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The authors have been examining the diagnosis method for partial discharge (PD) occurring inside gas insulated switchgear (GIS) using the UHF method with a focus on electromagnetic (EM) wave mode conversions at L-shaped, T-shaped pipes, and components exsiting in actual GIS. Based on these conversion characteristics, an on-site, high-accuracy PD diagnosis algorithm was constructed for use on high-order mode EM-waves. All the researches so far were targeted at an isolated-phase-type (single-phase-type) tank. The present study was targeted at a straight-pipe three-phase structure, the most basic form, in order to apply the evaluation method to a three-phase-common-type tank (three-phase-type tank). Through the implementation of EM-wave propagation experiments and the EM-field analysis, the presence of TEM, [TE.sub.11], and [TE.sub.21] modes was verified also in the three-phase-type tank. Next, the EM-field analysis was conducted on models equivalent to actual apparatuses of 72 to 550 kV classes. Then, formulas for simplified calculation of cutoff frequencies in [TE.sub.11] and [TE.sub.21] in a three-phase-type tank were proposed. Furthermore, the basic concept of the PD diagnosis method was examined and the work flow was presented. Index Terms--Partial discharge (PD) diagnosis, gas insulated switchgear (GIS), three-phase-type, ultra high frequency (UHF), electromagnetic wave (EM-wave), high order mode.

As a means of diagnosing partial discharge (PD) signals propagate inside a Gas Insulated Switchgear (GIS), a study for the leakage of electromagnetic waves (EM-waves) emitted from the insulating spacer was implemented. The EM-waves leaking out from the solid insulator have the resonance frequencies depend on the spacing between adjacent bolts in the direction of the flange circumference, because the leakage portion is the equivalent of a slot antenna. In this paper, using an electromagnetic analysis model which has a simulated spacer on a concentrically-shaped GIS tank, the output characteristics of the EM-waves that leaked out from the slit were analyzed under various conditions such as the spacing between adjacent bolts, the width of the spacer, the dielectric constant of the spacer and the form of the flange. Also the actual measurement by the experimental equipment used to simulate the model was implemented for comparison with the analytical results. Consequently, the optimal specifications of the sensor and the measurement method used to achieve highly-sensitive detection for practical use were summarized and proposed as well as evaluating the effectiveness of the electromagnetic analysis model adopted in this paper. Index Terms--Gas insulated switchgear (GIS), partial discharge (PD), electromagnetic wave (EM-wave), slot antenna, resonance frequency, highly-sensitive detection.

The diagnosis algorithm using ultra high frequency (UHF) method with the higher-order electromagnetic wave (EM-wave) was examined as the advanced partial discharge (PD) diagnosis method in a gas insulated switchgear (GIS) in the former paper. Previous studies evaluated the characteristics of EM-waves propagated in a simple coaxial cylindrical pipe, or through an L-shaped or a T-shaped pipe formed by combining these. In the present study, components (a hand hole, a post-type spacer, etc.) present in an actual GIS structure were identified, and an evaluation done mainly through an analysis on what effects their presence has on the mode transformation of EM-waves propagated inside a GIS tank. The direction in which a GIS tank is bent is horizontal (0[degrees]) in the L-shape structure. If there is either a post-type spacer or a shield ring inside, and the signal source is in the 0[degrees] direction, the distribution of the EM-wave strength varied according to through the L-shape structure. Also in the case of a T-shaped structure, the distribution of the output after signals are transmitted through the T-shaped junction varied depending on each component. Based on these characteristics, the diagnosis algorithm for PD with higher-order mode EM-waves was improved taking into account the effects of various components existing in an actual GIS, and a PD diagnosis method more suitable for an actual structure was established. Index Terms--Partial discharge (PD), UHF, diagnosis, electromagnetic wave (EM-wave), higher-order mode, actual GIS structure

