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This paper discusses electrical insulation characteristics of liquid nitrogen ([LN.sub.2]) / polypropylene (PP) laminated paper composite insulation system for the practical electrical insulation design of high temperature superconducting (HTS) cables. Partial discharge (PD) inception, propagation and breakdown (BD) characteristics under ac voltage application were obtained and discussed at atmospheric and pressurized conditions. Experimental results were analyzed quantitatively and revealed that void-type discharge was categorized as an important form of discharge to characterize the transition of PD activity leading to BD. Index Terms--Superconducting cables, dielectric liquids, paper insulation, partial discharges, electrical breakdown, laminated paper.

The electrical insulation characteristics under quench-induced thermal and electrical combined stress for High Temperature Superconducting (HTS) cables are discussed. Results suggest the activation of the quench-induced partial discharge generation at the larger thermal and electrical stresses.

The development of the second generation (2G) high-temperature superconducting fault current limiting transformer (HTc-SFCLT) with coated conductors is discussed. Results suggest the exhibition of excellent current limiting function by 2G HTc-SFCLT as a superconducting fault current limiter.

Partial discharge (PD) inception and breakdown (BD) characteristics of liquid nitrogen (L[N.sub.2]) / polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables are presented. In this paper, we measured PD inception characteristics under lightning impulse and ac voltage applications by electrical, optical and acoustic methods. The impulse PD inception characteristics were compared with those under ac voltage application, and the PD generation mechanisms are discussed. Experimental results reveal that PD detection sensitivity under lightning impulse voltage application is as small as 6 pC. PD inception at the lower stress was verified to be generated not only in the butt gaps but also in micro gaps between PP laminated paper layers under both lightning impulse and ac voltage applications. Alter PD inception, PD activity could be developed in the butt gaps at the higher stress, finally leading to BD. Index Terms--High temperature superconducting cable, liquid nitrogen, polypropylene laminated paper, partial discharge, breakdown.

Partial discharge (PD) inception characteristics of liquid nitrogen (L[N.sub.2])/polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables were investigated in terms of the volume effect and the V-t characteristics. The electrical and optical measurements of PD inception characteristics showed that initial PD could be generated between PP laminated paper layers, as well as in a butt gap. Using a parameter called statistically stressed liquid volume (SSLV) based on the discharge probability in both butt gaps and L[N.sub.2]-filled thin layers between PP laminated papers, we could systematically analyze and evaluate the volume effect on PD inception stress (PDIE). Furthermore, experimental results revealed that n values of V-t characteristics at PD inception were as high as 80-110. On the other hand, the lower n values obtained at breakdown were interpreted by the intensified PD development in thermal bubbles generated after the PD inception. Index Terms--High temperature superconducting cable, partial discharge, butt gap, volume effect, V-t characteristics.

A High Temperature Superconducting Fault Current Limiting Transformer (HT(sub c)-SFCLT) with functions of both superconducting transformer in normal operating condition and superconducting fault current limiter in fault condition is developed. The findings indicate the operational feasibility of HT(sub c)-SFCLT through no-load and short-circuit tests, current limiting test and recovery test.

The electrical insulation performance of resistive superconducting fault current limiters (SFCL) under quench condition, exposed to thermal and electrical stress, is examined and reported.

In this paper, we studied partial discharge (PD) inception characteristics and V-t characteristics of PD inception in liquid nitrogen (L[N.sub.2])/polypropylene laminated paper (PPLP[R]) composite insulation system for high temperature superconducting (HTS) cable. Experimental results revealed that the magnitude of initial PD was in the range of 2.0-30 pC irrespective of butt gap condition and the initial PD was generated at the first and third quadrant of voltage phase. PD inception electric field strength (PDIE) without butt gap was 5-10% higher than that with butt gap and thicker butt gap gave larger PDIE drop. Moreover, PDIE with butt gap depended on the butt gap thickness and the number of PPLP[R] layers. The reason is explained by the existing probability of weak points of electrical insulation at butt gap. Finally, lifetime indices n of V-t characteristics at PD inception were obtained as 80-100 irrespective of butt gap condition. These values showed enough flat characteristics of V-t phenomena of electrical insulation.

The partial discharge (PD) inception characteristics are studied in liquid nitrogen (L[N.sub.2])/polypropylene laminated paper (PPLP[R]) composite insulation system for high temperature superconducting (HTS) cable. Experimental results revealed that the magnitude of the initial PD increased as the PD inception electric field strength was increased, because the injected energy increased. Initial PD was generated at the first and third quadrant of applied ac voltage phase. The probability of initial PD at the positive and negative voltage phase was almost the same. The reason is because liquid nitrogen is nonpolar molecule and we used symmetric electrode configuration with uniform electric field distribution. Finally, it was pointed out that PD inception electric field strength (PDIE) depended on the volume of the butt gap because of the increasing probability of weak points of electrical insulation, and PDIE linearly decreased with increasing stressed volume of the butt gap in the log-log scale.

