Katsumi Uchida

Weibull statistical analysis of area and volume effects on the breakdown strength in liquid nitrogen

In order to examine the area and the volume effects on breakdown strength in liquid nitrogen (LN/sub 2/), we measured dc and ac breakdown voltages in LN/sub 2/ with sphere to plane and coaxial cylindrical electrode configurations. We also carried out statistical analysis of the experimental results using the Weibull distribution and discussed with the statistical stressed electrode area (SSEA) and/or liquid volume (SSLV). The dc and ac breakdown strength in LN/sub 2/ decreased to 1/5 with increasing SSEA and SSLV over the wide range from 10/sup 0/to 10/sup 5/ mm/sup 2/ and from 10/sup -1/ to 10/sup 5/ mm/sup 3/, respectively. The Weibull shape parameter m for the sphere to plane and the coaxial cylindrical electrodes was estimated to be 6 to 8 and 11 to 13, respectively; the breakdown strength in LN/sub 2/ saturated with increasing SSEA and SSLV. From these results, it was verified that the area and the volume effects definitely determined the breakdown strength in LN/sub 2/. The results of the Weibull statistics were compared with the conventional concept of the stressed electrode area (SEA) and liquid volume (SLV). Consequently, SSEA or SSLV was found to be nearly equal to 80 to 85% of SEA or SLV. >

Partial Discharge Endurance of Epoxy / SiC Nanocomposite

Partial discharge (PD) endurance of epoxy/SiC nanocomposite was investigated in comparison with that of epoxy/silica (SiO2) nanocomposite. Filler contents were 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt% in epoxy/silicon carbide (SiC) nanocomposite. Evaluation was made by erosion depth and eroded cross sectional area in its profile measured with the laser surface profilometory. It was confirmed that epoxy can be much improved in its PD erosion performance by adding SiC nano fillers to a similar degree as compared to SiO2 nano fillers. Additionally, close inspection of the cone-shaped erosion profile clarifies that the shape of erosion profile is broader in epoxy/SiC nanocomposite than in epoxy/SiO2 nanocomposite. This indicates that SiC fillers tend to remain less stuck on the surface after exposure to PDs than silica fillers. One of the authors has proposed that nanometric segmentation by inorganic nano-fillers is effective to increase PD resistance. It can be assured that such function is also operative in the case of SiC as nano-fillers.

Study on detection for the defects of XLPE cable lines

This paper reports the results of a basic study of testing methods to replace the DC after laying test that is presently most frequently used as an after laying test for XLPE power cable lines. Chosen as the alternative waveforms for DC voltage were damped oscillating wave (OSW) and very-low-frequency wave (VLF) voltages. Experiments were conducted on the ability to detect various types of line defects under these voltages. It was found that these alternative waveforms are promising as after laying test voltage waveforms to replace DC and that greater defect detection ability can be achieved by combining them when conducting tests.

Area and volume effects on breakdown strength in liquid nitrogen

We measured dc and ac breakdown voltages in liquid nitrogen (LN/sub 2/) with a sphere-to-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN/sub 2/ 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 LN/sub 2/ was verified quantitatively; the breakdown strength decreased when increasing the. >

Dependence of partial discharge characteristics at spacer surface on particle size in SF 6 gas insulated system

A metallic particle appeared in a gas insulated switchgear (GIS) sometimes adheres on a solid spacer surface. If the adhered metallic particle is exposed to a surge high voltage, a breakdown (BD) may be induced. Therefore, it is eagerly demanded to diagnose its risk correctly under the service voltage by partial discharge (PD) measurement. In this paper, particle-initiated surface PD characteristics were systematically studied in 0.4 MPa SF6 gas by changing the sizes of particles. PD inception voltage (PDIV), temporal change of PD current and the PD pulse number were analyzed in detail. Furthermore, comparing with PD characteristics of particles in a gas gap, the influence of the solid insulator on the PD characteristics was clarified. It was found out that PD characteristics greatly changed with time owing to electric charges deposited on a spacer surface.

