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ABSTRACT As blood tests provide information The diagnostic about a person’s state of health, oil analyses provide information about the condition of a . testing has proven to power of oil be an excellent tool for early defect LGHQWLͤFDWLRQ LQ SUHYHQWLYH PDLQWH- QDQFH RI RLOͤOOHG WUDQVIRUPHUV ,W analysis for power has permitted extended service life by giving a measure of both oil and solid insulation condition, and by identifying the presence of incipient : thermal and dielectric faults. When performed regularly, oil analysis will help the asset manager run a New cost-effective maintenance program for transformers. This column addresses some recent developments in oil analysis, such as developments sustainable alternatives for mineral oil, an alternative marker for analy- 1. Introduction lishing the status of ageing and corrosion sis of paper degradation, and online Over the last decades a large variety of oil for replacement and refurbishment issues transformer oil monitoring. analyses techniques have been developed [1]. In the majority of oil-filled transform- and standardized. These techniques in- ers mineral oil is used, and physical oil KEYWORDS volve physical or chemical oil properties, samples are taken in service for analysis electrical and environmental properties, in the laboratory. This column addresses oil analysis, ester, cellulose paper, and DGA (). The some recent developments in oil analysis. degree of polymerization application of oil analysis may be linked to The first one is related to sustainable alter- maintenance, lifetime extension, and estab- natives for mineral oil, in particular esters.

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The second one deals with methanol as an basis, and have built a history that serves properties, and interacts very well alternative marker for analysis of paper as a reference for trending and analysis. with mineral insulating oil. Under the degradation, and the third one with the For esters, the methods for assessing the combined effects of temperature, oxygen benefits and drawbacks of online trans- condition can only be applied to a limited and moisture, the cellulose polymer chains former oil monitoring. extent using IEC standards. IEC did issue will break up and result in a weak and a standard on new and unused natural es- brittle material that has lost its mechanical ters, such as FR3. But so far there are no strength. Solid insulation in transformers 2. Natural esters as an guidelines for esters in operating electrical not only serves as an , but also alternative for mineral oil equipment. New knowledge rules need to ensures mechanical stability, creation of From the start, mineral oils have formed be applied, and a new history needs to be space, the proper direction of oil flow, and the insulation and cooling medium of built up. IEC TC 10 is presently working so on. liquid-filled transformers. For most ap- on such a standard. It is to be expected plications, mineral oil is a proven, accept- that, in the longer run, a similar standard New insulating paper typically has a DP able, cost-effective insulating and cooling will be developed for the analysis of paper value between 1000 and 1200, which fluid. It has good thermal and dielectric ageing through ester transformer oil. corresponds to a high tensile strength. As properties, and there is a lot of experience paper ages, its electrical properties do not gained over the years. change significantly, but the mechanical 3. Methanol as a marker for properties are reduced drastically. As a In recent years, manufacturers have intro- paper degradation result, the mechanical strength of the duced transformers with biodegradable Transformer insulation paper is generally paper is reduced with time, and when a liquids, in particular synthetic and nat - recognized as the most significant lim- DP value of about 200 to 300 is reached, ural esters. The advantages are pri- iting factor in the thermal operating life the mechanical strength has gone. The marily related to safety properties of a transformer [2]. Not as much because rate at which the DP value decreases (thermal stability, flashpoint, smoke it is the dominant failure mechanism – it in time during service depends mainly generation) and environmental properties is not, but because paper degradation is on the temperature in the transformer, (biodegradable). It is further claimed that an irreversible mechanism that will even- on the amount of moisture and oxygen they can absorb water without having a tually destroy the transformer and does present, and on the acidity of the oil. The detrimental effect on dielectric properties, (practically) not allow repair or refurbish- main cellulose degradation process is acid and that they have a longer life expectancy ment. hydrolysis, which requires water and acids. due to the reduced ageing rate of solid in- The process is accelerated by temperature, sulation. Transformers with such fluids The winding insulation consists of oil degradation byproducts and possible are being introduced more and more, impregnated cellulose. Cellulose has electrical fields. Figure 1 schematically especially in applications where fire excellent electrical and mechanical illustrates the degradation process [3]. safety and environmental protection are important, and have reached a state of maturity. There is a trend to use them at ever increasing voltages and power ratings, and today there are examples of biodegradable fluids being used in high- voltage and high-power transformers.

Apart from fulfilling the primary functions of insulation and cooling, the oil serves as an important carrier of information. From physical and chemical analysis we assess whether the oil still meets the required specifications for continued use. With DGA (Dissolved Gas Analysis) dormant errors can be detected at an early stage. This gives us, as it were, a glimpse into the interior of the transformer without having to open it and even without having to take it out of operation. DGA is the most widely Figure 1. Schematic representation of the interaction between oil and paper used technique worldwide for monitoring the operating status of transformers. Oil testing gives a measure of both oil A drawback of changing from mineral oil to esters is that is has an impact on and solid insulation condition,dition, and helpshe transformer monitoring and diagnosis. Most utilities have experience with min- identify the presence of incipientncipient thermaltherm eral oil analysis as a standard diagnostic and dielectric faults www.transformers-magazine.com 6 7 COLUMN

