Transformer oil sampling – a dirty and dangerous mission
As in medicine, the sampling process importance and particularities are related to the sampling from transformers or other machines.
Fig. 1a shows an unfortunate consequence of a poor blood sampling. As for the medical case (Fig 1b), technicians sampling transformer oil need to be aware for their personal protection and also for equipment protection. For oil transformer sampling, personal protection is crucial not only for notorious PCB (polychlorinated biphenyl) but for all insulating liquids, like all minerals, and also for esters, where sampling technicians must carefully read the instructions and get organised before proceeding to the sampling process itself.
The primary goal of sampling is to get a representative sample that may be easily and safely obtained, transported and analyzed. Sampling processes need to be optimized for minimalizing their affect on samples and, at the same time to indicate correctly as many parameters of the sampled subject as possible of the sampled subject.
Isokinetic sampling is the extraction of a representative portion of the process stream without altering the physical and chemical properties of the sample. In isokinetic sampling, all phases (solid oxides and precipitates, liquid droplets, and vapour) of the sampled fluid enter the sampling nozzle with the same velocity vector (meaning the same velocity and direction of flow).
Sampling insulating oil from the bottom valve (Fig. 6) or Buchholz relay is not isokinetic sampling because the sampling direction is perpendicular to oil flow inside transformers and because of multi-phase expectancy that may appear at the bottom and top valves. This is a bigger challenge for online sampling, especially in online devices that use one port for oil entrance. In this case, the flow is restricted to a theoretical temperature gradient that may either exist or not. In case of two-port oil flowing DGA online device, other aspects should be considered, such as potential leaking, damage to thin tubing and above all, bubble formation due to a special flowing regime.