Phenomenon of copper sulphide deposition in the paper: influential factors,

precursors and metal passivators

By

J.Lukic, D.Nikolic, V.Mandic,

S.Milosavljevic, A.Orlovic

SECOND PAGE IS LEFT BLANK

PHENOMENON OF COPPER SULPHIDE DEPOSITION IN THE PAPER: INFLUENTIAL FACTORS, PRECURSORS AND METAL PASSIVATORS

J.Lukic

1, D.Nikolic

1, V.Mandic

1, S.Milosavljevic

1, A.Orlovic

2

1) Electrical Engineering Institute, Nikola Tesla

2) Faculty of Technology and Metallurgy, University of Belgrade

Abstract:

Copper sulphide deposition in the paper insulation is a question of high importance when risk of failure is

concerned. This paper attempts to analyze key influential factors and precursors which promote copper sulphide

deposition in the paper. Protective role and distribution of metal passivator between oil paper was investigated,

in order to achieve better understanding of the efficiency of metal passivators in suppressing copper sulphide

formation.

Modified IEC 62535 tests were performed in order to analyze behavior of different oils fortified with DBDS and

different aromatic compounds, including phenolic inhibitors, under different oxygen conditions. Decrease of

DBDS and inhibitor concentration in the oil, and copper content in the paper was monitored. After the addition

of different aromatic alcohols and phenolic inhibitor to uninhibited oils, copper sulphide deposition in the paper

increased. Aromatic compounds contributed to increased copper sulphide deposition in the paper. Copper

sulphide deposition in the paper was enhanced in conditions of higher oxygen content.

During prolonged IEC 62535 test, metal passivator in the oil and absorbed in the paper was determined using

HPLC technique. The absorption rate was found to increase with time. The protective role of paper-absorbed

TTA was investigated by heating experiments at 1400C of unused and used corrosive oils with metal passivator

present only in the paper and on the copper surface, in order to mimic the situation of units passivated in early

days of service, when the passivator is fully bonded to the conductors without interference from formed copper

sulphide layers and at a time when it is completely saturated with the oil. Passivated conductors were protected

from copper sulphide deposition for a certain period. Rise of furans was observed to be one of the side effects of

metal passivator absorption in the paper at temperatures of 1500C. This was attributed to changed equilibrium of

furans in the paper/oil system.

Key words: copper sulphide, paper, DBPC, oxygen, aromatics, metal passivator

Introduction Over the last decade problems encountered with the exploitation of corrosive oils and failures related to copper sulphide deposition arose to be very important issues in the evaluation of risk and reliability of units in service. Important issues related to understanding the mechanism and influence of temperature, oxygen, oil type, additives, in regard to test methods, evaluation of risk factors and application of adequate mitigation techniques are currently being investigated by researchers, and within CIGRE WG A2.40 which has the task of addressing these topics. Better understanding of key influential factors and the mechanism of copper sulphide deposition, with focus on the copper deposition in the paper is expected to improve evaluation of the risk in service and application of the appropriate mitigation action. Recent experiences in service showed that in the broader region of temperature and oxygen concentrations copper sulphide deposits on the bare metal and paper insulation can be detected. It was observed in practice that different oils exhibit different deposition patterns after being tested using the IEC 62535 corrosive sulphur test. This difference is attributed to the type of sulphur compound present, base oil composition and presence of additives. Usually uninhibited oils deposit copper sulphide on the copper plate only, while inhibited oils deposit copper sulphide also on the paper. It was observed that the oil oxidation process may enhance oil corrosiveness. Several classes of sulphur compounds, besides

