Effects of triazole additives in transformer oils. SCHAUT Annelore, EECKHOUDT Steve Laborelec Belgium

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1 21, rue d Artois, F PARIS D1-102 CIGRE 2012 http : // Effects of triazole additives in transformer oils SCHAUT Annelore, EECKHOUDT Steve Laborelec Belgium SUMMARY Recently an undeclared triazole additive, TAA was detected in some commercially available transformer oils. Several laboratory studies such as stability/degradability, impact on oxidation stability and corrosive sulfur were performed. The stability of the compound was determined by performing a heating test based on IEC61125C. After 24h, TAA could not longer be detected. In the presence of a Cu coil, this decrease was accelerated. The IEC61125C oxidation stability test was performed to verify the impact of this additive on oxidation stability, once without any pretreatment, secondly after IX-resin treatment and finally after IX-resin treatment and readdition of the compound. It was clear that the resin treatment removed this additive from the oil but also some other (natural) inhibitors. Readdition did not fully recover the oxidation stability of the oil. This could indicate that not only the triazole compound was removed from the oil by the IX-resin treatment but also that some other (natural) inhibitors were present in the oil. The impact on corrosive sulfur was examined by applying the IEC62535 before and after IX-resin treatment but it seemed that the oil was not protected against DBDS. From these laboratory studies it seems that only the oxidation stability of the oil was improved. KEYWORDS Transformer oil, oxidation stability, corrosive sulfur, antioxidant, metal passivator, metal deactivator 0

2 1. INTRODUCTION Oxidation of transformer oil leads to the production of many polar compounds such as acids, aldehydes, ketones, peroxides and alcohols affecting the insulating properties of the oil but also forming sludge which affects the heat transfer properties of the oil. This seriously damages the performance of power transformers, thereby reducing their life [1]. The main function of insulating liquids is to ensure high dielectric properties and compatibility with solid insulation, expressed as high dielectric constant, low and stable kinematic viscosity over ranges of temperatures to enable efficient heat removal. Change of dielectric properties, efficiency in gas absorption and heat removal, due to the formation of polar and conductive by-products are influenced by oil oxidation rates. Test methods for determination of oil oxidation stability, at defined temperatures, catalysts surface and oxygen content should correlate to existing transformer working conditions [2]. In order to improve the stability of mineral transformer oil against oxidation and thereby to improve the lifetime of the oil, additives known as antioxidants, metal deactivators and passivators can be added to the oil [1]. Antioxidants play an important role in the rate of formation of ageing products. The primary phenolic oxidation inhibitors act as radical scavengers by radical chain breaking during the first stage of the oxidation, the initiation formation of radicals. The best known phenolic inhibitor, 2,6-di-tert-butyl-pcresol (DBPC) is functional at temperatures up to 150 C. The secondary antioxidants like alkyl phenyl thio-ethers, disulfides and sulfurized alkyl phenols are commonly applied in different lubricant and industrial oil applications. These compounds play a very important role in the breakdown of the propagation of the oxidation process. Dibenzyl-disulfide (DBDS) is a well-known antioxidant but is also a compound with high corrosive potential [2,3]. Studies during the last several years have shown that DBDS in one of the main substances causing deposition of copper sulphide on insulating paper [4]. Another important triazole additive influencing the oxidation of the oil is an oil deactivator such as Irgamet 30 (or N,N-bis(2-ethylhexyl)-1,2,4-triazol-1-ylmethanamine, Irg30 of TAA). These oil additives are used to stabilize the fluid by deactivating metal ions which are mostly introduced by the action of the naturally occurring acids generated by the oxidative process. Thus, metal deactivators inhibit the catalytic effect of the metal ions. Corrosion phenomena may occur when chemical reactions between the different parts take of an oilimmersed transformer, such as copper, iron, cellulose, wood, rubber, glues, polymers and obviously oil take place. Three materials are mainly involved in corrosion reactions; oil, copper and paper. Corrosion may cause the formation of pollutants, both dissolved in the bulk oil and forming deposits in the paper or insoluble compounds that precipitate as sludge. Copper sulfide formation causes copper sulfide growth on the conductor surface and on solid insulation and on the other hand copper dissolution which forms dissolved and suspended copper compounds. Several mitigation techniques have been proposed to reduce the impact of corrosive sulfur in transformers [5]. The use of copper passivators is the most widely applied countermeasure. Irgamet 39 (or N,N-bis-(2ethylhexyl)-4- methyl-1h-benzotriazole-1-methanamine; Irg39), the most common used and commercially available metal passivator, may slow this process by passivation of the metal surfaces [2]. Passivation is a term used to describe the formation of non-permeable protective layers on metal surfaces. The formation of such layers prevents corrosion and dissolution by decreasing the exchange currents between the electrolyte and the metal. Thus, passivators create a chemical layer that protects the copper from the oil, and the oil from the copper. This prevents the formation of oil soluble copper complexes that can catalyze oil oxidation [6]. 1