Impulse partial discharge (PD) and breakdown (BD) characteristics of rod-plane gaps in [N.sub.2]/S[F.sub.6] gas mixtures were investigated for different gap geometry, gas pressure and S[F.sub.6] gas content. Experimental results revealed that the 50% probability breakdown voltage increased with gas pressure, which agreed with the theoretical values with consideration of discharge time lag for impulse voltage application. For the calculation of discharge inception voltage, the volume-time theory was successfully applied to the [N.sub.2]/S[F.sub.6] gas mixture. Furthermore, impulse PD light emission image was observed together with PD current pulse and light intensity waveforms. The long discharge with stepwise propagation and double-peak PD pulses could be observed, which corresponded to the streamer/leader transition leading to breakdown.

A superconducting fault current limiting transformer is proposed and the operating characteristics are reported.

Quench-induced dynamic breakdown strength of liquid helium was measured within the quench environment of superconducting coils.

Ac superconducting wires were studied to determine propagation characteristics at quench inception, and transient propagation velocity was found to reach 90km/sec.

We have been studying quench-induced 'dynamic' breakdown characteristics of [LHe.sup.*] and have already found that the electrical insulation performance of LHe was degraded drastically by the thermal bubble disturbance due to quench of superconductors. In this paper, in order to improve the insulation performance of LHe under quench conditions, we measured dynamic breakdown and prebreakdown characteristics of pressurized LHe. Experimental results revealed that breakdown voltage of LHe under quench conditions at 0.2 MPa was greatly improved, reaching 2 to 4x compared with that at atmospheric pressure under both uniform and non-uniform electric fields. Moreover, for practical and efficient insulation design of superconducting power apparatus, we investigated the dynamic breakdown voltage of LHe as functions of pressure and gap length.

We investigated characteristics of electromagnetic spectrum caused by partial discharge (PD) in air at three different types of high voltage substations. From the measured results, we characterized the electromagnetic noise spectrum depending on the type of substation. We also measured long term change of the average gain of noise spectrum and discussed the influence of atmospheric conditions on the results. Moreover, we introduced 'the equivalent charge qe' which was derived by converting the electric field strength estimated from the average gain into the charge magnitude of PD. We also proposed 'phase gate control method' for better understanding of the electromagnetic noise spectrum characteristics and the mechanism causing electromagnetic wave by PD.

We measured the temporal change of partial discharge (PD) characteristics leading to breakdown in [SF.sub.6] gas for ac voltage application. At the final stage close to the breakdown, positive PD pulses with relatively high magnitude began to take place in the phase region near the applied voltage peak. This event was interpreted in terms of the change of PD type in [SF.sub.6] gas from streamer to leader. Optical observation also revealed that PD type transition occurred. With the results considered, the mechanisms of PD were discussed. Moreover, we discussed the possibility of breakdown prediction in GIS. An attempt was made to find a breakdown prediction parameter which characterized the change of PD type: the ratio [R.sub.L] of maximum charge to the average charge of PD pulses appearing in the phase region near the peak in positive half cycle. It was found that [R.sub.L] allowed to predict the time to breakdown successfully within the error of 4 [approximately] 20%.

For the insulation design of high voltage apparatus, electric field optimization technique becomes indispensable tool. We developed a highly efficient electric field optimization technique based on Neural Network under the personal computer. Once the Neural Network learns the relationship between inputs and outputs, the system easily allows to get a target solution. Principle of the developed optimization technique are described with the calculation example. Moreover, normalization of learning data is introduced and the applicability is expanded.

We measured dc discharge inception voltage [V.sub.i] for various electrode configurations in He, Ar and air in vacuum range from [10.sup.5] to [10.sup.-1] Pa to determine discharge characteristics under space vacuum environment. By quantitative consideration of the [E.sub.i]/p distribution in the gap space at the discharge inception, the discharge inception mechanism under nonuniform electric field was investigated in vacuum. At the same time, we observed the discharge profile variation with residual gas pressure and quantitatively analyzed its characteristics, using an image processing technique. Based on the analysis, we found that the dependency of the discharge inception voltage and the length of the discharge path on the residual gas pressure under non-uniform field conditions agreed well with the dependency previously obtained for uniform field conditions.

A study was conducted on alternating current insulated multi-strand superconducting cables' quench current level-time (QCLT) characteristics. The QCLT characteristics of two types of superconducting cables were compared. Results attribute the difference in QCLT characteristics between the cables to the dissimilarity between current redistribution where each strand quenches.

