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ABSTRACT: In order to clarify the mechanism of electrical-tree inception from water-tree degradation which often determines the remaining life of power cables, we have been investigating electrical tree inception and propagation from water-tree degradation in polyethylene. Some of the results suggested that at higher temperatures the extent of water-tree degradation was much higher than that of water-tree degradation formed at lower temperatures. We, therefore, investigated the temperature dependence of water tree degradation and its effects on electrical tree inception. At 363 K, the water tree growth is much faster than low temperatures and the degradation is severer. When the water tree is wet, electrical tree inception voltage becomes higher, which is explained in terms of electrical-field moderation and suppression of electron acceleration due to the existence of water and so on. In the sample with water tree formed at 363 K, however, the increase in tree inception voltage is small. When the water tree is dry, water tree degradation tends to lower electrical tree inception voltage and to promote the degradation under prolonged ac voltage application. These are explained by structural change due to water tree degradation, such as scission of polymer chains and micro void formation.
Electrical Insulation and Dielectric Phenomena, 2009. CEIDP '09. IEEE Conference on; 11/2009
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ABSTRACT: Power output fluctuation of photovoltaic power generation systems (PVSs) may cause negative impact on the existing electric power system when the penetration of PVSs is quite large. A micro-grid consisting of clustered PVSs and a battery system would be one of the promising measures to the negative impact of clustered PVSs, while the maximum power and storage capacity of battery system should be reduced as much as possible from the economic point of view. Because of the difference in output fluctuation among PVSs in the various locations, the total output fluctuations of PVSs would be relaxed due to the so-called "smoothing-effect". Considering such the effect, the required performance of battery system can be reduced, even though small output fluctuation of each micro-grid still remains. By using data on solar irradiance observed in five points simultaneously, this study evaluates the total output fluctuation of several micro-grids and the required performance of battery system, taking the smoothing effect into account.
Sustainable Alternative Energy (SAE), 2009 IEEE PES/IAS Conference on; 10/2009
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ABSTRACT: Partial discharge (PD) in a void reflects degradation of insulating material and, therefore, is utilized for insulation diagnosis. Although it is well known that PD pattern changes with voltage application, it is not clear how the pattern change is related to material degradation or how it is related to final breakdown. We simultaneously observed PD characteristics and light emission from partial discharge and the results were compared with those of surface observation. Both in an open void where atmospheric condition does not change and in a closed void, PD activity become concentrated in a small spot after certain time of voltage application. This seems to be caused by field concentration due to deposit of degradation products. In an open void, the intensity of each PD pulse is strong but the time to final breakdown is long. This is explained by the fact that the PD spot does not remain in the same place but constantly moves around, leading to smaller local energy input. On the other hand in a closed void, though the intensity of each PD pulse is small, the time before final breakdown is relatively short. This is explained by the experimental result that the PD spot in this case remains in the same place and the local energy input seems large. Finally, by terminating voltage application just before the final breakdown it is confirmed that the breakdown starts at the spot of PD concentration where the deposit exists.
Electrical Insulation and Dielectric Phenomena, 2008. CEIDP 2008. Annual Report Conference on; 11/2008
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ABSTRACT: Distributed generators (DGs) that use renewable energy sources have an unstable output and this can negatively affect existing electric power systems. To reduce this impact, we developed an energy control system for controlling the output from individual DGs that were installed in a microgrid. Controlling the DGspsila outputs enabled the power fluctuation at the point where the microgrid was connected to the existing power grid to be reduced. The microgrid was installed and operated in two separate locations for a total of 672 days. The actual DGs were photovoltaic power generation (PV) systems, fuel cells, and a NaS battery. We demonstrated that 99% of the power imbalance between the supply and demand over 10-, 15-, and 30-minute periods was within a range of plusmn3% under normal operating conditions when using our power control system. Furthermore, we demonstrated that, by using our power control system, the power flow fluctuation of the PV systems output at the grid connection point was smoothed.