3 It has been shown that oil passivated with Irg39 will, on depletion of the passivator, revert to its original corrosive conditions, leading to the deposition of copper sulfide on the insulating paper and copper. It seemed that the presence of air in the oil, together with high temperature, decomposed the passivator much faster than in the absence of the air at the same or higher temperatures. Recently, a new additive was detected in some commercially available oils. Research into this new additive, being Irg30, is required as its structure is comparable to the one of Irg39. All compounds added to mineral insulating oil in order to improve certain characteristics must be declared in product data sheets or certificates of compliance according to IEC The aim of this study was to test the impact on oxidation stability and corrosion phenomena of this undeclared additive, as it was not communicated by the supplier. 2. MATERIALS AND METHODS The analysis of Irg30 [7] was performed with an UPLC-TQD (Ultra high liquid chromatography coupled to a triple quadrupole detector, Waters) in positive APCI (atmospheric pressure chemical ionization) mode. The applied UPLC configuration and gradient are shown in table 1 and 2, respectively. Multiple reaction monitoring (MRM) was chosen as MS method. During the development of the method an in-source fragmentation occurred. One of the in-source fragments, being m/z (cone voltage 35V) was used as parent ion. Fragmentation of this parent ion occurred with a collision energy of 20eV giving m/z 57.1 and m/z 71.1 as most abundant daughter ions. Linearity, using a serial dilution of a prepared standard ( mg/l) was excellent as well as repeatability with an RSD of 2.3%. Column Acquity UPLC HSS T3 C18, 2.1 x 100 mm, 1.7µm Column temperature ( C) 40 Flow rate (µl/min) 0.5 Mobile phase A Water + 0.1% formic acid Mobile phase B Methanol + 0.1% formic acid Injection volume 5 µl (partial loop with overfill) Table 1: UPLC configuration conditions Time (min) % A % B Table 2: Applied UPLC gradient An in-house method, the IX-resin treatment (IEC60296 Ed. 4, annex A, procedure 1) was developed to extract metals passivators from the oil. This method will be recommended in the IEC60296 Fluids for electrotechnical applications unused mineral insulating oil for transformers and switchgears draft 4. By using an adsorbent, IX-resin (a strong mixed mode polymer based cation exchanger for basic analytes, Chromabond HR-XC resin, Macherey-Nagel) it is possible to extract metal passivators from the oil. Therefore, 500µg IX-resin is added to 100mL oil. After 1h shaking and precipitation of the IXresin, the oil is filtered (0.8µm) and tested for corrosive sulfur according to IEC62535 Insulating liquids test method for detection of potentially corrosive sulfur in used and unused insulating oil. 2

