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Power transformers have been expected to work with safety and reliability over a period of more than 30 years. The life expectancy of transformers depends upon the degree of deterioration of insulating materials, particularly conductor insulating paper. Many attempts have bee made in recent years to estimate the degree of deterioration of transformers but no diagnostic techniques have been put into practical use because of the difficulty of taking into account the service conditions. It is well known that materials change color when they deteriorate, but no colorimetry study has been made on material deteriorations. According to the recent advances in instrumentation and data processing equipment, the color of materials has become easy to measure qualitatively. Therefore, the basic investigation on colorimetry of insulating papers has been made to clarify the applicability to power transformers. Test results showed a strong correlation between the color difference and the deterioration of insulating paper. This report discusses the effect of materials of papers and impregnating oils on colorimetry. [ABSTRACT FROM AUTHOR]

Large power transformers using perfluorocarbon liquid have advantages of high dielectric strength, excellent cooling ability, and nonflammability. To make the transformer more compact, it is very important to improve the insulation material. Recently, the authors succeeded in developing a new type of aramid insulation paper which has no fluff on its surface to avoid field concentration. In this study, an outline of the developed paper and the experimental results of the insulation strength on turn-to-turn and intercoil models are described. It was found that the new aramid paper increases the insulation strength 1.2 (turn-to-case) to 1.7 (intercoil case) times that of the existing insulating paper. [ABSTRACT FROM AUTHOR]

Though it is said that the paperless age arrived with the spread of personal computers, the amount of office paper used shows no tendency to decrease. In general, used paper is recovered and recycled in order to preserve the environment. But labor and expense are required for the recovery of used paper and the recycled paper is lower in quality. If we could reuse used paper repeatedly without these disadvantages, for example, by using copying machines that could print again by removing the toner on used paper, it would be very convenient and ecological. We therefore studied the removal of toner from used paper by the laser ablation technique. As an optical source, an SHG-YAG laser was chosen. The energy density of irradiation with an SHG-YAG laser for the removal of toner from used paper was examined. At an energy density of approximately 12 mJ/mm2, which was the average value, the toner was removed sufficiently to allow reuse of the paper. The conditions of the laser irradiation, specifically the laser scanning speed and the number of irradiations, were studied. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 179(1): 8-14, 2012; Published online in Wiley Online Library (). DOI 10.1002/eej.21214 [ABSTRACT FROM AUTHOR]

This paper describes the dielectric breakdown characteristics of oil and oil-impregnated paper for very fast transient (VFT) voltages. Blumlein circuits generate VFT voltages of 60 and 300 ns in a pulse width that simulates disconnecting switching surges in gas-insulated switch gears. We measured the breakdown voltages of needle-to-plane, plane-to-plane oil gaps and several pieces of paper between plane electrodes for VFT and lightning impulse voltages. The measured data were formulated in V-t characteristics and Weibull probability distributions. The inclination n of V-t characteristics of insulating paper is 150, which is less than n = 13.7 of the plane-to-plane oil gap in the VFT time range. The shape parameters of Weibull distribution obtained in this study show that the scattering of breakdown voltages of paper is much less than that of oil. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 141(4): 16–24, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10043 [ABSTRACT FROM AUTHOR]

In a multilayer insulator made of two or more materials with different dielectric constants and conductivities, internal space change accumulates at their interfaces. If the local field is increased by the accumulated charge, the electric durability of the insulator should be decreased. In this paper, we describe the space charge accumulation measured when oil-impregnated PPLP (polypropylene laminated paper), which consists of three layers, kraft paper, polypropylene (PP), and kraft paper, was subjected to a strong dc electric field. The experimental results suggest that internal space charge accumulates at both surfaces of a PP layer and that the electric field in the kraft paper layers disappears. © 1998 Scripta Technica, Electr Eng Jpn, 124(1): 1–6, 1998 [ABSTRACT FROM AUTHOR]

More and more electronics are intended for use in adverse environments at high temperatures with high humidity. The spatial distributions of internal charge carriers, mainly due to ionic impurities that appear in hot and humid environments, are considered to affect the reliability of bulk insulation. Therefore, the authors examined space charge behavior inside paper/phenol-resin composites for printed circuit boards under DC voltages, focusing on the effect of the water absorption temperature. Both the sample weight and thickness were increased monotonically by immersion in water with an increase in the water temperature from 24 to 85 , indicating that the water absorption by the sample was temperature dependent. In the early periods of water absorption (up to 10 hours), the electric field decreased near the two electrodes and increased in the other regions. Furthermore, heterocharge formation was observed near the cathode as the water absorption progressed, becoming more significant at higher water temperatures. Ion chromatography analyses detected numerous ions such as Na+, $NH_{4}^{+}$ and Cl- from the water, in which the sample had been immersed for 100 hours at various temperatures. It is highly possible that these ions are responsible for the heterocharge formation. 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 173(2): 1–7, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20992 [ABSTRACT FROM AUTHOR]

