Power Transformer Diagnostics, Monitoring and Design Features Issouf Fofana www.mdpi.com/journal/energies Edited by Printed Edition of the Special Issue Published in Energies Power Transformer Diagnostics, Monitoring and Design Features Power Transformer Diagnostics, Monitoring and Design Features Special Issue Editor Issouf Fofana MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade Special Issue Editor Issouf Fofana Universit ́ e du Qu ́ ebec ` a Chicoutimi (UQAC) Canada Editorial Office MDPI St. Alban-Anlage 66 4052 Basel, Switzerland This is a reprint of articles from the Special Issue published online in the open access journal Energies (ISSN 1996-1073) from 2015 to 2018 (available at: https://www.mdpi.com/journal/energies/special issues/power-transformer) For citation purposes, cite each article independently as indicated on the article page online and as indicated below: LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. 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Contents About the Special Issue Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Issouf Fofana and Yazid Hadjadj Power Transformer Diagnostics, Monitoring and Design Features Reprinted from: Energies 2018 , 11 , 3248, doi:10.3390/en11123248 . . . . . . . . . . . . . . . . . . . 1 Janvier Sylvestre N’cho, Issouf Fofana, Yazid Hadjadj and Abderrahmane Beroual Review of Physicochemical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers Reprinted from: Energies 2016 , 9 , 367, doi:10.3390/en9050367 . . . . . . . . . . . . . . . . . . . . . 6 Issouf Fofana and Yazid Hadjadj Electrical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers Reprinted from: Energies 2016 , 9 , 679, doi:10.3390/en9090679 . . . . . . . . . . . . . . . . . . . . . 35 Chao Tang, Bo Huang, Miao Hao, Zhiqiang Xu, Jian Hao and George Chen Progress of Space Charge Research on Oil-Paper Insulation Using Pulsed Electroacoustic Techniques Reprinted from: Energies 2016 , 9 , 53, doi:10.3390/en9010053 . . . . . . . . . . . . . . . . . . . . . . 62 Pawel Rozga Streamer Propagation and Breakdown in a Very Small Point-Insulating Plate Gap in Mineral Oil and Ester Liquids at Positive Lightning Impulse Voltage Reprinted from: Energies 2016 , 9 , 467, doi:10.3390/en9060467 . . . . . . . . . . . . . . . . . . . . . 97 Wojciech Sikorski, Krzysztof Walczak and Piotr Przybylek Moisture Migration in an Oil-Paper Insulation System in Relation to Online Partial Discharge Monitoring of Power Transformers Reprinted from: Energies 2016 , 9 , 1082, doi:10.3390/en9121082 . . . . . . . . . . . . . . . . . . . . 109 Jian Li, Xudong Li, Lin Du, Min Cao and Guochao Qian An Intelligent Sensor for the Ultra-High-Frequency Partial Discharge Online Monitoring of Power Transformers Reprinted from: Energies 2016 , 9 , 383, doi:10.3390/en9050383 . . . . . . . . . . . . . . . . . . . . . 125 Jingxin Zou, Weigen Chen, Fu Wan, Zhou Fan and Lingling Du Raman Spectral Characteristics of Oil-Paper Insulation and Its Application to Ageing Stage Assessment of Oil-Immersed Transformers Reprinted from: Energies 2016 , 9 , 946, doi:10.3390/en9110946 . . . . . . . . . . . . . . . . . . . . . 140 Lingjie Sun, Yingyi Liu, Boyang Zhang, Yuwei Shang, Haiwen Yuan and Zhao Ma An Integrated Decision-Making Model for Transformer Condition Assessment Using Game Theory and Modified Evidence Combination Extended by D Numbers Reprinted from: Energies 2016 , 9 , 697, doi:10.3390/en9090697 . . . . . . . . . . . . . . . . . . . . . 154 Chen Wang, Jie Wu, Jianzhou Wang and Weigang Zhao Reliability Analysis and Overload Capability Assessment of Oil-Immersed Power Transformers Reprinted from: Energies 2016 , 9 , 43, doi:10.3390/en9010043 . . . . . . . . . . . . . . . . . . . . . . 178 v Youyuan Wang, Kun Xiao, Bijun Chen and Yuanlong Li Study of the Impact of Initial Moisture Content in Oil Impregnated Insulation Paper on Thermal Aging Rate of Condenser Bushing Reprinted from: Energies 2015 , 8 , 14298–14310, doi:10.3390/en81212429 . . . . . . . . . . . . . . . 197 Kapila Bandara, Chandima Ekanayake, Tapan Saha and Hui Ma Performance of Natural Ester as a Transformer Oil in Moisture-Rich Environments Reprinted from: Energies 2016 , 9 , 258, doi:10.3390/en9040258 . . . . . . . . . . . . . . . . . . . . . 210 Chenmeng Xiang, Quan Zhou, Jian Li, Qingdan Huang, Haoyong Song and Zhaotao Zhang Comparison of Dissolved Gases in Mineral and Vegetable Insulating Oils under Typical Electrical and Thermal Faults Reprinted from: Energies 2016 , 9 , 312, doi:10.3390/en9050312 . . . . . . . . . . . . . . . . . . . . . 