Dissolved gas analysis (DGA) is widely used to monitor the condition of transformers and other pieces of oil-filled equipment in service. Technicians may draw the wrong diagnoses, however, if the DGA results coming from the lab are not accurate. For instance, an electrical fault could be mistaken as a thermal fault, which may have serious consequences for the equipment. Several CIGR… and IEC round robin tests have shown that the repeatability (precision) of DGA labs is generally good, but that their accuracy (deviation from true value) is often poor.

The only way to reliably improve this accuracy is by running gas-in-oil standards. Procedures for preparing gas-in-oil standards are described in ASTM Method D 3612-01, Annex A2 (2001), and in IEC Publication 60567, Clause 6.2 (1992), based on earlier work (M.Duval et al, Doble Conferences, 10C-01, 1984). For most routine labs, however, preparing gas-in-oil standards is time-consuming and costly, and requires highly skilled personnel dedicated to this preparation.

Gas-in-oil standards for DGA are prepared following the specifications and basic concepts of ASTM and IEC standards, containing certified amounts of dissolved gases. They have been used for CIGR… and IEC round robin tests, and have allowed participating labs to improve their accuracies by detecting their otherwise hard-to-find calibration biases.

Commercial DGA gas-in-oil standards are provided in glass syringes of 30 ml. The shelf life of batches of gas-in-oil standards has been tested thoroughly and no significant changes in gas concentrations were observed over a period of several months.

Accuracy should be checked each time a significant DGA lab condition has changed (new operator, new equipment or a major piece of equipment like a gas extractor) and ideally at regular, frequent intervals. When outsourcing DGA analyses, gas-in-oil standards also can verify that suppliers meet the accuracy specifications. This will ensure more accurate and reliable DGA results and diagnoses. DGA samples are available commercially from Morgan Schaffer Inc. (Montréal, Canada).
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