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Spectroscopic and imaging analyses using electromagnetic waves in the far IR or submillimeter region [0.1-20 terahertz (THz)] are emerging technologies in the field of optics research. Thanks to recent advances in related science and technology [l]-[6], it has been proposed that THz technology be applied in many fields such as early diagnosis of cancer, detection of counterfeit bills, and moisture analysis.

Electromagnetic waves are named depending on their wavelength range. For many years, the generation methods, detection methods and applications of almost all electromagnetic waves have been investigated. However, there remains an unveiled range owing to a lack of appropriate generation methods. This wavelength range is from several tens to several hundreds of micrometers, and has a frequency range from one to several tens of THz. Generally, waves in this range are called the THz waves. Some generation methods for THz waves have been developed recently, the characteristics of these waves are still under investigation, but some applications have been proposed. One of the important properties of THz waves is their high transmissivity in opaque dielectric materials. Generally, insulating materials are opaque to visible light, and it is difficult to observe internal defects externally. However, if THz waves show sufficient transmissivity and proper interaction with internal physical quantities, they are expected to be a useful measurement tool for insulating materials. In this study, polyethylene, which is a typical insulating material, is choosen as the measurement target, and void interfaces and mechanical strain are detected using THz waves.

As mechanical vibration usually exists in HV motors and generators, its influence on the partial discharge (PD) needs to be studied so as to improve the reliability of on-line PD monitoring method. In this report, the mechanical vibration frequency was synchronized to the power frequency and the effects of vibration phase and frequency were investigated. Because the length of the discharge gap changed with the vibration, obvious variation in the slot-PD patterns was observed at different vibration lead phase and frequency. Moreover, when the vibration amplitude was large, it was also found that the PD behavior at low voltage just above the PD inception voltage was different from that at higher voltage. This was explained in terms of the effects of the field variation in the gap due to the vibration and the charges left on the insulating surface due to PD.

Partial discharge (PD) inception characteristics of liquid nitrogen (LN_2)/ polypropylene (PP) laminated paper composite insulation system for high temparature superconducting (HTS) cables were investigated in terms of the volume effect and the V-t characteristics. The electrical and optional 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 satistically stressed liquid volume (SSLV) based on the discharge probability in both butt gaps and LN_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.

Although surface discharges are one of the principal kinds of phenomena associated with faults in electrical apparatus, they have not yet been fully elucidated. Electric field measurements in and around a corona streamer during the creeping advance of a surface discharge are very helpful in analyzing discharges. Pockels crystals have been used in the optical measurement of electric fields, and an optical waveguide technique has been successfully used to miniaturize the sensors. Electric field measurements are made just beneath a surface discharge with an optical waveguide Pockels sensor installed on a PMMA plate and output signals expressing the development of the discharge are obtained. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(2): 28–34, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10185

The present paper describes two kinds of techniques aiming at improving THz imaging for detecting rust under coated steel plates. First, it is revealed that a THz camera considerably shortens the THz-imaging time, but fringe patterns due to high coherence light source sometimes prevent image concatenation. Second, a program flowchart to quantitatively estimate the degradation level of rust is also presented.

The applicability of terahertz (THz) waves to the detection of rust in coated steel plates has been examined. Spectral intensity measurements of reflected light from a sample coated with epoxy resin show that optical coherence tomography using near-infrared light and THz waves can detect the hidden rest region. This region can be reproduced by so-called THz imaging without contact. Since rust exhibits strong absorption in the frequency range from 1 to 5 THz, the absorption coefficient can be used to estimate the rust state with high sensitivity. In contrast, neither X-ray fluorescence nor Fourier transform infrared spectroscopy can detect the rust, because of the strong X-ray and IR peaks of the coating. Electro-chemical impedance measurement, which is one of the promising diagnostic methods, judges the water blocking ability of the coating. The impedance value and THz absorption due to rust shows good correlation until the amount of water in the film reaches its maximum.

One of the recent topics of optical measurement techniques is the generation and utilization of the “Terahertz wave (THz wave)”. In the long history of the research on electromagnetic waves, it remains as the last unrevealed area because its generation and detection techniques have not been sufficiently developed. In recent years, some useful and convenient devices have become commercially available, and much related research is being carried out. One important feature of THz wave is the high transmission probability through some opaque materials, many of which are dielectric materials through which a visible light beam cannot pass. Solid insulating materials are usually opaque dielectric materials, and it is difficult to measure their interior by conventional optical measurement techniques. If THz wave can pass through an insulating material sufficiently, the same measurement technique as with a visible light beam, already developed and practically applied, such as the technique of utilizing polarization, will become applicable. In this chapter, the research work on the introduction of THz wave techniques to the internal measurement of solid insulating materials is reviewed [1,2].

6 to 60 kV class XLPE cables are widely used and playing important roles in power distribution and transmission systems in Japan. The aging degradation is dominated by water trees in these XLPE cables. Therefore, water tree diagnosis with high precision is quite important for operation, maintenance and replaces planning of these XLPE cables. The authors have been investigating the applicability of the Damping oscillating high voltage (DOHV) to the water tree detection as well as insulation diagnosis for these XLPE cables. This paper firstly proposes the nondestructive insulation diagnosis using DOHV application. Then, some empirical results obtained by its application to water-tree degraded XLPE cable are discussed.