4 3. EXPERIMENTAL STUDIES AND RESULTS During the last 2 years some significant amounts of Irg30 were detected in commercially available mineral oils of different oil suppliers, such as oil 1 and 2 used during some laboratory studies. In a first test the stability or degradability of Irg30 was determined by performing a heating test based on IEC61125C (oxidation bath 120 C, 150mL/h air flow) in the presence of a copper coil. Periodically sampling of the oil and thus, measurement of Irg30 was performed within 48h. The same test was also repeated without the presence of a copper coil. The results are summarized in table 3. The results showed that the amount Irg30 in oil 1 decreased to undetectable within 24h. These results also showed that the rate of decrease of Irg30 seemed higher in case a Cu coil was present but this difference was rather limited and should be further investigated during a long-term stability test. The evolution of Irg30 in oil 2 confirmed the former results. Time (h) OIL 1 OIL 2 Irg30 (ppm) 1 Irg30 (ppm) 2 Irg30 (ppm) 1 Irg30 (ppm) < 0.5 < 0.5 < 0.5 < < 0.5 < 0.5 < 0.5 < < 0.5 < 0.5 < 0.5 < < 0.5 < 0.5 < 0.5 < 0.5 Table 3: Results of the periodically measurement of Irg30. 1 Test with Cu-coil, 2 test without Cu-coil In a second test the impact of Irg30 on the oxidation stability was studied. Again, 2 commercially available uninhibited oils (oils 1 and 2) with a known amount Irg30 were chosen. The oils were subjected to the IEC61125C oxidation stability test, once without pretreatment (test 1), secondly after IX-resin treatment (test 2) and finally after IX-resin treatment and readdition of the initial Irg30 concentration (test 3). This was performed to prove or verify that the IX-resin treatment only removed Irg30 and no other oxidation stability improving additives or natural molecules. The results are summarized in table 4. Volatile acidity (mg KOH/g) Soluble acidity (mg KOH/g) Total acidity (mg KOH/g) DDF 1 Sludge (%) S content 2 (ppm) OIL1 test test test OIL2 test test test Table 4: Summary of the results of the impact of Irg30 on oxidation stability. Test1 is according to IEC61125C, test2 is IEC61125C after IX-resin treatment and test3 is IEC61125C after IX-resin treatment and readdition of Irg30. 1 Dielectric Dissipation Factor at 90 C. 2 Total sulfur content according to ASTM D5185 The results of the acidity of oil 1 after the 3 tests are shown in figure 1. The total acidity in test1 after the oxidation test according to IEC61125C was 0.35 mg KOH/g. The total acidity increased to 0.73 mg KOH/g after the IX-resin treatment. After readdition of Irg30 the total acidity decreased again to 0.56 mg KOH/g. It could be concluded that the acidity was clearly highest after IX-resin treatment 3

5 which indicated that this treatment removed Irg30 from the oil. After readdition of Irg30 the total acidity was lower, but not as low as the original value (test1). Perhaps this is due to the fact that the IX-resin treatment not only removed Irg30 from the oil but also some other (natural) inhibitors present in the oil. An additional test, total sulfur content according to ASTM D5185, was performed with oil 2. But the total S-content remained stable during the 3 tests indicating that no secondary S-containing antioxidants were removed by the IX-resin treatment. This could signify that some other non S- containing (natural) inhibitors were present in the oil. Figure 1: Evolution of the acidity of oil 1 Also the impact of Irg30 on corrosive sulfur was studied. A known amount of DBDS (approximately 50ppm) was added to the same 2 commercially available oils containing Irg30. Two tests were performed, determination of corrosive sulfur according to IEC62535 before and after IX-resin treatment. It seemed that oil 1 was corrosive after both tests. Similar results were obtained for oil 2. As Irg30 is a metal deactivator, it will not form protective layers on the copper surface. Some additional tests with oil 2 were performed. Higher amounts Irg30, approximately 100 and 300ppm were added to the oil. The oil still tested positive before as well as after resin treatment. 4. CONCLUSIONS The oxidation stability is one of the most important insulating oil properties affecting all application properties during its service life. Additives such as antioxidants, metal passivators and metal deactivators are used in order to improve the stability of mineral transformer oil against oxidation and thereby, to improve the lifetime of the oil [1]. Metal passivators, such as Irg39, are widely used to inhibit corrosion by passivation of the copper. The effect of corrosion is reduced by the formation of a molecular layer onto the copper surface. Metal deactivators are used to deactivate the metal ions produced in the oil by oxidation by-products like acids. During this study the impact of an undeclared additive Irg30 on oxidation stability and corrosion phenomena was examined. During a heating test based on IEC61125C with and without copper coil the stability or degradability of the compound was studied. It seemed that the concentration of Irg30 decreased to undetectable within 24h. In the presence of a copper coil the rate of decrease was higher which could be expected as Irg30 acts as a metal deactivator in the oil. The impact of Irg30 on the oxidation stability was studied according to IEC61125C. Commercially available oils with a known amount Irg30 were subjected to the IEC61125C oxidation stability test, first without any pretreatment, secondly after IX-resin treatment and finally, after IX-resin treatment and readdition of Irg30. The 4

