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DKE German Commission for Electrical, Electronic & Information Technologies of DIN and VDE

Overview

Sulfur can be present in insulating liquids in various forms, including elemental sulfur, inorganic sulfur compounds and organic sulfur compounds. The number of diverse sulfur species comprised of different isomers and homologous can run into hundreds of compounds. The total sulfur (TS) concentration in insulating liquids depends on the origin of the liquid, refining processes and the degree of refining and formulation including addition of additives to the base oils. Base oils include mineral based paraffinic and naphthenic oils, synthetic iso-paraffins obtained through gas to liquid conversion process (GTL-Fischer-Tropsch), esters, poly alpha olefins, poly alkylene glycols, etcetera. Additives can be comprised of electrostatic discharge depressants, metal deactivators, metal passivators, phenolic and sulfur containing antioxidants such as the polysulfides, disulfides, dibenzyl disulfide (DBDS), etcetera. Certain sulfur compounds present in the insulating liquids exhibit antioxidant and metal deactivating properties without being corrosive. Other sulfur compounds have been known to react with metal surfaces. Specifically, sulfur compounds such as mercaptans are very corrosive to metallic components of electrical devices. Presence of these corrosive sulfur species has been linked to failures of electrical equipment used in generation, transmission and distribution of electrical energy for several decades. Therefore, the IEC standard for mineral insulating oils states that corrosive sulfur compounds shall not be present in unused and used insulating liquids. Recently, the serious detrimental impact of corrosive sulfur has been linked to the presence of a specific highly corrosive sulfur compound (DBDS). This substance has been found in certain mineral insulating oils. The presence of this compound has been shown to result in copper sulfide formation on the surfaces of copper conductors under normal operating conditions. Current standard test methods for detection of corrosive sulfur (ASTM D1275, methods A and B, and DIN 51353) and potentially corrosive sulfur in used and unused insulating oil (IEC 62535) are empirical and qualitative. These methods rely on visual and subjective perception of colour profiles and do not yield quantitative results in regard to the concentration of DBDS or other corrosive sulfur compounds present in insulating liquids. Furthermore, methods for corrosive sulfur and potentially corrosive sulfur in insulating liquids (ASTM D1275, method B and IEC 62535) are applicable only to mineral insulating oils that do not contain a metal passivator additive, the methods otherwise can yield negative results even when corrosive sulfur compounds are present in the insulating liquids - thus providing a false negative test result. On the other hand, the test method when used with aged insulating oils (for example those with relative high acidity), may give ambiguous results and lead to a false positive test result. Further analysis of insulating liquids is stipulated, for example IEC 62535 specifies that if there are any doubts in the interpretation of the results of inspection of paper, the composition of precipitate should be analyzed by other methods (for example by SEM-EDX). For this reason, IEC TC 10 WG 37 was set up to prepare test methods for the unambiguous quantitative determination of corrosive sulfur compounds in unused and used insulating liquids. Because of the complexity of such determinations, the test methods are divided into three parts: Part 1 - Test method for quantitative determination of dibenzyldisulfide (DBDS). Part 2 - Test methods for quantitative determination of total corrosive sulfur (TCS). Part 3 - Test methods for quantitative determination of total mercaptans and disulfides (TMD) and other targeted corrosive sulfur species. This International Standard does not purport to address all the safety problems associated with its use. It is the responsibility of the user of this standard to specify and apply appropriate health and safety practices and determine the applicable legal provisions and limitations prior to its use. The synthetic organic esters which are the subject of this standard shall be handled with due regard to personal hygiene. Direct contact with eyes may cause slight irritation. In the case of eye contact, irrigation with copious quantities of clean running water should be carried out and medical advice sought. Some of the tests described in this standard involve the use of processes that could lead to a hazardous situation. Concerning this matter, explicit reference is made to the corresponding standard or procedure to be applied for guidance. This standard involves the handling of mineral insulating oils, natural esters, insulating liquids, chemicals and used sample containers. The disposal of these substances should be carried out in accordance with applicable national regulations with regard to the environmental compatibility. Every precaution should be taken to prevent the release of chemicals used during the test into the environment. This part of DIN EN 62697 specifies a test method for the quantitative determination of corrosive sulfur compounds-dibenzyl disulfide (DBDS) in used and unused insulating liquids over a 5 mg kg^-1 to 600 mg kg^-1 concentration range. The responsible committee is DKE/K 182 "Flüssigkeiten und Gase für elektrotechnische Anwendung" ("Fluids and gases for electrotechnical applications") of the DKE (German Commission for Electrical, Electronic and Information Technologies) at DIN and VDE.

Document: references other documents

Responsible national committee

DKE/K 182 - Flüssigkeiten und Gase für elektrotechnische Anwendung