This International Standard deals with the techniques for sampling free gases from gascollecting
relays from power transformers. Three methods of sampling free gases are
described.
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 no
longer covered by this standard, but are instead 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 and, on the other hand, of the problems imposed by monitoring equipment
in the field, where transport of samples may be by un-pressurized air freight and where
considerable differences in ambient temperature may exist between the plant and the
examining laboratory.