Relationship between static electrification of transformer oils with turbidity and spectrophotometry measurements I. Fofana ⇑ , Y. Bouslimi, H. Hemmatjou, C. Volat, K. Tahiri Canada Research Chair on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME), Université du Québec à Chicoutimi, QC, Canada article info Article history: Received 17 April 2012 Received in revised form 28 June 2013 Accepted 30 June 2013 Keywords: Dissolved decay products Turbidity Conductivity Dielectric dissipation factor Spinning disk system Electrification current abstract The paper presents the research results on electrostatic charging tendency (ECT) of transformer oil in a spinning disk system. The measurements were performed at different aging severity. Changes in static electrification were related to some classical aging indexes (conductivity, dissipation factor, water content, resistivity, etc.). Fast, inexpensive and reliable laboratory testing procedures developed by ASTM (D 6802 and D 6181) were also used to monitor the decay products as trace impurities. The obtained results show that static electrification currents increase with temperature, oil flow velocity, coating disk material prop- erties and oil’s aging byproducts. The polarity, the amplitude and the time constant of the streaming elec- trification currents are also affected. This contribution is not only intended to provide a fresh review in this domain of research, but also contains a substantial amount of new material with a view of closing some gaps in the present state of knowledge of transformer oil streaming electrification phenomenon. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Transformers are key equipments, required throughout modern interconnected power systems. Their insulation system consists mostly of hydrocarbon oil and paper. The solid insulation materials commonly used as wrapping and spacers are cellulose papers and boards made with special care from wood pulps, have to be carefully processed under heat and vacuum to remove moisture and air, before they are impregnated with oil [1,2]. This procedure allowed reducing the water content in the paper down to 0.1%. In practice, the life of a transformer can be as long as 60 years with appropriate maintenance [3]. Practicing engineers currently use a number of modern diagnostic techniques to assess the insulation condition of aged transformers [e.g. 3–7]. To prevent these failures and to maintain transformers in good operating condition is a very important issue for utilities. In the last decades, static electrification has been recognized as one of the important factors threatening the safety of power trans- formers [8,9]. The measurement of electrostatic charging tendency in power transformers is as important now as about 15 years ago, when research on static electrification problems began. This phe- nomenon only occurs when there is relative motion between oil and the surface of the solid insulation when partnered. Such sur- face can be stationary, as are the pressboard, paper, metal and pipe-work of a transformer or in motion, as are the moving parts of a pump [10,11]. Static charges are produced at the interface solid/liquid flow of oil causing a portion of the electrical double layer formed by preferential absorption of negative ions by the cel- lulose surface [12,13]. Thus, paper has generally a negative charge and oil positive charge [14]. The static electrification processes are complex and depend on the properties of the oil/solid material interface constituting the electric charge double layer seat and its evolution in time. The characteristics of this latter depend in turn on the condition of oil (aging, moisture, etc.) and the intrinsic prop- erties and surface condition (geometry, roughness, porosity, etc.) of the cellulose paper as well as the external parameters such as the streaming velocity of oil and temperature. Oxidation by-products (peroxide gas, water soluble acids, low molecular weight acids, fatty acids, water, alcohols, metallic soap, aldehydes, ketones, lac- quers, and sludges of asphaltene) which change the chemical make up of the oil [15] are also contributing factors. Because oil is the most readily replaceable element in a trans- former, efforts have been made to correlate flow induced failure with oil properties. The influence of the type of solid insulation, the velocity of oil flow, temperature and aging condition of oil and paper (assessed by conductivity/resistivity, dissipation factor, water content, capacitance, turbidity and dissolved decay products content) on the static electrification phenomena is investigated. 2. Theoretical background on the electrification current in a spinning disk system When a liquid is in contact with a solid (charged or not), the complex liquid–solid system polarizes. The system can be 0142-0615/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijepes.2013.06.037 ⇑ Corresponding author. Tel.: +1 418 545 5011x2514; fax: +1 418 545 5012. E-mail address: ifofana@uqac.ca (I. Fofana). Electrical Power and Energy Systems 54 (2014) 38–44 Contents lists available at SciVerse ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes