Hitoshi Okubo

A novel technique for partial discharge and breakdown investigation based on current pulse waveform analysis

A novel technique for partial discharge (PD) measurement and analysis (PD-CPWA; PD current pulse waveform analysis) is developed and introduced in this paper. PD-CPWA is expected to be utilized to discuss PD mechanisms and physics in electrical insulating materials, focusing on the PD current pulse waveform and its time transition from PD inception to breakdown (BD). In this paper, the concept and principle of PD-CPWA are described, and the applications of PD-CPWA to (1) epoxy spacer samples under thermal and electric combined stresses in GIS, (2) creepage PD on epoxy spacers in SF/sub 6/ gas and (3) liquid nitrogen/polypropylene laminated paper composite insulation system for high temperature superconducting cables are introduced and discussed.

Streamer and leader discharge propagation characteristics leading to breakdown in electronegative gases

This paper describes the propagation characteristics and mechanisms of partial discharge (PD) leading to breakdown (BD) in electronegative gases such as SF6/N2 and C2 F6/N2 gas mixtures for AC voltage application. An established PD measuring system of highly time-resolved PD current and light intensity waveforms, streak and light emission images enabled us to discuss the physical mechanisms of PD generation and propagation in terms of space charge behavior. Experimental results revealed that the time-resolved PD generation was divided into different groups composed of the first large PD and the subsequent small PDs; the former was identified as the leader discharge, and the latter corresponded to the streamer discharges. The successive generation of the streamer and leader discharges was interpreted by the change of critical volume size. The time-resolved PD generation characteristics leading to BD were also measured and discussed. The leader discharge would be obliged to take a detour along the periphery of the positive space charge area in the vicinity of the needle electrode, and finally result in BD in electronegative gases

Progress in Development of Superconducting Fault Current Limiting Transformer (SFCLT)

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT), which has multifunction of both a superconducting transformer in a steady state as well as a superconducting fault current limiter in a fault condition. This paper introduces the progress in our SFCLT project since 1998, from Step-1 to the latest Step-5, with the concept, design, fabrication and test results of SFCLT. In the latest Step-5, we developed a 2 MVA, 22/6.6 kV class SFCLT with YBCO coated conductors and verified the fundamental function as a transformer, effective current limiting function as a fault current limiter, and recovery characteristics after the fault clearance with its operational criterion.

High-Tc Superconducting Fault Current Limiting Transformer (HTc-SFCLT) With 2G Coated Conductors

We developed a 3-phase, 100 kVA, 6600 V/210 V high temperature superconducting fault current limiting transformer (HTc-SFCLT) with functions of both superconducting transformer and fault current limiter. The HTc-SFCLT is characterized by the application of 2G coated conductors with the higher current limiting performance and flexibility than that of the HTc-SFCLT with 1G Bi2212/CuNi composite bulk material. Fundamental tests of the HTc-SFCLT were carried out, and the design parameters as a superconducting transformer and as a superconducting fault current limiter were verified.

Current Limiting and Recovery Characteristics of 2 MVA Class Superconducting Fault Current Limiting Transformer (SFCLT)

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT) with the function of both superconducting transformer in steady state and superconducting fault current limiter in fault condition. As the Step-5 of SFCLT project, 2 MVA class HTS-SFCLT using YBCO coated conductors with the ratings of 22 kV/6.6 kV has been developed. One of the important concerns for the development of HTS-SFCLT is the current limiting and recovery characteristics as well as the transformer function. Thus, in this paper, current limitation and recovery tests of the HTS-SFCLT are carried out. The current limiting test have revealed that the HTS-SFCLT exhibits the excellent current limiting function as a superconducting fault current limiter, where the prospective short-circuit current IPRO=784 Apeak is reduced to 34% at the first peak and 18% at the 5th cycles. The HTS-SFCLT has been able to recover into superconducting state immediately after the fault clearance. According to the transition of generated resistance of the HTS-SFCLT, the criteria of recovery into superconducting state are quantified for different combinations of load current before the fault and prospective short-circuit current during the fault.

