T. Paillat

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

Electrokinetic phenomena in porous media applied to soil decontamination

The feasibility of using electrokinetic phenomena for contaminant removal from low-permeability porous media is investigated by laboratory experiments and theoretical modeling. The main processes by which contaminant transport takes place under a dc electric field in soil are electromigration, electro-osmosis and electrophoresis. They are particularly effective in fine-grained soils, which is not the case for pressure-driven flushing. Meanwhile, although the results of various studies on a laboratory scale suggest that electrokinetics is a promising technology for water soluble contaminant removal, further research is needed at both laboratory and field scales in order to determine the effectiveness of this technology in situ and in the case of non-soluble organic matter such as heavy hydrocarbons. First, this paper describes the electrochemical processes occurring in soil under an electric field. Then the electro-osmosis phenomenon is more particularly investigated in a theoretical and experimental way. Furthermore, the main previous work on the subject is summarized, and experiments conducted in the laboratory on decontamination by electric field of clay samples polluted by motor oil, are presented.

Space charge density in dielectric and conductive liquids flowing through a glass pipe

Abstract Flow electrification phenomena in the case of conductive and dielectric liquids flowing through an insulating pipe are compared. A theoretical model, which takes into account the development time of the diffuse layer in the case of a laminar flow, is compared with streaming current measurements. Experiments are conducted with water (electrical conductivity 0.2 mS/m ) and heptane ( 53 pS/m) flowing through a 32 μ m-radius glass pipe. Evolution of the space charge density at the wall ρw in function of the liquid conductivity σ is analysed. Results show that the intensity of flow electrification depends mainly on the space charge density at the wall and the liquid electrical conductivity.

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.

Flow electrification in high power transformers: BTA effect on pressboard degraded by electrical discharges

The flow electrification process occurring at the oil-pressboard interface in high power transformers seems to cause electrical discharge incidents and may cause failures. The goal of this experimental study is to measure and to compare the streaming current obtained from unused pressboard, pressboard artificially degraded by electrical discharges, and pressboard obtained from a damaged power transformer. The influence of additive 1,2,3 Benzotriazole (BTA) in oil is also analyzed. The results show that flow electrification is greatly increased by the degraded pressboards compared with the new one. On the other hand, adding of BTA significantly reduces flow electrification.

Flow electrification process: the physicochemical corroding model revisited

Even if the phenomenon of flow electrification has been observed for several decades, the physicochemical process appearing at the solid/liquid interface creating the double layer is not yet totally understood. In particular, returns from experiments (Cabaleiro, 2008) made with oil and pressboard seems to show that the wall current density at the interface for a diffuse layer under development is not only a function of the chemical behaviour of the interface but also of the flow wall shearing stress. The present work concerns analyze of experiments made with heptane flows through a stainless steel capillary of varying length. In that case also, even if the wall material is conductive and not porous, divergences appear with the classical physicochemical model predictions, while a model taking into account the effect of the wall shearing stress on the interfacial process seems to have a much better agreements with the experiments, especially, for high laminar Reynolds numbers.

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.

Influence of pressboard physico-chemical composition on static electrification in power transformers

As part of a research program in the field of flow electrification in power transformers, some fundamental studies have been engaged on the physical chemistry of this phenomenon. The two main constituents involved in the process of charge generation are the oil and the pressboard. As oil composition is difficult to deal with, the choice has been made to understand the overall chemistry through the role played by the pressboard whose composition is better mastered. In order to identify the chemical species in the pressboard responsible for the charge generation, in view of perfecting additives to neutralize the phenomenon, the EFPG laboratory carried out modifications on commercial pressboards, and manufactured some others. Charge accumulation, measured from capacitive currents on insulated electrodes facing the pressboard ducts, has been observed for modified and manufactured pressboards with regard to oil flow rate and temperature. All the experiments have led to various behaviors in terms of kinetics and magnitude of the charge accumulation. The obtained results tend to prove that chemical composition of pressboards, with regard to their content in chemical functional groups (hydroxyl or carboxyl groups) or radical nature (lignin), as well as board manufacturing parameters (pulp refining, layer composition), or surface properties seem to be at the origin of the electrification phenomenon.

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.

Flow electrification process: The physicochemical corroding model revisited

Though the phenomenon of flow electrification has been observed for several decades, the physicochemical process appearing at the solid/liquid interface creating the double layer is not yet totally understood. In particular, returns from experiments made with oil and pressboard seems to show that the wall current density at the interface for a diffuse layer under development is not only a function of the chemical behaviour of the interface but also of the flow wall shearing stress. The present work concerns analysis of experiments made with heptane flows through a stainless steel capillary of varying length. In that case also, even if the wall material is conductive and not porous, divergences appear with the classical physicochemical model predictions, while a model taking into account the effect of the wall shearing stress on the interfacial process seems to have a much better agreement with the experiments, especially, for high laminar Reynolds numbers.