M. Hikita

Measurements of partial discharges by computer and analysis of partial discharge distribution by the Monte Carlo method

The mechanism of partial discharge (PD) occurring in the CIGRE Method II (CM-II) electrode system, which is a representative closed-void model system is discussed in the context of a computer-aided PD measuring system. Measurements of PD are made for the CM-II electrode system. Effects of the pressure and gas inside the void on the PD are examined. Taking into account the experimental results, a model for the PD mechanism is proposed. This model assumes that the statistical time lag of discharge depends on the overvoltage and that the residual voltage depends on the PD magnitude. A Monte Carlo simulation of the PD distribution is made on the basis of this model. The computed results agree well with the experimental data and the appearance of swarming pulsive micro discharges. The physics of the model are discussed. >

High-field conduction and breakdown in insulating polymers. Present situation and future prospects

We review the results of electrical conduction and breakdown research reported mainly after 1980 in Japan, and comment on the present situation and future prospect of research work in this field. Effects of introduction of polar groups into polymers on electric strength F/sub b/ are summarized. Influences of copolymerization and modification of morphology and chemical structure on F/sub b/ of polyethylene are also reviewed. Finally, we show currently used experimental techniques for direct observation of space charge, prebreakdown current and localized heat generation. >

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. >

High voltage insulation performance of cryogenic liquids for superconducting power apparatus

This paper describes high voltage insulation technology of cryogenic liquids for superconducting power apparatus. The insulation of a superconducting transformer is classified into several insulation components. Moreover, we investigate fundamental insulation characteristics peculiar to the superconducting and cryogenic circumstances: area and volume effects on breakdown strength, V-t characteristics, quench induced dynamic breakdown characteristics and so on. Finally, we systematize the electrical insulation of cryogenic liquids, and propose a flow chart for the practical insulation design of superconducting power apparatus.

Breakdown mechanism of liquid nitrogen viewed from area and volume effects

We investigated the area and volume effects on the breakdown strength in liquid nitrogen (LN/sub 2/) to discuss the breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in LN/sub 2/ with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that the breakdown mechanism changed from an area dominant to volume effective region at larger electrode configurations in LN/sub 2/. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in LN/sub 2/. It was suggested that a mutual contribution of area and volume effects appeared in breakdown characteristics in LN/sub 2/ under thermal bubble conditions, as a phenomenon peculiar to cryogenic liquids. Consequently, we pointed out that it is very important to consider both thermal bubbles and electrode surface condition for HV insulation of superconducting power apparatus.

Kerr electro-optic field measurement and charge dynamics in transformer-oil/solid composite insulation systems

We measured the electric field in transformer oil/solid composite insulation system under dc voltage application using a Kerr electro-optic measurement system. When a pressboard was inserted at the center between two parallel-plane electrodes, the electric field at the mid-point between the pressboard and the cathode decreased with time. On the other hand, the electric field at the mid-point between the pressboard and the anode increased initially with time, reached a maximum value and then fell down. We measured also the electric field before and after polarity reversal of the dc voltage. The results revealed that the electric field after polarity reversal reached a magnitude 2.7/spl times/ as much as the average dc applied field. We interpreted the above results on the time transition of the electric field in terms of a charge dynamic model, considering the difference in charge accumulation speed on the pressboard between positive and negative charges. This model was verified to be applicable to the time transition of the electric field in oil/solid composite system and BTA added oil/solid composite system as well.

Partial discharge measurements in SF/sub 6/ and air using phase-resolved pulse-height analysis

Carries out a study on partial discharge (PD) phenomena in SF/sub 6/ gas and tries to construct a diagnostic system capable of separating noise from observed PD signals for assessing the condition of insulation performance in power apparatus. The authors have constructed a computer-aided PD measuring system which permits a time-sequential data acquisition and phase-resolved pulse height analysis. Using the PD measurement system, the authors measured 60 Hz AC PD in SF/sub 6/ gas and in air at atmospheric pressure for needle-plane electrodes with three different radii at the tip of the needle, 3, 30 and 500 /spl mu/m. They proposed particular PD statistical parameters, unbalance ratios U/sub T/ and U/sub q/ of the pulse repetition rate and average of charge, respectively, between positive and negative PD pulses occurring per cycle; these parameters proved to be good indicators to distinguish PD occurring in SF/sub 6/ from that in air. The mechanism of PD inception was also discussed. >

Kerr electro-optic field mapping and charge dynamics in impurity-doped transformer oil

We measured dc electric field distribution in transformer oil mixed with different impurities and additives so as to simulate practical degradation conditions of transformer oil. The electric field in the oil doped with asphalt was reduced near the cathode and enhanced near the anode. On the other hand, electric field in oil with copper oleate was enhanced near both electrodes and reduced at the center between the electrodes. The space charge density was estimated from the measured electric field profile using a 1-dimensional form of Gauss law; negative ions occurred more than positive ions by 5 to 40 pC/cm/sup 3/ in the oil with asphalt from 10 to 30 ppm. It was also found that heterocharges with 100 to 300 pC/cm/sup 3/ existed near both electrodes in the oil with copper oleate. Consequently, the electric field and the charge distribution in the oil proved to change depending on the kind and content of impurities or additives and thus depending on the degradation of the oil.

Dielectric breakdown and electrical conduction of poly(vinylidene-fluoride) in high temperature region

Dielectric breakdown of poly(vinylidene-fluoride) (PVDF) is studied over the temperature range from room temperature to 150 degrees C, by applying a linearly rising voltage. The electric strength is in excess of 8 MV cm-1 at room temperature, but above 50 degrees C it rapidly falls inversely with temperature and depends on the rate of rise of the electric field but not on the sample thickness. An attempt is made to apply the impulse thermal breakdown theory to the data obtained above 50 degrees C. Assuming that the conduction is ionic the temperature rise of the sample is calculated numerically using impulse thermal breakdown theory. By fitting these theoretical electric strengths to the experimental values, the conduction parameters are estimated as follows: the activation energy is 0.9 eV, the jump distance is 9 AA and j0 is 1.2*107 A m-2.

Development of Kerr electro-optic 3-d electric field measuring technique and its experimental verification

We have developed a method to measure 3-dimensional electric field distributions in liquid dielectrics using the Kerr electro-optic effect combined with ac field modulation and circularly polarized light. We derived theoretical relations between an arbitrary electric field vector and a transmitted light intensity. The derived equations were verified by comparing the theoretical electric field profile with the experimental one in transformer oil with an axisymmetric electrode configuration, i.e. measured electric field vectors proved to agree well with theoretical ones. The light intensities experimentally obtained were transformed into axisymmetric electrode configuration by Abel transformation to calculate the electric field distribution.