In the last decade, research has indicated It does, by its nature, not allow immediate feedback or alert. that methanol is a promising alternative as a tracker for paper degradation These days, however, modern sensing and diagnostic equipment allows to re- place sampling, transport and laborato- ry analysis of oil samples. Mobile on-site test methods are presently commercially available. By applying continuous on-line oil testing, in combination with intelligent electronic devices which can record and trend results and trigger alarms, we now have predictive diagnostic tools which can eliminate the risks associated with interval testing. On-line testing also elimi- nates the necessity of oil sampling and the associated issues related to contamination during sampling and storage. After all, the test results are only as reliable as the sam- ples obtained. One might suggest that this will eventually lead to the end of sampled, interval-based, oil testing practice [7]. Will we exclusively use on-line predictive oil analysis in the future? To answer this question we need to understand the prin- )LJXUH7\SLFDOFRUUHODWLRQVEHWZHHQ'3YDOXHDQGPDUNHUFRQFHQWUDWLRQVUDWLRV>@ ciples of both methods.

The off-line analysis provides a wealth For the above reasons, operators are The CO2 and furfural concentrations are of information from all kinds of analyses keen to know the degree of polymeri- more sensitive to DP variations in the low (physical and chemical properties and zation (the DP value) of the insulation DP range (below 400, late degradation DGA), and needs to be performed on a paper. Sampling paper for analysis is stages), whereas the MeOH concentration regular basis. According to CIGRÉ [7], a not a feasible option, not only because is more sensitive to DP variations in typical maintenance interval for DGA, it requires opening the transformer, but the high DP range (above 400, earlier dependent on load, age and importance, more importantly, because the sample degradation stages). The methods may varies from three months to two years. would have to be taken from the hot- be regarded complementary. For early Once a defect is observed, the same sam- spot location. The breakdown strength warning, the MeOH approach would be ple may be used for additional analysis, of the insulating paper is determined better, but close to the end of paper life, the and the sample frequency may be in- by the weakest link in the chain, which furfural content may give a more accurate creased. Interpretation is performed by an is the hotspot location. This is located picture. It should be noted, however, expert (or a team of experts). inside the winding package close to the that research is still ongoing to arrive core. at a reliable empirical relation between The on-line analysis is predominantly MeOH and DP value. At present, all three based on DGA monitoring, and ranges Therefore, the DP value is determined methods have some challenges based from simple fault detectors to full DGA indirectly from the byproducts in the on their advantages and disadvantages. monitoring systems, and may be performed oil that result from breaking up of the Table 1 gives an overview of the three continuously. The first interpretation can cellulose chains. The byproduct para- methods discussed [3]. be made by knowledge rules embedded in meters used so far for assessing the DP software. In order to fully benefit from this value are the CO2 concentration, the 4. Online monitoring of feature it is important to ensure that oil is CO2/CO ratio and the furfural content. drawn from and returned to chosen points In the last decade research has indicated transformer oil in the oil circulation circuit so as to ensure that methanol (MeOH) is a promising For critical transformer units where pre- representative sampling. It is also impor- alternative as a tracker for paper degra- mature or unexpected failure would have tant that the oil is suit ably con ditioned dation [4, 5]. significant consequential costs one may prior to testing to ensure measurement consider to increase the oil sampling consistency. As mentioned earlier, on-line The assessment is based on the strong frequency for coping with defects that analysis may be used for fault detection or correlation between CO2 concentration, develop fast. This allows maintenance for a full DGA analysis. the CO2/CO ratio, the furfural content engineers to timely recognize incubating (2-FAL) and the MeOH content on the defects and take corrective action before Fault detectors usually operate on only one side, and the DP value on the other they mature, however at the expense of one or two key parameters. As hydro- [4]. This is illustrated in Figure 2. cost, manpower and logistic complexity. gen is produced in all fault mechanisms,

68 TRANSFORMERS MAGAZINE | Volume 5, Issue 4 | 2018 monitoring will find its way on a limited It is anticipated that on-line oil analysis will scale, focusing on strategic applications. increasingly be applied for high-value stra- It is unlikely that on-line monitoring will replace off-line analysis at a significant tegic transformers, but will not be able to scale in the foreseeable future, unless the replace laboratory oil analysis in the fore- parameters of the business case dramatic- ally change. seeable future Conclusion measuring dissolved hydrogen serves sampling and laboratory analysis) and Transformer oil testing has proven to be as an excellent fault indicator. Similarly, the benefits (e.g. the failures prevented an excellent tool for early defect iden- moisture is a very useful indicator of the by timely detection). Expensive and stra- tification in preventive maintenance of overall condition of (and risks associated tegic high-voltage transformers will have oil-filled transformers. It has permitted with) both oil and solid insulation. These a better monitoring business case then extended service life by giving a measure are therefore regarded as key parameters. medium-voltage commodities. of both oil and solid insulation condition, Fault detectors combined with intelligent and by identifying the presence of incipi- logic systems allow to timely intervene in Also, one has to take into account that on- ent thermal and dielectric faults. When critical situations, but do not have the pre- line systems need maintenance and cali- performed regularly, oil analysis will help dictive power of a full analysis. bration, and tend to have a lower accuracy the asset manager run a cost-effective then laboratory analysis. maintenance program for transformers. Complete DGA monitoring benefits from the DGA analysis advantages to the full. Transformer oil testing is firmly entren- New sustainable fluids are being de- However, it does not monitor all physi- ched in the condition-based maintenance veloped as an alternative for mineral oils. cal and chemical properties, the costs rise practice. The results of oil analyses form Esters have become available with sim- with the number of key gases to be ana- an important part of the condition infor- ilar dielectric and thermal properties, but lysed, and complex situations still requi- mation feeding maintenance and replace- with superior safety and environmental re an expert interpretation. The decision ment decisions. As such, oil analysis is in- properties. Further work is needed in or- to use an on-line monitoring system in- creasingly integrated in decision support der to develop guidelines for using ester volves a trade-off between the costs (costs systems such as health and risk indexing analysis for diagnostic and maintenance of automated monitoring versus interval software [8]. It is very likely that on-line applications.

