Diagnostic of Silicon Carbide Surge Arresters using Leakage Current Measurement

Nowadays, the zinc oxide surge arresters (ZnO) are widely used in power systems, however, a large number of silicon carbide surge arresters (SiC) are still in service in the utilities. On the other hand, it is not possible to replace all SiC surge arresters in a short time period, being necessary to review the maintenance program taking into account the surge arresters that are more degraded. In this context, a research project was established between the University of Sao Paulo and the electrical utility CTEEP, aiming the investigation of its SiC surge arresters. This work shows that the leakage current measurement, a diagnostic method for the ZnO surge arresters, can provide useful information related to the condition of the SiC surge arresters. Analysis of the amplitude and distortion of the leakage current, also considering thermovision measurements, resulted in better evaluation of the SiC surge arresters.

Comparative degradation of ZnO- and SnO(2)-based polycrystalline non-ohmic devices by current pulse stress

The degradation behaviour of SnO(2)-based varistors (SCNCr) due to current pulses (8/20 mu s) is reported here for the first time in comparison with the ZnO-based commercial varistors (ZnO). Puncturing and/or cracking failures were observed in ZnO-based varistors possessing inferior thermo-mechanical properties in comparison with that found in a SCNCr system free of failures. Both systems presented electric degradation related to the increase in the leakage current and decrease in the electric breakdown field, non-linear coefficient and average value of the potential barrier height. However, it was found that a more severe degradation occurred in the ZnO-based varistors concerning their non-ohmic behaviour, while in the SCNCr system, a strong non-ohmic behaviour remained after the degradation. These results indicate that the degradation in the metal oxide varistors is controlled by a defect diffusion process whose rate depends on the mobility, the concentration of meta-stable defects and the amount of electrically active interfaces. The improved behaviour of the SCNCr system is then inferred to be associated with the higher amount of electrically active interfaces (85%) and to a higher energy necessary to activate the diffusion of the specific defects.

Temperature impact on the tunnel fet off-state current components

In this work, the temperature impact on the off-state current components is analyzed through numerical simulation and experimentally. First of all, the band-to-band tunneling is studied by varying the underlap in the channel/drain junction, leading to an analysis of the different off-state current components. For pTFET devices, the best behavior for off-state current was obtained for higher values of underlap (reduced BTBT) and at low temperatures (reduced SRH and TAT). At high temperature, an unexpected off-state current occurred due to the thermal leakage current through the drain/channel junction. Besides, these devices presented a good performance when considering the drain current as a function of the drain voltage, making them suitable for analog applications. (C) 2012 Elsevier Ltd. All rights reserved.

Performance of electronic devices submitted to X-rays and high energy proton beams

In this work we have studied the radiation effects on MOSFET electronic devices. The integrated circuits were exposed to 10 key X-ray radiation and 2.6 MeV energy proton beam. We have irradiated MOSFET devices with two different geometries: rectangular-gate transistor and circular-gate transistor. We have observed the cumulative dose provokes shifts on the threshold voltage and increases or decreases the transistor's off-state and leakage current. The position of the trapped charges in modern CMOS technology devices depends on radiation type, dose rate, total dose, applied bias and is a function of device geometry. We concluded the circular-gate transistor is more tolerant to radiation than the rectangular-gate transistor. (C) 2011 Elsevier B.V. All rights reserved.

Detection of partial discharges at field in SF6 insulated equipments

This paper presents the results of research aiming to develop partial discharge detection techniques in high voltage equipment, at substation environment. Measurements of high frequency components of leakage current, at equipments' grounding conductor, were performed. This procedure was performed with the equipment energized and without disconnecting it from the system. The partial discharge generated current pulse is picked up by a high frequency CT, and is detected by an oscilloscope. The partial discharge identification was made considering previously obtained laboratory results, where partial discharges were characterized by means of its time domain signatures. This paper focuses measurements in SF6 circuit breakers. Encouraging results were obtained, showing the feasibility of detecting partial discharges in energized equipment in the laboratory and in the field, in a substation environment, using this method.