Overview

This International Standard describes the techniques for sampling free gases from gas-collecting relays from power transformers. Three methods of sampling free gases are described. The choice between the methods often depends on the available equipment and the quantity of oil required for analysis. The techniques for sampling oil from oil-filled equipment such as power and instrument transformers, reactors, bushings, oil-filled cables and oil-filled tank-type capacitors are described in 4.2 of IEC 60475:2011. Before analysing the gases dissolved in oil, they are first extracted from the oil. Three basic methods are described: one using extraction by vacuum (Toepler and partial degassing), another by displacement of the dissolved gases by bubbling the carrier gas through the oil sample (stripping) and the last one by partition of gases between the oil sample and a small volume of the carrier gas (headspace). The gases are analysed quantitatively after extraction by gas chromatography. A method of analysis is described. Free gases from gas-collecting relays are analysed without preliminary treatment. The preferred method for assuring the performance of the gas extraction and analysis equipment, considered together as a single system, is to degas samples of oil prepared in the laboratory and containing known concentrations of gases ("gas-in-oil standards") and quantitatively analyse the gases extracted. Two methods of preparing gas-in-oil standards are described. For daily calibration checks of the chromatograph, it is convenient to use a standard gas mixture containing a suitable known amount of each of the gas components to be in a similar ratio to the common ratios of the gases extracted from transformer oils. The techniques described take account, on the one hand, of the problems peculiar to analyses associated with acceptance testing in the factory, where gas contents of oil are generally very low. On the other hand, they take account of the problems imposed by monitoring equipment in the field. This includes that transport of samples may be by un-pressurized air freight and considerable differences in ambient temperature may exist between the plant and the examining laboratory.

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Responsible national committee

DKE/K 182 - Flüssigkeiten und Gase für elektrotechnische Anwendung