223 vi About the Special Issue Editor Issouf Fofana , Professor, IET Fellow, held the Canada Research Chair on insulating liquids and mixed dielectrics for electrotechnology (ISOLIME) from 2005 to 2015. At his university, he is the Director of the international research Centre on Atmospheric Icing and Power Network Engineering (CENGIVRE) and holds the research chair on the Ageing of Power Network Infrastructure (ViAHT). Dr. Fofana is/was a member of technical/scientific committees of international conferences (e.g., IEEE ICDL, IEEE CEIDP, IEEE ICHVE, CATCON, ISH, etc.), IEEE DEIS, and CEIDP AdComs. He also serves on the scientific committee as associate editor for IET GTD, IEEE TDEI, a Guest Editor of Energies, and chair of the IEEE DEIS Technical Committee on Dielectric Liquids. He is also a member of several working groups (i.e., CIGRE and ASTM). Dr. Fofana’s research in the area of HV engineering has emphases on insulation systems relevant to power equipment. His lifetime publication record is more than 270 scientific publications and 3 patents. vii energies Editorial Power Transformer Diagnostics, Monitoring and Design Features Issouf Fofana 1, * and Yazid Hadjadj 2 1 Aging of Power Network Infrastructure (ViAHT), Universit é du Qu é bec à Chicoutimi, Chicoutimi, QC G7H 2B1, Canada 2 Measurement Science and Standards, National Research Council (NRC), Ottawa, ON K1A 0R6, Canada; yazid.hadjadj@nrc-cnrc.gc.ca * Correspondence: ifofana@uqac.ca; Tel.: +1-418-545-5011 Received: 12 October 2018; Accepted: 13 November 2018; Published: 22 November 2018 1. Introduction The reliability of the power grid system directly contributes to the economic well-being and the quality of life of citizens in any country. In the electric power distribution and transmission systems, it is essential that key equipment such as transformers function properly for many years. In these important equipment, the Achilles’ heel is the insulation system, i.e., (a) insulation between the high voltage (HV) winding and the tank; (b) insulation between the HV and the low voltage (LV) windings; and (c) inter-phase insulation. Over the past decade, various types of insulating materials have been introduced in these machines. Assessment methods have allowed monitoring their conditions with the aim of providing the basic information required for power grid operators and maintenance planners to understand the issues related to aging. Also, aging indicator indices have been the aim of a large number of investigations. Many transformers around the world are now approaching the end of their theoretical design life. In this context, managing the aging population of power transformers has become one of the most critical issues today’s maintenance planners and engineers have to face. With increasing age, there are potential risks of extremely high monetary losses due to unexpected failures and outages. A simple solution would be to replace all aging and risky transformers at once with new ones. Such an approach is obviously not a fiscally realistic solution. The main objectives are: to extend their service life and optimize their performance through increased availability. For these reasons, transformer life management in the past few decades has gained an ever-increasing interest due to economic and technical reasons. The greatest challenges are related to the need for methods to assess the condition and life expectancy along with the improvement of transformer efficiency by noble designs and/or application of new materials. The special issue was focused on theoretical and practical developments with a special emphasis on new research and development (R&D) trends in transformer designs/diagnostics and maintenance. Additionally, today, “sustainable development” has become a world-wide concept for the scientific community. The focus is now on harvesting renewable resources instead of fossil fuels, and using environmentally friendly materials. In this context, new materials are emerging for the design of electrotechnical insulation systems including biodegradable insulating materials, with properties that are at least equal to conventional materials. Esters with excellent high temperature performance, enhanced fire safety, increased environmental protection and increased moisture tolerance are gaining importance. Some aspects regarding the application of biodegradable fluids in these important machines were therefore also of particular interest. Listed hereafter, among others, were some of the topics of interest considered: • New or emerging diagnostic/monitoring technologies; Energies 2018 , 11 , 3248; doi:10.3390/en11123248 www.mdpi.com/journal/energies 1 Energies 2018 , 11 , 3248 • Mineral oils of improved characteristics, additives; • Nanofluids and synthetic/vegetable dielectric liquids; • New materials for transformers; • Transformer life management. 