A sudden capacity increase in demand during the summer peak, or in contingencies such as malfunctioning transformers, may cause an overload in normal transform-ers. In this paper, on the basis of overloaded transformer operation in distribution substations, thermal aging testing in oil was carried out under various overload patterns, such as short-term overload and long-term overload, but with the winding insulation paper's lifespan loss kept constant. From the results, various characteristics, such as the mean degree of polymerization and production of furfural and (C02+C0), and their effects on the lifespan loss of the insulation paper, were obtained. ©2012 Wiley Periodicals, Inc. Electr Eng Jpn, 182(2): 1-8, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). [ABSTRACT FROM AUTHOR]

Power capacitors are highly reliable due to their completely enclosed configuration. They are relatively trouble-free over the long term. However, their dielectric strength can be reduced due to some stresses, including overvoltage application or thermal degradation of the insulation material during long periods of operation; these events can result in dielectric breakdown of capacitors in extreme cases. Since almost all power capacitors with oil-impregnated paper have been operated for more than twenty years, some kind of failure should be expected. To prevent these incipient accidents, it is important to examine the insulation deterioration mechanism and evaluate the degree of deterioration before capacitor breakdown. To establish a diagnostic method of deterioration of the capacitors with oil-impregnated paper, we have conducted thermally and electrically accelerated deterioration tests and investigated the changes of their characteristics. Based on the experimental results we obtained, it can be concluded that the behavior of the oil-dissolved gases can be used as a useful index to diagnose the insulator deterioration of these capacitors. [ABSTRACT FROM AUTHOR]

We show that a logical error is contained in some epidemiologic papers which intend to verify the hypothesis that there is an association between magnetic fields of power lines and childhood leukemia. For instance, the Swedish paper by Feychting and Ahlbom in 1993 presented a leukemia association related to calculated magnetic fields in one-family houses other than apartments, from the results obtained from stratified analyses according to type of building; this was contradictory to their premise that exposure assessment was calculated from magnetic fields irrespective of type of building. However, the authors stated in conclusion that their hypothesis could be verified, although it needed to be supplemented with other data, as was customary in epidemiologic analysis, in order not to miss any possible causes of cancer. Our views are as follows: Engineers in the field would be unlikely to accept any procedure that needs supplementary data, because it is employed at the author(s)'s will for the intended verification of the hypothesis; it would be natural to suppose that the hypothesis could not be verified. © 2000 Scripta Technica, Electr Eng Jpn, 134(3): 53–59, 2001 [ABSTRACT FROM AUTHOR]

In June 2001, the International Agency for Research on Cancer (IARC) classified ELF magnetic fields as a possible human carcinogen based on the results of two recent pooled analyses by Ahlbom et al. and Greenland et al. on epidemiological papers for childhood leukemia. Examining their data, this author read information other than theirs: in the paper by Ahlbom et al., for instance, 9 cases were prepared for verifying the hypothesis that magnetic fields are associated with childhood leukemia, and this author considers that the general conclusion derived by inductive inference is that the hypothesis is not supported, because 2 cases of the 9 support the hypothesis and 7 cases do not, although the hypothesis has been accepted by many people for about 20 years; furthermore, the author does not consider that the IARC's evaluation that risk doubles in excess of 3 or 4 mG is based on the general conclusion, because it is derived from pooling two kinds of data, one of which is the 7 cases bringing the “general conclusion,” and the other is the remaining 2 cases being the “particular fact.” © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(4): 59–64, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10344 [ABSTRACT FROM AUTHOR]