6 total acidity was highest in the second test, after IX-resin treatment, indicating that this pretreatment removed Irg30 from the oil. After readdition of Irg30 the total acidity was better. The total S-content remained stable during the 3 tests, meaning that no secondary S-containing antioxidants were removed by the resin treatment. These findings could indicate that other (natural) oxidation inhibitors were present in the oil. As the structure of Irg30 is very alike the structure of the well-known metal passivator Irg39 it could be possible that Irg30 would also have an impact on the corrosive sulfur problem. Therefore, a known amount DBDS was added to the commercially available oils containing Irg30. The oils were subjected to IEC62535 before and after IX-resin treatment. It could be concluded that before as well as after the IX-resin treatment the oils tested positive according to IEC62535, independent of the amount Irg30 present in the oils (Figures 2, 3 and 4). From these results it seemed that Irg30 was not able to protect the copper from corrosive sulfur. Figure 2: Result of IEC62535 on oil 2 containing approximately 100ppm Irg30 without addition of DBDS Figure 3: Result of IEC62535 on oil 2 containing approximately 100ppm Irg30 and 50ppm DBDS before resin treatment Figure 4: Result of IEC62535 on oil 2 containing approximately 100ppm Irg30 and 50ppm DBDS after IX-resin treatment The overall conclusion was that Irg30 can inhibit the oxidation of the oil because this compound acts as a metal deactivator in the oil. Thus, Irg30 improved the oxidation stability results of the oil. 5

7 Conversely, Irg30 did not have a protective effect towards the copper. This undeclared additive is not able to form protective layers on the surface. 6

8 BIBLIOGRAPHY [1] P.R. Krishnamoorthy, S. Vijayakumari, S. Sankaralingram, Effect of antioxidants and metal deactivators on the oxidation of transformer oil, IEEE Trans. Electr. Insul., 1992, Vol 27, pages [2] J. Lukic, Oxidation phenomena and role of oil inhibitors, My tranfo 2008, Turin, Italy [3] F. Scattigio, V. tumiatti, R. Maina, M. Tumiatti, M. Pompili, R. Bartnikas, Corrosive sulfur in insulating oils : its detection and correlated power apparatus failures, IEEE Trans. Power Del., 2008, Vol 23, pages [4] T. Amimoto, E. Nagao, J. Tanimura, S. Toyama, N. Yamada, Duration and mechanism for suppressive effect of triazole-based passivators on copper-sulfide deposition on insulating paper, IEEE Trans. Dielectric. Electr. Insul., 2009, Vol 16, pages [5] R. Maina, Corrosion phenomena, My transfo 2008, Turin, Italy [6] B. Pahlavanpour, K. sindkvist, Mineral insulating oil passivation; effectiveness of passivation to stop copper deposition, Nynas naphthenics, 2007 [7] A. Schaut, S. Autru, A. De Rop, S. Eeckhoudt, Effects of Irgamet30 as additive in transformer oil, IEEE Trans. Dielectric. Electr. Insul., accepted 7