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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 5 • Date Oct 2001

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Displaying Results 1 - 16 of 16
  • Three-dimensional PEA charge measurement system

    Page(s): 845 - 848
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (440 KB) |  | HTML iconHTML  

    The pulsed electroacoustic (PEA) method already has been used widely to measure the space charge distribution in the pulsed electric field direction of sheet specimens. We have developed a new PEA measurement system which can observe the space charge distribution in three dimensions. Since the PEA system is not damaged if the specimen breaks down, the new PEA system can measure the space charge distribution of dielectric materials safely and nondestructively within a short time View full abstract»

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  • Electrostatic properties of ultra-thin polyimide Langmuir-Blodgett films biased in a needle-plane electrode system

    Page(s): 832 - 837
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (456 KB) |  | HTML iconHTML  

    Surface potentials of polyimide (PI) Langmuir-Blodgett (LB) films on metal (Al, Ag and Au) electrodes, charged at various voltages, were examined under a needle-plane electrode system. It was found that PI LB films were negatively charged when the biasing voltage applied to the needle-electrode increased. The surface potentials saturated when the number of deposited layers was 20 to 30, and they were dependent on the nature of the metal electrodes. The temperature dependence of the surface potential also was examined, and it was concluded that the tendency to accept electrons increases as the temperature increases. These results suggest that the presence of interfacial electrostatic space charges in as-deposited PI LB films at the metal/film interface made a significant contribution to the creation of the additional electrostatic potential when the films were biased under a needle-plane electrode system View full abstract»

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  • Deep trapping centers in crosslinked polyethylene investigated by molecular modeling and luminescence techniques

    Page(s): 744 - 752
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (848 KB) |  | HTML iconHTML  

    Space charge in crosslinked polyethylene (XLPE) has been detected under both ac and dc fields. Its role in electrical aging and breakdown is recognized, but not deeply understood. It is thought that identification of the trapping centers in this material would help improve the modeling of conduction and electrical aging as well as making possible the design of crosslinked materials with improved properties. We have developed theoretical and experimental approaches to this problem which emphasize the role of chemical traps acting as deep trapping centers. Molecular modeling is used to estimate the trap depth for negative and positive charge carriers associated with the main by-products of crosslinking reactions (using dicumyl peroxide as a crosslinking agent) since their aromatic structure makes them candidates for deep traps. Calculations on acetophenone, n-methyl styrene and cumyl alcohol show that they indeed can act as deep traps. Because such deep traps can act as recombination centers, their involvement in charge trapping can be checked in specially designed luminescence experiments. In our experiments, charges of both polarities are generated at the surface of the material under study by using a non-reactive cold plasma in helium. The analysis of the decay kinetics and emission spectrum of the subsequent luminescence allows us to define unambiguously the time range in which charge recombination is the dominant excitation process of the luminescence. The emission spectra obtained within this time range provide the optical fingerprint of chromophores acting as deep traps in the material. The low-density polyethylene (LDPE) doped with crosslinking byproducts and XLPE (film and cable peeling) have been investigated. Their role in charge trapping is apparent in the luminescence experiments and in space charge distribution analysis. In thermally treated XLPE, it is shown that other species strongly bonded to the polymer chain are also able to trap electrical charges View full abstract»

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  • Pulsed power production of ozone in O2/N2 in a coaxial reactor without dielectric layer

    Page(s): 826 - 831
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (472 KB) |  | HTML iconHTML  

    Very short duration pulsed streamer discharges have been used to produce ozone in a gas mixture of nitrogen and oxygen at atmospheric pressure. The ratio of nitrogen to oxygen in the mixture was varied in the range from 2.5/0.5 to 0.5/2.5, while maintaining a total flow rate of 3 l/min. The production of ozone was found to be higher for a specific mixture ratio of N2/O2 than that in oxygen or in dry air. The production of ozone in O2 was higher than that in dry air. The production yield of ozone (g/kWh) increased with decreasing nitrogen in the O2/N2 mixture. It has been found that the peak of the streamer discharge current decreased with time after application of the pulsed power. This decrease in the current corresponded with the increase in the ozone production and is attributed to the loss of electrons in the discharge current due to their attachment to ozone to form negative ions View full abstract»

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  • A new method for space charge measurements in dielectric films for power capacitors

    Page(s): 753 - 759
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (616 KB) |  | HTML iconHTML  