The bushing of a transformer is one part of the transformer system, but is as vital as the transformer itself because it forms a part of the main circuit. Deterioration and abnormal condition of the bushing possibly cause its destruction, shortage of the main circuit to the Earth, and fire in the systems, so that diagnosis of the bushings is thus very important. At present, mainly visual inspection from outside of the bushing is carried out. Inspection of the inside condition of the bushing by dissolved gas analysis of the inside oil is done only as sampling for selected bushings because it is difficult due to the sealed construction of the bushings. Therefore, a simple diagnostic method for bushings has been expected for a long time. In Eastern Europe, the RVM (Return Voltage Measurement) method based on scanning the polarization spectra was applied to the oil–paper insulation systems inside the transformer as a diagnostic method. In this paper we investigate theoretically the principle of the RVM method applied to bushings with the same oil–paper insulation systems as the transformer by using the equivalent circuit, and the possibility of using this novel diagnostic method for condenser-type bushings is demonstrated by a model experiment. © 1999 Scripta Technica, Electr Eng Jpn, 130(1): 21–29, 2000 [ABSTRACT FROM AUTHOR]

Power capacitors are highly reliable equipment due to their completely enclosed configuration. They have usually worked with very little trouble for a long time. Their dielectric strength can, however, be reduced due to some stresses such as overvoltage application or thermal degradation of the insulation material during their long period of operation, and these events can result in the dielectric breakdown of the capacitors in extreme cases. Power capacitors with oil-impregnated paper/film are commonly used in substation. Therefore, it is very important to diagnose the deterioration and predict the remaining service life of such equipment, to assist in plant maintenance. To establish the diagnostic method of the deterioration of the capacitors with oil-impregnated paper/film, we have conducted thermally and electrically accelerated deterioration tests and investigated the changes of their characteristics. Based on the experimental results obtained, it can be concluded that the behavior of the oil-dissolved gases can be used as a useful index to diagnose the insulation deterioration of these capacitors. © 1997 Scripta Technica, Inc. Electr Eng Jpn 119(2): 1–9, 1997 [ABSTRACT FROM AUTHOR]

The Swedish (Karolinska Institute) epidemiology investigation, which has been carried out on cancers in children residing near high-voltage power lines in and outside Stockholm county during 1960 to 1985, makes the conclusion that there is a significant association between the historical magnetic fields calculated and childhood leukemia. Employing the results of the stratum-specific analyses, and assuming that the relative risk of childhood leukemia is significant whenever the lower limit of its 95% confidence interval is above unity, we learn that the association is seen for children diagnosed during 1975 to 1985 residing outside Stockholm county, while it is not seen for those diagnosed 1960–1974 residing in and outside Stockholm county. We thus conclude that the association is not a causal relation, because consistency of association was not always seen throughout the areas and periods investigated. Therefore, we have no alternative but to consider that the conclusion which suggests that the same association is to be observed in every area and during every period is not valid. © 1999 Scripta Technica, Electr Eng Jpn, 129(4): 39–44, 1999 [ABSTRACT FROM AUTHOR]

Much of oil-filled (OF) cable has been used for a long time as 66- to 500-kV extrahigh-voltage cable. Sometimes we can see extremely degraded tan δ oil (several tens of percent of tan δ, for example) in joint boxes, etc. The values of tan δ calculated by a simple combination model of paper/oil show that the oil-impregnated paper with such a high tan δ oil likewise has an extremely high tan δ, which must result in thermal breakdown. However, such an event has not taken place up to the present in actually operated transmission lines. This fact suggests that some mechanism for the suppression of tan δ has acted in the paper insulation impregnated with degraded tan δ oil. Therefore, we investigated in detail the tan δ characteristics of paper impregnated with extremely high tan δ oil. In addition, based on the investigation results, we developed a method for simulation of heat generation by dielectric loss in an OF cable joint (which has degraded tan δ oil). © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(1): 1-10, 2012; Published online in Wiley Online Library (). DOI 10.1002/eej.21184 [ABSTRACT FROM AUTHOR]

This paper proposes a method for detecting the angle of a bearingless motor with less magnetic interference due to the current. Bearingless motors have suspension windings and motor windings that generate leakage flux. Owing to this electromagnetic phenomenon, the signal of the linear Hall‐effect sensor for angle detection is disturbed, and it has to be avoided to place Hall sensors near the winding conventionally. To solve this interference problem, this paper clarifies the ideal combinations between sensor positions and the number of poles and slots of concentrated winding bearingless motors. [ABSTRACT FROM AUTHOR]

We propose an educational tool for foundation of electricity using conductive pens. Electrical leads for an experiment were drawn on the inkjet printing paper using silver ink marker. Electrical resistances were drawn by two kinds of pens; one was the same silver ink marker and the other was a carbon ink brush pen. A combined resistance was measured and compared with the calculated value. The relative error was less than ±1% in both series and parallel connection with the resistances drawn by the carbon ink brush. Therefore, this is useful to experience and learn the foundation of electricity. [ABSTRACT FROM AUTHOR]

