Water-tree characteristics in low-density PE under simulatedinverter voltages

Water tree characteristics have been investigated for low-density polyethylene by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter. If we compare the water-tree length among single-frequency voltages, the water tree grows faster if the frequency is higher. However, if we superpose a power-frequency voltage on the high-frequency voltage, tree growth is suppressed. The tree length becomes shorter if the superposed power-frequency voltage is higher. This surprising result is explainable by considering that the zero-crossing of the applied voltage plays an important role in tree propagation     

Flashover phenomena in positive rod-to-plane air gaps under impulse and DC voltages

Discharge phenomena in positive rod-to-plane air gaps under lightning impulse and dc voltages were investigated. Under impulse voltages, the 50 percent flashover voltages agree with the 50 percent corona inception voltages only in a gap shorter than a certain value d_c which increases with the rod diameter φ. The 50 percent flashover voltage at d_c rises at a rate of about 14 kV/cm with d_c in the range of d_c < 2.8 cm (φ ? 1.5 cm), and then it rises at a rate of about 5 kV/cm. Experimental results show that the flashover process in the former region (termed the Gp process) is different from that in the latter region (the Lp process). Under dc voltages, the critical gap length, at which the first corona may lead to flashover without any succeeding streamer corona, corresponds to d_c under impulse voltage. In a gap shorter than d_c flashover occurs through the Gp process (φ ? 1 cm) or Lp process (φ ≥ 2 cm), but in gap longer than d_c flashover may occur through another process which cannot be seen under impulse voltages. In this paper, those flashover characteristics and processes are described on the basis of experimental results and photographic observations with an image converter camera.     

Electroluminescence in polyethylene subjected to impulse voltages

Electroluminescence (EL) from HDPE (high-density polyethylene) films subjected to repetitive rectangular-shaped impulse voltages has been investigated at electric fields <80 kV/mm in order to clarify the temporal behavior of carriers injected into the polymer. EL emission was distinctly observed during the voltage rise and fall phases. During the voltage plateau and the zero voltage phases, the EL intensity did not instantly decrease to zero but a very weak EL emission lasted for at least 3 ms. The number of EL pulses was determined by the amplitude of the impulse voltage, whereas the EL intensity strongly depended on the temporal variation rate of the voltage. The total number of EL pulses during both phases of the voltage rise and fall was almost equal. The EL spectra did not depend on the phase and the amplitude of the impulse voltage. The above behavior of EL emission can be explained qualitatively by the electron-hole recombination in the metal-polymer interface region and showed the picture of temporal behavior of carriers, carrier injection and transport, in the interface region for the field relaxation at the interface     

Streamers and partial discharge mechanisms in silicone gel under impulse and AC voltages

Silicone gels are widely used to encapsulate power electronics modules. The objective of this paper is to study the partial discharge (PD) mechanism in a silicone gel, using electrical measurements and fast visualization. Experiments are carried out in a pointplane geometry, using either impulse voltage or ac. Under impulse voltage, the visualization shows that the first discharges recorded in a new sample are due to the initiation and propagation of streamers, with features close to those previously reported in viscous liquids. Under ac, a stable PD regime rapidly establishes: a streamer creates a cavity with a long lifetime, and PDs occur in this cavity during the subsequent ac waves. Since slow streamers develop during long times (some ms), classical PD measurements do not provide an adequate evaluation of the actual discharge magnitude: very small PDs (1 pC) are recorded, whereas large cavities with a total charge in excess of 10 pC are actually present in the gel. When PDs are repeated permanent degradations of the gel rapidly occur, showing the limited self-healing capabilities of silicone gels.     

Characteristics of creeping discharge along aerial insulated wire under impulse voltages with various wave front durations

When lightning occurs in the neighborhood of outdoor high‐voltage distribution lines, creeping discharges propagate along the wire surface from the binding wire tip just after insulator flashover. These discharges give rise to various faults on distribution lines, for instance, disconnection and melting of wire, punch‐through breakdown, and so on. We must clarify the creeping discharge characteristics associated with various inductive lightning surges from the viewpoint of safety in high‐voltage distribution systems. In our previous paper, it was reported that the lengths and aspects of the negative creeping discharges were influenced by the wave front durations of impulse voltages applied to the central line with a grounded binding wire. The present study was performed to obtain more information on such creeping discharges. This paper describes the distinctive characteristics of a creeping discharge along the insulated wire surface when impulse voltages with various wave front durations are applied to the binding wire. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(3): 29–37, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20430     

快速振荡电压下SF6气体间隙放电的光电检测

利用光电倍增管及分幅式变象管高速相对放电产生的光信号进行了检测。实验证明,光电检测是研究快速振荡冲击（FOI）作用下SF6气体放电过程的有效手段。由检测结果可看出,SF6气体间隙在FOI下的放电特征与标准雷电波（LI）下的放电特征是不同的,负极性FOI作用下的放电机理遵循茎先导机理。     

