Kazuyuki Tohyama

Dielectric Properties of LDPE/MgO Nanocomposite Material under AC High Field

Low density polyethylene (LDPE) is widely used as a dielectric insulation of power cable. It has already reported that an incorporation of MgO nanoparticles in LDPE matrix could provide dielectrics with new and improved properties under dc high field. This paper describes high field dielectric properties of LDPE/MgO nanocomposite material under ac high voltage application. From the results of dissipation current waveform observations under ac high field, the volume resistivity and the relative permittivity for various contents of MgO nanoparticles in LDPE were estimated at the applied frequency 50 and 200 Hz. Though, the dependences of MgO content on both volume resistivity and relative permittivity under ac high field at 50 and 200 Hz are recognized, but it seems not to be drastically improved by adding a few percent of nano-size filler like those under dc high field.

Dissipation Current Waveform of Water Tree Deteriorated Low Density Polyethylene Sheet

Crosslinked polyethylene (XLPE) is widely used as the insulator for power cable. Water tree degradation phenomenon is the one of typical problem for the electrical insulation of power cable. In this paper, to confirm whether a signal of a fundamental component and the third harmonics peculiar to the water tree deteriorated sample appears by changing the applied frequency, the dissipation current waveform observations was carried out at 50 and 500 Hz. As the results of dissipation current waveform observation, it was found that the trend of the electric field dependence of both the fundamental component and third harmonics for the water tree deteriorated LDPE sheet sample considerably varied from that for LDPE without water tree at 50 Hz. However in the case of observation at 500 Hz, it was not admitted that a signal peculiar to water tree emerged in electric field dependence of both the fundamental component and the third harmonics in the water tree deteriorated LDPE sheet sample.

High field dielectric properties of nonpolar polymeric films

Nonpolar polymeric materials, such as LDPE (Low Density Polyethylene), PP (Polypropylene), and so on, are widely used for an insulator of power cable, a dielectric material of power capacitance, etc. due to their high insulating performance. In high field region, it is known that the dissipation current waveform shows nonlinearity and the electroluminescence (EL) is also observed at the initial stage of degradation, that is, just after the high field application [1]. These phenomena are considered that it is relating with the charge injection from the electrode and the instantaneous space charge formation at the vicinity of metal electrode. Thus the investigation of the relation between the dissipation current waveform and EL is important on to clarify the conduction mechanisms under ac high field, which is connecting to the developing the further high performance insulation material and the studying the degradation phenomena [2]. As the space charge formation is a time dependence phenomena, in this study, the electric field dependences of dissipation current waveform and EL have been investigated at the frequencies 50, 200, 400 and 1,000 Hz, respectively, by using LDPE and PP film. As the results of experiments, the inception field of EL between LDPE and PP were examined. The inception field for PP was slightly higher than that for LDPE. Similar trend for these samples was also confirmed on the electric field dependence of the third harmonic component of dissipation current waveform. However, the relation between the dissipation current and EL pulse number between LDPE and PP were similar at each frequency. When the frequency varied from 50 to 1,000 Hz, as the dissipation current increase with the frequency rise was large compared with the increase of the number of EL pulse, the properties themselves were shifted to the large dissipation current in the higher frequency. It is indicated that the frequency rise makes the quantity of space charge layer small, and it may cause the internal field small compared with the applied field. Then it is considered that the contribution to the EL decreases as the results of frequency rise and most of injected charges contribute the conduction. In this study, the conduction mechanisms of nonpolar polymer under ac high field are discussed from the results of dissipation current waveform and the number of EL pulse distribution.

Electroluminescence and dissipation current due to impulse superimposed on AC voltage in ldpe

LDPE is widely used as an insulating material for power cable. In order to clarify the conduction mechanism of LDPE under ac high field, the simultaneous observation of the dissipation current waveform and electroluminescence (EL) is carried out. As the results of experiments, it was found that when the EL was observed above the certain electric field, the dissipation current waveform became nonlinear and become to include the harmonic component. Power cable in service are inadvertently subjected to impulses generated by lightning and switching surges that are superimposed on the ac voltage at which they operate. Analysis of EL can help to clarify the degradation mechanisms that occur at points of electrical stress enhancement and lead to cable breakdown. Though the EL due to impulse voltage in XLPE has already reported by S.S. Bamji et.al., here it has been reported only the behavior of EL under various impulse voltage application by using needle-plane electrode system geometry. By using the film sample and three-terminal electrode system, the simultaneous observation of EL pulse number and dissipation current waveform due to impulse voltage superimposed on ac voltage in LDPE was carried out. From the results of experiments, it has confirmed that the EL properties due to impulse superimposed on ac voltage coincide with Bamjis previous work. In this study, the relation between the dissipation current waveform and EL pulse due to impulse voltage superimposed on ac voltage will be discussed.