The service restoration to sound sections within a short period as much as possible, performed after the permanent fault occurs in a distribution system, is significant for maintaining power supply reliability. This paper proposes an efficient method to find service restoration procedures based on fault searching and the optimal work scheduling of crews, considering the power supply priorities of loads and the probability of occurrence of faults. The proposed method is composed of two‐stage problems to reduce the outage sections efficiently: (i) the problem to find a series of searching procedures for unknown fault locations in the first stage and (ii) the optimization problem of work schedules of crews in the second stage. In the first stage, the order of operation of switches opened for searching for the fault direction is determined, constituting the searching tree. After that, the proposed optimization method can efficiently solve the work scheduling problem by formulating it as a mixed‐integer linear programming problem in the second stage. The computational experiments using a large‐scale distribution system model with many remote and manual switches show that the proposed method can provide efficient service restoration procedures within a reasonable computational time. [ABSTRACT FROM AUTHOR]

This paper addresses semiconductor piezoresistive materials selection in MEMS engineering design. From the practical engineering point of view, it is important to understand piezoresistance properties of semiconductors even if less accuracy under feasibility design phase. However, piezoresistance is frequently analyzed based on first principle electronic band structure simulations by sophisticate physicists. Practical engineers not familiar with this simulation cannot directly apply useful information derived from the result of simulation. This paper provides practical prediction method for piezoresistance based on electronic band parameters obtained from the state‐of‐the‐art solid‐state physics. It is demonstrated that the crucial parameters which control the p‐type shear piezoresistance coefficient π44 in diamond and zinc‐blend single crystals are the inverse of square of bond length in unit cell atom, the square root of valence light hole mass, and the shear elastic compliance coefficient S44. [ABSTRACT FROM AUTHOR]

Photovoltaics (PV) output prediction, which is indispensable for power system operation, can affects demand and supply adjustment adversely when large prediction error occurs. Thus, the reduction of large error as well as average error is required in PV power prediction. In 2019, the operation of the Meso‐scale Ensemble Prediction System (MEPS) of numerical weather prediction started from the JapanMeteorological Agency, and the amount of forecasting information would be potentially useful for the improvement of PV power prediction. However, very few studies on inputting multiple meteorological elements of the MEPS have been reported. In this paper, we newly develop the prediction model for an area day‐ahead PV power output composed of Just‐In‐Time Modeling (JIT Modeling) with multiple elements of theMEPS. The developed method achieves precise forecasts with low computational load by both selecting meteorological elements valid for improving prediction accuracy and adequately devising the structure of JIT Modeling. Some numerical examples demonstrating the effectiveness of the developed method are also presented. In particular, the proposed method reduces large error significantly. [ABSTRACT FROM AUTHOR]

A separable connector is used for the power cables and apparatuses in renewable energy systems. The shield of the collector cable is sometimes an open circuit at one end to reduce power loss due to the shield current. However, this has a disadvantage from the viewpoint of lightning protection. The semiconductive part of the separable connector is grounded for safety. The ground potential rise due to a direct lightning strike to a wind tower sometimes damages the collector cables and power apparatuses in the wind tower. Combustion in a cable is one of the most serious types of lightning‐caused damage. This paper describes an experimental study on fire ignition in a collector cable caused by a ground potential rise due to a direct lightning strike to a receptor. The mechanism leading to fire ignition is discussed based on the experimental results. This paper also proposes a countermeasure against such fire ignition. [ABSTRACT FROM AUTHOR]

Electrochemical machining (ECM) is a processing method for metals that uses a DC power supply and flowing electrolyte. In some cases, discharge occurs during ECM when the gap between the electrode and workpiece, whose distance is rather small, is filled with hydrogen generated via electrolysis. We attempt to utilize a DC inductor placed between the DC supply and the ECM load in order to limit di/dt of the short or discharge current, which damages both the electrode and workpiece. When the output current surges, that is, when discharge occurs, the output voltage is suppressed by the DC inductance. In this paper, we propose a method for designing a PWM rectifier to reduce the discharge current, establish a linear approximation model for use in the PWM rectifier, describe a control method of the PWM rectifier, and demonstrate the efficacy of the model via simulations and experiments. [ABSTRACT FROM AUTHOR]

