Dissolved gas analysis began as a simple “sniff” test: a technician would open the valve and smell the head-space gas, hoping not to detect the sharp odor of acetylene that could signal a problem. Today, there are guidelines and procedures for DGA testing (namely the IEEE C57.104 and IEC 60599 interpretation guides), based on the conventional method of using percentile limits to determine whether or not the transformer has an issue.
Gas concentration and rate of increase limits are based on the idea that an unusually high fault gas concentration or rate of change should be a sign of trouble. To assess severity, additional limits and considerations of rates of increase are used to produce a simple “report card” ranking with either a three- or a four-tier condition code to indicate the transformer’s status. Typically, the 90th, 95th, and 98th percentiles are used as limits to categorize transformers. While this system has been widely used, it does have some challenges. The IEEE guide itself comments that DGA is “more art than science”.
In the past few years, a new, more scientific method of interpreting dissolved gas analysis called Reliability-based DGA has been introduced. Instead of assessing severity in terms of limits exceeded, statistical models of the fault energy index distribution in transformers about to fail are used to estimate prior risk exposure and risk of near-term failure.
These statistical models (one for the hydrocarbon gas fault energy index, NEI-HC; and one for the carbon oxide gas fault energy index, NEI-CO) were derived from a large DGA database with additional information about transformer failures. The information provided by the model of the failure-related values of the hydrocarbon gas fault energy index (NEI-HC) is summarized by the failure rate graph shown in Figure 1. The four vertical dotted lines represent (left to right) the 90th, 95th, 98th, and 99th percentiles of cumulative hydrocarbon gas fault energy index (NEI-HC) in a large DGA database. The peak failure rate occurs at about the 82nd percentile, well below the 90th percentile, suggesting that waiting for something to exceed the 90th percentile before investigating may not be a good idea!
If a transformer with very little fault gas begins to produce hydrocarbon fault gas, it should raise immediate concern since the associated failure risk is increasing very steeply. As NEI-HC increases further, the failure rate decreases, indicating that – contrary to how higher DGA limits are often interpreted – continued gassing does not necessarily imply worsening reliability. This means one of three things: either whatever is causing the gassing is not very harmful to the transformer and may continue indefinitely; or the transformer is gassing because it is damaged or defective, and the next through fault may kill it; or something between the extremes of (a) and (b) is going on.
The new Reliability-based DGA approach was evaluated by a large US utility on 7,200 transformers. It performed so well, identifying many previously undetected serious problems, that the utility immediately adopted it as a key part of its transformer condition assessment system.
Delta-X Research advertisement in the issue: “Would you rather use art or science to assess your transformers’ health?”