Voltage distribution analysis of 500-kV DC transmission-line voltage divider under impulse voltages: stray parameter extraction

It is essential to extract the stray parameters accurately and efficiently in analyzing impulse responses of a power apparatus. In this paper the stray capacitances and stray inductances of a 500-kV dc transmission-line voltage divider are extracted by the hybrid finite element-analytical approach, and the corresponding equivalent circuit for analyzing the impulse voltage response of the voltage divider is established. According to the discussion of the results of the extracted stray parameters of the voltage divider, reasonable simplification of the equivalent circuit of the voltage divider is made. In addition, the influence of the insulating housing and the electrical insulation oil on the stray capacitances of the voltage divider is discussed.     

A FinFET SRAM cell design with BTI robustness at high supply voltages and high yield at low supply voltages

In this paper, a SRAM cell structure which uses pMOS access transistors and predischarged bitlines is presented. By using the strained pMOS transistor technology, the degradation of the read static noise margin (SNM) at high supply voltages due to the aging, especially in the presence of symmetric stress, is suppressed. In contrast to conventional cell, the write margin of the proposed cell does not degrade considerably at low supply voltages. To assess the efficacy, the proposed cell is compared with conventional cell for two cases of unstrained and strained pMOS. A comparative study is performed using mixed mode device/circuit simulations for a gate length of 22 nm. The results show that the read SNM degradation due to the symmetric aging at the supply voltage of 1 V is about 6% after three years for the proposed strained structure, while degradations are 14%, 12%, and 11% for the unstrained proposed structure, unstrained, and strained conventional structures, respectively. In addition, the proposed cell has both read and write cell sigma yields higher than six for supply voltages ranging from 1 V down to 0.5 V while the other structures have read or write yields less than six at the minimum supply voltage. Through some work function tuning, the cell sigma yields of the other structures reach above six for both read and write while being still lower than those of the proposed structure. Copyright © 2015 John Wiley & Sons, Ltd.     

Breakdown and prebreakdown phenomena in liquids under positiveimpulse voltages

We examine the breakdown and prebreakdown behavior of two insulating liquids: benzyl-toluene and mineral oil in a nonuniform electric field at large distances (up to 20 cm) under a positive impulse voltage. We evaluate and compare the breakdown strengths of both liquids in these conditions as a function of interelectrode distance via Weibull type plots, a method commonly used when dealing with insulating gases and solids. The comparative behaviors of the liquids change greatly when the cumulative probability of breakdown is decreased from 100%. The second part of this study concerns streamer stopping lengths and the corresponding current and light emission signals     

Voltage-time characteristics for steep-front impulse voltages ofparticle-contaminated spacers in SF6 gas-insulated switchgear

The voltage-time characteristics of spacer surfaces for steep-front impulse waves are investigated under a particle-contaminated condition in SF₆ gas. The characteristics are measured as a function of particle length, particle position, and space shapes. Flashover voltages monotonically increase in the submicrosecond region as time to flashover is shorter, and are a minimum in the 1 μs region. Applicability of the equal voltage-time area criterion for estimating the voltage-time characteristics is discussed and the estimation is clarified. Moreover, it is demonstrated that an optimized spacer with ribs greatly improves flashover voltages in the submicrosecond region as well as in the 1 μs and power frequency region     

Surface charge accumulation and decay on a spacer stressed with DC voltages at atmospheric conditions

Contents Surface charge accumulation and its decay at a GIS spacer are studied at atmospheric conditions employing a two step-calibration method. The results show that the accumulation saturates within 30 minutes; the dominant mechanism of surface charge accumulation is the drift of charged particles via air, while that of the decay is surface conduction. The characteristics are similar at both polarities.

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Sättigungskonzentration und Abklingverhalten der Oberflächenladung an einem mit Gleichspannung beanspruchten Abstandshalter unter atmosphärischen Bedingungen

Übersicht Die Konzentration und das Abklingen von Oberflächenladungen an einem GIS Abstandshalter wurden unter atmosphärischen Bedingungen mit Hilfe eines zweistufigen Kalibrierungsverfahrens untersucht. Die Ergebnisse zeigen eine Sättigung der Ladungskonzentration innerhalb von 30 Minuten. Als Hauptursache der Ladungskonzentration ist die Drift geladener Teilchen in Luft anzusehen, während das Abklingen auf die Oberflächenleitung zurückzuführen ist. Ähnliche Charakteristiken ergeben sich mit beiden Polaritäten.