In recent years, the aggregation business has gained a lot of attention in Japan. Aggregators will make contracts with customers with photovoltaic (PV) power systems and battery energy storage systems (BESSs) including electric vehicles (EVs) to participate in electricity markets. Aggregators might have to pay electricity supply‐demand imbalance charges when generated and consumed energies contracted at the day‐ahead supply‐demand market are different from those at the current day operation due to distribution network constraints such as voltage and power flow limitations. Therefore, the information on network constraints is very important for aggregators to determine their day‐ahead schedules. In this paper, we evaluated the relationship between aggregator supply‐demand schedules and the distribution system operation. It was assumed that the reverse power flow limitations due to network constraints are notified to the aggregators by distribution system operator (DSO). Two cases of the aggregators' schedules of PV systems and stationary BESSs were compared in the simulations. In addition, aggregator strategies for making adequate schedules were evaluated. [ABSTRACT FROM AUTHOR]

The paper proposes a method to select a power factor for an inverter of distributed power generation systems to keep appropriate voltage of a power system with the high penetration of inverter‐interfaced generation. The increase of inverter‐interfaced power sources, such as wind and photovoltaic power generation, arouses concerns on the voltage fluctuations in a power system. While constant power factor control of an inverter has been studied as a means to stabilize the voltage of a power system, there still remains room to improve the control. The proposed method in the paper selects the power factor based on the R/X ratio of the looking‐back impedance into the interconnected power system. By normalizing the related variables, a general formula is derived for the power factor, and the resultant relationship between the apparent power and voltage of an inverter becomes identical irrespective of the R/X ratio with the control. Sample studies show that, by operating an inverter with the power factor, voltage fluctuations are effectively suppressed even for a power system with the high penetration of inverter‐interfaced generation. [ABSTRACT FROM AUTHOR]

Single‐phase loads and photovoltaic generation cause three‐phase imbalance in distribution systems, and prospective growth of normal chargers of electric vehicles may even increase the imbalance. The analysis of such unbalanced systems requires the three‐phase power‐flow calculation. Since existing methods require the admittance matrices of three‐phase transformers, they must be derived for all possible winding connection patterns in advance to its computer‐code implementation. This paper proposes a three‐phase power‐flow calculation method which formulates circuit equations using the modified nodal analysis, making it possible to automatically handle any winding connection. The power‐flow constraints are then embedded into the circuit equations using a fixed‐point iteration. Newton‐Raphson, backward/forward sweep and fixed‐point iteration methods are the existing three categories of solution methods. Newton‐Raphson methods may show convergence problems due to the high R/X ratios of distribution lines. Backward/forward sweep methods cannot be used, because loops are temporarily formed in a distribution system during circuit switching. These justify the use of a fixed‐point iteration. In this paper, the proposed method is validated by practical examples. [ABSTRACT FROM AUTHOR]

Electric power generated by photovoltaic (PV) systems is generally unstable because of uncertain meteorological conditions. Stably maintaining the supply‐demand balance of power systems requires an accurate prediction of the performance of PV systems. Especially for predicting the performance of the PV systems containing modules with different electrical characteristics or installed in different angles, it is necessary to calculate the generated power of the system by combining the current‐voltage (I‐V) characteristics of each module in series and parallel. Therefore, an accurate method is required for estimating the I‐V characteristics in various meteorological conditions. In this paper, a simple modeling method of the I‐V characteristics for crystalline silicon PV modules is proposed. The I‐V characteristics can be simulated in a good accuracy for practical use based on a simplified equivalent circuit of PV modules and a temperature correction of the short‐circuit current, open‐circuit voltage, and the current and voltage at maximum power point. The validity of the proposed method is examined by comparing simulated and measured I‐V characteristics of a PV module. Furthermore, to demonstrate the effectiveness of the proposed modelling method, it was applied to simulating the power generation of a PV system containing modules installed in different angles. [ABSTRACT FROM AUTHOR]

The axial gap structure of a half‐wave rectified variable field flux motor (AG‐HVFM) has been proposed, which has a double stator structure and field winding short‐circuited by a diode. Although the AG‐HVFM can achieve a high‐efficiency drive in wide speed and torque range via variable field flux control, its structure deteriorates the average torque and torque ripple. In this paper, we sequentially design the core shape, aspect ratio, and current control method for an AG‐HVFM to improve its torque characteristics. [ABSTRACT FROM AUTHOR]

