Kenichi Haji

Surface Degradation of Silicone Rubber Exposed to Corona Discharge

This paper describes the surface degradation of unfilled high-temperature vulcanized silicone rubber (HTV-SR) resulting from creeping corona discharges under atmospheric pressure. In this paper, HTV-SR specimens were exposed to corona stress generated by a parallel needle-plate electrode system; furthermore, physicochemical analyses were conducted on the surface layer of SR before and after corona discharge treatment. The results showed that the plasma impingement from the corona discharge can cause physical and chemical damages to the SR surface. In addition, it was demonstrated that instead of hydrophobic CH 3 groups, hydrophilic OH groups that are by-products of aging may be formed on the surface of aged SR; furthermore, the corona discharge plays an important role in the loss of hydrophobicity

Electrohydrodynamic behaviour of water droplet on an electrically stressed hydrophobic surface

This paper describes the electrification characteristics of water droplets on a hydrophobic surface and their influence on the induced discharge in an ac electric field. Tests were conducted by placing water droplets with different conductivities and volumes on an electrically stressed silicone rubber (SR) sheet, and their electrohydrodynamic behaviours were observed using a high-speed video camera. It is demonstrated that a locally high electric field at the tip of a droplet can trigger corona discharges, and droplets are always charged negatively during a corona discharge process. The deposited droplets are deformed and synchronized with the ac field. Once the deformation becomes noticeable, it increases rapidly until the droplet becomes mechanically unstable and ejects water filaments from its vertices. This can bridge the electrode gap and result in a flashover. In addition, the volume and conductivity of the water droplets have a marked effect on the mode of corona discharge and flashover development.

Fundamental research on characteristics of hydrophobicity disappearance of silicone rubber surface in Dynamic Drop Test

A study on the development of deterioration / performance evaluation method for outdoor electric insulation of polymer materials is pushed forward now in the International Council on Large Electric Systems (CIGRE). The small scale docimasy (dynamic drop test) which could evaluate a disappearance characteristic of the hydrophobicity easily was suggested [1]. This test method is to evaluate the loss of hydrophobicity to drop electrically-charged water droplets on the sample surface continuously. We applied this dynamic drop test (DDT) to polymer materials which differ in the amount of alumina tri-hydrate (ATH) filled and investigated factors about the change of hydrophobicity, in which the surface roughness and difference of the quantity of migration of the low molecular weight (LMW) were examined.

Distribution of Leakage Current on Polluted Polymer Insulator Surface

This paper describes the results of experiments made to examine the leakage current distribution on the polluted polymeric material surface. In this study, a plate-shaped silicone rubber specimen or a real polymer insulator energized with ac voltage was exposed to a polluted liquid spray after an artificially hydrophobic removing, and its surface state was observed by a high-speed video camera. Simultaneously, waveforms of the leakage current flowing along different surface areas were measured by several separate detecting systems during the experiment. Because of irregular profiles and nonuniform environmental stresses, the polluted polymer insulator appears different surface conditions in different areas. The information extracted from the leakage current waveforms measured through the separate detection system can well reflect the surface insulation states in each area.

Characteristics evaluation of the Partial Arc Discharge and the Silicone Rubber surface by Dynamic Drop Test

This paper describes the results of experiments made to examine characteristics of hydrophobicity of ATH-filled silicone rubber specimen surface. As the examination method, we used a dynamic drop test (DDT), in which electrolyte solution droplet from an electrode continuously dropped on a sample and the AC voltage was supplied between electrodes. Hydrophobicity loss times of samples for various conductivities of electrolyte solutions were measured. The electric fields before and after hydrophobicity reduction were evaluated. The results showed that the hydrophobicity loss time became short with the increase of conductivity and the electric field increased after hydrophobicity reduction.

Erosion resistant characteristics of ATH-filled silicone rubber exposed to dry band arc discharge

This paper describes the results of experiments made to examine the erosion resistant characteristics of ATH-filled silicone rubber exposed to dry band arc discharge. In this study, one simulated electrolyte electrode system was used to trigger the dry band arc discharge and the weight loss of filled silicone rubber specimens due to erosion in cases of different ATH filler level was investigated. In addition, surface chemical changes were examined by FT-IR. It is demonstrated that the erosion due to dry band arc discharge can be well suppressed by liberating the crystalline water in ATH and the depression effect of ATH filler on erosion can be improved with filler amount increasing.

Influence of Recrystallization of Alumina Trihydrate Filler on Leakage Current Development in Salt-Fog Ageing Test for Polymeric Materials

In this study, the influence of the recrystallization of an alumina trihydrate (ATH) filler on the leakage current development in a salt-fog ageing test for polymeric materials was investigated. The possibility of the recrystallization of ATH was confirmed by differential thermal analysis (DTA). The intensity of ATH in X-ray diffraction (XRD) profiles showed a fluctuant change in salt-fog and dry periods, and the measured leakage currents during different ageing cycles also showed a fluctuant change with the dehydration and recrystallization of the ATH filler. Thus, the leakage current can be well suppressed by the recrystallization of the ATH filler, and the lifetime of ATH-filled polymeric materials can be prolonged when they are exposed to electrical stress under wet conditions.

Relation between leakage current and NSDD on the surface of polymer insulator

This paper describes relation between leakage current and non solubility deposits density (NSDD) on the surface of a polymer insulator. Until now, we have studied outdoor polymer insulators in the coastal region in west Japan. In addition we proposed a novel on-line monitoring system, adopting differential techniques to separate the leakage current of outdoor polymer insulators into three components such as a conductive current, a corona discharge and a dry band arc discharge current. The results indicated that leakage current was influenced by NSDD rather than equipment solubility deposits density (ESDD). In this study, the test to clarity the above stated factor was performed.

Relation between dry band arc discharge development and erosion shape on the silicone rubber

This paper describes the results of experiments made to examine the erosion of alumina trihydrate filled silicone rubber exposed to dry band arc discharges. The eroded shape and weight loss of each sample, caused by the dry band arc discharges, were evaluated. The eroded amounts of the silicone rubbers aged in dry band arc discharge by changing in electrolyte conductivity were measured. The average weight loss was also experimentally obtained. The weight loss was compared with that calculated by considering the eroded shape and the density of the rubber aged in different conditions. Based on the results, the relation between the erosion shape and the dry band arc discharge was discussed.

Harmonic Analysis of Leakage Current in Salt-Fog Aging Test for Polymeric Material

In this paper, we describe the waveform of the leakage current that flows along the surface of a silicone rubber (SR) specimen exposed to a salt fog. In this study, a plate-shaped SR specimen energized with an ac voltage was set in a fog chamber, and its surface state was observed using a high-speed video camera. Simultaneously, the waveform of the leakage current with elapsed time was measured. In accordance with frequency analysis, it was confirmed that the ratio of the third-harmonic component in the leakage current waveform increased with elapsed time. Information on the frequency characteristics of the leakage current was extracted and correlated with the insulation surface condition. The harmonic component of the leakage current can be considered as a diagnostic index for the electrical characteristics and insulation state of polymer insulators under wet conditions.