Serveron Technical Note: Hydrogen Detection A Detector vs. Diagnostic Tool (warning vs. analysis) Detector – Detector – a means to indicate that a fault is occurring without quantification of an incipient fault. There are no specific tools/methods available for use with a detector type system to specifically identify the type of transformer fault occurring. Diagnostics – a means to identify and determine the type of incipient fault occurring. A key element with a diagnostic type system is the availability of various tools/methods that can be utilized to determine the type of transformer fault occurring. A single gas (i.e. hydrogen, H 2) monitoring solution product that provides only a warning of an impending fault is inadequate compared to a multi-gas solution that is capable of supporting a diagnosis of the most most critical faults – partial discharge, thermal faults and arcing. A hydrogenonly solution merely provides alarm on an unknown problem (critical or otherwise) with absolutely no diagnosis. Sources have shown that many types of faults evolve the generation of H 2 gas. In addition to the traditional fault types H 2 has been shown to be generated by; •
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Rust inside a transformer, where the moisture in the oil reacts with any uncoated surfaces. The combination of H 2O & galvanized metal (i.e. a pipe fitting added to the transformer) produces huge quantities of H 2 (that have confounded DGA results over the years). Exposure of oil to sunlight, during installation/inspection when the transformer tank is opened.
H2 is the tiniest molecule, the lightest fault gas and by far the least soluble in oil of any transformer gases including N 2 & O2. CO is the next lightest but over twice as soluble as H 2, then CH4 that is 9 times more soluble. This means two things; •
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H2 does not like to dissolve into the transformer oil and is more likely to accumulate in small gas pockets at the top of a transformer. H2 tends to ultimately escape to the atmosphere through gaskets & welds (that oil won’t leak through).
Partial Discharge types: Hydrogen does identify or provide an early warning of Partial Discharges (PD), sort of. There exist two types of PD that can be identified by Hydrogen (H 2)- Corona or Ionization type of PD and Sparking. Along with Hydrogen (H 2), Methane (CH 4) is also an early detector of Corona types of PD.
© 2006 Serveron Corporation. All rights reserved. Printed in USA. PN 810-1716-00. Rev. B
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Corona Type – is early PD that occurs anywhere within a transformer, an infrequent occurrence. Normally a small amount of H 2 is generated (approx. 100 ppm) along with a smaller amount of CH 4 is generated (approx. 7 to 10 ppm or 7% of the amount of hydrogen). For all other faults outside of Corona PD the ratio of hydrogen/methane is 10:1. Sparking Type – is much hotter and more damaging than the Corona type, and is indicated by a number of other gases than H 2 alone, including C 2H2, CH4 and C2H4. It is the more dangerous of the two types of PD.
Hydrogen – Other Causes….Published References; IEC 60599:1999 page 17 4.3 Other sources of gas Gases may be generated in some cases not as a result of faults in the equipment, but through rusting or other chemical reactions involving steel, uncoated surfaces or protective paints. Hydrogen may be produced by the reaction of steel with water, as long as oxygen is available from the oil nearby. Large quantities of hydrogen have been reported in some transformers that have never been energized. Hydrogen may also be formed by reaction of free water with special coatings on metal surfaces, or by catalytic reaction of some types of stainless steel with oil, in particular oil containing dissolved oxygen at elevated temperatures. Hydrogen may also be formed in new stainless steel, absorbed during its manufacturing process, or produced by welding and is released slowly into the oil. Hydrogen may also be formed by the decomposition of the oil film between overheated core laminates at temperatures of 140 deg. C and higher. Gases may also be produced by exposure of oil to sunlight or may be formed during repair of the equipment. Internal transformer paints, such as alkyd resins and modified polyurethanes containing fatty acids in their formation, may also form gases. These occurrences, however, are very unusual, and can be detected by performing DGA analysis on new equipment which has never been energized, and by material compatibility tests. The presence of hydrogen with the total absence of other hydrocarbon gases, for example, may be an indication of such a problem. Statements from “New Guidelines for Interpretation of Dissolved Gas Analysis in OilFilled Transformers”, by A. Mollmann and B. Pahlavanpour, ELECTRA No. 186, 1999 5.3.7 Hydrogen from Chemical Reactions 5.3.7.1 Rusting of steel in a water-phase at the bottom of the tank or in other cavities, as long as oxygen is available, generated Hydrogen. This means that even oxygen can be taken from the oil nearby. Large quantities of Hydrogen, up to 1,000,000 ppm may be produced in the transformer that had never seen voltage.
© 2006 Serveron Corporation. All rights reserved. Printed in USA. PN 810-1716-00. Rev. B
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5.3.7.2 The reaction between zinc chloride and free water on a zinc coated surface generates hydrogen too. Zinc chloride is always present on fire plated steel ware and should be removed by washing. 5.3.7.3 Sometimes aluminum castings are pre-tightened by painting with water-glass. In the presence of moisture water-glass is hydrolyzed and reacting with the aluminum resulting in free hydrogen. 5.3.7.4 Some sorts of stainless steel react at elevated temperature with the hydrocarbon oil, together with dissolved oxygen, to liberate free hydrogen. New Mechanism of moderate Hydrogen Gas Generation in Oil-Filled transformers by T.V. Oommen, R.A. Ronnau, R.S. Girgis, CIGRE 1998, 12-206. Section 3.0 Non-PD Sources for Hydrogen 1. Catalytic decomposition of oil: evolution of hydrogen in the temperature range 100 – 180 Deg. C under the catalytic influence of certain stainless steels has been demonstrated in laboratory tests by Japanese researchers. After some time the metal surface becomes passivated and the evolution of hydrogen ceases. 2. Decomposition of oil in presence of oxygen has been shown to produce hydrogen at relatively low temperatures. Once oxygen is depleted below 2000 ppm, the gas generation stops. 3. Corrosion of iron parts such as core steel in presence of water and air could lead to hydrogen. This may happen when liquid water is collected near outlet valves. Galvanic action due to dissimilar metals (brass valve, iron tank wall) may also play a part. 4. Electrolysis of water has been suggested as a possible source for hydrogen, when free water is present. 5. Mixing of gases from tap changer compartment: this is possible if the transformer and the tap changer compartment use the same expansion vessel, or if the seal separating the two is faulty. 6. Leftover hydrogen in supply oil: occasionally it has been reported that if the oil has not been degassed by vacuum well, some hydrogen may be detected in the oil later. This hydrogen is believed to be the product of oil cracking at the refinery.
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© 2006 Serveron Corporation. All rights reserved. Printed in USA. PN 810-1716-00. Rev. B
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