DBDS may become reactive even at moderate to low temperatures; below 800C in low oxygen environment, while in

uninhibited oils build-up of oxygenated corrosive sulphur species was detected. In general terms, oil oxidation process and corrosion phenomena are in many aspects the same process, i.e. two sides of one story. Copper in oil dissolution and deposition in the paper as an intermediate step of the reaction of copper sulphide formation in the paper is driven by temperature and oxygen. Propensity of oil for copper sulphide deposition in paper insulation was studied. Influence of oxygen, base oil composition and presence of additives were investigated, as it was observed that copper sulphide deposition pattern of the same oil may change under impact of oxygen and antioxidants. Several experiments were conducted in order to analyze copper sulphide deposition pattern in different oils fortified with DBDS and a few different aromatic compounds: phenolic inhibitors (DBPC and DBF) and two aromatic alcohols, which were used for the course of investigation on the type of intermediate compounds able to carry copper and promote copper sulphide deposition in the paper. Mitigation techniques applied to counteract the problem of corrosive oils in service are: addition of metal passivators, oil treatments applied to remove corrosive suphur from the oil, oil change, modification of working regime and improvement of cooling systems of transformers. Service experiences and long-term effects of applied mitigation techniques have being collected and analyzed within CIGRE WG A2.40. Addition of metal passivators is the mitigation technique applied to highest extent. However, side effects were observed after several years of extensive application in service. Rapid depletion of metal passivator from the oil, stray gassing effects and mainly the production of hydrogen limit efficiency in some cases (scant reports of failure cases after passivation) were observed in practice. Metal passivator efficiency, absorption in the paper and the protective function of paper absorbed metal passivator once it has been consumed by the oil was investigated. Besides the protective function,the addition of metal passivator is accompanied by certain side effects: stray gassing, fast depletion of the metal passivator from the oil and interference with paper ageing. Significant rise of furan compounds was observed with oil containing metal passivator after IEC 62535 tests, in comparison to the same oil without metal passivator. In order to evaluate the effect of paper absorbed metal passivator on paper ageing DP (degree of polymerization of the paper) measurements were also performed.

Copper sulphide deposition in the paper: influence of oil type, oxygen content and aromatic compounds The postulated mechanism of copper sulphide formation and deposition in the paper relies on formation of dissolved copper complex compounds which are absorbed in the paper, where in the final step reaction with corrosive sulphur compounds takes place and copper sulphide is deposited on the paper. Better understanding of the mechanism of copper sulphide formation and deposition in the paper includes improved knowledge about oils inherent propensity for copper dissolution and deposition in the paper. Copper in oil dissolution is a necessary intermediate step in the reaction of copper sulphide formation and deposition in the paper, therefore a affinity of the oil to make complex compounds with copper is an important issue. Previous studies showed that oil affinity to transport copper to the paper is related to base oil composition, oxygen content and presence of specific aromatic compounds, namely oil oxidation inhibitors

1.

Comparison of uninhibited with inhibited oils in various ageing tests at 120

OC with varying oxygen contents, from

breathing conditions, to oxygen flow of 1 lit./h and a high amount of available copper surface, revealed that copper deposition in the paper is higher with inhibited oils. Differences between uninhibited and inhibited oils become more significant when oxygen content is increased when inhibited oils deposit more copper in the paper then uninhibited ones. In conditions of excess amount of oxygen (conditions of IEC 61225 B; oxygen flow 1lit/h), suppressed copper deposition in the paper takes place contributing to precipitation of insoluble copper compounds into sludge

1. This could be explained

by very intensive oil degradation of uninhibited oil, which is not adequately protected with inhibitors in such severe conditions, and consequently the transport of copper through the oil to the paper is hampered (Figure 1).