2. An Outlook of the Special Issue This special issue from Energies , has been successfully organized with the support extended by the editorial team of the journal and the MDPI publishing team. The average processing time of the articles was noted to be 75.67 days. The guest editor is very much grateful to all reviewers for reviewing and revising the manuscripts. Devoting their valuable time to reviewing papers is essential for upholding the voluntary peer review process and is highly commendable. Their constructive comments and suggestions to the authors and confidential reports to the editors ensured that the high standard of this journal is maintained. Response to our call was excellent, with the following statistics: • Submissions: (21); • Publications: (12); • Rejections: (9); • Article types: review articles (3); research articles (9); • Authors’ geographical distribution (published papers): China (6); Poland (2); Australia (1); Canada (1); Cote D’Ivoire (1); UK (1). The summary of the articles published in this issue is discussed in the subsequent sections of this editorial. 3. A Review of the Special Issue A number of articles involve several subjects about power transformer diagnostics, insulation characterization, and new materials for transformers have been published in this issue. Sun et al. [ 1 ] proposed a new decision-making model for transformer condition assessment. The new model integrates the merits of fuzzy set theory, game theory and modified evidence combination extended by D numbers. It was shown that compared to the evidential reasoning-based method, the final evaluation result of the presented method could clearly show the health condition of the transformer. Li et al. [2] introduced a new intelligent sensor for ultra-high-frequency (UHF) partial discharge (PD) online monitoring in power transformers. The statistical characteristic quantities of UHF PD signals were acquired by means of a new method, namely the level scanning method which is the base of the intelligent sensor. The experimental results of the proposed sensor under laboratory conditions showed that the intelligent sensor could accurately acquire statistical characteristic quantities of the UHF PD signal, which indicated that the proposed intelligent sensor was qualified for UHF PD online monitoring. In recent years, a lot of research has been directed towards environmentally friendly insulating liquids, as an alternative to mineral oils. However, as the chemical compositions of these fluids are very different than those for mineral oils, new specification standards for non-mineral oils have been 2 Energies 2018 , 11 , 3248 produced. In this Issue a comparison study of streamer propagation and breakdown between Ester liquids and mineral oil was conducted by Rozga [3]. The work was focused on the comparison of light waveforms registered using the photomultiplier technique. The results indicated that both esters demonstrated a lower resistance against the appearance of fast energetic streamers than mineral oil. Xiang et al. [ 4 ] also presented a comparison study of the formation of dissolved gases in mineral- and vegetable- insulating oils. The authors used four interpretation dissolved gas analysis (DGA) methods and they confirmed that the diagnosis methods developed for mineral oil were not suitable for the diagnosis of electrical and thermal faults in vegetable-insulating oils and needed some modification. Thus, the proposed modified Duval Triangle method based on Duval Triangle 3 is used to diagnose the thermal and electrical fault of FR3 oil and camellia oil through redefining zone boundaries of Duval Triangle 1 and obtains more accurate diagnostic results. Furthermore, the generation mechanisms of gases in vegetable oils have been interpreted by means of unimolecular pyrolysis simulation and the reaction enthalpies calculation. Bandara et al. [ 5 ] investigated the performance of natural ester (NE) in moisture-rich environments. They have compared the aging behaviour of NE and mineral oil impregnated pressboard (PB) insulation. While NE insulating oil possesses resistance to the aging of PB insulation, it was noted that the acidity and the color of NE oils could increase rapidly due to the pronounced hydrolytic degradation in a moisture rich environment. On the other hand, dielectric dissipation factor (DDF), viscosity, and the dielectric breakdown voltage, were suitable for the assessment of the overall condition of NE insulation oils. Wang et al. [ 6 ] have introduced a new aspect for overload capability assessment of power transformers. In their article they estimated the running time of a power transformer under overload conditions by means of the hot-spot temperature. The overloading probability was then fitted by the Weibull distribution, in which the desired parameters were computed according to a new proposed objective function. Wang et al. [ 7 ] investigated the influence of initial moisture contents in oil impregnated paper of the condenser bushing. The results of their experience indicated that the initial moisture content has appreciable impact on the degradation