    Space charge measurements form a recognized test for the characterization of insulating materials. The present paper is concerned with the determination of space charge in dielectric films for capacitors. We develop a new measuring technique inspired by thermal step method (TSM). Its originality is the use of an alternative thermal excitation so that the technique can be called alternative thermal wave method (ATWM). The equipment employed to generate alternative thermal excitations (sine or triangle) is described. The upper limit of the efficient frequency range is 10 Hz. Signal processing is made easier and more accurate because of the periodicity of the measured phenomena View full abstract»

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  • Evaluation of diffusion-driven material property profiles using three-wavelength interdigital sensor

    Page(s): 785 - 798
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1392 KB) |  | HTML iconHTML  

    The response of a three-wavelength interdigital dielectrometry sensor to the moisture diffusion process in oil-impregnated transformer pressboard has been simulated using an empirical relationship between the moisture concentration and the dielectric properties of the pressboard. A benchmark test of the moisture diffusion process has been developed with the purpose of comparing alternative parameter estimation algorithms used in ω-κ (frequency wavenumber) dielectrometry. The results of simulations highlight characteristic features of the multi-wavelength sensor response, such as the sensor response delay from the start of the transient moisture diffusion process as a function of sensor wavelength, the influence of moisture boundary conditions, and the relation between the signal magnitude and variations of dielectric properties. One of the parameter estimation algorithms, linear calibrated admittance-based estimation (LCABE), has been applied to both simulated and measured data. Adequate performance of the LCABE approach in the absence of strong discontinuities of dielectric properties in the electric field penetration region is demonstrated and contrasted to an electrically shielded region case, in which the signal response becomes nonlinear. The proposed approach offers significant potential for the measurement of diffusion processes in various dielectrics, especially for cases with highly irregular geometry or material structure. Measurement results from moisture diffusion process monitoring are included. Parameter estimation of measurement results with LCABE confirm its applicability to the monitoring of moisture dynamics in transformer pressboard View full abstract»

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  • The fractal analysis of water trees: an estimate of the fractal dimension

    Page(s): 838 - 844
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (960 KB) |  | HTML iconHTML  

    Water trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 μm), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (~3 μm). Each photograph was analyzed to compare regions with and without water trees View full abstract»

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  • On the transient potential in insulators

    Page(s): 760 - 770
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (928 KB) |  | HTML iconHTML  

    The methods currently used to study the bulk and surface transport properties of insulators usually consist of charging the sample by corona or electrons, and monitoring the natural decay of the surface potential or its buildup after a temporary short circuit (return potential). These measurements have become quite easy since the advent of reliable potential probes, but their interpretation still raises delicate problems concerning, among others, the sample conductivity and its field dependence, the sample polarization and the interfacial injection efficiency. A discussion of these contributions shows that a strict experimental protocol is required if significant results are to be obtained. An alternate technique proposed here uses a scanning electron microscope (SEM), but no potential probe. Electrons from the gun of the SEM are injected below the surface of a thin insulating sample, having its rear face grounded, then a beam of lower energy, acting as probe, is scattered by the trapped charge and forms on the screen a mirror-image of the gun. If enough charge is trapped, the field from the image charge carries the injected charge down across the sample depth. This causes the mirror to contract at a rate which is related to the mobility of the electrons in the sample. Therefore, the mobility is obtained with the resolution of the microscope. Preliminary results indicate that the mobility of electrons injected in LDPE ranges by at least two orders of magnitude, depending on the local field and the sample morphology View full abstract»

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  • Moisture solubility for differently conditioned transformer oils

    Page(s): 805 - 811
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (952 KB) |  | HTML iconHTML  

    It is important to monitor the moisture content of transformer oil in a transformer. One parameter of particular interest is the moisture solubility of transformer oil. It has been reported that transformer oils under different conditions have different solubility. Measurements of solubility for four different types of conditioned oil are presented in this paper: fresh Shell Diala AX oil, lab-aged Shell Diala A oil, Texas Utility used transformer oil, and Ramapo Substation used transformer oil. To avoid the difficulty of achieving full saturation, this paper proposes an alternative method of measuring the moisture solubility in transformer oil using a relative humidity sensor. It utilizes the linearity between the relative humidity of the oil and the moisture content of the oil, to measure the solubility indirectly. The measured results of fresh oil, lab-aged oil, and the Texas Utility oil are very close, and only Ramapo oil shows different sensor response characteristics and solubility View full abstract»

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  • Molecular mobility in amorphous and partially crystalline PEN after isothermal crystallization

    Page(s): 776 - 784
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (728 KB) |  | HTML iconHTML  

