber and epoxy resin) with conductive and non-conductive (distilled) water droplets were measured. These measurements showed that inception voltage makes a step whenever the oscillation mode of water droplet changes. Conductivity of the water droplets was found to have no effect on the inception voltage on silicone rubber surfaces having a high degree of hydrophobicity, whereas conductive droplets on epoxy resin with lower hydrophobicity lead to lower inception voltages.

Experimental investigations on water droplet oscillation and partial discharge inception voltage on polymeric insulating surfaces under the influence of AC electric field stress

Hydrophobicity is a prominent characteristic of silicone rubber for polymer insulators. However, due to the environmental influence, the decrease even the loss of hydrophobicity will take place, which causes the essential concern on the electrical accidents induced by the ageing of polymer insulators. The accurate evaluation on the insulator hydrophobicity is increasingly required for the long-term performance of outdoor insulators in service. In this paper, based on dynamic dropping test (DDT), pattern characteristic of surface discharges generated by the dynamic behavior of water droplet was investigated to evaluate the hydrophobic properties of silicone rubber insulator at different ageing levels. The specimens were prepared by using the method of corona degradation and determined by HC levels from HC1 to HC6. Both the dynamic behavior of water droplet and the induced discharge phenomena were captured by using a high-speed camera fixed vertically to the sample surface. In order to quantify the surface discharges, methods of image processing and fractal dimension were employed to establish the relationship between the pattern characteristic of discharge light and the HC levels. The morphology of discharge region and the distribution of luminous brightness in relationship with the lapse time can provide an optical evaluation method for the insulator hydrophobicity. It is found that the number and duration of discharge light increase with the increase of HC level. The increasing tendency in brightness intensity and fractal dimension of discharge pattern can reveal the decrease of hydrophobicity. Therefore, the results obtained indicate that the pattern characteristic of discharge light in DDT method is sensitive to the hydrophobic properties, which can be applied as a non-contact method for hydrophobicity evaluation of polymer insulators.

Pattern analysis of discharge characteristics for hydrophobicity evaluation of polymer insulator

Understanding the interaction behaviors between sessile droplets under imposed high voltages is very important in many practical situations, e.g., microfluidic devices and the degradation/aging problems of outdoor high-power applications. In the present work, the droplet coalescence, the discharge activity and the surface thermal distribution response between sessile multidroplets and chloride salt droplets under high tangential electric fields have been investigated with infrared thermography, high-speed photography and pulse current measurement. Obvious polarity effects on the discharge path direction and the temperature change in the droplets in the initial stage after discharge initiation were observed due to the anodic dissolution of metal ions from the electrode. In the case of sessile aligned multidroplets, the discharge path direction could affect the location of initial droplet coalescence. The smaller unmerged droplet would be drained into the merged large droplet as a result from the pressure difference inside the droplets rather than the asymmetric temperature change due to discharge. The discharge inception voltages and the temperature variations for two salt droplets closely correlated with the ionization degree of the salt, as well as the interfacial electrochemical reactions near the electrodes. Mechanisms of these observed phenomena were discussed.

/pdf/sessile-multidroplets-and-salt-droplets-under-high-4yg0hf73nd.pdf

Sessile multidroplets and salt droplets under high tangential electric fields

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

This paper describes the assessment of the resistance to tracking of various polymers after (i) flashover voltage (FOV) and flashover gradient (FOG); and (ii) partial discharges (PDs) measurements in dry, clean and salt fogs on one insulator model using standardised electrodes; the aim being the development of optimised methods based on flashover and PDs to the conception of new generation of piercing connectors. The investigated materials belong to two distinct classes widely used in the electrical industry: thermoplastics (namely polyphenylene sulfide and high density polyethylene) and thermosetting cycloaliphatic epoxy resins namely one unfilled material (UnCEP) and another unfilled and hydrophobic resin (UnHCEP). FOV measurements are based on the electro-geometrical parameters of each specimen and the resistance to tracking, both may be assessed through the slope of FOV curves against the leakage distance as an indicator of surface damages. Regarding PDs, the suggested technique is devoted to clean and salt fogs and provides quicker and more accurate information about PDs effects on materials deteriorations rather than IEC60109 where the exposition time to discharges is much longer at lower voltage levels by using only liquid contaminants such as sulfuric or nitric acids or ammonium chloride as described in IEC60587 or ASTM D2303.

Assessment of the resistance to tracking of polymers in clean and salt fogs due to flashover arcs and partial discharges degrading conditions on one insulator model

The behavior of a water droplet on a silicone rubber plate was observed with a high-speed camera under ac voltage application. It was confirmed that a water droplet vibrated and that a remarkable deformation from the original droplet form occurred at the mechanical resonance frequency. Vibration frequency of an uncharged water droplet was double the frequency of the applied voltage. On the contrary, in the case of an artificially charged droplet, vibration frequency was the same with that of the applied voltage. A macroscopic mechanical model was devised, to which the experimental data were well correlated. Flashover voltage via the water droplet was also measured. Lower flashover voltage was obtained at the mechanical resonance frequency.

Khalil Honsali

Papers

Vibration of a water droplet on a polymeric insulating material subjected to AC voltage stress