of the insulation paper during the initial aging period. They found that it was possible to evaluate the aging degree and moisture of solid insulation of bushing by doing some analysis of the DDF. As discussed earlier, this Special Issue also reported on three comprehensive surveys [ 8 – 10 ]. A snapshot look at some significant developments and applications over the last decades were addressed and future research hotspots and notable research topics were also discussed for the benefit of researchers. Two reviews about physicochemical and electrical-based diagnostic techniques for insulation condition assessment were presented by Fofana et al. [ 8 , 9 ] considering 149 and 137 references, respectively. In the article written by N’cho et al. [ 8 ], in addition to traditional diagnostic techniques, some modern physicochemical diagnostic techniques such as Fourier transform infrared spectroscopy (FTIR), UV/visible spectroscopy, turbidity analysis were introduced. The benefits of using alternative insulating materials has also been discussed. In [ 9 ], Fofana and Hadjadj reported detailed descriptions and interpretations of traditional and advanced electrical diagnostic techniques. Online condition monitoring of power transformers were also discussed. Finally, the authors presented some suggestions/recommendations related to the nature of the defect or fault in the power transformer’s main component. The third review article by Tang et al. [ 10 ], reported on the space charge behavior in an oil-paper insulation system. Research progress during the last two decades was critically reviewed considering 62 references. The influences of applied voltage, temperature, moisture content and aging on the space charge evolution in the oil-paper insulation has been demonstrated. This review ends with future work on space charge measurement of oil-paper insulation materials. Sikorski et al. [ 11 ] reported on important aspects regarding PDs monitoring on power transformers. They reported that PDs activity under thermal runaway should be associated with moisture changes in the insulating system. 3 Energies 2018 , 11 , 3248 Zou et al. [ 12 ] proposed a new way for assessing the aging condition of oil-paper insulation based on confocal laser Raman spectroscopy (CLRS) in conjunction with principal component analysis (PCA) and multi-classification support vector machine (SVM). The investigations were performed in laboratory conditions using 160 oil-paper insulation samples and the approach validated with 105 oil-paper insulation samples. The results reported demonstrated the feasibility of using CLRS in conjunction with the PCA-SVM technique for aging stage assessment of oil-paper insulation. 4. Closing Remarks The contributions in this Special Issue discussed a wide range of subjects relevant to power transformer monitoring and applications. Power transformers are amongst the costliest equipment in the power grid. Maintaining these important machines in a pristine condition is therefore very important for power grid reliability. Even though the articles reported very interesting applications and monitoring techniques developed thus far, there are still many gaps to close to improve service reliability. Combined with the ever-increasing global demand for energy, it has become essential to find new solutions, based on the ability to properly diagnose the condition of transformers and to delay/slow down the aging process. According to Victor Hugo “what to foresee is the unforeseen”. An important strategy consists in reducing the risk of failures through remote monitoring and optimized maintenance. However, to be cost-effective, this requires an accurate assessment of the condition of the transformers. The general trend towards smart-grids, digital systems and the continued reduction in the cost of these technologies, combined with the existence of a communications infrastructure in a growing number of facilities, will facilitate the implementation of monitoring technologies. In today’s grids, power transformers must withstand not only transients due to lightning, and switching operations with load changes or fault occurrences but also the increase penetration of renewable energy sources/plug-in vehicles. These machines have a common denominator; they consist mainly of a large core of magnetic sheets around which are wound insulated conductors immersed in oil. However, their design philosophy and manufacture (based on years of R&D and experience) require a considerable number of calculations, verifications and precautions. However, in a world where everything is changing rapidly, the key strategy must be based on continuous improvement of every aspect of the manufacturing elements. Innovative strategies/concepts that can improve the quality of these important machines are therefore required. Author Contributions: