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The effect of passivator additive used in transform ers and reactors’ mineral oil to neutralize the sulphur corrosion, and its influe nce on low thermal defects.

André Vita – Furnas – Brasil (*)

Paulo R. T. Patrocinio – Furnas – Brasil (*) Sérgio A. Godinho – Furnas – Brasil Edilson G. Peres – Furnas – Brasil

João Baudalf – Areva - Brasil (*) Principal Authors.

1. SUMMARY 1.1. In mid 1998, Brazilian manufacturers of transformers and reactors began to use napthenic insulating mineral oils, made of Venezuelan crude, Type Nitro 10 GBA or 10 GBN. All of the reactors that failed since 2004, from a number of different manufacturers, used this type of oil which was used also in the 550 kV Furnas reactors connected in the Ibiúna/Bateias transmission line and that till this moment have not failed. 1.2. The failures that occurred in the equipment filled with Nitro 10 GB oil were associated with the corrosive sulphur present in these oils. 1.3. The transmission utilities in Brazil, including Furnas due to the importance of the transmission line Ibiúna/Bateias to the Brazilian system, decided to investigate what was happening to the reactors - on one hand concerned with the integrity of the equipments, on the other hand concerned about the high costs involved in losing commercial operation. Due to the long transmission line lengths involved, the Brazilian transmission utilities cannot operate without reactors and their failure could result in both loss of revenue and penalties imposed by the National Regulating Agency of Brazilian System. 1.4. In all the equipment where the presence of corrosive sulphur was confirmed, Furnas passivated the insulating oil, together with the reactor manufacturers, using a passivator derived from benzotriazol recommended by the oil supplier. 2. KEYWORDS Passivator - Sulphur Corrosion - Thermal Defect - Mineral Oil – Reactors - Combustible Gases - Hydrogen

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3. History: 3.1. Characteristics and installation site: - Installation site: Ibiúna Substation ( Ibiúna/Bateias transmission line ) - Equipment: Single Phase Shunt Reactor - Line Voltage: 550 kV - Frequency: 60 Hz - Power: 35 MVAr - Serial number: 111281-5, 111-281-7, 111281-1 - Manufacturing year: 2002 - Energization year: 2003 3.2. Due to the fact that all the reactors in the Ibiúna/Bateias transmission line were filled with Nitro 10 GBN, they were passivated ( adding one liter of passivator for each 1000 liters of oil) between 02/09/2005 and 17/09/2005. In the table below are indicated the ppm of H2, using the DGA test some days after the passivation, and the ppm of passivator found in the oil of the Ibiúna reactors almost one year later: Reactor Sample H2 (ppm) Passivator (ppm) in 08/2006 111281-1 28/09/2005 55 58 111281-2 28/09/2005 6 28 111281-3 28/09/2005 6 23 111281-4 28/09/2005 10 23 111281-5 28/09/2005 368 >150 111281-6 28/09/2005 2 27 111281-7 28/09/2005 6 28 3.3. Two years after initial energization, reactor serial number 111281-5 of the Ibiúna Substation began to display an abnormal evolution of combustible gases in the insulation oil. The main gas was hydrogen and the gas increase in this reactor was much higher than in the other identical units installed in the same substation. The gas evolution before the equipment was removed from service was: Test date H2 O2 N2 CH4 CO CO2 C2H4 C2H6 C2H2 24/02/2003 3 11560 24666 1 32 78 0 0 0 25/03/2003 4 20384 44059 1 55 202 0 0 0 27/03/2003 5 11024 25532 1 46 141 0 0 0 12/06/2003 21 15046 47875 4 214 587 2 10 0 19/11/2003 20 10580 59153 37 381 1524 6 46 0 22/01/2004 17 8171 54124 37 402 2127 6 55 0 22/11/2004 17 2744 59665 49 120 1858 4 87 0 22/11/2005 714 3279 57467 43 393 1505 3 101 0 07/12/2005 545 5505 57231 36 345 1224 2 85 0 3.4. Analyzing the evolution of the key gases CH4, C2H6 (which existed prior to passivation) and H2 in the table above, the diagnostics using the IEC and LABORELEC methods they indicate a thermal problem and partial discharges in oil. After an internal investigation in the field, made in January 2006, where nothing was found, before return to commercial operation the oil was degassing and the reactor was energized

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again and after a few days in operation the gases increased again with a predominance of the hydrogen as can be seen in the follow table: Test date H2 O2 N2 CH4 CO CO2 C2H4 C2H6 C2H2 16/01/2006 9 1785 5354 1 13 61 0 1 0 26/01/2006 220 12052 42643 9 162 672 0 10 0 03/02/2006 220 1529 12948 8 144 391 0 8 0 23/02/2006 315 1620 20405 14 216 655 0 14 0 03/03/2006 349 4387 31729 22 426 888 0 19 0 23/03/2006 399 1396 23464 23 169 799 0 18 0 The gas analysis were still indicating a thermal defect in the reactor and the unit was returned to the factory for a thorough inspection and investigation. 4. Inspections 4.1. In the field: 4.1.1. Two internal inspections were made at the substation: one as previously mentioned and another prior to returning to the manufacturer. No non conformity was found in the equipment that could explain the gas evolution. Also an acoustic detection was made and it did not indicate any partial discharge activity that could be associated with the internal anomaly. 4.2. In the factory: 4.2.1. After a factory partial discharge test that did not show any problem, the reactor was opened and the winding removed with the presence of Furnas. See the pictures below:

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4.3. During the inspection it was verified: 4.3.1. Darkening of the paint in the middle of the core pressing structure ( under the winding ) in the bottom of H1 side , caused by a strong heating due to a modification of the nonmagnetic characteristics of the stainless steel plate as a consequence of the welding process. The fenolic insulation material used between the pressing structure and the core showed also heating signs as it can be seen in the picture:

4.3.2. Darkening with less intensity in the same zone of the side H0 and the upper pressing structures. 5. Solution: 5.1. The core clamping structure used in the reactor are all made of nonmagnetic stainless steel to avoid excessive heating due to the winding stray flux. The reason why one of the lower plates become warmer than the other three ( including the upper structure where the oil is warmer) can probably be attributed to the welding process that was used because high temperatures can alter the characteristics of the nonmagnetic stainless steel making it become magnetic. 5.2. As a solution to the problem, to avoid heating of the core clamping structure, aluminum shields were introduced to reject the leakage flux. To avoid double earthing the shield was insulated with aramid.

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5.3. Besides the removal of the paint in the affected zone and the introduction of the shield, all the insulation material with heating signs or damaged during disassembling was substituted. 6. Conclusion: 6.1. Analyzing the DGA results of the reactors it can be seen the higher the quantity of passivator, the higher the quantity of hydrogen (H2). This verifies studies presented in some papers that link the passivator quantity to the hydrogen, which tends to increase even under normal operating conditions. 6.2. Some minor thermal faults do not require the user of transformers and reactors to deenergize the equipment to make repairs. These faults may be tolerated with regular monitoring of the evolution of the gas in oil. 6.3. The presence of a metallic part with high temperature, even without any risk of failure, after the addition of passivator in the oil caused a significant increase in the quantity of hydrogen in a very short time obliging Furnas and the manufacturer to return the reactor to the factory and make a repair. 6.4. The heated metallic parts were the cause of the increase of CO, CO2, CH4 and C2H6. 6.4.1. Normally an increase of hydrogen is due to: -Partial discharges -Paint not well cured -Galvanized metal -Unpainted stainless steel -Thermal faults 6.5. The experience described above, showed in this specific case, that metal passivator used in the Ibuna-Bateias 550 kV reactors made sudden increase the hydrogen (H2) gas in the oil where there was a thermal defect. 6.6. Metal passivator did not have any influence on thermal defect, but was the reason of hydrogen increase and anticipated necessity of repair the Furnas’ 550 kV reactors. 6.7. After the described repair the reactor is operating normally and displaying acceptable DGA results without increase in the hydrogen level as indicated in the next table:

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Test date H2 O2 N2 CH4 CO CO2 C2H4 C2H6 C2H2 06/11/2006 0 11676 32919 1 3 40 0 0 0 18/11/2006 0 2377 6388 1 10 41 1 0 0 20/11/2006 0 2448 7308 1 12 40 0 0 0 24/11/2006 1 1854 6727 1 19 51 0 0 0 19/12/2006 1 7772 25297 2 95 264 2 1 0 11/01/2007 0 12486 36358 3 156 467 2 1 0 13/02/2007 0 12421 44554 3 276 660 2 0 0 13/03/2007 0 19141 68950 6 451 1496 2 1 0 13/04/2007 3 12839 60493 6 579 1580 5 2 0 16/05/2007 5 17352 60991 6 546 1770 2 1 0 6.8. As a precaution Furnas decided to ask to the manufacturer to change the oil of all the reactors installed in Ibiúna and Bateias Substations. This preventive action has the objective of avoiding the increase of hydrogen in the oil that could lead to errors in the DGA interpretation and to ensure the avoidance of any problem due to the corrosive sulfur present in the original oil that could impact upon the commercial operation of the Ibiúna Bateias transmission line. 7. BIBLIOGRAPHY [1] The Copper Corrosion Phenomenon – Nynas ( May/2005 ) Mr. Kjell Sundkvist, Mr. Bruce Pahlavanpour [2] Corrosive Sulphur Its Origin Detection and Prevention – Siemens ( June/2005 ) Mrs. Martinato, Mr. Miethke, Mr. Asano, Mr. Alaor, Dr. Knorr, Mrs. Höhlein [3] Copper Strip Corrosion Standards – ASTM Method D130/IP 154 [4] ASTM D1275 – Standad Test Method for Corrosive sulfur in Mineral Oils [5] NBR 10505 – Óleo Mineral Isolante – Determinação de Enxofre Corrosivo [6] IEC 60296 – Fluids for Electrotechnical Applications Unused Mineral Insulation Oils for Transformers [7] IEC 60422 – Mineral Isulating Oils in Electrical Equipment – Supervision and

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