Uninhibited oil D

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Figure 1

Copper content in the paper after different ageing tests of uninhibited oil D - left and inhibited oil H - right Therefore uninhibited oil in conditions correlated to conditions of sealed units had achieved the highest copper content in the paper, while with an increase in oxygen this tendency decreased significantly. In inhibited oil in a condition correlated to free-breathing conditions of transformers the highest copper content in the paper was achieved. With an increase in oxygen, copper, content decreased, but in comparison to uninhibited oil much higher values of copper content were achieved, especially in conditions of extreme oxygen content ( 1lit. of oxygen flow /h). This observation was expected, since inhibited oil is better protected with inhibitors against severe oxidation, sludge formation and copper precipitation. In sealed conditions higher copper content was achieved with uninhibited oil, while in breathing conditions both oils will have similar values of copper content in the paper (figure 1). In a low to moderate oxygen range, copper deposition in the paper was higher in uninhibited oil; in moderate oxygen, free-breathing conditions values of copper content in the paper were similar for both oils, while in range of very high oxygen contents, copper content in the paper was higher after ageing of inhibited oil (figure 1).

Accelerated Ageing Test for Copper Sulphide Deposition in the Paper. In the course of the analysis of copper sulphide deposition in the paper, a set of experiments, modified IEC 62535 tests designed to simulate sealed and free-breathing conditions in terms of oxygen content, were performed at 150

0C during 7 days. One set of vials was

punched with a needle to provide the conditions of a free-breathing system (Figure 2) and another set was used according to standard procedure, simulating a non-breathing /restricted breathing system. Oxygen content in vials sealed with silicone septa was in the range of 2.5-10 ml/lit., while in the breathing system oxygen content was in the range of 7- 27 ml/lit. These values corresponded well to the levels of oxygen in sealed and free-breathing transformers. After ageing experiments copper content in the paper was determined using AAS technique, Thermo Sollar AAS. DBDS content was measured using GC-ECD method according to IEC 62697. Four different oils (D uninhibited and H inhibited high grade, P-L inhibited parrafinic, high grade, M uninhibited which originally contained DBDS) were used for the investigation (table 1.). Oils were different in carbon type composition, degree of refining and presence of oxidation inhibitor. Degree of refining is usually lower in uninhibited oils in comparison to inhibited ones. In uninhibited oils natural inhibitors are left in the oil, while inhibited oils have a better response to added antioxidant if they are more deeply refined. This was indicated by the lower values of refractive index and total aromatic content of inhibited oils in comparison to uninhibited ones. Three of four oils were spiked with DBDS and for the course of investigation oil D was additionally inhibited with DBPC (table 1).

Figure 2

Modified test conditions of IEC 62535

Table 1

Oil properties

Oils Ca/Cp/Cn DBPC, %m nd200C 40

oC, mm

2/s IR 39, ppm DBDS,

pmm

D-uninhib. 9.5/44.2/46.3 < 0.01 1.4809 8.30 nd 242*

H-inhibit. 3.2/43.8/52.5 0.30 1.4751 8.61 nd 249*

P-inhibit. 10.1/50.8/39.1 0.44 1.4741 11.10 nd 259*

M-uninhib. 14.0/34.7/51.3 < 0.01 1.4860 8.94 nd 255

nd: not detected * oils were spiked with DBDS Visual inspection of paper wrapped conductors after IEC 62535 test revealed that the four oils used in the experiment had significantly different copper sulphide deposition patterns. Increase in copper sulphide deposition in the paper was observed after the ageing test of uninhibited corrosive oils D and M in breathing conditions, while the same oils deposited copper sulphide only on the copper plate in non-breathing conditions (IEC 62535). Higher content of oxygen facilitated transport of copper sulphide from the copper plate to the paper (Figure 3).

Figure 3

Photographs of conductors after modified IEC 62535 test during 168h

Addition of phenolic compounds, primary inhibitors, DBPC (di-tert-butyl-para-cresol) and DBF (2,6 di-tert-butylphenol) to uninhibited oils D and M facilitated transport of copper sulphide from the copper plate to the paper (figures 3, 4a and 4b).