The authors contributed equally to this work. Funding: This research received no external funding. Acknowledgments: Issouf Fofana is grateful to the MDPI Publisher for the invitation to act as the guest editor of this special issue and is indebted to the editorial staff of “ Energies ” for their kind cooperation, patience and committed engagement. The guest editor would also like to thank the authors for submitting their excellent contributions to this special issue. Thanks is also extended to the reviewers for evaluating the manuscripts and providing helpful suggestions. Conflicts of Interest: The authors declare no conflicts of interest. References 1. Sun, L.; Liu, Y.; Zhang, B.; Shang, Y.; Yuan, H.; Ma, Z. An Integrated Decision-Making Model for Transformer Condition Assessment Using Game Theory and Modified Evidence Combination Extended by D Numbers. Energies 2016 , 9 , 697. [CrossRef] 2. Li, J.; Li, X.; Du, L.; Cao, M.; Qian, G. An Intelligent Sensor for the Ultra-High-Frequency Partial Discharge Online Monitoring of Power Transformers. Energies 2016 , 9 , 383. [CrossRef] 3. Rozga, P. Streamer Propagation and Breakdown in a Very Small Point-Insulating Plate Gap in Mineral Oil and Ester Liquids at Positive Lightning Impulse Voltage. Energies 2016 , 9 , 467. [CrossRef] 4. Xiang, C.; Zhou, Q.; Li, J.; Huang, Q.; Song, H.; Zhang, Z. Comparison of Dissolved Gases in Mineral and Vegetable Insulating Oils under Typical Electrical and Thermal Faults. Energies 2016 , 9 , 312. [CrossRef] 4 Energies 2018 , 11 , 3248 5. Bandara, K.; Ekanayake, C.; Saha, T.; Ma, H. Performance of Natural Ester as a Transformer Oil in Moisture-Rich Environments. Energies 2016 , 9 , 258. [CrossRef] 6. Wang, C.; Wu, J.; Wang, J.; Zhao, W. Reliability Analysis and Overload Capability Assessment of Oil-Immersed Power Transformers. Energies 2016 , 9 , 43. [CrossRef] 7. Wang, Y.; Xiao, K.; Chen, B.; Li, Y. Study of the Impact of Initial Moisture Content in Oil Impregnated Insulation Paper on Thermal Aging Rate of Condenser Bushing. Energies 2015 , 8 , 14298–14310. [CrossRef] 8. N’cho, J.S.; Fofana, I.; Hadjadj, Y.; Beroual, A. Review of Physicochemical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers. Energies 2016 , 9 , 367. [CrossRef] 9. Fofana, I.; Hadjadj, Y. Electrical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers. Energies 2016 , 9 , 679. [CrossRef] 10. Tang, C.; Huang, B.; Hao, M.; Xu, Z.; Hao, J.; Chen, G. Progress of Space Charge Research on Oil-Paper Insulation Using Pulsed Electroacoustic Techniques. Energies 2016 , 9 , 53. [CrossRef] 11. Sikorski, W.; Walczak, K.; Przybylek, P. Moisture Migration in an Oil-Paper Insulation System in Relation to Online Partial Discharge Monitoring of Power Transformers. Energies 2016 , 9 , 1082. [CrossRef] 12. Zou, J.; Chen, W.; Wan, F.; Fan, Z.; Du, L. Raman Spectral Characteristics of Oil-Paper Insulation and Its Application to Ageing Stage Assessment of Oil-Immersed Transformers. Energies 2016 , 9 , 946. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). 5 energies Review Review of Physicochemical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers Janvier Sylvestre N’cho 1 , Issouf Fofana 2, *, Yazid Hadjadj 2 and Abderrahmane Beroual 3 1 Département Génie Électrique et Électronique, Institut National Polytechnique Houphouët Boigny (INP-HB), BP 1093 Yamoussoukro, Ivory Coast; nchosylvestre@yahoo.fr 2 Research Chair on the Aging of Power Network Infrastructure (ViAHT), Université du Québec à Chicoutimi, 555 Boulevard de l’Université, Chicoutimi, QC G7H 2B1, Canada; yazid.hadjadj@uqac.ca 3 Ecole Centrale de Lyon, Université de Lyon, Ampère CNRS UMR 5005, 36 Avenue Guy de Collongue, 69134 Ecully, France; Abderrahmane.Beroual@ec-lyon.fr * Correspondence: ifofana@uqac.ca; Tel.: +1-418-545-5011; Fax: +1-418-545-5012 Academic Editor: Enrico Sciubba Received: 23 March 2016; Accepted: 3 May 2016; Published: 13 May 2016 Abstract: A power transformer outage has a dramatic financial consequence not only for electric power systems utilities but also for interconnected customers. The service reliability of this important asset largely depends upon the condition of the oil-paper insulation. Therefore, by keeping the qualities of oil-paper insulation system in pristine condition, the maintenance planners can reduce the decline rate of internal faults. Accurate diagnostic methods for analyzing the condition of transformers are therefore essential. Currently, there are various electrical and physicochemical diagnostic techniques available for insulation condition monitoring of power transformers. This paper is aimed at the description, analysis and interpretation of modern physicochemical diagnostics techniques for assessing insulation condition in aged transformers. Since fields and laboratory experiences have shown that transformer oil contains about 70% of diagnostic