In recent years, wind power generation has been promoted as a countermeasure to global environmental problems. However, the introduction of many wind power generators into the power system may cause system frequency fluctuations. This paper proposes a control method to reduce system frequency fluctuations by using the rotor kinetic energy (RKE) of a variable speed wind turbine (VSWT). When the frequency falls, this method suppresses fluctuations by consuming RKE to increase VSWT power generation, and when it rises, it decreases VSWT power generation by accumulating RKE. The effectiveness of this method is confirmed through simulations using PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper proposes a high power density inverter utilizing SiC metal-oxidesemiconductor field-effect transistor (MOSFET) modules and an interstitial via hole (IVH) printed circuit board (PCB) for a motor drive system. The inverter also includes a power circuit, heatsink, cooling fan, gate drive circuit, and DC capacitors. The output power density of the proposed inverter is 81 kW/L. The inverter outputs 37 kW formotor drive applications. To achieve this superior output power density, this paper explains a prototype SiC MOSFETmodule without an antiparallel schottky barrier diode, and a unique multilayer laminate IVH PCB to connect the modules and DC capacitors. This paper describes the experimental results to verify the motor drive performance and the increase in temperature under the rated load operation. [ABSTRACT FROM AUTHOR]

Sodium-immersed electromagnetic pumps will be critical for commercialization of fast breeder reactors in the near future. In order to operate the pumps in molten sodium satisfactorily, it is indispensable to have electrical insulation that can withstand temperatures of above 500°C for a prolonged period of time. Several mica-alumina composite insulation types were aged under a high electric field in nitrogen at high temperatures. All composite insulations aged at 750°C experienced dielectric breakdown after around 1000 hours. However, it was possible to detect the onset of dielectric breakdown by monitoring the g current. Mica splittings insulation was found to be superior in dielectric performance to mica paper insulation. Thermal degradation contributed to the dielectric deterioration of these insulations. [ABSTRACT FROM AUTHOR]

We are developing a small and lightweight direct‐drive system toward the realization of in‐wheel electric vehicles (EVs). To increase the power density of the motor, we developed a direct‐cooling motor that immersed the core, coils, and magnets in cooling oil and improved cooling efficiency. Moreover, we performed measurements of frictional loss, measurement of pressure drop, and a continuous heat‐run test on the test bench. The results demonstrated that direct cooling is effective and continuous operation can be achieved. Additionally, it was found that the coil temperature can be controlled by the flow rate of the cooling oil. In this paper, we report on the concept of direct cooling and the measurement results. [ABSTRACT FROM AUTHOR]

Silicone gel is widely used to encapsulate power modules, and improvement of its dielectric strength has been required. So, the purpose of our research is to improve dielectric strength of silicone gel encapsulant, and we focus on crosslinking degree of silicone gel. Previous studies have shown that growth mechanism of electrical tree changes with crosslinking degree of silicone gel. This suggests the possibility that the presence of the interface by different crosslinking degrees inhibits the tree growth. In this paper, we have investigated the tree growth and breakdown characteristics in silicone gel—crosslinking degrees graded layer materials. The interfaces in our study are arrange as being vertical to the line of electric force. Consequently, it was clarified that barrier effect of interface by different crosslinking degrees and the relaxation of electric field in low crosslinking degree region retards on tree growth, which improves the dielectric strength. [ABSTRACT FROM AUTHOR]

The paper is proposed approximate formulas of the ac resistance of lead wires and coils which made by using copper clad aluminum (CCA) wires. There are the frequency range that the ac resistance of CCA wires is lower than it of cupper solid wires for lead wires and coils of an electrical equipment. It is shown that the application of CCA wires is economical as comparing with copper solid wires, if the design is due to successful calculation in the electrical equipment. The measured resistance is compared with the formula. CCA wires is an example of multiple conductors which shall be expected to apply in future. [ABSTRACT FROM AUTHOR]

A novel high step‐up DC‐DC converter suitable for the power interface of fuel cells (FC) is presented in this paper. The proposed converter features a multi‐resonant tank for achieving high‐step up ratio under zero current soft switching (ZCS) without any snubber circuits. The simple and minimized‐component topology is beneficial for megahertz driving of gallium nitride (GaN) power transistors with effective reductions of switching loss, electromagnetic emission interference (EMI) noise, and ripple‐free DC input current. Mode‐transitional power stages are described in detail, following that the high step‐up principle is revealed by means of frequency‐domain analysis. The essential performances of the proposed circuit topology are demonstrated by simulation and experiment of 1.8 MHz‐120W prototype whereby effectiveness of the snubberless and multi‐resonant topology are verified from the practical point of view. [ABSTRACT FROM AUTHOR]