Rate of DBDS consumption was lower in highly refined, originally inhibited oils, H and P in comparison to uninhibited oil D with lower degree of refining. Rate of DBDS decrease was followed by the rate of copper increase, indicating increased rate of copper sulphide formation (figure 4.). It is noteworthy to mention that total copper content in the paper is the sum of copper bonded in sulphides and oxides, and consumed DBDS is not in total converted to copper sulphide, but also is consumed as a secondary antioxidant and may be degraded into other oxidation products. This effect was pronounced in breathing conditions, copper content in all oils was higher then in non-breathing conditions, as expected, but DBDS consumption was lower, indicating that more copper oxides are formed instead of copper sulphide. This was very pronounced in the case of oil D + DBPC (figure 4c and 4d) and another good example was oil P-L-inhibited which in breathing conditions deposited reddish-brown/rust color of copper oxide deposits on the copper and in the paper. Significant decrease of DBDS concentration was not followed up by an expected rise of copper content in the paper, as shown in figures 4 a-d. It was interesting to notice that decrease of DBDS was highest in oil D, but increase of copper content in the paper was lowest (Figure 4). On the other side, when DBPC was added to oil D, similar rate of DBDS decrease was followed by substantial increase of copper content in the paper. This result indicated that copper sulphide was formed on the copper plate in oil D, while after addition of DBPC (oil D + DBPC) deposits were formed on the paper and this was confirmed by visual inspection of paper and copper after test (Figures 3, 4).

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Figure 4a Figure 4b Decrease of DBDS in the oil Copper content in the paper

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Decrease of DBDS in the oil Copper content in the paper

On the basis of results from the previous experiment, additional experiments were conducted in order to better understand the mechanism of copper sulphide formation and the type of intermediate compounds which are able to carry copper to the paper. Three naphthenic oils were used, D and H (oils D-uninhibited and H-inhibited) fortified with DBDS and oil M-uninhibited which originally contained DBDS. Oils were different in carbon type composition and degree of refining (table 1). Oils were additionally spiked with the following compounds: di-terc.bytil phenol (DBF is a primary antioxidant, like DBPC), 1-phenyl-1propanol and 2-phenyl-1propanol in equal quantities of 0.40% and tested using modified IEC 62535 tests for 7 days at 150

0C.

Uninhibited oils D and M deposited copper sulphide on the copper plate a after regular IEC 62535 test. In breathing conditions, copper sulphide deposits were transferred to the paper (figure 3). Furthermore, after addition of DBPC and DBF to uninhibited oil, D and M copper sulphide deposition pattern changed - deposits migrated to the paper (Figure 3). Compared to the initial oils, copper content in the paper increased with the addition of aromatic compounds. The effect was pronounced when DBPC and DBF inhibitors were added (figure 5). Obtained results imply that certain aromatic compounds have affinity to bond copper by the formation of copper complex compounds. These copper sulphide intermediates facilitate transport of copper from the conductor to the paper, where reaction with sulphur compounds can occur. It was observed that oils D and M, both naphthenic uninhibited oils had different affinities to transfer copper to the paper (figures 3 and 5). These differences were related to chemical composition of the base oil, but carbon type composition and refraction index, pointing at oil degree of refining cannot provide answers related to this issue. Oil M had much lower affinity to dissolve and transfer copper to the paper after addition of inhibitors, DBPC and DBF and aromatic alcohols than oil D, but the shape of the curves were similar (figure 5, oils D and M) with apparent differences in concentrations of copper in the paper. Oils had similar aromatic content and refraction index, only the Cp fraction in oil M was much lower then in oil D (table 1). Inhibited oil H-high grade was more refined than previous uninhibited oils, but contained DBPC inhibitor and had much higher affinity to transfer copper to the paper in comparison to oil M.

