Naoki Hayakawa

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.

Technical Development on Partial Discharge Measurement and Electrical Insulation Techniques for Low Voltage Motors Driven by Voltage Inverters

This paper reviews the up-to-date research activities on electrical insulation techniques under repetitive surge voltages with steep wavefront for low voltage inverter-fed motors. Partial discharge (PD) inception, propagation, ageing and breakdown characteristics of magnet wires, including recently developed PD-resistant wires such as nanocomposite enameled wires, are described. PD measurement techniques under the steep-front voltages and IEC activity toward their standardization are also introduced.

Partial discharge characteristics of inverter-fed motor coil samples under ac and surge voltage conditions

This work investigates the partial discharge (PD) characteristics of twisted pair samples for inverter-fed motor coil under ac and surge voltage conditions. It also discusses the difference in the PD inception characteristics of a twisted pair under 60-Hz voltage and surge voltages with a steep wave front. The PDIV under ac voltage was measured and was found to agree well with the theoretical value using electric field analysis and Paschens law. The PDIV under surge voltage was measured for different repetition rates and the PDIV for 60-pps repetition was found to be lower than those for 6 pps and for the single shot, which suggested that the electrical insulation performance would be influenced by the repetition rate. The PDIV under surge voltages was 1.5 to 1.8 and 2.3 to 2.7 times higher than those under AC voltage application for the used and virgin samples, respectively.

Common insulating properties in insulating materials

Insulating materials exist in a variety of different forms, including gas, liquid, solid, and vacuum, and different forms of insulating materials have significantly different dielectric strengths and insulating properties. Power transmission/substation facilities for UHV AC and 500 kV DC power transmission and 500 kV underground power transmission facilities have been designed and constructed by taking advantage of the insulating properties of various insulating materials and structures so that they conform to the demanded insulation specifications. Conventionally, insulating material technology has been developed by making a close study of apparatuses and insulating materials individually. In this paper, as a new attempt, the insulating properties of various insulating materials/apparatuses are cross-sectionally compared and examined from an entirely new viewpoint through the assessment of commonalities and differences between them. A listing of lightning impulse and AC dielectric strength data is prepared and systematically positioned from the standpoint of material and structure, and compared and evaluated in a unified way from the viewpoint of typical indices of insulating properties including the n value of the V-t characteristic, Weibull parameters, and impulse ratio. Further, an insulation assessment of apparatuses is systematically made with consideration given to their sizes and structures as well as the insulating materials themselves. As a result, it was found, for example, that the design field strengths of power capacitors were high and those of generators were not, and there was a noticeable difference in design field strength between SF/sub 6/ gas insulated apparatuses and oil-immersed transformers. It is expected that the analysis conducted from an entirely new angle may provide a new approach to the development of apparatus insulation technology and to the rationalization of insulation design.

Partial discharge and breakdown mechanisms in ultra-dilute SF6 and PFC gases mixed with N2 gas

Because of the high global warming potential of SF6 gas, research on alternative gases for electrical insulation with a lower environmental impact is essential. Gas mixtures composed of electronegative gases and N2 gas have the advantage of the reduction of the amount of SF6 gas and of utilizing the synergistic effect in electrical insulation performance. We investigated the partial discharge (PD) and breakdown (BD) characteristics of SF6/N2 and PFC (C3F8/N2 and C2F6/N2) gas mixtures under non-uniform electric field conditions, by changing the dilute content of electronegative gases. As a result, the synergistic effect in SF6/N2 gas mixtures was verified to be higher than that in PFC/N2 gas mixtures. The physical mechanism from PD inception to BD was discussed with consideration of the difference in electronegativity of SF6 and PFC gases. Furthermore, we found that PD inception and PD-to-BD mechanisms changed at a content of 10xa0ppm for SF6 due to the electron attachment activity of SF6 gas. The change in the PD and BD mechanisms in C3F8/N2 and C2F6/N2 gas mixtures appeared at 0.1% content for C3F8 and at 1% content for C2F6.

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

Evaluation of UHF method for partial discharge measurement by simultaneous observation of UHF signal and current pulse waveforms

The partial discharge - current pulse waveform analyzing (PD-CPWA) system can measure partial discharge (PD) current pulses continuously in high speed, and is being used for the study of PD mechanisms in SF6 gas in a laboratory. On the other hand, the ultra high frequency (UHF) method is becoming popular for the on-site monitoring of gas-insulated switchgears (GIS). The former can detect PD with high accuracy and make it possible to analyze and understand discharge physics in detail. The latter is suitable to detect PD in actual power equipment, even in noisy site condition. Clarification of coincidence of PD characteristics measured with both methods is very important for further application of the UHF method. In this paper, PD current pulses generated from artificial defects in SF6 gas were synchronously measured with both methods. Measured data were analyzed and compared in many PD characteristics. As a result, many characteristics coincided well between them. However, the scatter of PD and the limited equipment performance of the UHF method may cause some detection failures. An appropriate and advanced use of the UHF method was pointed out.

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.

Permittivity characteristics of epoxy/alumina nanocomposite with high particle dispersibility by combining ultrasonic wave and centrifugal force

This paper proposes a novel technique to fabricate epoxy/alumina nanocomposites with nanoparticle composite process by combination of ultrasonic wave and centrifugal force. The particle dispersion effect of the nanoparticle composite process and its influence on dielectric permittivity were discussed quantitatively. Experimental results clarified that the combination of ultrasonic wave and centrifugal force was effective to increase dispersed nanoparticles and as well as to separate residual agglomerates. We verified that the improvement of particle dispersibility in the nanoparticle composite process by combination of ultrasonic wave and centrifugal force could bring about lower permittivity of the nanocomposites, especially than that of unfilled epoxy material.

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.