A.L. de Oliveira

Practical approaches for AC system harmonic impedance measurements

AC supply system harmonic impedance measurements are discussed. On the basis of a given and known switchable shunt impedance and/or nonlinear load and site measurements, three methods are presented. The first approach assumes a no-load distorted busbar and uses basic circuit theory to calculate the required harmonic impedance. The second method disregards any previous busbar distortion and the harmonic injection but it takes into account any busbar distortion prior to the nonlinear load connection. Using a laboratory model and by comparing harmonic impedance results it is shown that both no-load busbar distortions and the correct measurement of harmonic phase angles may strongly affect the accuracy of calculation. >

181Ta gamma-gamma angular correlation study of oxygen self-diffusion in heavily defective oxides

Abstract The results of gamma-gamma angular correlation experiments performed on the 181 Ta nucleus in calcia-and yttria-stabilized zirconias and in calcia-doped cerias between 20–1150°C are reported. At high temperatures, the angular correlation spectra indicate a relaxation of the 181 Ta spins induced by the diffusive motions of oxygen vacancies. The slow and fast relaxation regimes are successively observed when the temperature is raised. The behaviour of the relaxation constant is fairly well reproduced by assuming the existence of fluctuations in the potential energy barrier which controls the migration of oxygen vacancies. For stabilized zirconias the mean activation energies deduced from angular correlation experiments agree with the conductivity data. The disagreement observed in doped cerias is attributed to impurity effects produced by the 181 Ta probe itself. The local investigation of oxygen self-diffusion give evidence for the homogeneous behaviour of these highly defective oxides at a microscopic level. The 181 Ta relaxation data, especially the comparison of the respective data relative to aged and deaged calcia-stabilized zirconia, strongly suggest the existence of short-range ordering phenomena in these solid solutions.

Electric and magnetic properties of polymer electrolyte/carbon black composites

Abstract Measurements of 1H Nuclear Magnetic Resonance (NMR) relaxation times, Electron Paramagnetic Resonance (EPR) and AC Impedance Spectroscopy (IS) are reported for composites based on PEO8:LiClO4 and carbon black (CB), prepared by two methods: solvent and fusion processing. Three nuclear relaxation processes were identified for 1H nuclei: (i) belonging to the polymer chains in the amorphous phase, loosely bound to the CB particles, whose dynamics is almost the same as for unfilled polymer, (ii) belonging to the polymer chains which are tightly attached to the CB particles, and (iii) belonging to the crystalline phase in the loose polymer chain. The paramagnetic electronic susceptibility of the composite samples, measured by EPR, was interpreted by assuming a contribution of localized spin states that follow a Curie law, and a Pauli-like contribution of delocalized spins. A significant change of the EPR linewidth was observed at 40 K, which is the temperature where the Curie and Pauli susceptibilities equally contribute to the paramagnetic electronic susceptibility. The electrical properties are very sensitive to the preparation methods of the composites, which conditions the interaction between carbon particle–carbon particle and carbon particle–polymer chain. Classical statistic models to describe the conductivity in these media were not satisfactory.

181Ta perturbed angular correlation study of hydrogen duffusion in hafnium hydride

Abstract The quadrupole coupling parameters of the 181Ta probe measured by Perturbed Angular Correlation spectroscopy in the f.c.c. phase of hafnium hydride (HfH1.64). Spin relaxation phenomena associated with the hydrogen diffusion are observed above 100°C. The activation energy deduced from the temperature dependence of the quadrupole relaxation constant is in agreement with the values previously observed by 1H NMR spectroscopy. No significant perturbation of the hydrogen diffusion due to the presence of Ta substitutional impurity is detected.

On the study of losses in cables and transformers in nonsinusoidal conditions

This paper presents a study on losses in cables and transformers considering nonsinusoidal conditions in commercial facilities. A mathematical development based on classical definitions available in the literature supports the proposal of a factor that is suitable to indicate where losses are significant and must be compensated. A case study is also presented and meticulously analyzed in order to validate the theoretical assumptions.

Complex impedance study of KDP protonic conductivity

Abstract The protonic conductivity in KDP (KH2PO4) pellets prepared by fusion are studied in air by complex impedance spectroscopy in the temperature range 404–471 K. Impedance spectra are plotted over a wide range of frequencies (10−2 – 106 Hz) at each sample temperature. The KDP protonic conductivity is found to obey an Arrhenius law with Ea = 0.99 eV and σ o = 6.89 × 10 5 Ω −1 cm −1 . The low frequency part of the impedance spectra tends towards a perfect 45° line typical of an interfacial diffusion process which was interpretated as being due to the migration of H species in and out the silver electrodes.

Specific conductivity of lithium perclorate dissolved in poly (ethylene glycol-400) distereate

Abstract We have measured, at different concentrations, the conductivity of lithium perclorate dissolved in poly (ethylene glycol-400) distereate. Heating-cooling cycles from 40°C to 100°C have shown reversible and reproducible data. In liquid phase the concentration-dependence of the conductivity passes through a sharp peak (1.42 × 10−4 s/cm at n=10 and T=50°C). A fitting model to this behaviour is suggested. The temperature dependence both of the conductivity and of the viscosity is shown and discussed.

Etude par correlation angulaire γ-γ perturbee des aspects statiques et dynamiques de la structure du heptafluorohafniate de sodium

Abstract We have studied the temperature dependence of the perturbation of the differential γ-γ angular correlation of the 181 Ta 133–482 keV cascade in sodium heptafluorohafnate. In the 20–300°C temperature region, we observed a drastic variation of the quadrupole interaction parameters when raising the temperature from 100 to 180°C. Such a variation is due to the existence of a phase transition corresponding to an orthorhombic-quadratic transformation of the lattice. In the low temperature phase ( T T > 180°C) the perturbation arises from the combination of static and dynamic interactions. The dynamic part of the interaction could be analysed within the isotropic perturbation model and was ascribed to jumps of the fluorine atoms onto the vertices of the cubic coordination polyhedra of the hafnium in the HfF 7 3− ion. The activation energy of the process is 0.13 ±0.03 eV. The idea of an order-disorder phenomena occuring in the course of the phase transition is consistently supported by these observations. Our results yield also valuable informations about the nature of the hafnium-fluor bond in this compound.

Expert system for power quality improvement

The use of capacitors for correcting the power factor is a very well known approach in industry. However, the increase of harmonic sources in power systems is heightening the risk of resonance and, consequently, a drastic decrease in electric power quality. In order to keep the rating voltage, the power factor, and the voltage and current harmonic distortions within acceptable limits, an expert system approach was implemented. Based on this approach, a software program that is able to decide about the optimal distribution of the capacitor banks has also been developed. The program decision is based on the rating voltage, the power factor, end the whole voltage and current harmonic distortions. When necessary, the computer program decides on the use of filters for harmonic compensation, indicating the best filter and the corresponding tuning frequency. Furthermore, the approach is able to deal with some aspects concerning the electric losses, both on the rating frequency and on the harmonic components.

Disorder model for specific conductivity of lithium perchlorate dissolved in poly(ethylene glycol-400) distearate☆

Abstract Impedance Spectroscopy (IS) and Differential Scanning Calorimetry (DSC) measurements, which we have performed at different concentrations and temperatures, support a peculiar order-disorder transition model for lithium perchlorate dissolved in Poly(ethylene glycol-400) distearate systems ( LiClO 4 PEGD ). Our model has the advantage of encompassing both critical and non-critical features displayed by the specific conductivity dependence of LiClO 4 PEGD systems on concentration and temperature, respectively.