The barrier effect in dielectrics: the role of interfaces in the breakdown of inhomogeneous dielectrics

The barrier effect in high voltage engineering is usually understood to mean an increase in the breakdown voltage of the insulation gap due to the use of additional insulation layer (a barrier) which is placed between two layers of the main dielectric. This effect has been known for more than 70 years and it is widely applied in high voltage engineering with a needle-plane configuration to increase the breakdown voltage. The noticeable progress in the barrier effect study was achieved over the last 15 years, when applying new composite polymeric materials with specific properties, the so-called nanomaterials, as the barrier materials. An analysis of existing models of the barrier effect is presented. The influences of various factors such as the barrier position in the gap, the ratio of permittivities and conductivities of the barrier and main dielectric, space charge and inhomogeneous polarization are demonstrated in this review.     

The effect of electrode material and additives on the breakdownstrength of a vacuum gap

The influence of additions of tungsten and cobalt into CuCr contact materials on the dielectric strength of a vacuum gap is investigated. Experimental results show that the dielectric strength increases significantly when the chromium phase in CuCr materials is selectively strengthened. The proper preparation method that determines the distribution of alloying elements is also suggested in this paper     

Influence of liquid temperature and electrode size on insulated breakdown characteristics in saturated superfluid helium

The dc breakdown strength and pulsed breakdown time lag of saturated superfluid liquid helium (HeII) at 1.4 and 1.85 K are measured over a wide range of electrode sizes and applied field strengths. The results of the measurements are statistically analyzed by using Weibull distribution functions. It is found that the area effect is dominant for dc and pulsed breakdown rather than the volume effect as in the case of liquid helium of 4.2 K (HeI). The shape parameter in the Weibull distribution function for dc breakdown strengths of HeII is smaller than that of HeI. As a result, dc breakdown in superfluid helium can take place in a wider electrode area than in normal liquid helium, while no remarkable differences in breakdown areas are observed between 1.85 and 1.4 K. On the other hand, the statistical time lag in HeII becomes shorter with a decrease in temperature. According to the Fowler‐Nordheim theory, it is suggested that a higher electron mobility in HeII may increase the field‐emitted electron kinetic energy to trigger liquid breakdown with a higher probability. © 1999 Scripta Technica, Electr Eng Jpn, 128(3): 16–23, 1999     

Relationship between electrode surface roughness and impulse breakdown voltage in vacuum gap of Cu and Cu-Cr electrodes

The relationship between the impulse breakdown voltage of vacuum gaps and electrode surface roughness was investigated for the purpose of controlling the surface roughness on HV conductors. The roughness of mechanically polished Cu and Cu-Cr electrodes was measured with a roughness meter, and the relationship between the breakdown voltage and surface roughness was obtained for plate-to-plate gaps. The discharge-conditioning effect increased with reduction in the surface roughness. The breakdown voltage depended not only on the roughness of the cathode but also on the anode surfaces. Reducing the surface roughness was found not to be an effective way to increase the breakdown voltage for non-uniform field gaps after discharge conditioning.     

The sequence of events in mid-gap laser-triggered electricalbreakdown in liquid dielectrics

Recent experiments on mid-gap laser-triggered electrical breakdown in liquid dielectrics have been scrutinised. The sequence of events that leads to the disruptive discharge is described and accompanied by electric field calculations     

Dielectric-breakdown-induced epitaxy: a universal breakdown defect in ultrathin gate dielectrics

The breakdown phenomena in SiO/sub x/N/sub y/ (EOT=20 /spl Aring/) gate dielectric under a two- stage constant voltage stress in inversion mode are physically analyzed with the aid of transmission electron microscopy. The results show that dielectric-breakdown-induced epitaxy (DBIE) remains as one of the major failure defects responsible for gate dielectric breakdown evolution even for a stress voltage as low as 2.5 V. Based on the results, the same failure mechanism i.e., presence of DBIE would be responsible for the degradation in ultrathin gate dielectrics for gate voltage below 2.5 V. It is believed that DBIE will be present in MOSFETs failed at nominal operating voltage.     

Propagation and structure of streamers in liquid dielectrics

Our purpose is to present a critical review of the current understanding of streamer propagation in dielectric liquids in order to help define the direction of future research. We show that the molecular structure has a significant effect on streamer propagation. The main parameter affecting propagation is the electronic affinity of the liquid molecules     

Hot carrier degradation of HfSiON gate dielectrics with TiN electrode

Hot carrier reliability of the HfSiON dielectric with the TiN metal gate electrode has been studied in the nMOS and pMOS short channel transistors. Hot carrier induced degradation of the high-/spl kappa/ gate stack devices are severe than the one in the SiO/sub 2//poly devices. It is determined that total threshold voltage shift during a hot carrier stress is a sum of contributions from both hot carrier and cold carrier effects. The hot carrier contribution induces permanent damage while cold carrier contribution is shown to be reversible. The contribution from the cold carrier can be evaluated by applying a de-trapping (opposite polarity) bias after the stress.     

Impulse breakdown voltages of triggered multistage vacuum gap

Several sealed-off triggered vacuum gaps are connected in series to improve hold-off voltage. The characteristics of impulse breakdown voltage of these series-connected gaps are investigated experimentally. The sum hold-off voltage of series-connected gaps decreases to a unit hold-off voltage when the maximum value of voltage division ratio across the gaps increases to unity. Self-breakdown probability of the series-connected gaps is always higher than that of a single gap under the same conditions. Hence, stage efficiency of the multistage gap decreases with increasing number of stages. Its value is 90 percent with 2-stage gap and 75 percent with 5-stage gap, respectively, under the same voltage division ratio and the same gap length (2.0 mm) in each stage. Triggered breakdown voltage of 2- or 3- stage gap is several hundred volts when all gaps are triggered simultaneously at the peak of the main impulse wave and a working voltage range is nearly 100 percent in this case. The working voltage range decreases with number of stages. Its value is 45 percent with 3-stage gap and 15 percent with 5-stage gap, respectively, when one triggered gap is fired for switching.     