information, the physicochemical analyses of oil samples can therefore be extremely useful in monitoring the condition of power transformers. Keywords: power transformers; insulating oil/paper; diagnostics; color/visual examination; particle count; inhibitor content; moisture; DGA; acidity; interfacial tension; viscosity; DP; furan; HPLC; gas chromatography-mass spectrometry coupling; FTIR spectroscopy; UV/visible spectroscopy; dissolved decay products; turbidity; methanol; free radicals 1. Introduction Today’s power transformers constitute a major part of the capital equipment of power utilities all over the world. It is therefore important they function reliably. They are indispensable equipment for power generation plants, transmission systems and large industrial plants. Outage of power transformers while in service usually lead to significant revenue loss to the utility, potential environmental damage, explosion and fire hazards and expensive repairing or replacement costs. The cost of replacing the transformers varies from a few hundred dollars to several million dollars [ 1 ]. Early detection of problems can reduce repair costs by 75 percent and loss of revenue by 60 percent, and that annual cost savings equal to two percent of the price of a new transformer— i.e. , approximately 40,000 to 80,000 US dollars (USD)—can be achieved [ 2 ]. Hence, it is desirable that the transformers should be utilized to the maximum extent consistent with adequate service life [ 3 ]. In order to reach such a device management, diagnostic techniques and condition monitoring are becoming Energies 2016 , 9 , 367; doi:10.3390/en9050367 www.mdpi.com/journal/energies 6 Energies 2016 , 9 , 367 increasingly important in assessing the condition of transformers and prevent incipient electrical failures. Any improved preventive maintenance procedures should help extending their life. In these important machines, the Achilles heel is the insulation system, i.e. ,: (a) insulation between the high voltage (HV) winding and the tank; (b) insulation between the HV and the low voltage (LV) windings; and (c) inter-phase insulation. These parts are the most sensitive to the insulation deterioration as they usually have the smallest margins in the dielectric strength. The life of the transformer is actually the life of the internal insulation system. Analysis of the insulating system, consisting of oil and paper provides information not only on the quality of the latter, but also can detect the warning signs of failure. The monitoring of the solid and liquid insulation in these machines is therefore of utmost importance [ 3 ]. By monitoring accurately the condition of the insulation, it is possible to detect on time incipient defects and avoid potential failures. Consequently, an effective approach to maintenance can be adopted and the optimum intervals determined for replacement. Common diagnostic techniques for transformers rely on testing based on physical, chemical, and electrical parameters. Physical measurements, in general, involve measurement of temperature, vibration, acoustic emission, etc. However, the diagnostic methods that give useful information on transformation insulation condition are chemical and electrical tests [ 3 ]. The term “diagnostics” indicates incorporation of advanced analysis that are capable of performing reliable assessment of equipment condition and suggesting actions to be taken. As each diagnosis method is developed and applied in real-life situations, it is always weighed up against other methods [ 3 ]. Methods that have been established over the years satisfy important criteria, among which [ 4 ]: sensitivity to important parameters of transformer condition, reproducibility of results over time and for different testing personnel, compensation of raw data for significant environmental effects like temperature, good correlation with other established methods availability of valuable information for the time and expense involved. The purposes of diagnostic testing are threefold: (a) to identify increased aging aspects; (b) to identify the cause of aging; and (c) to identify, if possible, the most appropriate corrective actions. The life of the transformer being connected with that of its insulation, the evaluation of the insulation system condition is essential to assess the condition of the transformer when new and after several years of use. This evaluation necessarily involves both electrical and physicochemical techniques/diagnostic methods. The currently used techniques include modern methods and improved conventional techniques, allowing providing additional information on the condition of insulation. Figure 1 adapted from [ 5 ], sketches the functional based classification of oil properties. This review encompasses physicochemical-based diagnostic techniques for assessing insulation condition in aged transformers, while electrical-based diagnostic techniques are for concern in a companion paper submitted in this journal [ 6 ]. In addition to electrical methods, the physicochemical diagnostic methods are very important for the condition monitoring or for studying the degradation of electrical insulation in power transformer. This review is subdivided into traditional and modern diagnostic methods. 