     

Streamer propagation under impulse voltage in long point-plane oilgaps

A 67 mm point-plane gap immersed in transformer oil, at atmospheric pressure, stressed with a 1/180 μs impulse, has been studied by recording gap current and light emission. Time-resolved images were obtained by use of an image converter camera. Minimum breakdown voltages were twice as high for negative as for positive points. All streamers at these and higher voltages were supersonic, with negative streamer velocities ⩽50 km/s. The speed of the positive streamer gradually increased to 19 km/s with increasing voltage, where it saturated. At high stresses the propagating positive streamer has been observed to switch to a faster mode which propagates with speeds in the 65 to 200 km/s range. Positive streamers were bush shaped to 2× the minimum breakdown voltage, the branching diminishing with increased voltage. Negative streamers were coarsely bush shaped or usually tree shaped. For both polarities, light emission consisted of a background light, with superposed pulses corresponding to 15 ns wide current pulses of up to 10 A. The pulses are caused by very bright, brief re-illuminations of single streamer channels. The tips of positive streamers are considerably more luminous than the channels, indicating that electron multiplication may take place at the tips. A model of the streamers as being plasma filled channels may explain the re-illuminations     

Dielectric breakdown characteristics of an insulator with a sharp edge in SF6 gas under lightning impulse voltage

This article discusses the dielectric breakdown characteristics of an insulator with a sharp edge that forms inhomogeneous gaps in SF₆ under a lightning impulse voltage. The high voltage electrode is a plane electrode, and the grounded electrode is a plane electrode with a column insulator, which has an inside electrode. The column insulator has a height of 50 mm and a diameter of 50 mm. The inside electrode in the column insulator has a height of 20 mm or 30 mm or 40 mm. The inside electrode changes the electric field on the surface of the column insulator. The SF₆ gas pressure is 0.225 MPa. Dielectric breakdown does not occur at the edge of the column insulator, but rather at the top plane of the column insulator. The dielectric breakdown voltage is defined by the electric field at the center of the column insulator     

Understanding the partial discharge activity generated due to particle movement in a composite insulation under AC voltages

The UHF signals are generated due to partial discharges formed by particle movement in Gas Insulated Switchgears (GISs). It is observed that the bandwidth of UHF signals generated due to partial discharges formed by particle movement in GIS is independent of operating pressure and the applied voltage. The levitation voltage of the particle sitting on the dielectric barrier is high compared to that of it on the ground electrode. When a particle is present in between the barrier and an electrode, at certain voltage level the movement of particle will be in pseudo resonance with the applied voltage frequency. When the particle is on the barrier, on application of voltage the particle moves to the center of the electrode axis causes discharges thereby radiating UHF signals, with its dominant frequency in the range of 3–4 GHz, which is much different from that of the UHF signal generated due to partial discharges formed by particle movement. When the particle moving between the high voltage electrode and the barrier, at some time its motion get collapses and the particle settles in the low field region away from the axis of the electrode and generates UHF signals having characteristics of UHF signals generated due to corona discharge.     

PD characteristics in an air-filled void at room temperature undersuperimposed sinusoidal voltages

Partial discharge in an artificial air-filled void under superimposed sinusoidal voltages is investigated at room temperature in order to clarify the effect of voltage superposition. The applied voltage waveform is composed of a 60 Hz fundamental sinusoidal wave and a high frequency sinusoidal wave of 300 Hz to 1.2 kHz. It was found that PD started when the peak value of the superimposed voltage reached the PD inception voltage under 60 Hz sinusoidal voltage. Also, PD occurrence frequency increased remarkably when the peak value of the high frequency component exceeded a critical value, which is smaller than the PD inception voltage under 60 Hz sinusoidal voltage. PD characteristics under such conditions were discussed as the effect of the residual voltage, caused by the surface charge deposited in the void by the preceding PD. The obtained experimental results suggest that the superposition of high frequency component accelerates the degradation of the solid insulator containing void defects     

Steep fronts at transient recovery voltages appearing with the interruption of inrush currents of transformers

High lightning overvoltages do not appear in underground substations connected to transmission cables. Consequently, it is very important to thoroughly investigate switching overvoltages and to achieve rational insulation coordination for apparatus installed in such underground substations. This paper discusses the occurrence of steep fronts at transient recovery voltages (TRV) appearing at circuit breakers when the inrush currents of transformers are interrupted. Caused by a steep front at the TRV, reignitions occur at circuit breakers, resulting in the generation of high overvoltages with high frequencies. The overvoltages are among the highest switching overvoltages appearing at the terminal of a transformer. The authors clarified the mechanism of the generation of steep fronts at TRV by means of EMTP analysis, as well as by carrying out tests in a high‐power laboratory. © 2001 Scripta Technica, Electr Eng Jpn, 135(1): 24–32, 2001