Effect of electrode surface roughness on electrical breakdown in high voltage apparatus

This paper considers the statistical theory of electrical breakdowns in high voltage (HV) devices. The extended probability distribution of breakdown is deduced for nonuniform gaps. The breakdown of high voltage apparatus is sensitive to local irregularity of the electric field which may result from the presence of defects such as surface roughness. The surface roughness leads to existence of localized microscopic projection with local electric field strength at the projection top larger than the average field at the electrode surface. The link between parameters determining the probability of electrical breakdown and the parameters determining microscopic geometry of electrode surface is obtained. In this paper a simplified breakdown model is used as a basis for statistical treatment. According to the model the breakdown proceeds if the electric field strength at the projection top exceeds the critical value. The method of dielectric strength calculation for HV devices using a simplified model of breakdown is presented. The paper gives an example of calculation of the breakdown voltage for vacuum switch TVS-43. Results of calculations are compared with available experimental data and Weibull distribution     

Electronic conduction and breakdown in liquid helium and liquid neon

New data of the electronic properties of liquid helium and liquid neon are discussed with respect to their relevance for the initiation of electric breakdown. It is concluded that under ideal conditions the cathode plays a decisive role in the initiation of the breakdown process. It is speculated that a Townsend type breakdown mechanism is absent in the liquid phase of these elements.     

Method for repetitive measurement of the electrostatic chargingtendency of liquid dielectrics

The construction of an apparatus for the measurement of the electrostatic charging tendency (ECT) of a dielectric liquid is described. A sealed reciprocating piston arrangement permits a liquid to be repetitively passed through a charge separating element. The use of a gold element is shown to minimize the effects of moisture and control temporal effects. The system is evaluated by testing samples of transformer oil drawn from the field. In general, the new system ranks the oils in the same way as a conventional assessment against a paper filter, although anomalies have been found which are an impediment to adoption     

Relation between breakdown voltage and prebreakdown current in vacuum gap

Two different surface treatments (mechanical polishing, thin film deposition) were performed on cathode surfaces, and the field emission currents from the cathodes were measured with a microchannel plate. In order to discuss the relationship between the breakdown voltage and prebreakdown current in the vacuum gap, the breakdown voltage was measured after field emission measurement. The V–I characteristics of the field emission and breakdown voltage were influenced by surface treatment, and the breakdown voltages of mechanical polished cathodes were lower than those of the thin film deposited cathodes. It was found that the probability of breakdown increased when the field emission current reached 10^–11 A. Atomic force microscope (AFM) measurements showed numerous protrusions on the cathode surface in the case of thin film deposition treatment, but we estimated by the finite element method that these protrusions make the field enhancement effect low. It was inferred that the breakdown voltage in vacuum gaps could be increased by the thin film deposition method. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 11–18, 2000     

Comparative tests of tape dielectrics impregnated with liquid nitrogen

Superconducting power cables with so-called "cold dielectric" insulation system normally use tape dielectrics impregnated with pressurized liquid nitrogen (LN2). The AC and lightning impulse (LI) dielectric strength of four commercially available dielectrics have been measured under nitrogen pressures in the range 0.15-0.35 MPa, absolute. A planar test cell is used, and slits were made in the dielectric sheets to simulate butt gaps. Polypropylene laminated paper (PPLPreg) and mica filled polyamide based paper (Nomexreg 418) show AC and LI breakdown field in the range 50-80 kV/mm and 130-160 kV/mm, respectively, whereas Kraft paper and polyethylene paper (Tyvekreg) are found to have a significantly lower dielectric strength. Most of the results are reasonably consistent with previous studies. However, as opposed to other investigations, the dielectric strength for PPLPreg and Nomexreg is not found to increase with increasing nitrogen pressure     

Area and volume effects on breakdown strength in liquid nitrogen

We measured dc and ac breakdown voltages in liquid nitrogen (LN ₂) with a sphere-to-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN₂ did not increase monotonously but partially decreased as the sphere diameter increased at a constant gap length. Thus, the existence of the area and the volume effects on the breakdown voltage in LN₂ was verified quantitatively; the breakdown strength decreased when increasing the {SEA}₉₀ (90% stressed electrode area) and the {SLV}₉₀ (90% stressed liquid volume). By changing the experimental conditions, it was verified that both area and volume effects, having a mutual correlation, simultaneously lead to the degradation of the breakdown strength in LN₂. In order to examine the area and the volume effects for the larger {SEA}₉₀ and {SLV}₉₀, we also measured the breakdown voltage with a coaxial cylindrical electrode. It was concluded that the dc and ac breakdown strengths in LN₂ decreased as the {SEA}₉₀ and the {SLV}₉₀ increased varying widely from 10⁰ to 10⁵ mm² and from 10 ^-1 to 10⁵ mm³, respectively     

Characteristics of creeping discharge developed in a narrow gap between insulators and effect of a backside electrode

Discharge characteristics have been investigated for a needle–plane electrode configuration containing insulating barriers with a narrow gap which has been placed between the needle and the plane. The characteristics of creeping discharge developed in the narrow gap of the barrier filled with SF₆ are especially considered. In the case of a configuration with a backside electrode below the needle, the corona generated from the needle easily extended to the gap. On the other hand, for a configuration without a backside electrode, the corona has hardly extended to the gap, and on increasing the applied voltage the corona developed greatly in the gap. This difference in corona extension should affect the flashover characteristics in the present system. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(2): 27–36, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10248