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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.