Figure 5 Copper content in the paper after addition of different aromatic compounds to oils and 7 days of IEC 62535; lower oxygen

content left, higher oxygen content - right

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Metal passivator absorbed in the paper: Efficiency in suppressing copper sulphide deposition and side effects One of the most frequent side effects observed in practice is rapid depletion of metal passivator (IR 39) from the oil. This effect was observed to occur not only in aged insulation systems, but also in new ones and was not only related to units that had thermal problems or a severe working regime. However, in many cases detailed temperature profile of the transformer was not known. Depletion of metal passivator from the oil can be attributed to bonding of metal passivators to different metal surfaces inside a transformer, absorption of metal passivator in cellulose materials, degradation at elevated temperatures and under electrical stresses, and attack by oil degradation products, hydro-peroxides and acids. Depletion rate of approx. 25 % of initial concentration has been attributed to the normal bonding and absorption of metal passivator on metal surfaces and in cellulose materials. However, it is not clear yet and is not determined quantitatively how much passivator is needed to cover and saturate all metal surfaces and cellulose materials in a transformer. The process of diffusion and absorption of the metal passivator into cellulose materials is governed by temperature. It may be the case that passivated units operating at very low temperatures are not yet passivated fully, meaning that passivator has not yet being absorbed in cellulose materials completely. Therefore when temperature is increased, rapid decrease of metal passivator in the oil could be attributed to further absorption in insulation of metal passivator in thick cellulose materials. Diffusion and absorption of metal passivator in the paper and long-term effectiveness of paper absorbed TTAA in suppressing copper sulphide formation and deposition in paper-wrapped conductors was studied on lab scale

2.

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Figure 6

Metal passivator concentration in the oil, relative concentration in the paper and copper content in the oil at 150OC

2.

Migration of metal passivator from the oil to the paper occur during heat treatment of insulating oil (figure 6)

2.

Experiments for the evaluation of protective function of paper absorbed metal passivator were conducted in order to simulate conditions of passivated transformers with completely depleted metal passivator in the oil, when metal passivator is present only in the paper and on the copper surface, in order to mimic situation of early passivated units with no interferences from copper sulphide layers. Results of experiments indicated that passivated conductors resisted copper sulphide deposition at 140

0C for 120 h in non-

oxidized corrosive oil, while in the same oxidized corrosive oil copper sulphide formation on the copper plate started at 96 h, and after 120 h deposits were formed on the paper

2. Resistance to copper sulphide deposition in aged oil was lower

than in new oil. In conditions of low oxygen, copper sulphide was deposited on the copper plates, while in conditions of high oxygen content copper sulphide deposition was on the paper. This correlated well with the measured increase of dissolved copper in the oil during oxidative heating and postulated a mechanism regarding the role of oxygen in pronounced deposition of copper sulphide on the paper, via oil soluble copper complex intermediates (Figure 7)

3,4 .

Increase of copper content in the oil occurred sooner in the case of an absence of metal passivator in the paper (48 h and 72 h for CII vs. 72 h and 96 h for CI), at both low and high oxygen conditions (figure 7). These results confirm a previously postulated protective role of metal passivator absorbed in the paper insulation and its long-term effectiveness

5-9. The

increase of copper content in the oil occurs simultaneously with copper sulphide deposition on the paper. This might be correlated to increased concentration of copper sulphide intermediates released in the oil, as soon as metal passivator has been depleted from the copper plate and paper.

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Figure 7 Copper dissolved in new and aged oil in tests with CI (paper wrapped conductors saturated

with metal passivator, absorbed in the paper and bonded to the copper plate); CII conductors (conductors wrapped with new paper and metal passivator bonded only on the copper plate)

2.

Obtained results indicate that metal passivator is absorbed in the paper and has a protective function in suppressing copper sulphide deposition. Paper absorbed metal passivator function could be in transfer of metal passivator to the copper plate, where the paper is a kind of reservoir of metal passivator. Another possibility is the decomposition of copper complex compounds absorbed in the paper (copper sulphide intermediates) by metal passivator

