BS EN 60567:2011 pdf download – Oil-filled electrical equipment — Sampling of gases and analysis of free and dissolved gases — Guidance
This International Standard deals with the techniques for sampling free gases from gas-collecting relays from power transformers. Three methods of sampling free gases aredescribed.
The techniques for sampling oil from oil-filled equipment such as power and instrumenttransformers,reactors,bushings,oil-filled cables and oil-filled tank-type capacitors are nolonger covered by this standard, but are instead described in 4.2 of lEC 60475:2011.
Before analysing the gases dissolved in oil, they are first extracted from the oil.Three basicmethods 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 oilsample (stripping) and the last one by partition of gases between the oil sample and a smallvolume of the carrier gas (headspace).The gases are analysed quantitatively after extractionby gas chromatography; a method of analysis is described. Free gases from gas-collectingrelays are analysed without preliminary treatment.
The preferred method for assuring the performance of the gas extraction and analysisequipment, considered together as a single system, is to degas samples of oil prepared in thelaboratory and containing known concentrations of gases (“gas-in-oil standards”) andquantitatively analyse the gases extracted.Two methods of preparing gas-in-oil standards aredescribed.
For daily calibration checks of the chromatograph,it is convenient to use a standard gasmixture containing a suitable known amount of each of the gas components to be in a similarratio to the common ratios of the gases extracted from transformer oils.
The techniques described take account,on the one hand,of the problems peculiar toanalyses associated with acceptance testing in the factory,where gas contents of oil aregenerally very low and, on the other hand, of the problems imposed by monitoring equipmentin the field,where transport of samples may be by un-pressurized air freight and whereconsiderable differences in ambient temperature may exist between the plant and theexamining laboratory.
2Normative references
The following documents, in whole or in part, are normatively referenced in this document andare indispensable for its application.For dated references, only the edition cited applies. Forundated references, the latest edition of the referenced document (including anyamendments) applies.
IEC 60296,Fluids for electrotechnical applications – Unused mineral insulating oils fortransformers and switchgear
IEC60475:2011,Method of sampling insulating liquids
ISO 5725(all parts),Accuracy (trueness and precision) of measurement methods and resultsASTM D2780,Standard Test Method for Solubility of Fixed Gases in Liquids
3 sampling of gases from gas-collecting (Buchholz) relays3.1 General remarks
lt is important to bear in mind that receiving a qualitative and a representative sample iscrucial for obtaining a reliable diagnosis of the electrical equipment. Even the mostsophisticated extraction or diagnosis methods cannot overcome faulty samples.
Gas samples from relays should be taken from the equipment with the minimum delay aftergas accumulation has been signalled.Changes in composition caused by the selective re-absorption of components may occur if free gases are left in contact with oil.
Certain precautions are necessary when taking gas samples.The connection between thesampling device and the sampling vessel shall avoid the ingress of air. Temporaryconnections should be as short as possible. Any rubber or plastic tubing used should havebeen proved to be impermeable to gases.
Gas samples should be properly labelled (see Clause 4) and analysed without undue delay tominimize hydrogen loss (for example,within a maximum period of one week).
oxygen, if present in the gas, may react with any oil drawn out with the sample.Reaction isdelayed by excluding light from the sample,for example, by wrapping the vessel in aluminiumfoil or suitable opaque material.
Of the three methods described below, the syringe method is recommended.The other twomethods are alternatives to be used exclusively in case of serious hindrance.
Sampling into a sampling tube by liquid displacement using transformer oil as a sealing liquidis simple,but the different solubilities of the gas components may need to be taken intoaccount if the gas quantity is such that some oil remains in the tube.
The vacuum method requires skill to avoid contaminating the sample by leakage of air into thesystem. It is particularly true where the gas to be sampled may be at less than atmosphericpressure (for example,some sealed transformers).