Telecommunications Energy Conference, 2008. INTELEC 2008. IEEE 30th International; 10/2008
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ABSTRACT: In order to clarify the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we have investigated temperature dependence of water tree and electrical tree inception and propagation from water-tree degradation. In the samples with water tree of the area above ca. 5000 mum<sup>2</sup>, the ac electrical-tree inception voltage increased with increasing water-tree degradation in high temperature as well as room temperature. This is explained by the moderation of electrical field in the region of water tree. In the samples with water tree of the area less than 5000 mum<sup>2</sup> and in the samples without water tree, however, the ac electrical-tree inception voltage is higher at 70degC than at room temperature and ca. 15 kVrms regardless of water-tree area. On the other hand, the samples with dried water tree degradation show reduction in impulse tree inception voltage by ac prestressing at 70degC. Moreover, at 90degC the rate of water-tree growth and the extent of degradation is much higher than at 30degC. This suggests possible danger of water tree degradation at high temperatures.
Electrical Insulating Materials, 2008. (ISEIM 2008). International Symposium on; 10/2008
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ABSTRACT: Space charge formation in polyimide (PI) films heavily depends upon the water content. In order to clarify the effect of carrier injection on space charge properties in PI films, space charge distributions were observed in the PI films with different electrode conditions, such as aluminium (Al) or gold (Au) deposited on the both surfaces of the PI films. In each sample, positive and negative homo space charges were observed near the respective electrodes and their distribution profiles were similar to each other. No remarkable effects of electrode materials were observed. The space charge behavior in PI films may be explained by assuming surface states and carrier traps or surface region where carrier generation takes place.
Solid Dielectrics, 2007. ICSD '07. IEEE International Conference on; 08/2007
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ABSTRACT: Life-cycle management based on the degradation diagnosis is useful for preventing an unexpected failure and extending service life of electric power apparatuses, minimizing life-cycle cost. In our previous study, we formulated a numerical model of life-cycle management based on time-based maintenance and condition-based maintenance. Then, we applied the model for evaluating the economic effect of degradation diagnosis of power cables, assuming that the maximum length of water tree as a replacement criterion can be measured nondestructively. To carry out reliable life cycle management, however, accurate data on the relation between extent of degradation and failure probability or remaining life are necessary. In this study, we examine the influence of accuracy of the data used to determine the optimum diagnostic parameters and evaluate how the life-cycle cost is affected by the employment of inaccurate data. The results show that the condition-based maintenance with degradation diagnosis can be less subject to the accuracy of the back data and is possible to realize the reliable life-cycle management.
Solid Dielectrics, 2007. ICSD '07. IEEE International Conference on; 08/2007
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ABSTRACT: The transitions of partial discharge (PD) behavior with aging in a void under ac voltages with sine wave and square wave were observed. The transition of the PD magnitude with aging under square wave showed the same tendency as that under sine wave. Moreover, the experimental results also showed some distinctive characteristics of PDs under square voltage (e.g. more than two pulses after the voltage rise time and their transition with aging). These phenomena were explained in terms of the effect of discharge area and the change in surface condition due to PD degradation
IEEE Transactions on Dielectrics and Electrical Insulation 05/2007; · 1.09 Impact Factor
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ABSTRACT: Space charge formation in polyimide films, and therefore, charge injection from the electrodes largely depend upon the water content. In order to clarify the injection properties of positive and negative charge carriers, space charge distributions were observed in the polyimide films with different electrode conditions, such as aluminium was evaporated on a surface of the samples. Only when the rear electrode is a semi-conductive material, the evaporated aluminum electrode enhances charge injection. The positive and negative charge carriers had similar electrode effects on injection each other
Properties and applications of Dielectric Materials, 2006. 8th International Conference on; 07/2006
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ABSTRACT: In order to clarify the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we carried out simultaneous observation of partial discharge characteristics, electrical-tree inception and propagation and associated light emission on both samples with a wet water tree and a dried water tree. The ac electrical-tree inception voltage increased with increasing water-tree degradation in a sample with a wet water tree, which was attributed to field moderation. On the other hand, under impulse voltage application the increase in tree-inception voltage is much higher than ac-tree-inception voltage, suggesting increase in the intrinsic breakdown strength in and/or near water-tree region. On the basis of these results, effects of structural change in water tree region on electrical tree inception were discussed by investigating effects of drying of water tree and ac prestressing
Properties and applications of Dielectric Materials, 2006. 8th International Conference on; 07/2006
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ABSTRACT: The current form of partial discharge (PD) in a void was divided into fast and slow components, with short and long fall times, respectively. Comparing these with the current shape of PD in a void between two metal surfaces, in which the slow component was not observed, it was suggested that the formation of fast component in a usual void was due to the instantaneous discharge between the discharge areas of the PD, i.e. the region to which the carriers in a PD process can instantaneously propagate, and the slow one might be due to the low conductivity in the regions outside the discharge areas. An equivalent circuit was proposed to describe the effect and numerical simulations were carried out. It was suggested that the fall part of the first PD after polarity reversal might reflect the surface conductivity change in aging process.