7 Energies 2016 , 9 , 367 Figure 1. Functional based classification of oil properties adapted from [ 5 ]. Specifications in brackets are ASTM (American Society for Testing and Materials) standards. 2. Traditional Diagnostics Techniques 2.1. Color/Visual Examination Color [ 7 ] is often used as a qualitative method. The technique is based on the comparison of oil color to a standard colored and numbered disc [ 8 ]. An oil’s color comes from the light transmitting through it. Different colors are formed depending on the concentration and type of light-absorbing groups dissolved species in oil. Color of new oil is generally accepted as an index of the degree of refinement. For oils in service, an increasing or high color number is an indication of contamination, deterioration, or both. Oxidation is a common cause of an over-all darkening to occur. Comparisons between oil condition and color are reported in Table 1. Table 1. Oil condition based on color comparisons [9]. Color Comparator Number Color Oil Condition <7 Pale yellow Good oil 7–10 Yellow Proposition A oil 10–11 Bright yellow Service-aged oil 11–14 Amber Marginal condition 14–15 Brown Bad condition 16–18 Dark brown Severe condition (reclaimed oil) >18 Black Extreme condition (scrap oil) The visual examination [ 10 ] is applicable to electrical insulating liquids that have been used in transformers, oil circuit breakers, or other electrical apparatus as insulating or cooling media, or both. An oil sample is visually examined by passing a beam of light through it to determine transparency 8 Energies 2016 , 9 , 367 and identify foreign matters. Poor transparency, cloudiness, or the observation of particles indicates contamination such as moisture, sludge, or other foreign matter. 2.2. Particle Count It is recognized that particles have a harmful effect on the dielectric strength of insulating liquids [ 11 ]. Large amount of particle contaminations can lead to transformer failure. It was reported that moisture in combination with particles reduces significantly the breakdown voltage of the oil and increases the risk of static electrification, partial discharge activity and tracking [ 12 ]. Particle size, type and shape are also contributing factors. The most detrimental particles are the conductive ones (metal, carbon, wet fiber, etc. ). Particle identification and counting is an important procedure for condition monitoring [11,13]. Miners [14] reported the effect of particles and moisture on the breakdown voltage of insulating transformer oil using Verband der Elektrotechnik, Elektronik, Informationstechnik (VDE) electrodes. This author has considered different sizes and concentrations of iron, copper and cellulose fibers. He noticed very low breakdown strength for the combined effects of moisture and particles type, size and concentration. In a recent work presented at a The Council on Large Electric Systems (CIGRE) session [ 15 ], a comparison between the performance of ester liquids and mineral oil have shown that the breakdown voltages of both ester liquids and mineral oil decreased with the increase in cellulose particle-based content. However, it was found that the breakdown voltage of mineral oil is more sensitive to the particle contamination than ester liquids. This might be due to the higher viscosity of ester which slower the motion of metallic particles and therefore reduces breakdown occurrence [ 16 ]. Sarathi and Archana [ 17 ] investigated the role played by conducting particle in partial discharge activity under alternative current (AC) voltages by using an ultra-high frequency (UHF) technique. They have observed a partial discharge current pulse formation and frequency signal radiation due to particle movement, and when the applied voltage increased, the UHF signal magnitude formed due to particle movement increased. A multiple sources of particles contamination have been reported by CIGRE Working Group 12.18 [13]. In new equipment, the insulating liquid may contain cellulose fibers, iron, aluminum, copper and other particles from the manufacturing process. In used transformer, sludge particles forms slowly during utilization at normal and overload temperatures. Carbon particles due to the localized overheating may also produce and migrate by leakage or other accident errors from the on load tap changer (OLTC) diverter to the bulk liquid compartment and contaminate the active parts. Pump bearing wear is