Different voltage endurance characteristics of epoxy/silica nanocomposites prepared by two kinds of dispersion methods

It was elucidated that treeing lifetime can be drastically extended especially under the condition of electric field as low as design field in epoxy/silica nanocomposites, if nano fillers are properly dispersed in good contact with surrounding polymer matrices. It was actually found that there is a distinctive difference in V-t characteristics between two kinds of epoxy/silica nanocomposites. It is certainly surprising that similar epoxy/silica nanocomposites exhibit dissimilar behaviors. This will bring about in-depth insight on how nano fillers and their interfaces with surrounding polymer would work for voltage endurance. Specimens of the former kind were prepared in a way that hydrophilic fumed silica nano particles were dispersed in uncured epoxy, while specimens of the latter kind were produced from nano silica fillers densely dispersed uncured epoxy. Voltage endurance characteristics were evaluated by treeing growth and breakdown time.

Quench-induced breakdown characteristics of liquid helium and optical observation of thermal bubbles

Abstract This paper describes the breakdown characteristics of liquid helium (LHe) caused by quench of a superconducting wire exposed to a high electric field. The quench-induced breakdown of LHe took place at a considerably lower stress than the intrinsic level which could be withstood without the quench. A delay time of the order of milliseconds existed from quench onset to the breakdown. We also observed the behaviour of thermal bubbles in LHe generated by the quench using a fibrescope and a high speed video system. The optical observation clarified that the high electric field enhanced the propagation velocity of the bubbles, triggering the quench-induced dynamic breakdown of LHe.

Particle size identification in GIS by ultra high speed measurement of partial discharge

Gas insulated switchgear (GIS) is reliable equipment. However, when a metallic particle is in it, the insulation reliability of GIS sometimes reduces. Partial discharge (PD) measurement is usually performed to detect a metallic particle in GIS. After the PD detection, the criticality of PD must be estimated. For this purpose, identification of a particle size is very important. In this paper, every PD pulse of various particle sizes was measured sequentially with an ultra-high speed measuring system. And physical PD parameters were systematically analyzed. As a result, it was clarified that identification of the particle size was possible by considering the PD pulse number and current at positive and negative polarities. Finally, correspondence between electromagnetic wave emitted from PD and PD current was discussed.

Partial Discharge Detection Techniques under the Condition of Metallic Particle Adhering to Solid Spacer in SF6

Among the insulation defects in GIS under the operating condition, metallic particles adhered to the solid spacer would be the most critical situation against the surge voltage applications. Nevertheless, because of the complicated partial discharge (PD) mechanisms, PD detection can be recognized very difficult in GIS diagnostic techniques. In this paper, we have successfully simulated the condition of particle adhering on the spacer surface by an experiment applying an AC sudden step voltage. We firstly measured the time dependent PD characteristics, from immediately after the attachment of the particle, by using PD-CPWA (partial discharge current pulse waveform analysis) technique. The PD activities at the particle tip have different characteristics depending on the particle length and the diameter, as well as the gas pressure. Next, we have discussed that depending on the particle conditions, the PD characteristics can be changed with time and with AC half cycle of the polarity and have different sensitivity. Therefore, we need to identify the assessment criteria of GIS, based on the above mentioned phenomena and the facts. Finally, from analyses of PD mechanisms, we proposed the optimum PD detection techniques for adhering particles on solid spacer in GIS.

Fundamental study on relaxation processes of water tree in aged power cables aimed for diagnosis with spatial resolution

Water tree is a degradation mode of power cable with polymeric insulation. A water tree is composed of small droplets filled with water. As the conductivity in water tree is very high, it leads to dielectric breakdown when it grows up. As inside the water tree is filled with trap sites, it is polarized with a certain distribution of relaxation time when a DC poling voltage is applied. Although its depolarization process after removing the poling voltage is determined by ambient temperature, applying a ldquodepolarizing voltagerdquo with the opposite polarity can accelerate the process. If a short pulse propagating through the cable is employed as a depolarization voltage, we may locate the water tree by looking at the time-resolved pulse response. This would lead to a diagnosing method with spatial resolution. In order to retain 100 m of spatial resolution, the response should be as sharp as 1 mus. As a preliminary study, a coaxial communication cable was aged to form water trees. A DC poling voltage was applied followed by a pulse voltage with opposite polarity. The rising time of the pulse was several hundreds of microseconds. A sharp pulsive current response with 50 mus in width was observed, suggesting that a rapid depolarization took place. No such response was seen when the cable specimen was not aged. We concluded that the technique is quite feasible.