Polypropylene (PP) film is generally used as insulating material for many kinds of electrical and electronic equipments because of its excellent electrical insulation performance. We investigated the electrical conduction and the charge storage of the thin PP films by means of electron beam irradiation with the energy lower than 10 keV as one new attempt. As results, in the case of electron irradiation with higher energy than 2.5 keV, PP film charged up negatively. From the negative charge decay characteristics after electron beam irradiation within this energy region, the volume resistivity of the order of 10¹⁶-10¹⁷ Wm was obtained and decreased with increase of electric field in the bulk of the PP film. On the other hand, in the case of electron irradiation with lower than 2.5 keV, positive charging occurred. This threshold energy of about 2.5 keV is thought to be the energy at which secondary electron emission yield becomes one.

View full abstract

Nowadays, trials to use even higher frequencies are pursued to realize high-speed wireless communications. In order to respond to such a trend, dielectric properties of insulating polymers at high frequencies have to be clarified, although the importance of clarifying low-frequency properties is still very high, especially for use as high voltage insulators. In this regard, complex permittivity spectra from 10 mHz to 4.0 THz were obtained for various insulating polymers. The spectra obtained in the very wide frequency range are important, because they can be a valuable database and they provide various pieces of important information on dielectric characteristics.

View full abstract

Poly(cyclohexyl methacrylate)s (PCHMAs) were grafted on carbon nanotubes (CNTs) using surface-initiated polymerization technique. Electrical properties of the PCHMA-grafted CNTs (PCHMA-CNTs) were systematically measured under direct current (DC) and alternating current (AC), and compared with those of conventional nanocomposites prepared by blending PCHMA with CNT (PCHMA/CNT). At a comparable volume fraction of CNT, PCHMA-CNT is 15 orders of magnitude higher DC volume resistivity than PCHMA/CNT. This excellent electrical insulation of PCHMA-CNT is achieved by the perfect isolation of individual CNTs owing to the high molecular weight PCHMAs densely grafted on the CNTs. The highly insulated PCHMA-CNT showed the same AC frequency dependence of impedance as neat PCHMA. This means PCHMA-CNT is a dielectric. The dielectric performance of PCHMA-CNT was evaluated by comparing with that of a barium titanate-filled PCHMA composite.

View full abstract

Space charge behavior and conduction current in polyethylene under DC stress were investigated. One of the reasons for the different breakdown property in XLPE from that in LDPE may be based on the existence of cross-linking by-products in XLPE. Furthermore, a thermal history in cross-linking process for XLPE may also cause of the difference. It is generally accepted that the existence of the cross-linking by-products increase the conduction current in XLPE under DC stress. It is also said that an anneal treatment in air atmosphere may affect to the electrical properties under DC stress. Therefore, we investigated the effect of the cross-linking by-products and the anneal treatment on space charge behavior and conduction current in polyethylene under DC stress. In our research, it is thought that the increasing dissipation power in XLPE is the cause of the breakdown in it under DC stress. Therefore, to calculate the dissipation power in the bulk of test sample, we measured the space charge distribution and the external circuit current simultaneously. Based on the results, we discussed the reason of the difference of the space charge properties in XLPE and LDPE focusing on the cross-linking by-products and the oxidation of the test samples.

View full abstract

Complex permittivity spectra were obtained in a wide temperature and frequency range from 20 to 200°C and from 10^-2 to 10⁵ Hz for polydicyclopentadiene resin (PDCPD). For comparison, two epoxy resins, one with a similar glass transition temperature (T_g) to PDCPD and the other with a lower T_g, were used. As a result, PDCPD shows much smaller values of ε_r' and ε_r'' than the two epoxy resins in nearly the whole temperature and frequency range. The differences of ε_r' and ε_r'' between PDCPD and the epoxy resin with a similar T_g are also significant at low frequencies and at high temperatures. The increase in ε_r' is attributable to the accumulation of hetero space charges in front of the electrodes. On the other hand, the increase in ε_r'' is an indication of the increase in Joule heat caused by the enhanced charge transport. Therefore, the experiental results indicate that PDCPD possesses better electrical insulating properties than epoxy resin. Furthermore, analysis using complex electric modulus, which is the inverse of the complex permittivity, has revealed very clearly that both charge transport and dipolar orientation are much more difficult in PDCPD than the two epoxy resins.

View full abstract

In order to investigate space charge behavior and charge transport characteristics of epoxy resin, complex dielectric permittivity and conduction current were measured at various temperatures from 20 to 200°C. Space charge distributions polarized at these temperatures were measured at room temperature. At temperatures above 100°C, significant accumulation of heteropolar space charges, most likely due to ions, is seen around the cathode/sample interface. Furthermore, both the real and imaginary parts of complex permittivity increase to remarkably high values at these high temperatures. It seems that such increase in permittivity is ascribable to the electrode polarization due to the hetero-charge accumulation. These results indicate that the dielectric behavior in epoxy resin is mainly governed by the electronic charge injection at relatively low temperatures below 100°C and by the transport of ionic carriers at high temperatures above 100°C.