    Poly(ethylene-2,6-naphthalene dicarboxylate) (PEN), a new aromatic polyester, presents high-performance physical and chemical properties and may be considered as a worthy substitute for polyethylene terephthalate (PET). Dynamic dielectric measurements were performed under isochronal conditions for twelve different frequencies between 1 and 105 Hz. The aim was to follow the dielectric properties and their dependence on temperature, ranging from -100 to 200°C, with a heating rate of 2°C/min. Based on experimental considerations by using differential scanning calorimeter (DSC), a thermal cycle of crystallization was carried out. Different specimens were obtained in this way starting with as-received amorphous polymers. Crystallinity saturation, accompanied by a microstructure change, was observed with a second melting peak and a dual lamellar stack model was adopted. Amorphous and semi-crystalline PEN samples were compared. The thermal instability of PEN may be shown through dielectric relaxation before melting. A study was undertaken to understand the different dielectric relaxations present in PEN and the effect of thermal treatment on these relaxations. PEN mobility has been characterized by the presence of four relaxations: β, β*, α and ρ. When frequency increased, the β and α relaxations moved towards higher temperature, while the β* process disappeared gradually under the a maximum. The two secondary relaxations B and β* were found to obey Eyring's law while the primary one obeyed the empirical Vogel-Fulcher-Tamman (VFT) relation. Characteristic of the amorphous PEN relaxation is the presence of the ρ peak, at a temperature>Tg. In this temperature range, it was concluded that this peak, not observed in semicrystalline specimens, was due to cold crystallization of the amorphous structure. To assign the occurrence of this peak to the mechanism of detrapping of free charges in the material seems inappropriate View full abstract»

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  • Electroluminescence measurements to detect accumulated charge at the electrode-insulator interface

    Page(s): 771 - 775
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (424 KB) |  | HTML iconHTML  

    Electroluminescence (EL) spectrum analysis is proposed to detect charge accumulation in insulators. The objective is to find the nature of accumulated space charges at the electrode-insulator interface by using only EL measurements recorded under a moderate ac 50 Hz field. The differences between spectra obtained with virgin samples and treated samples are analyzed. Some experience obtained with 500 μm thick low-density polyethylene (LDPE) samples are described to illustrate the expected modification of the EL spectrum vs, interfacial space charges. Direct space charge measurements, using the laser induced pressure propagation (LIPP) method are used to verify our assumptions. Good agreement is found between EL analysis and experimental data of space charges View full abstract»

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  • Space charge injected via interfaces and tree initiation in polymers

    Page(s): 733 - 743
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1056 KB) |  | HTML iconHTML  

    Space charge is a recent hot topic in the field of electrical insulation, since it can be measured nondestructively as standard practice. It has been long suspected that space charge might be critically involved in local dielectric breakdown or tree initiation. It is widely accepted that space charge is formed under dc stress in dielectrics. The author demonstrated a hypothetical space charge-dominant model for tree initiation in 1979 in Japanese and in 1991 in English, and proposed that charge can be accumulated inside polymer dielectrics near the divergent high field region even under ac stress. The model consists of charge injection and extraction, charge trapping and space-charge forming, polymer scission due to high energy injected and extracted electrons, oxidation, electric field enhancement, and finally tree initiation. The author will explain his old hypothetical space charge dominant model for tree initiation, various relevant phenomena and data that could prove the validity of the model, try to interpret several recently discovered phenomena with his model, and re-evaluate it as compared with modern concepts for tree initiation View full abstract»

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  • Carrier mobility in ethylene-vinylacetate copolymer estimated by transient space charge

    Page(s): 849 - 853
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (416 KB) |  | HTML iconHTML  

    Carrier mobility in an ethylene vinyl acetate (EVA) copolymer film was measured by a new technique. The time-dependent charge distribution was monitored by the pulsed-electroacoustic method when a pulse voltage was superposed on a dc bias voltage. A positive charge packet was injected from the anode soon after the pulse application, and moved towards the cathode. It was suggested from the movement of the charge packet that the mobility of the positive carrier at 10 to 60 MV/m was in the order of 10-13 m2/Vs at room temperature. When the pulse voltage was very high, a charge packet appeared within the bulk, suggesting that carrier dissociation takes place View full abstract»

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  • Immunochemical measurement of polychlorinated biphenyl compounds in transformer oils

    Page(s): 799 - 804
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (624 KB) |  | HTML iconHTML  