Though the Gas Insulated Switchgear (GIS) insulation diagnosis method using ultra high frequency (UHF) band signal is widely used as a method to detect partial discharge (PD) sensitively, diagnosis methods for the electromagnetic wave mode propagation behavior in GIS tanks have not been investigated enough. Especially, diagnosis methods based on the electromagnetic wave distribution of the circumferential direction have been rarely discussed, in the preceding studies by the authors comparing investigations were made by experiments and analyses on the electromagnetic field distribution patterns of the GIS circumferential direction in front of or behind the L-shape or T-shape structures. It was found that the electromagnetic field distributions that are characteristic of mode transformations are seen when the electromagnetic waves of the PD signals are propagated through the L-shape and/or T-shape structures in the GISs. In this study, the diagnosis algorithm is constructed by extracting the characteristics in the output of the high order mode electromagnetic waves transmitted through the L-shape part and/or T-shape part. If the output distributions of the signals detected before and/or after the transmitting have those characteristics, the signals can be determined to be the ones propagated inside and the information on the circumferential direction position of the signal source can be obtained. A simple diagnosis method with high precision has been developed as a GIS-PD diagnosis technology in the field based on the diagnosis algorithm as above. The diagnoses in the field can be classified into the first order diagnosis for high sensitivity detection and the second order diagnosis for high precision judgment by using the algorithm to do more reliable evaluation. Index Terms--Partial discharge (PD), gas insulated switchgear (GIS), ultra high frequency (UHF), diagnosis, electromagnetic wave (EM-wave), high order mode.

To investigate the transmission of electromagnetic waves excited by partial discharge through T branch section and L-shaped sections of a gas-insulated switchgear (GIS) bus structure, electromagnetic fields were numerically simulated. The modes of the simulated electromagnetic wave were found to transform to different modes, e.g., from TEll to TE21, after transmission and reflection through T branch or L-shaped sections of a GIS. These characteristics found in the simulation, mode transformation and dependence of resonance frequency on excitation direction of partial discharge, were also confirmed experimentally. In the case of 90-degree bent direction, the characteristics of mode transformation in a T branch and those in an L shaped section are similar. In an obliquely angled tank although electromagnetic-wave propagation behavior becomes similar to that for the L shaped section, the ratio of mode transformation becomes smaller, and the transmitted component of the input mode tends to increase, as the tank shape becomes closer to straight. Index Terms--Partial Discharges, gas insulated switchgear, UHF, diagnosis, electromagnetic wave.

Numerical simulation of electromagnetic fields was performed to investigate the transmission and reflection characteristics of electromagnetic waves excited by partial discharge through spacers, L-shaped sections of a GIS bus structure. The resonance frequencies of a cavity between spacers agreed roughly with the experiment. The modes of electromagnetic wave are shown to transform to different modes, e.g., from TE11 to TE21 mode, after transmission and reflection through L-shaped sections of a GIS. The frequency intervals of the resonance in tank's longitudinal direction for a L-shaped section are different between horizontal and vertical excitation of partial discharge source. The two characteristics, mode transformation and the dependence of resonance frequency on excitation direction, were confirmed experimentally. Index Terms--Partial discharges, gas insulated switchgear, UHF, diagnosis, electromagnetic wave.

Gas insulated switchgear (GIS) has been widely applied in power equipment since the late 1960s due to its reliability and compactness. Because epoxy insulators in GIS are important insulation components that affect the dielectric withstand level and the equipment lifetime, high voltage tests and sensitive partial discharge (PD) tests have been carried out on them in the factory. If a latent defect occurs inside an actual epoxy insulator, its size is predicted to be small, such as a micro-defect, because it will have passed rigorous tests and multiple inspections at the factory. Although many studies have clarified the fundamental phenomena of deterioration caused by PD occurring due to defects in inner epoxy, little is known about the actual size of such defects and their effect on insulation properties. Therefore, to assess the risks associated with aging GIS equipment, it is essential to understand the actual size of latent defects in inner epoxy insulators of GIS and the impact the defects have on the insulation performance. The puncture properties and PD characteristics of artificial micro-defects of cracks, voids, and delaminations produced using an accurate estimation technique and precise production control were investigated and are described. The results suggest that an estimation method using electric field analysis is useful for designing the defect shape and that the breakdown voltage depends on the three-dimensional shape of the defect. Techniques for accurate estimation of small defects in epoxy resin and stable production of micro-defects have thus been achieved. [ABSTRACT FROM PUBLISHER]