| Research Abstract |
Polymers are extensively used as insulating material in underground power cables. It has been well established that electroluminescence (EL) and charge injection, which gives rise to space charge in the polymeric insulation, occur at ac field above a certain threshold value. Space charge can cause dissipative energetic processes such as photon and phonon emission, increase the local electric field, lead to the reduction of the insulation withstand voltage and ultimately to premature failure of the power cable. It is considered that EL is emitted even before the first partial discharge, EL occur continuously above a certain threshold field and has been attributed to the injection of electric charge form the electrodes. The injected charge accumulates in the insulating material to form a space charge, which plays a major role in do voltage applications, but cannot be completely ignored in ac applications. The role of space charge in polymeric materials subjected to AC high field is still
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not well understood.
In order to clarify the conduction mechanism of polymers, such as Low Density Polyethylene (LDPE), LDPE/MgO nanocomposite materials, and so on, under ac high field, the simultaneous observation system of dissipation current waveform and electroluminescence have been constructed for three years since 2005. As the developed dissipation current waveform detecting circuit, which called as"Displacement Current Bypassing Method(DCBM)", does not include any inductance for canceling the capacitance component, it is possible to detect the harmonics with high accuracy. Furthermore it is not only detecting the harmonics of the dissipation current in high field, but also estimating the volume resistivity and relative permittivity under AC high field. The observation of EL under AC high field is also very useful to understand the charge behavior. The developed EL observation system is just for a polymeric film sample. In 2002, the sample holder for a film sample was developed at the collaboration with NRC. To realize the simultaneous observation of the dissipation current waveform observed by DCBM and EL observed by PMT and CCD, the several software, which make possible perfectly controlling the digital oscilloscopes for the dissipation current waveform and EL observation, the triggering control for high sensitivity CCD camera, and the arbitrary function generator, has also developed by using Lab VIEW. In order to clarify the conduction mechanism for polymer under AC high field, the various kinds of experiments were carried out, such as estimations of dE/dt by using trapezoidal waveform application, effect of MgO nano filler on LDPE/MgO nanocomposite materials, the time variations of fundamental and third harmonics of dissipation current and EL, and so on, by using the developed system. Less
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