Vibration of a water droplet on a polymeric insulating material subjected to AC voltage stress
19 Apr 2010-IEEE Transactions on Dielectrics and Electrical Insulation (IEEE)-Vol. 17, Iss: 2, pp 566-571
TL;DR: In this article, the behavior of a water droplet on a silicone rubber plate was observed with a high-speed camera under ac voltage application, and it was confirmed that water droplets vibrated and that a remarkable deformation from the original droplet form occurred at the mechanical resonance frequency.
Abstract: 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.
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TL;DR: In this article, the 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.
Abstract: 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.
64 citations
Cites background or methods from "Vibration of a water droplet on a p..."
...According to [22] the water droplet on the surface can be modeled using a mass-spring-damper mechanical model....
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...In case of a charged droplet and considering the Coulomb force, we have [22]:...
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...By converting the radius of the (spherical) water droplet to its volume and considering that uncharged water droplets oscillate with twice the frequency of the applied electric field, whereas charged droplets oscillate with the original frequency only [22], a relation of resonance frequency versus volume of water droplet in different modes can be plotted....
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...Studies about oscillation and break up of water droplet under uniform electric field were published [3, 22-23]....
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...F is the main external force which can be expressed by Maxwell or Coulomb force respectively for uncharged and charged droplets [22]....
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TL;DR: In this article, a dynamic dropping test (DDT) was used to evaluate the hydrophobic properties of silicone rubber insulator at different ageing levels, which can be applied as a non-contact method for hydrophobicity evaluation of polymer insulators.
Abstract: 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.
43 citations
TL;DR: 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.
Abstract: 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.
26 citations
TL;DR: In this article, an electrostatic force acting as the domain driving force plays a key role in water droplet motion on a super-hydrophobic surface, and the simulation for surfaces with different wetting properties are in good agreement with the experimental results.
Abstract: 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.
20 citations
TL;DR: In this paper, the authors describe the assessment of the resistance to tracking of various polymers after flashover voltage (FOV) and flashover gradient (FOG) 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.
Abstract: 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.
19 citations
References
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TL;DR: In this paper, a theoretical model to determine the effect of dry band discharges on material performance was presented, and good agreement between the predicted behavior and the experimental findings was shown.
Abstract: The materials evaluated in fog produced from low (250 mu s/cm) and high (1000 mu s/cm) conductivity water include cylindrical rod samples of high-temperature-vulcanized silicone rubber and ethylene-propylene-diene monomer rubber (EPDM) containing various amounts of either alumina trihydrate or silica fillers, or both. Comparison is made with material performance results obtained with AC, which was reported in an earlier study. In both low- and high-conductivity fog, the time to failure with AC and +DC was very similar, but a reduction by a factor of about four was observed in the time to failure with -DC. For both AC and DC, silicone rubber performed better than EPDM samples in low-conductivity fog, while the order of performance was reversed in high-conductivity fog. A theoretical model to determine the effect of dry band discharges on material is presented. Good agreement between the predicted behavior and the experimental findings is shown. >
222 citations
"Vibration of a water droplet on a p..." refers background in this paper
...In the case of polymer insulators with silicone rubber sheds, it is said that hydrophobicity on the surface is maintained for a long period because of migration of low molecular silicone from the bulk of silicone rubber to the surface [ 1 , 2]. When polymer insulators are wetted by fog or rain, water droplets are formed on the insulator surface....
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TL;DR: In this article, the fundamental principles of water drop corona as an aging mechanism for nonceramic insulators are presented, and it is demonstrated that water drops in the shed and sheath regions enhance the electric field and may cause corona which can play an important role in long-term performance.
Abstract: 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.
181 citations
08 Jul 1991
TL;DR: In this paper, the surface free energies due to London dispersion and hydrogen bonding forces on the surface of RTV coatings were calculated from the contact angle measurements using both water and methylene iodide.
Abstract: Room temperature vulcanizing (RTV) silicone rubber coatings are applied to electrical insulators to improve their subsequent insulation strength, particularly under wet conditions. Under prolonged wetting the hydrophobicity of the coating is reduced temporarily and the insulator protection is lost. After a dry period, a recovery of the hydrophobicity takes place. To investigate this phenomenon, the surface was subjected to various wetting conditions with and without electrical stress. Measurements of the contact and the sliding angles were used to determine the state of the surface. Using water, the surface free energy of the RTV was calculated from the contact angle measurements as a function of time of exposing the surface to salt-fog. The surface free energies due to London dispersion and hydrogen bonding forces on the surface of RTV coatings were calculated from the contact angle measurements using both water and methylene iodide. Good agreement was obtained with the literature value for a virgin RTV surface. The chemical changes of the surface are shown to be responsible for the loss of hydrophobicity. >
127 citations
"Vibration of a water droplet on a p..." refers background in this paper
...In the case of polymer insulators with silicone rubber sheds, it is said that hydrophobicity on the surface is maintained for a long period because of migration of low molecular silicone from the bulk of silicone rubber to the surface [1, 2]....
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TL;DR: In this article, it was shown that certain regions of commercially available 500 kV non-ceramic insulators have surface electric fields that are above the threshold for water drop corona.
Abstract: It is shown that certain regions of commercially available 500 kV non-ceramic insulators have surface electric fields that are above the threshold for water drop corona. Observations made in a full scale accelerated aging chamber, and in service, demonstrate that water drop corona occurs on such insulators. A correlation between the magnitude of the surface electric fields and the condition of the silicone rubber non-ceramic insulator surfaces due to water drop corona is illustrated for two different insulator types in an accelerated aging chamber. Other examples of degradation of nonceramic insulators due to water drop corona are illustrated.
118 citations
"Vibration of a water droplet on a p..." refers background in this paper
...Moreover, investigations on degradation of polymer insulating material by corona discharge from water droplet have been carried out [ 8 , 9]. Permanent loss of hydrophobicity was reported to occur....
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TL;DR: In this article, a correlation was found between the distribution of the spectrum of surface discharges and hydrophobicity via the joint time and frequency analysis of discharges on various hydrophobic surfaces.
Abstract: According to hydrophobicity and hydrophobicity transfer studies on silicone rubber surfaces, although the silicone rubber can transfer its hydrophobicity to various contaminants deposited on its surface, the hydrophobic states of water droplets on clean silicone rubber surfaces and on various contaminated silicone rubber surfaces were found to be different based on evaluation by contact angle measurement. Furthermore, surface discharge and aging of materials decreases the hydrophobicity of the surface. Focused on the difference of hydrophobicity and the effect of water droplets on the hydrophobic contaminated surfaces, some contrastive computations and tests of water droplets on various surfaces were carried out in this study. The results showed that the water droplets existing on the surface of silicone rubber materials might change the electric field and voltage distribution along the surface, and even reduced the initial voltage of the corona discharge, which was verified with a comparison test on a high voltage side of polymer insulator monitored by a photomultiplier tube (PMT) system. Since the leakage current provides much useful information to diagnose the state of outdoor insulation, this study investigates the time and frequency distributions of surface discharges on ceramic insulators, new room temperature vulcanized (RTV) silicone rubber coated insulators and aging RTV insulators with various hydrophobic states with the short time Fourier transform and Gabor transform methods. Then a correlation was found between the distribution of the spectrum of surface discharges and hydrophobicity via the joint time and frequency analysis of discharges on various hydrophobic surfaces.
47 citations