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This study focuses on charge transport in epoxy bulk, an insulating spacer material. We discuss the relationship between the space charge behavior and the external current determined by conduction and displacement currents. One-dimensional space charge behavior in epoxy is simulated using a bipolar charge transport model that considers charge trapping, de-trapping and recombination. The simulated results show that the charge accumulation in epoxy is successfully modeled by considering high-density deep traps at the epoxy/electrode interface and optimizing the parameters, whereas the simulated external current is inconsistent with the experimental results where the external current decreases in power-law behavior. In the simulation, the external current gradually increases and reaches a steady state as the charge accumulation gets saturated. Charge accumulation does not contribute to the decrease in the external current, which is contrary to the prediction based on the experimental results. We then show that the power-law decrease of the external current is due to the presence of bipolar initial charges before voltage application and multi-depth traps in epoxy. The results strongly indicate that multi-depth trapping/de-trapping, like hopping transfer, rather than single-depth trapping/de-trapping should be taken into account when modeling charge transport in epoxy. [ABSTRACT FROM AUTHOR]

With the miniaturization of electrical devices, the insulation width and the separation between electrodes have been accordingly reduced. Consequently, electrical breakdown across micrometer-scale gaps is of great practical interest for insulation design. In this paper, electrical breakdown across micrometer scale surface gap in atmospheric air was investigated under inhomogeneous field. The breakdown was measured under negative impulse voltage and the generation and movement of neutral and charged particles were simulated based on PIC-MCC (particle-in-cell with Monte Carlo collision). The result shows that the breakdown phenomena across micrometer scale gaps is mainly determined by field emission current from the cathode surface, similar to the discharge process in vacuum. The positive ions generated by collision of electrons to neutral particles enhance the electric field near the cathode, resulting in the increase of the field emission current from the cathode. [ABSTRACT FROM AUTHOR]

AlN substrate is widely used along with silicone gel to encapsulate power electronic circuits. It is known that the weakness of the insulation system is surface discharges propagating at the gel-substrate interface. In this research, the nature of surface discharges in gel, on various substrates was investigated. The results are as follows: the maximum stopping length of cavities on AlN substrate was more than twice than that on other substrates, and light emission due to discharges in cavities on AlN substrate was different from that on other substrates; for AlN substrate, tip of the cavity emitted light while other part did not emit light, and for other substrates, all the path along the cavity emitted light. These results indicated that the surface of the AlN substrate is degraded by discharges and becomes conductive. We have confirmed this assumption by measuring the conductivity of the cavity path: 5 kΩ /100 μm for that on AlN and above 1 MΩ /100 μm for Al2O3 and glass substrates. The high electric field at the tip of conductive path elongates the cavity stopping length on AlN substrate compared with others. In order to investigate the degradation process of AlN substrate surface, energy dispersive X-ray spectroscopy analysis was carried out. Reduction of nitrogen component on the cavity path was seen, indicating that generation of Al increases the conductivity of cavity path. We have experimentally shown that oxidation treatment of the AlN substrate significantly reduces the cavity stopping length. [ABSTRACT FROM PUBLISHER]

Protrusions are generated with random distribution on the contacts surfaces of Vacuum Circuit Breaker (VCB) after arc interruption. They will degrade the vacuum insulation level by Joule Heating Vaporization while withstanding rated voltage. In order to investigate the behavior of protrusions in VCB after interruption, the point-to-plane insulation structure is constructed in the vacuum chamber. And, the pre-discharge phenomena are observed using high speed video camera. It is observed that successive current pulse cascades before the explosion of the point electrode, which results in craters on the surface of the point electrode. Through the analysis of the images, the speed of the particle is estimated to be less than 125m/s. This research also shows the vaporization is non-uniformly distributed in some channels around the point electrode, which can be the possible reason for the later discharge in VCB. Through the computation of the Fowler-Nordheim (F-N) Curve, two patterns of field emission are revealed on the F-N plot, which are the rising and falling pattern. In the voltage rising pattern, the field enhancement factor is 11 times larger than that in the voltage falling pattern; however, the effective field emission area is only 1/1000 of the voltage falling pattern. Moreover, the two patterns are enclosed with the non-field emission half cycle to form a cycle on F-N plot. It is concluded that the point electrode is under periodic conditioning process with the observation of the cycle, which is considered to be the key reason for late discharge in VCB. [ABSTRACT FROM AUTHOR]

Silicone gel is widely used to encapsulate power electronic circuits. It is known that the weakness of this insulation system is the surface discharges between gel and substrate. These discharges give rise to the growth of cavities in gel, which may lead to eventual failure of the insulation. Despite the fact, surface discharges in gel has not yet been studied enough. In order to clarify the nature of streamers in the cavity and relation between streamer propagation and charge accumulation at the interface of the cavity, the relation between apparent charge, light emission, motion of the cavities, and dynamic potential distribution of surface discharge in silicone gel was investigated. The results showed that conductive streamers initiate at the electrode, propagate through the cavity and accumulate charges on the cavity surface. In addition, it was confirmed that conductivity of the cavity surface is low. [ABSTRACT FROM AUTHOR]

While interrupting the large current in vacuum circuit breaker, the surface temperature of both electrodes will rise due to the arc current Joule heating process; the surface profile gets rough due to the vacuum arc burning on electrodes nonuniformly; and protrusions are simultaneously generated, which may potentially lead to late discharge. In this paper, the random discharge after current zero is reported, and the particle phenomenon is observed as considered to be responsible for the late discharge phenomenon previously. The authors develop a synthetic experiment system, which consists of current source, voltage source, control units, and the observation system. The high-speed video camera is used to record the particle motion in the interelectrode space. Using this system, the motion of small particles is tracked. From the particle information in each frame, the size and speed of the particle are calculated to be within \(160~\mu \) m and 100 m/s, respectively; the charge quantity of small particles is derived as less than \(10^{-11}\) C. Furthermore, the surface field enhancement factor is deduced from the particle detachment model, which lays in hundred orders. Since observed particles cannot directly lead to the observed current, possible reasons for the late discharge would be Townsend type metal vapor discharge, and the vapor comes from high-temperature electrode surface and protrusions. [ABSTRACT FROM PUBLISHER]

The performance of high-voltage apparatus is seriously restricted by the surface discharge on dielectric materials. A surface charge density distribution scanning system is developed with a dc feedback-type electrostatic probe based on the field-nullifying technique. The residual charge density distribution after the occurrence of a positive leader discharge is measured. Using the 2-D Fourier transformation in the spatial domain and Tikhonov's regularization method, the surface potential distribution and electric field distribution are arrived at. The leader stem and radial lament streamer zone located at the head of the leader can clearly be distinguished from the distributions. There is an obvious knee point that divides the main channel of the discharge into a leader channel and a streamer channel. The knee point is at about 9 kV, which corresponds to about 320 pC/mm^2. [ABSTRACT FROM PUBLISHER]

The residual surface charge density distribution under 12.5–16.0 kV application impulse voltage is measured. From these charge distributions, the potential distributions are calculated. The characteristics of the potential gradient in streamer and leader channel and the mechanism of streamer-to-leader transition under positive impulse voltage is analyzed. When the charge density is as high as 210–320 pC/mm \(^{\mathrm {2}}\) , the streamer channel will transit into a leader channel. [ABSTRACT FROM PUBLISHER]