Timing and modality to obtain representative and beneficial DGA sample for liquid-filled device
For Transformers Magazine readers, the importance of DGA to transformer health and transformer owner’s reputation does not have to be described or promoted. But most of the readers are attracted by new algorithms, software and methods for improving the reliability of the DGA diagnosis. There is an abundance of diagnostic methods within new and well-established companies that intend to gain more credibility for the DGA methodology by new investments and offering sophisticated diagnostics. Much more significant and clear advancement of DGA may be achieved by paying more attention to the sampling stage. This can be attained with the right decision on when to sample, from which valve and by whom. A transformer maintenance engineer needs to hire and employ only competent, experienced and well-trained staff. The same criteria as for a DGA diagnosis expert needs to be observed when hiring the sampling personnel. The DGA value is the same as the sampling quality. Thus, no sophisticated measurements or diagnosis may overcome a poor sampling procedure.
Firstly, it is necessary to stabilize the DGA sampling timing and frequency. For some transformer types, it is important to establish a maximum of 5 years between two samplings, and for others, a frequency for a routine DGA of three months will be the maximum frequency. The frequency test is also relevant for an online device with the real implication on a diagnostic assessment.
Is the trend magnitude obtained by an online and offline DGA device comparable? The quick answer is yes, but the correct one is, of course, no, and the reasons for this will be described below.
After deciding on the best time for sampling as a compromise between the transformer loading regime, transformer owners, sampling team availability, and external parameters such as weather, holidays, budget etc., are needed to determine the laboratory identity. The laboratory must be selected according to its capabilities, performance, methods and standards, and above all, the prices. It is a well-known fact that in our epoch, the final decision is made by finance departments. Here, the discussion is focused on is it better to be faithful to our previous testing lab or is it possible to switch to a suitable one and / or the one with the best capabilities.