Simultaneous observation of electroluminescence and dissipation current waveform in LDPE film

Polyethylene (PE) is widely used as an insulating material for power cable. Recently, the simultaneous observation system for electroluminescence (EL) and dissipation current waveform has been developed by authors. By using this system, the simultaneous observation in low-density polyethylene (LDPE) film was carried out at 50, 100 and 200 Hz, respectively. From the results of experiments, the relation between the time and spatial distributions of EL and the dissipation current waveforms were carefully examined. At the 1st 50 Hz electric field application, the plenty number of EL pulses have occurred. Also, at the consecutive experiments at 100 and 200 Hz, the number of EL pulses shows the positive frequency dependence. However, at the 2nd 50 Hz electric field application just after that at 200 Hz, the number of EL pulses extremely decreased compared with 1st application. On the other hand, the nonlinearity of dissipation current at the 1st 50 Hz application seems to small compare with that at the 2nd application. The fundamental component itself at 1st one is smaller than that at 2nd one. The EL images showed that EL occurred at the multiple weak points within the circular observation area. The spatial distributions of EL are almost same for all frequencies. It is considered that a lot of recombination center in material will exist in initial. But the long period ac high field application makes the number of recombination center decrease. This decrement due to occupation by carrier leads the carrier density increase and it may cause the nonlinearity of dissipation current waveform.

Dissipation current waveform observation of water tree deteriorated LDPE

Water tree degradation phenomenon is the one of typical problem for electrical insulation of power cable. In this paper, in order to examine the water trees fundamental properties, the dissipation current waveform observations are carried out by using the both of water tree deteriorated and un-deteriorated low-density polyethylene (LDPE) sheet samples. As the results of experiments, when the length and the number of water tree is short and small in the specimen, the difference of the detected dissipation current waveforms between the water tree deteriorated and un-deteriorated LDPE was small. By using FFT technique, the electric field dependences of fundamental and third harmonics are examined. And then it is found that the trend of the electric field dependence of third harmonics between the water tree deteriorated and un-deteriorated LDPE is slightly different.

Dissipation current and electroluminescence of LDPE/MgO nanocomposite material under trapezoidal waveforms application

Low density polyethylene (LDPE) is widely used as a dielectric insulation of power cable. It has already reported that an incorporation of MgO nanoparticles in LDPE matrix could provide dielectrics with new and improved properties under dc high field. However under ac high field, it has already reported that the effect of the addition of MgO nanoparticles was relatively small compared with that under dc high field. This paper describes the dissipation current waveform and the electroluminescence (EL) of LDPE/MgO nanocomposite material under the high field trapezoidal waveform application by using the simultaneous observation system. From the results of experiments, the EL image and the relation between the dissipation current and the EL under high field trapezoidal waveform application were introduced.

Film Thickness Dependence of Dissipation Current for LDPE Film under Trapezoid Waveform Application

Polyethylene is widely used as an insulating material for power cable. We have already reported that the dissipation current waveform depends on not only the electric field but also the changing rate of electric field. It is considered that to clarify the influence of the changing rate of electric field into the dissipation current waveform, and to examine whether this is the bulk dominant role or the surface dominant role are very important. By carrying out the film thickness dependence of dissipation current under trapezoid waveform application, the electric field E versus the current density J properties under the constant changing rate of electric field is estimated. As the results of experiments, not only the effect of changing rate of electric field but also the film thickness dependence was confirmed. Though the conduction mechanism under AC high field is pretty complex, it will be discussed based on the results which mentioned above.

Charge behavior observation in LDPE/MgO nanocomposite material under AC high field

Recently, authors have developed the simultaneous observation system which is combined a photomultiplier tube (PMT), a high performance CCD camera and the dissipation current waveform detecting circuit. In this study, the observations of the dissipation current waveform and EL of LDPE/MgO nanocomposite material under AC high field were carried out. LDPE and the concentration of LDPE/MgO nanocomposite were 1 and 5 phr respectively, were prepared. Electric field dependence and time variations of EL pulses were examined by using the system. As the results of these experiments, the incorporation of MgO nanoparticles in LDPE matrix makes elevate the EL inception level and makes increase the number of EL pulses, especially negative half cycle. But there were no dependences of the relaxation time tau at 60 kV/mm 50 Hz with MgO contents under AC field.

Frequency Dependence of Electroluminescence and Dissipation Current Waveform in LDPE film

Polyethylene (PE) is widely used as an insulating material for power cable. The simultaneous observation system for electroluminescence (EL) and dissipation current waveform of polymeric film has been developed by authors. By using this system, the simultaneous observations in low-density polyethylene (LDPE) film were carried out at 50, 100 and 200 Hz, respectively. The frequency dependence of the inception level of EL and the 3rd harmonic components, which has been estimated from the results of the electric field dependences of EL and the dissipation current, was relatively small. On the other hand, the estimated relaxation time from the results of the time dependences of EL and the dissipation current, which has been carried out just after the electric field dependence at each frequency, seems to be in inverse proportion to the applied frequency. It seems to be considered that the duration time of EL at each frequency will depend on the number of polarity reversal under AC high field.