We investigated the area and volume effects on the breakdown strength in liquid nitrogen (L[N.sub.2]) to discuss breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in L[N.sub.2] with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that breakdown mechanism changed from area dominant to volume effective region at the larger electrode configurations in L[N.sub.2]. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in L[N.sub.2]. It was suggested that mutual contribution of area and volume effects appeared in breakdown characteristics in L[N.sub.2] under thermal bubble condition, 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.

For practical insulation design of superconducting power apparatus, it is necessary to take into account an inherent phenomenon known as quench. The transition from the superconducting to the normal state. We investigated quench-induced dynamic breakdown and prebreakdown characteristics of liquid helium (LHe). Experimental results revealed that the quench of the superconductor drastically reduced the breakdown voltage of LHe in the thermal bubble disturbance. Moreover, dynamic breakdown characteristics in quasi-uniform field using superconducting coil were investigated. The results revealed that the breakdown was induced in the quench-initiated region where the largest thermal energy was generated.

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 press-board 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 x 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.

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 the flow chart for the practical insulation design of superconducting power apparatus.

In order to examine the area and the volume effects on breakdown strength in liquid nitrogen (L[N.sub.2]), we measured dc and ac breakdown voltages in L[N.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 L[N.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. Weibull shape parameter m for the sphere to plane and the coaxial cylindrical electrodes were estimated to be 6 to 8 and 11 to 13, respectively; the breakdown strength in L[N.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 L[N.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.

The performance of a 'prospective power transmission model system integrated under superconducting environment' (PROMISE) is discussed. The PROMISE, which was built to evaluate the technical feasibility of deploying superconducting cables for power transmission, consists of a transformer, a fault current limiter and five-meter power line. All three superconducting equipment are enclosed in a cryostat maintained at 4.2 K.

This paper described the discharge classification under nonuniform electric field in air and He at low gas pressures for the application of HV technology to the power apparatus in space. In order to discuss quantitatively the change of the discharge, an image processing method was introduced. In the first place, 4 shape parameters were proposed to characterize the discharge pattern: the area [S.sub.H], the flatness rate H/V, the location g of the center of luminosity of the luminous area, and the length L of the positive column. The analysis with the image processing for air revealed that [S.sub.H], H/V and g continuously increased as the pressure decreased. Consequently, the discharge type in air was successfully classified into 3 regions with two boundary pressures 200 and 2000 Pa-over the pressure range from 1.3x[10.sup.4] to 27 Pa. The discharge in He was also analyzed in the same way. Another parameter, the effective current density [J.sub.e], was introduced, which was defined as a ratio of the discharge current to the luminous area viewed from the vertical direction. [J.sub.e] proved to be independent of the discharge current [I.sub.d], so that there is the possibility that [J.sub.e] was suggested as a universal parameter to classify the discharge in a nonuniform electric field in vacuum.

We measured dc dielectric breakdown characteristics of cryogenic nitrogen gas above a liquid nitrogen surface for needle-to-plane and sphere-to-plane electrode configurations. Experimental results revealed that the dc breakdown voltage of cryogenic nitrogen gas increased, as the distance from the gap axis to the liquid nitrogen surface diminished, i.e. as the liquid nitrogen surface got closer to the gap. The breakdown voltage proved to be enhanced not only by the temperature drop of nitrogen gas due to the existence of liquid nitrogen, but also by an effect of vapor mist arising from vaporization of liquid nitrogen. For the quasi-uniform electrode configuration, the relation between breakdown voltage and the gas density times the gap spacing agreed well with the Paschen curve for nitrogen with both the temperature falling and the vapor-mist density considered. For the nonuniform needle electrode, positive breakdown voltage was higher than negative one; the polarity effect was interpreted in terms of the electric field relaxation at the tip of the positive needle resulting from partial discharges observed only for the positive needle.

With the goal of reducing [SF.sub.6] gas usage, we investigated partial discharge (PD) and breakdown (BP) mechanisms in ultra-dilute (0 to 1%) [SF.sub.6]/[N.sub.2] gas mixtures. The experimental results and discussions are given in this paper. In particular, we focused our research on the transition characteristics of the electrical insulation performance with an extremely small amount of [SF.sub.6] content, the PD behavior on the applied voltage, and the relation among prebreakdown streamer, return stroke and BD. Moreover, we discussed the PD and BD mechanisms with reference of space charge behavior From these results, the transition threshold at which the influence of [SF.sub.6] gas on the discharge characteristics began to appear, was clarified to be [SF.sub.6] content k=10 ppm for a total pressure of 0.1 MPa.