IEEE Transactions on Dielectrics and Electrical Insulation 01/2006; · 1.09 Impact Factor
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ABSTRACT: In order to clarity the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we carried out simultaneous observation of partial discharge (PD) characteristics, electrical-tree inception and propagation and associated light emission. The AC electrical-tree inception voltage increased with increasing water-tree degradation, which was attributed to field moderation due to space charge formed in the water-tree region. To investigate the intrinsic tree inception process without the effect of field moderation, tree inception and propagation under impulse-voltage application and under dry condition was studied. The increase in tree-inception voltage under impulse voltage was much higher than AC-tree-inception voltage, suggesting increase in the intrinsic breakdown strength in and/or near water-tree region. The drying of water-tree region led to lower tree inception voltage and this is consistent with the above presumption.
Electrical Insulation and Dielectric Phenomena, 2005. CEIDP '05. 2005 Annual Report Conference on; 11/2005
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ABSTRACT: In order to clarify the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we have investigated effects of water-tree degradation condition and impulse-voltage application on electrical-tree propagation from water tree. As for the condition of water-tree degradation, we have investigated electrical-tree inception voltage by using NaCl solution with different concentrations. Regardless of concentration, the inception voltage becomes higher with the water-tree size. This is explained by electrical-field moderation. On the other hand, under impulse-voltage application where less field moderation is expected, the increase in tree inception voltage is much higher. This suggests increase in intrinsic breakdown strength in and/or near water-tree region.
Electrical Insulating Materials, 2005. (ISEIM 2005). Proceedings of 2005 International Symposium on; 07/2005
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ABSTRACT: Life-cycle management combined with degradation diagnosis is useful for preventing an unexpected failure, extending service life of an electric power apparatus and minimizing life-cycle cost. In our previous paper, we proposed a method of life-cycle-cost evaluation based on TBM, and evaluated the economic effect of degradation diagnosis. To carry out reliable life cycle management, however, accurate data on the relation between extent of degradation and failure probability or remaining life as well as a well-established diagnostic method are necessary. Therefore, we have examined the influence of accuracy of the data used to determine the optimum diagnostic parameters and have evaluated how the life cycle cost is affected by the employment of inaccurate data. As a result, we have confirmed that the optimized diagnosis interval is influenced by change in failure probability, and the life cycle cost increases greatly when the inaccurate measured data give longer diagnostic interval and looser diagnostic criterion as the optimum diagnostic parameters.
Electrical Insulating Materials, 2005. (ISEIM 2005). Proceedings of 2005 International Symposium on; 07/2005
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ABSTRACT: In order to clarify the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we carried out simultaneous observation of partial discharge (PD) characteristics, electrical-tree inception and propagation and associated phenomena. The ac electrical-tree inception voltage increased with increasing water-tree degradation, which was attributed to field moderation due to space charge formed in the water-tree region. The relations among ac tree propagation, gas formation and PD as well as water tree growth from electrical-tree tips were observed and discussed. Impulse voltage application, which usually does not lead to significant space charge formation, showed more increase in the tree inception voltage than ac voltage application, which suggests some other mechanisms to increase breakdown strength in and/or near the water tree-region. The effect of the ion concentration in the water was also studied.
