Demystifying online DGA monitor specification Advertorial Hitachi ABB Power Grids
For detecting desired faults it is essential to combine the right types of the online DGA monitors
Introduction
It has now been firmly established that dissolved gas analysis (DGA) is one of the best methods to detect internal faults within power transformers. DGA can be applied to transformers with mineral oil, ester oil etc. In the latest IEEE C57.104-2019 standard [1] it states that voltage class, MVA, and volume of mineral oil in the transformer do not significantly contribute to the determination of 90th percentile values of dissolved gas levels. This has been a significant change from earlier standards such as IEC 60599 – 2015 [2], where it stated that oil volume did affect DGA 90th percentile values. However, it is to be noted that IEC 60599 – 2015 used around 20,000 DGA samples, whereas IEEE c57.104-2019 used 1,391,436 samples. Thus, the statistical analysis is bound to be different. It is also to be remembered that both standards are written for manual oil sampling and offline DGA analysis in a laboratory. On the other hand, online DGA has gained popularity over the years, and statistical fitting techniques applied for offline DGA can be theoretically applied to online DGA as well, albeit considering different trend periods –1 day or 10 days or 1 month or 6 months or 1 year, etc., depending on the capability of the used online DGA monitor.
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For detecting desired faults it is essential to combine the right types of the online DGA monitors
Nevertheless, online DGA monitoring makes it possible to detect or diagnose, nearly in real-time, any incipient failure occurring in a transformer, thereby giving the operator time to intervene and avoid a major failure. However, the operator must choose the right online DGA monitor to be able to take correct decisions, but there are always some “budget” constraints. Many vendors come around the end-user budget constraints by offering 3/4/5 gas online DGA monitors instead of a 9-gas monitor or equivalent and sell them as online DGA monitors. This article is an attempt to address some of the practical experiences gained by the author while talking about online DGA monitors to end-users. This article discusses the following – choice of DGA monitor, confidence in the DGA monitor, important specifications to consider while choosing a DGA monitors and avoiding false positives while setting gas alarm levels.
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