T. Tokoro

High field dissipation current waveform of polyethylene film obtained by new method

Non-polar polymers, such as polyethylene (PE), polypropylene (PP) and so on, are widely used as insulators for power cable and capacitor. In ac high field region, it is known that carrier injection from electrodes is getting dominant, and then the dissipation current waveform shape begins to show a nonlinear property due to complex conduction mechanism, that is, charge injection, space charge effect, recombination and so on. To study the conduction mechanism under ac high field, a new dissipation current waveform observation method which enables detection of the harmonies with high accuracy is developed. By using low density polyethylene (LDPE) film, high field dissipation current waveform observations from room temperature to 90/spl deg/C are carried out. Through these series of experiments, very unique properties which indicate the space charge effect under ac high field in high temperature are obtained.

Effect of water salinity and temperature on the hydrophobicity of ethylene propylene diene monomer insulator

Ethylene propylene diene monomer (EPDM) is extensively used in outdoor composite polymer insulators. The effects of water salinity and temperature on the hydrophobicity of EPDM rubber were studied The hydrophobicity was determined by measuring the contact angle using a droplet of distilled water. The specimens were immersed in saline water solutions for up to 576 h. The salinity levels used were 5, 10/sup 3/, 10/sup 4/ and 10/sup 5/ /spl mu/S/cm. The aging temperatures were 0, 25, 50, 75 and 98/spl deg/C. The weight was also measured to determine the amount of water absorption in the polymer as might occur during a heavy rain. The change in the contact angle is related to the amount of water absorption, the salinity level of solution and the temperature. The absorption of saline water into the bulk is larger at higher temperatures and decreases with increasing salinity. At 25/spl deg/C and higher the absorption of saline water at all salinity levels was saturated during the aging period Generally, after long immersion the contact angle decreased. At 0/spl deg/C the decrease of contact angle was larger at higher salinity. At high temperatures (/spl ges/75/spl deg/C), the sample surface gradually changed its color from the original dark gray to white.

Effects of water salinity, electric stress and temperature on the hydrophobicity of nylon

The loss of hydrophobicity of nylon caused by water salinity, electric stress and temperature is investigated. The hydrophobicity is determined by measuring the contact angle using a droplet of distilled water at room temperature. To investigate the loss of hydrophobicity, nylon specimens are immersed in saline water solutions. The salinity is varied from 5 to 100,000 /spl mu/S/cm. The aging temperatures are 0, 23.5 (room temperature), 50 and 75/spl deg/C. The time of immersion is up to 336 hours. Electric field is applied in air at room temperature. From this study it is found that the contact angle of water on nylon decreases with increasing salinity of water. The contact angle decreases rapidly with increasing temperature of the saline water. The contact angle decreases with increasing time of subjecting the specimen to a fixed value of high dc electric stress in air at 23.5/spl deg/C.

High-field dissipation current waveform in e-beam-irradiated XLPE film at high temperature

We developed a detection method of ac dissipation current waveforms combined with a fast rising ac ramp voltage application as a new technique to evaluate the high-field dielectric properties and the dissipation current waveform over a very short time range. This measurement was carried out on electron beam irradiated crosslinked polyethylene film. At high temperature the high field tan /spl delta/ and ac dissipation current show nonlinear electric field dependence and the dissipation current during decreasing field becomes larger than that during increasing field, suggesting the contribution of carrier injection from electrodes. The dissipation current waveforms in the high temperature region show the peak in the first part of each half cycle of a full waveform, and fast decrease in the latter part. This result is probably due to the hetero and home space charge formation caused by the injected carriers from the electrode.

Recovery of hydrophobicity of nylon aged by heat and saline water

The recovery of hydrophobicity of nylon insulators after aging by long exposure to a stress of saline water at different temperatures is investigated. The hydrophobicity is determined by measuring the contact angle of a droplet of distilled water on nylon. The aging of nylon was performed by immersing it for up to 336 h in saline water solutions in the range 5.0-10/sup 5/ /spl mu/S/cm. The aging temperatures were 0 to 98/spl deg/C. After aging, the specimens were kept in air at room temperature for up to 4500 h during which the recovery of hydrophobicity and weight changes were measured. Specimens were also kept in high vacuum for 20 h to determine the changes in the contact angle and in the weight with the absence of air.