Electrical Insulation and Dielectric Phenomena, 2004. CEIDP '04. 2004 Annual Report Conference on; 11/2004
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ABSTRACT: The effectiveness of diagnosis method is closely related to the correlation between the parameter used for diagnosis criterion (degradation parameter) and the degree of degradation. In this paper, as an example, two parameters, the maximum water-tree length and dielectric loss tan δ, for water-tree degradation in power cables were compared. Assuming that both the parameters could be measured non-destructively and the costs for diagnosis were the same, the minimum total life-cycle costs which corresponded to the optimized diagnostic parameters, e.g. diagnosis interval, criterion, etc., were calculated. As a result, the total cost for the case of diagnosis based on the maximum water-tree length was much lower than that for the case of tan δ. This suggested that the maximum water-tree length reflects the degradation degree much better than the dielectric loss (tan δ), and practically the non-destructive diagnosis method based on the measurement of parameters related to water-tree length, e.g. residual-charge measurement, is expected to be more efficient.
Solid Dielectrics, 2004. ICSD 2004. Proceedings of the 2004 IEEE International Conference on; 08/2004
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ABSTRACT: An earlier model for simulation of the phase-resolved pattern of partial discharge (PD) showed that the range of PD magnitudes could be produced by a variation of discharge area. Here we present a further development of that model. When the field in the void exceeds a certain value E<sub>c</sub>, PD occurs and extends along the void surface. In the PD propagation, the fields in the discharge paths and in the perimeter segments around the PD paths are reduced little by little by gradually changing the charge distribution. And the PD is terminated when the internal field in the PD path becomes lower than a critical value E<sub>in</sub> and the field in the perimeter segments around the PD paths becomes less than another critical value E<sub>p</sub>. The simulation results became much closer to the actual PD patterns. And the large fluctuation of PD magnitude can be simulated even without consideration of any stochastic mechanisms. This model provides an approach to explain the transition of PD pattern in the aging process in terms of the change of surface condition.
Electrical Insulation and Dielectric Phenomena, 2003. Annual Report. Conference on; 11/2003
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ABSTRACT: A new negative-resistance device which had a structure of anode/insulating layer/hole transport layer/cathode was fabricated by thermal CVD and vacuum deposition. Its current-voltage characteristics at room temperature showed a remarkable negative resistance. A current peak was observed around 5∼6V with increasing voltage. It decreased with increasing the width of an insulating layer. The peak voltage also increased with increasing the ionization potential of dye or the barrier of hole tunneling. The current peak decreased with the difference between Fermi level of the anode and ionization potential of insulating layer. These suggest that the negative resistance around 5∼6V may be explained by the tunneling phenomenon.
Properties and Applications of Dielectric Materials, 2003. Proceedings of the 7th International Conference on; 07/2003
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ABSTRACT: Very narrow artificial tubes have been used to investigate the behaviour of discharges in conditions similar to those encountered in electrical trees. In the present study a discharge sequence measured prior to the formation of an electrical tree at the tube tip has been analysed to see if there was any evidence for the existence of deterministic chaos. The results show that the voltages at which the discharges occur do follow a sequence governed by deterministic chaos. The discharge amplitude sequence, however, contains a lot of noise and this makes it unclear whether or not they have an underlying chaotic pattern. Some speculation as to the meaning of these results for the processes controlling the discharge sequence will be made.
Properties and Applications of Dielectric Materials, 2003. Proceedings of the 7th International Conference on; 07/2003
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ABSTRACT: In order to clarify the mechanism of electrical tree propagation from a water tree within polyethylene, we have carried out simultaneous observation of partial discharge (PD) characteristics, electrical-tree propagation and associated phenomena. The electrical tree inception voltage increases with the area of water tree degradation. An electrical tree is initiated from a water needle tip. When the electrical tree connects with water needle, PDs become large abruptly. There are two types of electrical-tree growth, in one of which only small PDs occur only at the electrical-tree tips. After removing the applied voltage, water fills the electrical-tree channels, and water trees propagate from the electrical-tree tips when the voltage is applied again.
Properties and Applications of Dielectric Materials, 2003. Proceedings of the 7th International Conference on; 07/2003