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Test Advantages 'LVDGYDQWDJHV Challenges

Laboratory or online test; Limited information on cellulose 'LI¿FXOWWRGHWHUPLQHVRXUFH ageing; location, type of degradation, degree of egress/ingress;

CO2 & CO &DQEHXVHGDVDÀDJWR Origin can be solid insulation or oil; May be dependent on indicate cellulose ageing manufacturer / design Gases may escape, depending on WKHEUHDWKLQJFRQ¿JXUDWLRQ Laboratory test; Thermal instability furans; Proper interpretation requires knowledge on the insulation Estimates DP value; Detection limited at higher DP system values 2-FAL Empirical relation between 2-FAL and DP; Sensitive for late degradation (low DP) Laboratory test; Requires knowledge on paper (VWDEOLVKLQJXQLWVSHFL¿FHPSLULF type, partitioning factors, size, equations still in development Estimates DP value; GHVLJQEUHDWKLQJFRQ¿JXUDWLRQ Methanol Empirical relation between MeOH and DP; Linear & sensitive for early degradation (high DP) www.transformers-magazine.com 6 9 COLUMN

For determining the ageing status of paper [3] N. Perjanik, Weidmann Introduces [6] CIGRÉ, Guide for transformer insulation, methanol analysis has been Methanol and Ethanol in Oil Testing, maintenance, CIGRÉ brochure 445, developed as an alternative for furfural NETA Powertest Conference, 2018 WG A2.34, 2011 analysis. It turns out that methanol has superior properties for identifying [4] M. Duval, Transformers with low [7] M. Kuises, W. Morse, Has transfor- the early phases of paper degradation. degree of polymerization, Transformers mer oil testing reached its sell by date?, However, further development is Magazine, Vol. 1, Issue 3, 2014 Energize, April 2005 needed to arrive at empiric relations for interpretation. [5] A. Schaut and S. Eeckhoudt, Iden- [8] J. Wetzer, Transformer health and tification of early-stage paper degrada- risk indexing, Transformers Magazine, It is anticipated that on-line oil analysis tion by methanol, CIGRÉ Conference, Vol. 5, Issue 2, 2018 will increasingly be applied for high-value 2012, paper A2-107 strategic transformers, but will not be able to replace laboratory oil analysis in the foreseeable future: for large numbers of medium-voltage commodities the bene- Author fits will not balance the costs. Dr. Jos Wetzer from the Netherlands is senior principal consultant “Asset Management” with DNV GL - Energy, stationed in Arnhem, The Netherlands. He obtained Acknowledgement a M.Sc. degree in Electrical Engineering (cum laude) The author gratefully acknowledges the and a Ph.D. degree in Technical Sciences, both from the support of Walter Hulshorst and Pieter Eindhoven University of Technology. After obtaining his de Bijl of DNV GL. They have delivered Ph.D. degree he worked as an associate professor at the valueable contributions to the field of Eindhoven University of Technology for 14 years. His pow er transformer oil analysis. activities included research, education and consultancy in the field of High- Voltage Engineering and EMC. In 1998 he joined KEMA (now DNV GL) as a senior consultant, in the field of condition assessment, asset management and Bibliography EMC. From 2001 to 2006 he held a management position, and from 2006 on [1] N. Robinson, Transformer oil ana- he has been active as a (senior) principal consultant, and has served numerous lysis: a basic introduction, in: Transfor- clients in Europe, North America, the Middle East and Asia Pacific on asset mer and substation handbook 2014, management issues. From 2004 to 2007 he also held a position as a part-time Electricity &Control Spot On, Crown professor at the Eindhoven University of Technology in the Netherlands, Publications engaged in Asset Management of Electrical Infrastructures. His expertise includes asset and maintenance management, condition assessment and [2] CIGRÉ, Thermal performance of diagnostics, dielectrics and electrical insulation, electromagnetic fields and transformers, CIGRÉ brochure 393, EMC. He has published over 150 scientific and professional publications. WG A2.24, 2009

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