    Polychlorinated biphenyl (PCB) contaminated insulating oils represent a serious environmental problem to the power-transmission industry and there is much ongoing work directed at decommissioning such oils. The correct identification of contaminated oils is a key aspect of any decommissioning program. However, current analytical approaches are costly, complex, slow or are non-specific. This paper describes a combined sample preparation and PCB-specific immunoassay method for the measurement of these compounds in transformer oils that overcomes these shortcomings. A key aspect of the work centered on manipulating the sample such that it was compatible with the immunoassay process. Sample pretreatment using octadecyl carbon (C18) sorbents or washing with KOH/ethanol-sulfuric acid proved suitable with PCB recoveries of ⩽100%. Aroclor (PCB mixture) contaminated transformer oils were assayed using both an in-house immunoassay and a commercially available test kit for PCB in soil and water. The in-house method proved reliable for the measurement of oils containing >35 μg mL-1 Aroclor, but overestimated Aroclor levels in oils containing <10 μg mL-1 analyte. The commercial kit tended to underestimate Aroclor concentrations yielding values between 23 and 63% of that expected. These differences were attributed to the different antibody preparations employed View full abstract»

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  • Nuclear qualification of PVC insulated cables

    Page(s): 818 - 825
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (752 KB) |  | HTML iconHTML  

    Cables insulated with eleven different PVC compounds representing fire-retardant, non-fire retardant, 60, 75 and 90°C temperature ratings were evaluated. The specimens were subjected to accelerated thermal, radiation, and sequential radiation and thermal exposure simulating 15, 20, 30 and 40 yr in-service environments in Ontario Power Generation Nuclear Plants. The radiation level was limited to 12 to 30 Mrad. Two sets of samples were evaluated. The first contained 15 cm insulation specimens in tubular form with the conductors removed. The material performance of the insulations was assessed using the 15 cm specimens during the various stages of aging exposures by conventional elongation measurement. The second set was made from 4.3 m single or twisted pair of cable samples (unjacketed) wound on mandrels. The cable samples were sequentially irradiated and thermally aged and subjected to an additional 5 Mrad accident radiation exposure, followed by a simulated loss of coolant accident (LOCA). Similarly thermal only aged specimens with 2 Mrad of background radiation representing powerhouse environment were subjected to a main steam line break (MSLB) steam test. During the steam tests, samples were energized and their insulation resistance (IR) values were continuously monitored using an on-line setup and periodically with a megohm meter at 500 Vdc. The results from the insulation samples were compared with the electrical performance of the cable samples wound on mandrels that had been subjected to the LOCA/MSLB steam test. The test results indicate that elongation values were not a good indicator of electrical performance of PVC insulated cables subjected to radiation and thermal environment. With remaining elongation values ranging from 50 to 150% absolute, four out of eleven compounds performed poorly electrically during the steam test. All thermal only aged specimens functioned satisfactorily. In addition, two additional compounds rated for 60°C applications were investigated and functioned unsatisfactorily despite high elongation values. It was found that specific additives were responsible for poor electrical behavior. During radiation exposure ionic salts were formed as a result of break down of PVC and of some plasticizers. Upon thermal aging or later in the steam test these ionic compounds migrated to the conductor surface. Then, under high temperature and pressure steam conditions, the salts were dissolved and created an electrical leakage path. Similarly, in thermal only aged specimens, breakdown products of specific plasticizers caused poor electrical behavior View full abstract»

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  • Surface profile effect on streamer propagation and breakdown in air

    Page(s): 812 - 817
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (512 KB) |  | HTML iconHTML  

    In a uniform field arrangement, under direct voltage, positive streamer propagation and breakdown are investigated along cylindrical insulators with different profiles, inserted perpendicularly between two parallel plane electrodes. The basic properties of streamer propagation and breakdown, namely the electric field required for a stable propagation together with the associated velocity and the breakdown field together with time to breakdown, are measured as influenced by the pulse voltage amplitude used for the streamer initiation and by the insulator profile. It is shown that a strong relation between streamer propagation and breakdown exists, because the insulator profile exerts a similar influence on the breakdown and propagation fields. The effect of a shed on an insulating surface, forming an `obstruction' to streamer progress, is to increase the stability for propagation and breakdown fields, and to reduce the propagation velocity at all applied fields compared with those in the case of a smooth insulator. Along the surface of a smooth insulator, a streamer system propagates with a `surface' and an `air' component; however, a shed on an insulating surface modifies this system, resulting in only one component reaching the cathode View full abstract»

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Aims & Scope

IEEE Transactions on Dielectrics and Electrical Insulation contains topics concerned with dielectric phenomena and measurements with development and characterization of gaseous, vacuum, liquid and solid electrical insulating materials and systems.

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Reuben Hackam