Effect of temperature on the evaluation of hydrophobic condition of polymer surface

Hydrophobicity of polymer surface is one of the degradation indices of polymer insulator material and is usually evaluated by using an image of sprayed water on the sample surface. Surface free energy of both solid and liquid have negative temperature dependence. The decrease of surface free energy of solid increases the hydrophobic property; however, the decrease of the one of liquid decreases the hydrophobic property. In this study, the hydrophobic condition of the sample surface was measured at 5, 22 (Room temperature), and 50degC. During the temperature equalization to the room temperature, changes in the hydrophobic condition were measured. Comparing of these results can define the degradation of hydrophobicity of polymer insulator more accurately.

Image analysis of hydrophobicity and dielectric property of polymer insulating material

Hydrophobicity of polymeric insulating material surface such as silicone rubber insulator was studied by using image data analysis of the sample surface. Hydrophobicity of polymer was evaluated by the motion of water droplets on the sample surface under AC high field application. The vibration image of the water droplets was taken by CCD camcorder. The video image was divided into each image frame and the image indices such as size and shape factor of the droplets were evaluated for each image frame. Then, the time variation of the image indices of sample surface during high electric field application was evaluated, where electric field deformed the droplets. The motion of the water droplet was also determined by measuring the dielectric property of silicone rubber surface using a pair of inter-digital electrodes. The motion of water droplets was detected by using unbalance operation of current comparator type Capacitance Bridge system. The changes in dielectric loss current and capacitive current by the deformation of the water droplets were measured for every one cycle of the applied AC field. To compare these results the degradation of hydrophobicity of polymer insulator is defined more accurately.

Recovery of hydrophobicity of ethylene propylene diene monomer aged by heat and saline water

The recovery of hydrophobicity of ethylene propylene diene monomer (EPDM) insulator after aging by long exposure to a stress of saline water at different temperatures is investigated. The hydrophobicity is determined by measuring the contact angle of a droplet of distilled water on EPDM rubber. The aging of EPDM rubber was done by immersing it for up to 576 h in saline water solutions in the range 5.0-10/sup 5/ /spl mu/S/cm. The aging temperatures were 0, 25, 50, 75 and 98/spl deg/C. After aging, the specimens were kept in air at room temperature for up to 2,500 h during which the recovery of hydrophobicity and weight changes were measured. From these results, surface free energy and diffusion coefficients of EPDM rubber were evaluated.

Detection of high field AC conduction loss current in polymeric insulating materials

A novel method for the detection of high-field AC conduction loss current in polymeric insulating materials is developed. It uses the unbalanced operation of a current-comparator-type high-field tan- delta -capacitance bridge and the deconvolution technique for signal processing. Examples of the transfer function of this detection system for different amplifier gain levels are shown. Under certain conditions, the high-field AC conduction current of LDPE (low-density polyethylene) film evaluated with this method contains both an in-phase fundamental sine wave component which correlates with tan delta and harmonics caused by the nonlinear electric-field dependence of the conduction current. In epoxy resin, however, only the fundamental sinusoidal wave component of the dissipation current is detected.<<ETX>>

Diagnosis of degradation condition of silicone rubber using hydrophobic surface analysis

The application of polymer insulator has been expanded in the field of power supply and distribution system all over the world. Compared to the porcelain insulator, however, polymer insulator has been demanded to solve the subjects on a long-term reliability and on a diagnosis techniques in the real application field. Hydrophobicity of polymer surface is one of the degradation indices of polymer insulator material and is usually evaluated by using an image of sprayed water on the sample surface, which is called STRI or spray method. In this study, the effect of parameters to the evaluation of hydrophobicity of silicone rubber are studied. The parameters include surface roughness, measurement temperature, amount of water absorption, weight percent of ATH filler and surface treatment of fillers such as ATH and silica. Hydrophobicity is evaluated by using the image analysis of sprayed water, the dynamic drop test and also the contact angle of a water droplet.