2. These two

possible routes do not exclude one another as both scenarios could be going on in parallel. Besides the protective role of paper absorbed metal passivator, interferences with other compounds - products of paper thermal degradation were investigated. One of the side effects of metal passivator that was observed on lab scale was the rise of furans in the oil. These are widely used as cellulose degradation markers for condition monitoring of transformers in service. This effect was observed after IEC 62535 test of new and service aged oils with added metal passivator, in comparison to IEC 62535 test of the same oils without metal passivator. New oils, D, H and M used in this study and service aged oils E (low acidity 0.04 lmKOH/g) and L (moderate acidity 0.13 mgKOH/g) were subjected to the IEC 62535 test after the addition of metal passivator in order to evaluate impact of metal passivator on paper ageing. In some oils a significant rise in furan derivates was observed in comparison to furan content in the same oils without metal passivator that had undergone the same heating tests in the same time (table 2), new oil D and two oils from service, E and L. Nevertheless differences in furan content were high, DP values were very similar in cases of oils E and L , with and without metal passivator.

The observed effect of a significant rise of furans in these oils where DP values were similar could be attributed mainly to absorption of metal passivator in the paper, and consequently changed partition of furans between paper and oil, due to competitive absorption/desorption process. This implicated decrease of furan content in the paper, followed by the increase in the oil, i.e. rise of furan oil/paper partition coefficients. It is noteworthy to mention that these two oils had borderline corrosiveness, low concentration of corrosive sulphur compounds. However, after ageing of new, uninhibited oil D, a drop of DP was observed with passivated oil, in comparison to the oil without passivator. Besides changed partition of furans it seems that paper degradation was intensified. This could be related to oil quality, evolution of oil ageing products which interact with paper and paper absorbed metal passivator in the presence of high DBDS concentration. These by products most probably influence the higher rate of paper degradation, as darkened paper was observed after IEC 62535 test (figure 8). Oil D had the lowest degree of refining and had the highest copper deposition tendency after addition of DBPC, as previous results indicated. Interestingly acid formation in the oil D and H after ageing was the same (total acidity for both oils D and H was 0.05 mgKOH/g). Other two new oils, H and M behaved differently, as there were no significant differences in DP values and furan content in the oil with and without metal passivator (table 2.). Moreover in oils H and M, DP values were slightly higher when metal passivator was added to oils with dried paper wrapped conductors (table 3).

Table 2 Ageing test of oils with wet paper (water content = 4.4 ), with and without metal passivator

Oils IR 39o/ IR 39

IEC 62535

2-FAL 5-HMF 5-MEF 2-FOL 2-ACF DP

New D nd/nd +++ 12.22 1.73 0.37 0.00 0.04 540

New D + IR39 105/nd +/-* 32.19 2.93 1.31 0.00 0.24 403

New H nd/nd +++ 13.82 1.29 0.37 0.00 0.00 388

New H +IR 39 103/nd + 11.42 2.37 0.29 0.00 0.00 442

New MO nd/nd +++ 1.10 0.42 0.01 0.00 0.00 608

New MO +IR 39 117/12.8 - 1.01 0.33 0.01 0.00 0.00 599

Service E nd/nd +/- 7.80 2.90 0.19 0.00 0.04 391

Service E +IR 39 96/nd - 23.50 5.47 0.57 0.00 0.08 365

Service L nd/nd +/-* 8.59 3.02 0.25 0.00 0.05 469

Service L +IR 39 127/nd - 21.22 5.09 0.52 0.00 0.10 427

nd: not detected; +/- minor deposits, edges of the paper; +/-* minor deposits on copper plate and edges of the paper

Figure 8 Paper and copper plates after IEC 62535 test of oils after addition of metal passivator

Table 3

Ageing test of oils, dried paper (paper water content 0.5 %), with and without metal passivator