considered as a typical source of metallic particles [13]. Actually, there are many methods for counting and determining the size and shape of particles. Such methods are based on the light extinction, light scattering, coulter principle or direct imaging. However, the automatic particle counters using light extinction are the most widespread for counting in hydrocarbons and lubricants. For transformer insulating liquids, the measurements are performed using standards such as American Society for Testing and Materials (ASTM) D6786 or International Electrotechnical Commission (IEC) 60970. These standards are based on the International Organization for Standardization (ISO) 11171 calibration. IEC 60422 [ 18 ] and CIGRE brochure 157 [ 11 ] provide some guidelines to judge the condition of oil based on the level of contamination. In 1983, Oommen published an interesting study on particle levels in 200 samples taken from field and factory units [ 19 ]. Atomic absorption spectroscopy was used to determine the content of iron, copper and others. 2.3. Inhibitor Content Although insulating oils are carefully refined, the impact of mechanical, electrical, thermal and chemical stresses, produces a variety of degradation products. Most of new brand insulating oil contain small amount of unstable hydrocarbons. The electronegative character of oxygen makes it very effective to attack vulnerable hydrocarbons [ 20 ]. The oxidation rate is accelerated by temperature, oxygen and 9 Energies 2016 , 9 , 367 catalysts such as copper. The results to such reactions are the formation of hydro-peroxides which brown the oil. Insoluble molecules which may be adsorbed at the surface of cellulose fibers are formed. These impurities reduce life expectancy and reliability of in-service transformers. To reduce the impact of oxygen and enhance the oxidation stability, synthetic oxidation inhibitors are usually incorporated in the oil. Therefore, as long as the inhibitors are present, the oil will be protected against oxidation and therefore increase the expected lifetime of the insulation. The antioxidants perform better in these cleaner oils since they do not have to counteract the negative aspects of contaminants. However, as the service life proceed, the inhibitor will be consumed and when is gone the oxidation rate become higher. Since the anti-oxidant is a consumable material, the initial chemical stability of new insulating oil gradually decreases. Their amount has to be monitored and must be replenished if necessary. Thus, the determination of the inhibitor content is an important factor in maintaining long service life of insulating oil. Actually, phenolic inhibitors are often used in transformers. The commonly used inhibitors are 2,6-di- tert -butyl-paracresol (DBPC) and 2,6-di- tert -butyl-phenol (DBP) [ 12 ]. In recent study, Mehanna et al . [ 21 ] examined the characteristics of several inhibitors dissolved in mineral insulating oils including 2,6-di- tert -butyl- p -cresol (DBPC), 2,6-di- tert -butyl-phenol (DBP), dibenzyl disulfide (DBDS), 2- tert -butyl- p -cresol (2- t -BPC), N -phenyl-1-naphthylamine, 1,2,3-benzotriazol (BTA) and methylated-BTA. The obtained results confirmed that the DBPC and DBP are the most suitable to be used as inhibitors in transformer mineral oils. ASTM D3487 defines two types of mineral oils according to the inhibitors content (0.08% for type I and 0.3% for type II). Detection and measurements of the inhibitor content shall be done according to IEC 60666 Standard [ 22 ]. Three used analytical methods are presented in this standard, related to infrared spectrophotometry (IR), high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). 2.4. Moisture in Oil Determination Moisture is considered enemy number one of transformer insulation. Each time the moisture is doubled in the solid insulation of a transformer, the life of the insulation is cut by one-half [ 23 ]. The presence of moisture in the solid and liquid insulation is known to play a critical role on transformer life [ 24 , 25 ]. The moisture in transformer is generated from several sources [ 12 ]: remaining moisture in insulation during manufacturing, humid air from outside during transportation and/or assembling in substation, humid air from outside through the breather (non-sealed), moisture ingress through gaskets, chemical decomposition of cellulose, moisture absorption from outside during some maintenance operations such as on site control of active part or bushing replacement, topping-up of oil level made with humid oil (non-dried). An accurate method of measuring very small amount of moisture in oil is the Karl Fischer Titration technique. This technique can indicate moisture content as low as 1–2 ppm [ 26 , 27 ]. As oils become very oxidized with increasing amounts of polar aging byproducts, their water solubility character