Dielectric characteristics of HTS cables based on partial discharge measurement

We have discussed dielectric 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. Focusing on the partial discharge (PD) inception characteristics and mechanism, the volume effect and its saturation on PD inception electric field strength (PDIE) was evaluated at atmospheric and pressurized condition. PD inception characteristics under lightning impulse voltage application were also investigated using an electrical and optical PD measuring system, and compared with those under ac voltage application.

Simulation of electrical and thermal behavior of high temperature superconducting fault current limiting transformer (HTc-SFCLT)

Superconducting Fault Current Limiting Transformer (SFCLT) is expected to perform functions both of transformer in the normal operating condition and of fault current limiter in the system fault condition. As the Phase-3 of the SFCLT project, we have been developing SFCLT based on Bi2212/CuNi bulk coils at LN2 temperature and verified its technical feasibility. In this paper, we developed a numerical model for evaluation of the electrical and thermal behavior of HTc-SFCLT such as current limitation and recovery characteristics. This model took into account E-J characteristics of Bi2212/CuNi bulk coil and its electrical and thermal transient phenomena during the operation of HTc-SFCLT. The simulated current agreed well with the experimental data with the error of less than 5%. The excellent current limitation and self recovery characteristics obtained by the experiments could also be reproduced. With the numerical model, current and thermal behavior of HTc-SFCLT was simulated for different parameters of conductor configuration, which would be useful for the future design and optimization of HTc-SFCLT.

Recovery characteristics after current limitation of high temperature superconducting fault current limiting transformer (HTc-SFCLT)

Superconducting Fault Current Limiting Transformer (SFCLT) is one of the promising superconducting power apparatus, since it has both functions of superconducting transformer in normal condition and superconducting fault current limiter in fault condition. In this paper, current limiting and the recovery tests of high temperature SFCLT (HTc-SFCLT) with MCP-BSCCO2212 bulk coils were performed at liquid nitrogen temperature. Experimental results revealed that the HTc-SFCLT could successfully recover into the superconducting state by itself immediately after the fault clearance. Moreover, the recovery criterion of HTc-SFCLT as a function of load current was clarified.

Development of 2 MVA Class Superconducting Fault Current Limiting Transformer (SFCLT) with YBCO Coated Conductors

We have been developing a Superconducting Fault Current Limiting Transformer (SFCLT) with the functions of both superconducting transformer in normal operating condition and superconducting fault current limiter in fault condition. As the Step-5 of our SFCLT project, in this paper, we designed and fabricated a 2 MVA class HTS-SFCLT using YBCO coated conductors with the ratings of 3-phase and 22 kV/6.6 kV. The developed HTS-SFCLT is characterized by a hybrid structure of HTS coils using YBCO, YBCO/Cu and Bi2223 tapes for design flexibility as both the superconducting transformer and the superconducting fault current limiter. Fundamental tests of the HTS-SFCLT were carried out, and its design parameters as a superconducting transformer were verified.

Development of superconducting fault current limiting transformer (SFCLT) with the functions of fault current suppression and system stability improvement

We have been developing a superconducting fault current limiting transformer (SFCLT) with the functions of both transformers and fault current limiters. In this paper, firstly, we analyzed the operating characteristics of SFCLT introduced into a model power system with 3-phase, 500 kV/275 kV, 625 MVA and optimized the main parameters by EMTP simulation. Secondly, we designed and fabricated an experimental scale-down model of SFCLT with 3-phase, 275/105 V, 6.25 kVA, using NbTi superconducting wire, on the basis of simulations. Thirdly, we discussed the experimental results of the model SFCLT introduced into a transient network analyzer. It was finally revealed that simulation results and experimental results clarified the effectiveness of SFCLT having both functions of the fault current suppression and the system stability improvement in a future superconducting power system.