Oils IR 39o/

IR 39 IEC 62535

2-FAL 5-HMF 5-MEF 2-FOL 2-ACF DP

New D nd/nd +++ 8.14 1.24 0.24 0.00 0.00 746

New D + IR39 147.1/nd +/- 23.88 2.79 0.68 0.00 0.10 556

New H nd/nd +++ 10.60 0.88 0.26 0.00 0.00 449

New H +IR 39 160.2/nd + 6.67 1.94 0.14 0.00 0.00 538

New MO nd/nd +++ 0.70 0.24 0.00 0.00 0.00 638

New MO +IR 39 166.3/23.2 - 0.69 0.21 0.01 0.00 0.00 678

Service B nd/nd ++ 9.60 1.25 0.24 0.00 0.00 617

Service B + IR 39 147.7/nd - 13.27 2.90 0.28 0.00 0.03 632

nd: not detected; +/- minor deposits on the edges of paper Since the problem of paper ageing is directly related to paper water content and with the aim of investigating conditions closer to real service application, IEC 62535 tests with dried paper and different new oils, uninhibited and inhibited were performed. Paper wrapped conductors were dried prior to IEC 62535 test to values 0.4 - 0.5%. Another service aged oil Service B was tested for effects on paper ageing using dried paper. Oil was inhibited, contained 38 ppm of DBDS and deposits of copper sulphide were on the paper dominantly, after IEC 62535. Degradation of cellulose and production of furans in oils with dry paper was significantly lower than in wet insulation, when the same oils were compared (tables 2 and 3), since water promotes paper degradation and dissolution of furans in the oil. However, the impact of metal passivators on paper ageing remained the same for each oil, as it was in the previous wet condition of the paper (table 2). It can be summarized that paper absorbed metal passivator did not interfere with paper ageing for most of the investigated oils, since DP values were similar regardless of the presence of metal passivator, with exception of oil D.

Conclusion Copper sulphide formation and deposition in the paper takes into account complex reactions and interactions with other materials, namely metals and paper, under varying parameters, like temperature, oxygen, oil chemical composition or additives. Postulated mechanism of copper sulphide formation in the paper is based on copper in oil dissolution, formation of copper complex intermediates which are dissolved in the oil, followed by absorption in the paper where reaction with sulphur compounds takes place and copper sulphide is precipitated on the paper. Oils tendency to dissolve and deposit copper in the paper is favored by oxygen and the addition of oxidation inhibitors. Base oil composition has strong influence on oils affinity to dissolve and deposit copper, but these relations could not be straightforwardly correlated to carbon type composition. Significant difference in the copper deposition patterns of uninhibited and inhibited oils were observed. In non-breathing conditions the copper content of uninhibited oil was higher than the copper content of inhibited oil. Copper deposition tendency of uninhibited oil decreased with a substantial increase of oxygen. This was attributed to copper precipitation into sludge after advanced oil oxidation in an highly oxidative environment when transport of copper to the paper is hampered. On the other side copper content of inhibited oil was increased by the increase of oxygen. In free-breathing conditions copper contents were similar for both, uninhibited and inhibited oil, while in conditions with high oxygen ingress, copper content in the paper was higher in inhibited oil. Copper sulphide deposition pattern on the paper was observed more with inhibited oils, while uninhibited oils deposited copper sulphide on the copper plate. However when uninhibited oils were tested in conditions of higher oxygen content, free-breathing conditions, copper sulphide deposition in the paper increased. After addition of phenolic inhibitors, DBPC and DBF to uninhibited, copper sulphide deposition in the paper also increased. This effect was attributed to increased copper in oil dissolution, favored in oxygenated atmosphere, by the formation of Copper-DBPC/DBF complex compounds. These compounds absorbed in the