Two types of resin-coated papers were investigated; kraft papers and heat-resistant kraft papers partially covered with epoxy resin; those were laminated to some thickness. They were thermally degraded at 120degC for 240 to 1320 hours, and their mechanical strength, i.e. tensile strength was measured. The tensile strength of the resin-coated kraft paper decreased with the heating time and became smaller than that of the pressboard after 500 hours while the tensile strength of the resin-coated heat-resistant kraft paper almost retained the initial value after 1320 hours of the heating time. Then, electrical breakdown characteristics of composite insulation systems with a resin-coated paper and insulation oil were investigated. In the system, an oil-filled gap was artificially introduced between the resin-coated paper and a plane electrode to induce partial discharge (PD) initiation at the same location. PDs occurred before breakdowns and their PD inception electric field strength was almost as high as pressboards and the effect of the heating time was negligible. Characteristics of electrical breakdown field strength were similar to the PD inception field strength; negligible effects of the type of the resin-coated papers and the heating time. Electrical breakdown occurred at the oil-filled gap and the edge of a high voltage electrode.

We discussed partial discharge (PD) inception characteristics of liquid nitrogen (LN/sub 2/)/polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables. In this paper, we measured PD inception characteristics under lightning impulse voltage application by both electrical and optical methods. The impulse PD inception characteristics were compared with those under ac voltage application, and the PD generation mechanisms were discussed. Experimental results revealed that PD detection sensitivity under lightning impulse voltage application reached as small as 6 pC. PD was verified to be generated not only inside the butt gap but also outside the butt gap under both lightning impulse and ac voltage applications. AE sensors could also be applied to PD measurement under ac voltage application.

With the aim of developing XLPE insulation for extra high voltage cable, we investigated the morphology of cross-linked polyethylene. We used a kind of base materials and additives, and controlled curing condition and amount of additives. The effect of addition of additives on morphology of XLPE such as lamellar density, orientation and additive layer were analyzed using TEM analysis and X-ray diffraction test. For evaluation of breakdown strength of XLPE insulation with additives and base materials, the mold-type XLPE specimens were made and evaluated with Weibull plot parameters, and compared with morphological characteristics. We applied this result to model cable and this model cable was found to have superior dielectric strength and morphological characteristics compared with non additive added model cable.

As mechanical vibration usually exists in HV motors and generators, its influence on the partial discharge (PD) needs to be studied so as to improve the reliability of on-line PD monitoring method. In this report, the mechanical vibration frequency was synchronized to the power frequency and the effects of vibration phase were investigated. Because the length of the discharge gap changed with the vibration, obvious variation in the slot-PD patterns was observed at different vibration phase. Moreover, when the vibration amplitude was large, it was also found that the PD behavior at low voltage just above the PD inception voltage was different from that at higher voltage. This was explained in terms of the effects of the field variation in the gap due to the vibration and the charges left on the insulating surface due to PD.

In this paper, we discussed partial discharge (PD) inception characteristics in liquid nitrogen (LN/sub 2/)/polypropylene (PP) laminated paper composite insulation system in terms of volume effect and V-t characteristics, both for PD inception strength (PDIE). Different kinds of butt gap condition were applied in the experiments, using parallel plane and coaxial cylindrical cable samples. Experimental results revealed that the volume effect on PDIE could be evaluated by the statistical stressed liquid volume (SSLV) taking account the discharge probability not only in the butt gap but also in the other thin layers between PP laminated papers. Furthermore, the indices n of V-t characteristics at PD inception were estimated to be 80/spl sim/100, irrespective of the butt gap condition.

In order to reduce construction cost of substations and to become more environmentally benign, we have been proposing the All Solid Insulated Substation as a next generation substation, in which the high voltage section like busbars and transformer windings are insulated by the solid insulating materials such as epoxy resin and silicone rubber. In the All Solid Insulated Substation, compact connection systems between each piece of power equipment have to be developed for the reduction of the volume and area of the substation. From these viewpoints, this paper introduces the concept of the All Solid Insulated Substation and describes basic experimental investigation for the development of the compact connection system.

Investigations were undertaken to focus on partial discharge (PD) and breakdown (BD) characteristics in SF/sub 6//N/sub 2/ gas mixtures under inhomogeneous electric field by the effect of an extremely small amount (1%-5/spl times/10/sup -4/% (5 ppm)) of SF/sub 6/ gas content. Experimental results and discussions are summarized in this paper in that the insulation characteristics of SF/sub 6//N/sub 2/ gas mixtures at SF/sub 6/ content k/spl ges/0.1% correspond to those of pure SF/sub 6/ gas, while at k/spl les/2/spl times/10/sup -3/% (20 ppm), to those of pure N/sub 2/ gas, respectively,.

We have proposed a concept of an environment-conscious all-solid-insulated substation system as a next generation substation. In order to make the system compact and its layout flexible, it is required to develop a compact connection system with solid insulating materials. Hard and soft insulating materials are to be used for this connection system. Therefore, there is a concern about air penetration into their interface at the time of connection, which reduces the insulation performance. In order to simulate this situation, a model connection part was made using an epoxy resin board as the hard material and a silicone rubber board as the soft material with a spacer inserted into their interface. The present paper reports experimental results on the breakdown strength under ac and impulse voltages. It is confirmed that the insulation performance decreases with an increase in the thickness of the air layer between the two boards.