Evaluation of transformer insulating oil quality using NIR, fluorescence, and NMR spectroscopic data fusion Mariana S. Godinho a , Marcos R. Blanco b , Francisco F. Gambarra Neto a , Luciano M. Lião a , Marcelo M. Sena c , Romà Tauler d , Anselmo E. de Oliveira a,n a Universidade Federal de Goiás, PO Box 131, 74001-970 Goiânia, GO, Brazil b DO-DPEM, CELG D, 74805-180 Goiânia, GO, Brazil c Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil d Department of Environmental Chemistry, IDAEA-CSIC, Barcelona, Spain article info Article history: Received 9 December 2013 Received in revised form 14 May 2014 Accepted 16 May 2014 Available online 27 May 2014 Keywords: Data fusion Power transformer Mineral insulating oil NIR Spectrofluorimetry NMR PLS VIP abstract Power transformers are essential components in electrical energy distribution. One of their most important parts is the insulation system, consisting of Kraft paper immersed in insulating oil. Interfacial tension and color are major parameters used for assessing oil quality and the system's degradation. This work proposes the use of near infrared (NIR), molecular fluorescence, and 1 H nuclear magnetic resonance (NMR) spectroscopy methods combined with chemometric multivariate calibration methods (Partial Least Squares – PLS) to predict interfacial tension and color in insulating mineral oil samples. Interfacial tension and color were also determined using tensiometry and colorimetry as standard reference methods, respectively. The best PLS model was obtained when NIR, fluorescence, and NMR data were combined (data fusion), demonstrating synergy among them. An optimal PLS model was calculated using the selected group of variables according to their importance on PLS projections (VIP). The root mean square errors of prediction (RMSEP) values of 2.9 mN m 1 and 0.3 were estimated for interfacial tension and color, respectively. Mean relative standard deviations of 1.5% for interfacial tension and 6% for color were registered, meeting quality control requirements set by electrical energy companies. The methods proposed in this work are rapid and simple, showing great advantages over traditional approaches, which are slow and environmentally unfriendly due to chemical waste generation. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Electricity transmission and distribution require the use of high-voltage power transformers. These devices have an insula- tion system which consists of Kraft paper immersed in insulating mineral oil [1]. Kraft paper is composed of cellulose, hemicellu- loses, and lignin [2]. The gradual depolymerization of Kraft paper inside a power transformer releases its degradation products into the insulating oil. Cellulose degradation involves breaking glyco- sidic bonds that hold glucose rings together [3]. During the decomposition reaction of cellulose chains, water and furanic compounds such as 2-furaldehyde, 5-hydroxymethyl-2-furalde- hyde, 5-methyl-2-furaldehyde, and furfurylalcohol are generated [4]. These compounds change the oil's physicochemical properties, such as color and interfacial tension. The latter, which measures the interfacial force required to separate insulating oil and water, is one of the most important parameters used for evaluating the degradation of the insulation system. It corresponds to an indirect measurement of polar substances, such as furanic compounds and water, so the more degraded the insulating system, the lower the interfacial tension [5,6]. Color is important when assessed together with other parameters. It is determined by a colorimeter and represented by a number (between 0.5 and 8, measured at 0.5 increments), which is compared to ASTM D1500-12 color standards [7] to assess whether or not the oil is degraded. Color increases along with the insulation system's degradation and has a reasonable correlation with interfacial tension. Different analytical techniques are used to quantify gaseous and furanic compounds, as well as physicochemical analyses. Dissolved gas analysis (DGA), degree of polymerization (DP) [8,9], and HPLC analysis of furans [10] have been commonly used to assess the degradation of paper oil's insulation system. All these techniques pose some drawbacks, e.g. destructive, time-consum- ing, and relatively costly analysis, sample pretreatment demand, solvent consumption, and waste generation. On the other hand, spectroscopic techniques such as molecular fluorescence and Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/talanta Talanta http://dx.doi.org/10.1016/j.talanta.2014.05.021 0039-9140/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: elcana@ufg.br (A.E. de Oliveira). Talanta 129 (2014) 143–149