paper facilitated reaction with sulphur compounds and formation of copper sulphide in the paper. Absorption process is governed by diffusion of copper intermediates, favored by increase of oxygen and temperature. The addition of metal passivators is an efficient mitigation technique when it is performed in the early stages of transformer and oil life. Metal passivator has an affinity to absorb into cellulose and this process is governed by increase of temperature. Paper absorbed metal passivator seems to have protective function, as passivated conductors can resist copper sulphide deposition for some time at certain temperatures, even in the case when it is completely consumed by the oil. This protective activity was longer in asystem with lower oxygen content and a low degree of oil ageing. Impact of paper absorbed metal passivator on paper ageing was investigated to certain extent. In most of the investigated oils there were no effects of passivator on paper ageing, although paper ageing markers increased significantly in some oils (oils from service) with addition of metal passivator. Rise of furans in these oils could be attributed to changed partition of furans between oil and paper, since the DP values were not affected by addition of metal passivator. However, it seems that addition of metal passivator during accelerated ageing test of low quality oil with a high content of corrosive sulphur compounds may interfere with paper ageing. This could be attributed to interactions of paper absorbed copper complex compounds, DBDS and metal passivator and generation of by-products which may increase paper ageing rate. It was apparent that different oils behaved differently in terms of metal passivator efficiency, impact on equilibrium of furans and paper degradation. More investigations with different oils are needed to analyze the impact of paper absorbed metal passivator and the degradation products of oil and metal passivator on paper ageing and paper ageing markers partitioning, maybe at lower temperatures over longer ageing periods. Nevertheless only one of six oils exhibited interference with paper ageing it would be desirable to test oil response to the addition of metal passivator in terms of efficiency and possible side effects, including interactions with paper ageing prior to application.

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Authors Biographies Jelena Lukic, is Head of Laboratory for Insulating Oil and Paper Testing, Department for Electrical Measurements of Electrical Engineering Institute Nikola Tesla. The Institute Nikola Tesla is an independent research organization, which is in one part responsible for testing and diagnostics of electrical apparatus equipment for utilities in production and distribution of electricity. Jelena Lukic earned Master degree in Chemical Engineering, currently working on PhD thesis, earned her university degree of Faculty of Chemical Engineering, University of Belgrade, Serbia. She is engaged in different IEC TC 10 working groups and CIGRE SC A2 and SC D1 WGs. She is the convener of CIGRE WG A2.40: Copper sulphide long-term Mitigation and Risk Assessment. Srdjan Milosavljevic is Manager of Department for Electrical Measurements in Electrical Engineering Institute Nikola Tesla in Belgrade, Serbia. The main area of activities of the Electrical Measurement Department is electrical measurement of electrical, magnetic and non electrical quantities and testing, monitoring and diagnostics conditions of power and instruments transformers and generators, also development and implementation of measurement methods and measurement devices. University degree earned at University of Belgrade, Faculty of Electrical Engineering. Draginja Nikolic is working in Electrical Engineering Institute Nikola Tesla, Serbia, in Laboratory for Testing and Calibration, as young associate, research engineer. She is working in the Department for Insulating Oil Testing. Her field of work is paper aging, gas and liquid chromatography. She is engaged in research activities in the field of natural ester oils. Draginja Nikolic has a university degree from the Faculty of Chemical Engineering, University of Belgrade, Serbia. She gained her B.Sc. in Chemical Engineering in November 2008. Valentina Mandic is working in Electrical Engineering Institute "Nikola Tesla", in the Laboratory for Testing and Calibration, Department for Mineral Insulating Oil Testing, in the position of Associate Research Engineer. Her field of work is oil analyses, high-performance liquid chromatography, infrared spectrofotometry, Karl Fisher coulometry, determination of viscosimetric degree of polymerization of paper for electro technical purposes, etc. She is engaged in research activities in the field of natural ester oils. In 2008 she gained her B.Sc. in Chemical Engineering at the Faculty of Chemical Engineering, University of Belgrade, Serbia. Aleksandar Orlovic, Associate Professor Department of Organic Chemical Technology and Chemical Engineering, responsible for teaching of chemical reactor design and petrochemical engineering. His Masters Thesis was on the subject of catalysts used in Fischer Tropsch, and PhD thesis in heterogeneous catalysis using aerogel in alkylation reactions. His main field of work cover the area of reactor engineering, reactor modeling and simulation, catalysis, petrochemical processing, oil re-refining.