The technical soundness of an oil-immersed power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. To establish clear ...control criteria for this PD test, it is necessary to study PD propagation characteristics inside the transformer windings. In this paper, an analysis model was constructed using constants in the oil for a 500 kV-class transformer to analytically obtain PD signal propagation characteristics. For this analysis model, a circuit model for high frequencies up to several MHz was used in order to obtain the characteristics of highfrequency PD propagating inside the winding in detail. Using this model, PD propagation characteristics were investigated in terms of the PD occurrence position, measurement position, and front time as parameters. As a result, it emerged that the propagation pattern was determined by the winding constants as viewed from the PD injection position and the terminal conditions. When the charge amount was the same, the steeper the PD signal front time, the higher the peak voltage value was likely to be because the waveform became more oscillatory. Conversely, assuming an actual test using the ERA device (PD detector developed by Electrical Research Association), it was found that the front time had little impact on the charge amount measured because the oscillatory high-frequency components were removed. As with the previous studies, it emerged that the PD signal transmission rate was lowest when a PD occurred between sections and the signal was detected as one significantly damped to a level of several percent depending on the conditions.
The lightning protection design of a substation is generally based on the surge overvoltage incoming to the substation, which is generated by the back-flashover across an insulator string of the ...transmission tower that is closest to the substation and struck by a lightning return stroke. In this case, surges propagate along non-horizontal or non-uniform lines such as the tower and the inclined lines incoming to the substation. Although the surge characteristics of a tower have been intensively studied as of today, those of inclined incoming lines to a substation have not yet been investigated. In this paper, experiments have been conducted using a reduced-scale model to clarify the surge characteristics of the conductor system that is composed of horizontal transmission line, two towers, inclined incoming line and a substation gantry. It turns out from the experiments that the potential (voltage) generated at the tower arm rises gradually, compared with the current injected in the tower top. Furthermore, it is found that the power line potential at the gantry is smaller than that that at the tower where the back-flashover occurs, while the opposite trend is obtained from a circuittheory- based simulation in which the inclined incoming line is represented by a horizontal line. The latter approximation has been usually employed in representing inclined incoming lines in lightning surge simulations using the Electromagnetic Transients Program (EMTP) or its alternatives. The fact that the power line potential at the gantry, evaluated by the present experiment, is smaller than the EMTPcomputed value would contribute to a more economical and rational design of a substation.
A widely used, effective method for diagnosing the insulation of a gas-insulated switchgear (GIS) is to detect partial discharges using signals in the ultrahigh frequency (UHF) band. To detect ...partial discharge signals that propagate inside a GIS, it is necessary to install a sensor inside or set on the outside of a GIS tank. For GISs that have already been constructed, it is effective to use external diagnosis using electromagnetic wave (EM-wave) signals that radiate (leak) from the insulation spacer of the GIS tank, and there is strong demand for such a diagnosis technology. Externally leaking EM-waves are often detected using an antenna, and multiple antennas supporting different frequency bands are commercially available. The accuracy of external diagnosis could be improved by comparing and evaluating the characteristics of various antennas. The insulation spacer of a GIS tank is usually installed on the flange and fastened with metal bolts. GIS tanks are made electrically continuous by these metal bolts and have the same grounding potential. EM-waves leak from the part of the solid insulator surrounded by these metal bolts and the GIS tanks. This leaking part forms a slot antenna and has a resonant frequency that depends on the spacing between the bolts in the circumferential direction of the flange. At this resonant frequency, the output of externally leaking EM-waves is higher and enables more sensitive measurement. In this study, the detection characteristics with different antenna positions and directions were compared in consideration of the number and positions of the bolts on the spacer, and the frequency characteristics of various antennas were obtained. To improve the detection sensitivity, a prototype of an improved dipole antenna was fabricated and its characteristics were evaluated. Based on the experimental results, an ideal diagnostic system based on the externally leaking EM-waves is proposed.
The phenomenon of accumulated charges on a solid insulator material surface is one of critical parameters at the insulation design stage for ac electric power equipment as well as dc equipment. ...Therefore, it is important to grasp charge amounts on an insulator surface with an appropriate resolution. Over the last few years, researchers have made tremendous progress on this subject, but reports are rather academic and not always easy for electric power engineers to understand and apply practically. This paper explains latest measurement methods plainly starting with the basic principle of the measurement, supposing mainly gas insulated switchgear (GIS) engineers. It includes inverse-problem-solving as signal processing technique, resolution evaluation from the standpoint of system geometry, sample frequency, and noise. Further, effects of various parameters on the resolution and also preferable measurement systems for, e.g., 500 kV actual GIS spacers are treated.
This paper describes discharge properties of N 2 and CO 2 -based gas mixtures including a perfluorocarbon (PFC) gas such as CF 4 , C 3 F 8 and c-C 4 F 8 under non-uniform field. The mixture ratio ...between a base gas of N 2 or CO 2 and the additive PFC gas was fixed as 9:1; namely, 90%N 2 /10%PFC or 90%CO 2 /10%PFC gas mixture. The PFC gases have even smaller global warming potential (GWP) than SF 6 gas and have good insulation properties as SF 6 gas. Thus, PFC gas mixture is expected to be a SF 6 substitute without highly pressurizing the gas over the conventional pressure of 0.5 to 0.6 MPa. In this study, in order to compare the partial discharge (PD) inception voltage V PD and breakdown voltage V B properties between N 2 and CO 2 -based gas mixtures, as well as between the additive gas of PFC and SF 6 gas, we investigated these properties of the gas mixtures with a needle to plane electrode under ac high voltage application. The gas pressure was changed from 0.1 to 0.6 MPa. As a result, it was found that V PD and V B characteristics of N 2 and CO 2 -based gas mixtures differed considerably, especially the gas pressure dependence of V B (so-called the N shape characteristics). V B characteristics of N 2 -based gas mixture including c-C 4 F 8 proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of V PD properties, CO 2 -based gas mixture had an advantage over N 2 -based gas mixture due to higher V PD . Furthermore, we discussed the synergy effects of V PD and V B for N 2 and CO 2 -based gas mixtures using the index R n which was defined to quantify the degree of the effect. R n for CO 2 -based gas mixture was higher than that of N 2 -based gas mixture.
A simplified model of corona discharge on overhead wire has been proposed for propagating surge computations using the finite-difference time-domain method. The radial progression of corona streamers ...from the wire is represented as the radial expansion of cylindrical conducting region whose conductivity is several tens of microsiemens per meter. Two wire radii are considered: 5 and 2 mm, in order to simulate two experimental con- figurations by Noda. The critical electric field on the surface of a 5-mm radius wire for corona initiation is set to E 0 = 1.8 or 2.9 MV/m. For a 2-mm radius wire, it is set to E 0 = 2.2 MV/m. The critical background electric field necessary for streamer propagation is set to E cp = 0.5 MV/m for positive voltage application, and E cn = 1.5 MV/m for negative voltage application. The computed waveform of radial current (including both conduction and displacement currents) agrees well with the corresponding measured waveform. Also, the computed relation between the total charge (charge residing on the wire and emanated corona charge) and applied voltage (qV curve) agrees well with the corresponding measured one, except for relatively low applied voltages. Additionally, the increase of coupling between the energized wire and another one nearby due to corona discharge is well reproduced.
Field study of lightning stroke effects, particularly the mechanisms that produce distribution line faults, is essential for designing rational lightning protection for distribution lines. The Tokyo ...Electric Power Company has conducted photographic research and observed voltage and current waveforms due to lightning in actual distribution lines since 1996, obtaining 284 datasets, including 62 for direct hits to the lines. These results permit a comprehensive study of each lightning flash; the photographs show the precise location of the lightning, and the waveforms provide detailed analyses of flashovers in the distribution systems. This paper evaluates typical observed datasets, including surge insulator voltage waveforms. In some cases, the direct lightning strokes did not cause distribution line faults because the power follow current extinguished spontaneously after flashover generation. These results provide a solid basis for examining the mechanisms underlying the production of distribution line faults, and can lead to effective lightning protection practices.
In order to attain reduction in insulation test voltages, voltage-time (V-t) characteristics under AC voltage and voltage-number (V-N) characteristics under lightning impulse and switching impulse ...voltages were experimentally investigated on the turn-to-turn insulation model, section-to-section insulation model and barrier-oil-duct insulation model of core- and shell-type transformers in EHV and UHV Classes. First, V-t characteristics for AC voltage were obtained in a short time range of a few tens of milliseconds to a long time range of three to four months. The n-values (inclination) of V-t characteristics are distributed around 40 for the short time and several hundreds for the long time characteristics. These values are of essence in determining the AC withstand voltage test. Next, V-N characteristics for both lightning and switching impulse voltages were acquired for up to 1,000 times of application. The n-values (inclination) of V-N characteristics are in most cases distributed around 70 for both impulse waveforms and about 40 in only turn-to-turn insulation models This result will be useful for evaluating the effect of frequent surges on apparatus insulation. Finally, insulation coordination studies were performed based on these experimental results.
A case of salvage supracricoid laryngectomy with cricohyoidoepiglottopexy after failed radiation therapy and vertical partial laryngectomy had successful oncological and functional outcomes. This is ...the first reported application of salvage supracricoid laryngectomy with cricohyoidoepiglottopexy after the failure of two major treatments.
A 65-year-old man was referred for salvage supracricoid laryngectomy with cricohyoidoepiglottopexy. The right recurrent hemilarynx was successfully resected. After pexis, the right lobe of the thyroid gland was repositioned to overlap and reinforce the pexis gap and fill the devoid portion of the strap muscular closure. Multiple scattered foci (recurrent tumour-node-metastasis stage T2) were identified around the arytenoid cartilage and beneath the musculocutaneous flap. Four years after supracricoid laryngectomy with cricohyoidoepiglottopexy, the patient's recovery was following a favourable course and he had satisfactory laryngeal function.
Appropriate case selection and proficient surgical skills were essential for a successful outcome. Head and neck surgeons should not be afraid to adopt functional preservation open surgical procedures in well-selected and well-motivated patients. A requirement for more challenging surgical procedures and meticulous rehabilitation processes should not exclude appropriate treatments from a surgeon's repertoire.
