O. Moreau

Flow electrification in power transformers: study of a potential remedy

Understanding the exact nature of the phenomena at the origin of the electrical charging tendency of transformer pressboards submitted to oil circulation in high power transformers is still a challenging problem. Former studies demonstrated that flow electrification depends in a large part on the chemical functional groups of the pressboard surface, and especially carboxyl groups. In this study, the action of several additives has been tested. Three experimental devices (conductivity cell, flow loop and electrostatic charging tendency tester) have been used to investigate the effect of the additives on the charge generation, accumulation and leakage currents. Moreover, experiments with demineralised pulps treated by carboxyl complexing agents such as methylene blue have confirmed the important role of ionisable groups in paper. Some additives were found to reduce the electrification phenomenon. A comparison between these results and some aspects of ionic chromatography could be done, and the related theory can be attempted to explain the chemical process which takes place between the oil and the pressboard

Electrical Double Layer's Development Analysis: Application to Flow Electrification in Power Transformers

A charge zone known as the electrical double layer exists at a solid-liquid interface. The liquid flow induces a phenomenon called flow electrification: it generates a streaming current (caused by charge convection) and a rise in the solids potential (if it is insulated from the ground). These potentials may reach values high enough to produce electrical discharges and cause accidents. Although this phenomenon was identified a long time ago, its physical description remains unknown (i.e., production and displacement of charges, equilibrium, etc.). We have modeled flow electrification phenomena occurring when transformer oil flows through a rectangular pressboard duct. The results of a parametric study made with this model are presented in this paper. The ducts geometry and the materials were selected to compare some of the numerical results to experimental ones. The facility used to obtain these experimental results was developed some years ago as a part of the research program of Electricite de France and the University of Poitiers. Another facility will be designed in the near future with the aim of reproducing surface electrical discharges. The results of this parametric study will be useful for its design.

3-D Stochastic Spectral Finite-Element Method in Static Electromagnetism Using Vector Potential Formulation

Stochastic spectral finite-element method can be used to take into account some random aspects in the input data (material characteristic, source terms) involved in static electromagnetism problems. Similarly to the deterministic case, two potential formulations can be used in the stochastic case. The vector potential formulation applied to static problems is developed and compared to the scalar potential one, previously developed.

Adsorption and surface properties of silica with transformer insulating oils

Abstract The presence of polar species in transformer insulating oil may cause degradation and electric discharges in the power transformer. Such oil polar fraction can originate either from the neat oil and/or from its oxidative degradation in the power transformer. The aim of this study is to examine the relation between the insulating oil and the electrical properties of its polar fraction in order to predict eventual failure in the power transformer. We investigate at ambient temperature the adsorption of the polar fractions of various transformers insulating oils (a new and two used oils) from the neat oils onto silica particles. The adsorbed amount was higher for the used oils as compared to the new one. Infrared spectra of the polar fraction indicate the presence of hydroxyl, aromatic and carboxyl functional groups that are found in the asphaltenes compounds. Microelectrophoresis study of the oil polar fraction covered silica particles gives negatively charged oil polar fraction. Such oil surface charge depends on the pH and results from the ionisation of the oil acidic surface groups. Finally, we obtain a good correlation between the amount of the oil polar fraction and the magnitude of the zeta potential at the water/oil–silica interface.

Adaptive Method for Non-Intrusive Spectral Projection—Application on a Stochastic Eddy Current NDT Problem

The Non-Intrusive Spectral Projection (NISP) method is widely used for uncertainty quantification in stochastic models. The determination of the expansion of the solution on the polynomial chaos requires the computation of multidimensional integrals. An automatic adaptive algorithm based on nested sparse grids has been developed to evaluate those integrals. The adapted algorithm takes into account the weight of each random variable with respect to the output of the model. To achieve that it constructs anisotropic sparse grid of the mean, leading to a reduction of the number of numerical simulations. Furthermore, the spectral form of the solution is explicitly identified from the constructed quadrature scheme. Numerical results obtained on an industrial application in NDT demonstrate the efficiency of the proposed method.

Physicochemical Analysis at the Interface Between Conductive Solid and Dielectric Liquid for Flow Electrification Phenomenon

At the solid-liquid interface, a charge zone called the electrical double layer (EDL) appears. It is constituted of two zones of opposite sign: one in the solid and another one in the liquid. When a liquid flows through a pipe, an axial streaming current is generated. This current is due to the convection of the charges coming from the EDL. The physicochemical reaction at the solid-liquid interface is one of the most important parameters which control the diffuse layer development inside the liquid and, consequently, the space charge density. In this paper, we present an analysis for the physicochemical reaction in the case of liquid containing additives or impurities partially dissociated into positive and negative ions. The addition of impurities, in our case, is associated with changing both the polarity and the value of the streaming current. Thus, from this difference in streaming current, we will be able to identify the reagent of the oil with the solid material. Furthermore, the effect of streaming potential on the EDL has been undertaken. Moreover, the fully developed space charge density at the wall is calculated with the help of streaming electrification experiments which are conducted in the case of nonfully developed EDL. This procedure can be conducted for the investigation of flow electrification in transformers with oil/metal configurations.

Flow electrification in power transformers. Explanation of the wall-current measurements

The aim of our work is to understand better the mechanisms of flow electrification due to oil circulating inside a rectangular pressboard channel. This research has been done because of technical incidents in power transformers in which oil is flowing past pressboard with a velocity in the range of several meters per second. A possible explanation of the wall current observed along the channel is proposed. >

Influence of pressboard ionizable groups on static electrification in power transformers

As a part of a research program in the field of flow electrification in power transformers, some fundamental studies have been engaged on the chemistry of this phenomenon. Previous works have shown that chemical composition of pressboards is highly involved in the kinetics and magnitude of the charge accumulation process. Present studies consist in focusing on the influence of the hydroxyl or carboxyl groups with regard to charge accumulation, measured from capacitive currents on insulated electrodes facing pressboard ducts. Special manufactured and modified pressboards have thus revealed that the phenomenon is highly related to carboxylic and hydroxyl groups content and also probably to the hydrophilicity of the oil/board.

Static electrification in power transformers: Correlation between charge generation and ECT-tan/spl delta/ parameters for different oil-pressboard couples

Up to now the ECT (Electrostatic Charging Tendency) measurement and, to a smaller extent, the continuous aging test for tan /spl delta/ are the most commonly applied measurements for transformer monitoring. So it is important that the reliability of these two tests involving only the oil should be established for different oil-pressboard couples. As part of the research program of Electricite de France in the field of static electrification in transformers, an experiment has been carried out, at the university of Poitiers, to study the most constrained parts in transformer as far as flow electrification is concerned: the insulated pressboards between windings. So, the generation process has been investigated with regard to time, temperature and velocity, for different oil samplings (used and new oils) flowing along a used or a new pressboard. In the meantime, ECT and tan /spl delta/ tests have been performed for each types of oil. It turns out that those two monitoring measurements and the charge generation in the pressboard are not always directly correlated but that diagnosis requires a deeper analysis taking into account transient kinetics aspects and pressboard state.

Current Calculation in Electrokinetics Using a Spectral Stochastic Finite Element Method

Electrical engineering often deals with global quantities as current, magnetic flux, difference of potentials, power or torque. Previous works have shown that spectral stochastic finite element method makes it possible to spread input data uncertainties (material characteristics) to the local output which are now random. The next step consists in deriving the random global quantities from the random distribution of the fields. It has been implemented in the extended stochastic version of a 3D FEM software. The approach has been successfully validated on a 3D electrical current density problem by comparing with Monte Carlo simulation method. Finally, this method has been applied to an industrial device.