View full abstract

Space charge distribution in a thin insulating layer of an enameled wire for motor windings was carried out using an improved PEA (pulsed electro-acoustic) measurement system under a dc and a square shape voltage. When a high voltage is applied to the windings of motor, a PD (partial discharge) sometimes occurs. Since the PD gives a physical damage to the insulating layer, many researchers have investigated the mechanism of PD generation. From their results, some researchers have pointed out the effect of space charge accumulation. Therefore, measurements of the space charge distribution in the insulating layer of the winding are required. The space charge accumulation in the insulating material is usually carried out using the PEA method. In this method, we observed the space charge distribution in film shape samples. However, the properties of specially-made film samples are different from those of the insulating layer with the actual configuration of the winding wire, and it is difficult to evaluate the actual influence of the space charge to the motor windings. In this research, we have improved the PEA system which is enable to measure the space charge distribution in the actual insulating layer of the winding wire.

View full abstract

Recently, it has been pointed out that the erosion of stress grading systems of converter-fed rotating machines may occur by partial discharges and heat generation. It is necessary to measure not only temperature distributions but also potential distributions to grasp the erosion process and to design reliable insulation systems even under inverter surges. Therefore, in this study, the potential distributions under repetitive pulses were measured by a Pockels sensor which covers wide frequency up to GHz. Consequently, it was observed that the field concentrates on the Corona Armor Tape (CAT) closer to the stator core under shorter rising impulse voltage. In addition to this, it has been confirmed that the field concentrates on the CAT during the wave-front of impulses and on Stress Grading Tape (SGT) in the wave-tail of impulses.

View full abstract

As technology of inverter-fed motors progresses in recent years, means of protection against inverter surge for the motor windings have been developed. It is strongly required for windings used in electric vehicles and hybrid electric vehicles to have excellent insulations properties together with high heat resistance, especially high partial discharge inception voltage (PDIV). This paper deals with PDIV of ribbon-type enameled wire which is fabricated to increase space factor in the motor windings compared with the conventional enamel wire.

View full abstract

In a bulk of cross-linked polyethylene (XLPE), there usually remain some chemical residues of cross-linking by-products such as cumylalcohol, acetophenone and α-methylstyrene. It is said that such cross-linking by-products are cause of the space charge accumulation under dc stress. Since the space charge accumulation affects the breakdown strength of the insulating materials, we have been investigating the effect of cross-linking by-products on space charge accumulation under high dc stress using pulsed electro-acoustic (PEA) method. In this report, we tried to observe the dependence of sample thickness on the space charge accumulation in XLPE including cumylalcohol under very high dc stress. It is found that the space charge accumulation in the thicker sample induces the breakdown rather than that dose not occur in the thinner sample even under the same stress.

View full abstract

The composite of epoxy resin and microvaristors shows the non-linear voltage-current characteristics because it has the chains of microvaristors which are formed by applying an electric field and work as current paths. These chains should be formed homogeneously to obtain the stable characteristics and their formation is strongly influenced by gravity. This paper reports the experimental results on the influence of diameter of microvaristors on the electric field to form and sustain the chains under the influence of gravity.

View full abstract

A numerical simulation has been done to evaluate the performance of the ionospheric tomography using the residual minimization training neural network (RMTNN) method. The results indicated that reconstruction with high-precision is possible when the standard deviation of the noise is about 2.5% or less of the average value of observed data (Slant TEC: STEC). Moreover, in the daytime when the value of STEC becomes large, the signal to noise ratio (SNR) increases and reconstruction accuracy becomes high; at night when the SNR falls conversely, it becomes low. Results of detectability tests show that the RMTNN method has a good performance around F-layer height with shape and peak intensity reconstruction. In conclusion, the developed RMTNN ionospheric tomography is effective in reconstructing 3D electron density distribution from realistic STEC data in the daytime, and is able to estimate images around F-layer.

View full abstract

The investigations into the degradation phenomena and the electric breakdown mechanisms in electrical insulating materials are very important to improve the long term reliability of the power apparatuses. Silicone rubber (SiR) has various excellent characteristics, and is widely used in various apparatus as insulating material. For example, SiR is used in cable joint of power transmission system. However, tree initiation process and degradation phenomena in SiR are not clear. It is commonly accepted that water in SiR influences its electrical insulating characteristic.  In this paper, we reported the influence of H₂O gas molecules and liquid water impregnation on electrical tree initiation in SiR. The followings are the main observations. 1. Removal of gaseous H₂O molecules from SiR by vacuum heating treatment increases tree initiation voltage of SiR. We consider there is a possibility that gaseous H₂O molecule forms the hydrogen bond with oxygen of the Si-O bond, and weakens the Si-O bond. This idea was qualitatively supported by a simulation where electric charge distribution and bond distance was calculated by “GAMESS”. 2. Liquid H₂O absorption by liquid water impregnating treatment increases tree initiation voltage of SiR. We consider that liquid H₂O removes the acceleration space for electron, and also relaxes the high electric field.

View full abstract

Spinel ferrite nano-particles, promising electrode materials for redox capacitors, were prepared with the chemical coprecipitation technique. In this work, the effects of x value on electrochemical properties of Zn_xFe_3-xO₄, Ni_xFe_3-xO₄, Cr_xFe_3-xO₄, (V_1/3Zn_2/3)_x Fe_3-xO₄ and V_xFe_3-xO₄ spinel ferrite electrodes were investigated. Additionally, the effects of occupied site and valence of Fe ion in spinel structure on capacitance were discussed. The results show that capacitance of the spinel ferrites increased with an increase in Fe²⁺ ion in 16d site of spinel structure.

View full abstract

[in Japanese]

View full abstract